Native Gastropods and Introduced Crabs: Shell Morphology And

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Winter 2006 Native gastropods and introduced crabs: Shell morphology and resistance to predation in the New England rocky intertidal zone Sarah Joanne Teck University of New Hampshire, Durham

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Recommended Citation Teck, Sarah Joanne, "Native gastropods and introduced crabs: Shell morphology and resistance to predation in the New England rocky intertidal zone" (2006). Master's Theses and Capstones. 249. https://scholars.unh.edu/thesis/249

This Thesis is brought to you for free and open access by the Student Scholarship at University of New Hampshire Scholars' Repository. It has been accepted for inclusion in Master's Theses and Capstones by an authorized administrator of University of New Hampshire Scholars' Repository. For more information, please contact [email protected]. NATIVE GASTROPODS AND INTRODUCED CRABS: SHELL MORPHOLOGY AND RESISTANCE TO PREDATION IN THE NEW ENGLAND ROCKY INTERTIDAL ZONE

SARAH JOANNE TECK B.A., Middlebury College, 2001

THESIS

Submitted to the University of New Hampshire in Partial Fulfillment of the Requirements for the Degree of

Master of Science in Zoology

December, 2006

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Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. This thesis has been examined and approved.

Thesis Director^r. Larry G. Harris, Professor of Zoology

Dr^anies E. Byers, Associate Professor of Zoology

Dr.^fliornas D. Lee,"Associate Professor of Forest Ecology

Dr. J^mes T. Carlton, Professor of Marine Sciences at The Maritime Studies Program of Williams College and Mystic Seaport

fa fif C Date

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. FUNDING

This work was supported by grants from the Center for Marine Biology, the

Department of Zoology, and the Graduate School of the University of New Hampshire.

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. ACKNOWLEDGEMENTS

Thank you, Dr. Larry Harris for all of your continued support, guidance, and

generosity over the past few years. I will always remember your encouragement to keep

exploring, observing, and writing everything down. These are valuable lessons that will

stay with me in all of my future studies. Thank you, Dr. Tom Lee for your inspiration in

community ecology class and also outside of class. Your enthusiasm for both the history

and present applications of ecology is contagious. Thank you, Dr. Jeb Byers for your

constructive comments and insights on this work, especially with an eye for the analyses.

Working with you has really taught me the value of taking broad-scale ecological

concepts and applying them to my research in a meaningful way. Thank you, Dr. Jim

Carlton; you inspired me to shift my path in 2000 and begin my exploration of science as

a career. I have learned that taking a holistic approach to research and teaching will yield

more meaningful results. I truly appreciate your unceasing support, kindness, and

generosity.

Thank you, graduate students from the Harris and Byers labs: Irit Altman, April

Blakeslee, Jenn Dijkstra, Aaren Freeman, Blaine Griffen, Wan Jean Lee, John Meyer,

Laura Page, and Erica Westerman for all of your detailed feedback throughout the

multiple stages of my research. Thank you to the following undergraduate assistants who

volunteered in the lab and in the field, creating their own projects and assisting with

aspects of my research: Jillian Armstrong, Scott Conley, Stephanie Doyle, Brian

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Donohue, Jilli Dur, Chris Hunt, Kathryn Lanza, Emily Strauss, Jessica Ware, Carolyn

Gordon, and Anthony Dutton.

Thanks go out to faculty, staff, and students from the Maritime Studies Program of

Williams College-Mystic Seaport in Mystic, CT. For assistance in the field, thank you

Rachel Fineman for your positive attitude and bright spirit even before the sun rises.

Thank you for assistance in providing housing, supplies, and a welcome home away from

home: Michelle Armsby, Nicole Dobrowski, Deniz Haydar, Mary O'Loughlin, and

Amanda Severin.

Thank you, Drs. Becky Warner, Jana Davis, Lara Gengarelly, Tim Miller, Pablo

Munguia, Michelle Scott, and Catherine deRivera for assistance on various matters

including statistical analyses and presentation of my thesis-work. Thank you, Dr. Andy

Rosenberg for motivating me to contemplate broad ecological questions that have

conservation implications, which I will carry with me to my future career. Thank you, Dr.

Steve Hale for introducing me to the art of teaching the ecology lab during my first year

at UNH.

Thank you, fellow researchers and staff at the Coastal Marine Lab at the

University of New Hampshire in Newcastle, NH including Noel Carlson and Nate

Rennels for your technical assistance. Thank you to Nancy Wallingford and Diane

Lavalliere for your assistance with many essential details pertaining to being a graduate

student in the Department of Zoology, and thanks to Dr. Jim Haney for providing

additional assistance especially with regard to my thesis defense.

Thank you to my friends who have not only spent countless hours talking about

science but also have been a network of support for me during the times when I needed it

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. most: Irit Altman, Michelle Chalfoun, Amy Cline, Ben Galuardi, Deniz Haydar, Tami

Huber, Kate Magness, Chris Rillahan, Lynn Rutter, Amy Wolf, and Erika Zollett. Thank

you to my family Patricia Teck, Julio Lorda Solorzano, Rose Marie and Eijk Van

Otterloo, and Myma Teck and Loren Amdursky for all of your love and kindness.

I am grateful to my parents Bruce and Susan Teck; your spirits will always live inside

me encouraging me to persevere and follow my dreams.

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. TABLE OF CONTENTS

FUNDING...... iii

ACKNOWLEDGEMENTS...... iv

LIST OF TABLES...... xi

LIST OF FIGURES...... xiii

ABSTRACT...... xv

CHAPTER PAGE

GENERAL INTRODUCTION...... 1

Predator-Prey Interactions ...... 1

Unsuccessful Predation ...... 3

Variability in Shell Form...... 4

Littorina saxatilis Variability in the Northeast Atlantic...... 6

Predators of Littorina saxatilis in the Northwest Atlantic...... 8

Littorina saxatilis Variability in the Northwest Atlantic...... 10

I. VARIATION IN LITTORINA SAXATILIS MORPHOLOGY ACROSS NEW ENGLAND: SHELL THICKNESS AND EVIDENCE OF CRAB PREDATION ATTEMPTS

Abstract...... 13

Introduction ...... 14

vii

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. History of Non-native Crabs in New England ...... 17

Objectives ...... 19

Methods...... 20

Field Sites ...... 20

Sample Sizes and Height Ranges for Examining Thickness of Unscarred Snails ...... 21

Snail Shell Thickness and Scarring Frequency ...... 22

Crab Density and Biomass ...... 23

Shell Thickness of Unscarred versus Scarred Snails ...... 24

Analyses ...... 24

Results...... 25

Shell Thickness and Height Differences across Regions ...... 25

Crab Density and Biomass across Regions ...... 26

Shell Thickness and Scarring Frequency ...... 27

Shell Thickness of Unscarred versus Scarred Snails ...... 28

Discussion ...... 28

II. VARIATION IN LITTORINA SAXATILIS MORPHOLOGY ACROSS THE GULF OF MAINE: SNAIL VULNERABILITY TO CRAB PREDATION

Abstract...... 49

Introduction ...... 49

Objectives ...... 51

Methods ...... 51

Snail Morphology ...... 51

viii

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Snail Vulnerability to Predation ...... 53

Results...... 54

Snail Morphology ...... 54

Snail Vulnerability to Predation ...... 55

Discussion ...... 55

III. DOES SUB-LETHAL INJURY PROVIDE PREY WITH A REDUCED RISK OF LETHAL PREDATION?

Abstract...... 67

Introduction ...... 67

Objectives ...... 69

Methods ...... 69

Results...... 71

Discussion ...... 71

SUMMARY...... 76

Future Directions ...... 79

LITERATURE CITED...... 81

APPENDICES...... 91

APPENDIX A: Field Site Details ...... 92

APPENDIX B: Shell Thickness Comparisons between Field Sites within each Region ...... 96

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. APPENDIX C: Additional Results Comparing Crab Predation of Snails Differing in Thickness...... 100

APPENDIX D: Comparison of Odiomes Point versus Wilbur Neck Snail Movement ...... 101

APPENDIX E: Comparing Claw Sizes between Hemigrapsus sanguineus and Carcinus maenas...... 102

APPENDIX F: Handling Time Details for Trials with Carcinus maenas and Hemigrapsus sanguineus Preying upon Scarred and Unscarred Snails ...... 103

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. LIST OF TABLES

TABLE PAGE

Table 1.1 Number of snails measured per size class per region ...... 34

Table 1.2a Summary of each site: unscarred snails 5-13 mm in height: shell thickness, shell height, and number of snails measured ...... 34

Table 1.2b Summary of each region: unscarred snails 5-13 mm in height: mean shell thickness, mean shell height, and number of sites in each region ...... 35

Table 1.3a Summary of each site: mean crab density, % C. maenas, % H. sanguineus, and number of quadrats ...... 35

Table 1.3b Summary of each region: mean crab density, % C. maenas, % H. sanguineus, and number of quadrats ...... 35

Table 1.4a Summary of each site: mean total crab biomass, mean biomass of C. maenas, mean biomass of H. sanguineus, and number of quadrats ...... 36

Table 1.4b Summary of each region: mean total crab biomass, mean biomass of C. maenas, mean biomass of H. sanguineus, and number of quadrats ...... 36

Table 1.5a Summary of each site: number of snails collected, number of scarred snails collected, and mean scarring frequency, number of samples collected per site, and percent of total snails collected blemished per site ...... 37

Table 1.5b Summary of each region: number of unscarred snails collected, number of scarred snails collected, mean scarring frequency, number of sites in each region, and average percent of total snails collected blemished per region ...... 37

Table 1.6 Mean scarring frequencies at the five most heavily scarred sites within the three southern regions ...... 38

Table 1.7 Number of snails measured per size class per region for scarred and unscarred snails ...... 38

Table 2.1a ANCOVAs for snail dry tissue and shell weight with shell height as the covariate ...... 60

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Table 2.1b Differences inZ. saxatilis dry tissue and shell weight between the sites 60

Table 2.2a ANCOVAs for shell width, thickness and spire height with shell height as the covariate ...... 61

Table 2.2b Differences in L. saxatilis shell width, thickness and spire height between the sites...... 61

Table 3.1 Scarred versus unscarred snails’ handling times for C. maenas and H. sanguineus...... 75

Table A.1 Site names and codes ...... 92

Table A.2 Individual sites’ coordinates ...... 93

Table A.3 Individual sites’ mean shell height and thickness across all sampled snails...... 93

Table A.4 Individual sites differences in mean crab density ...... 94

Table F .la Outcomes of 124 handling time trials for C. maenas and H. sanguineus preying upon scarred and unscarred snails matched for height ...... 103

Table F.lb Choice results for trials with C. maenas and H. sanguineus during which both unscarred or scarred snails were handled ...... 104

Table F.2 Relationships between handling time and crab to snail size ratio for unscarred and scarred snails handled by both C. maenas and H. sanguineus...... 104

xii

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. LIST OF FIGURES

FIGURE PAGE

Figure 1.1 Map of four regions where sampling occurred ...... 39

Figure 1.2 Scars as evidence of previous crab predation attempts on L. saxatilis shells...... 40

Figure 1.3a Aperture thickness against shell height for the four regions ...... 41

Figure 1.3b Aperture thickness against snails of particular size classes of shell height for the four regions ...... 42

Figure 1.4a Mean snail shell thickness at the aperture against latitude ...... 42

Figure 1.4b Mean snail shell thickness at the aperture against temperature ...... 43

Figure 1.5 Mean shell thickness at the aperture against mean height...... 43

Figure 1.6 Mean scarring frequency against mean crab density ...... 44

Figure 1.7 Mean shell thickness at the aperture against mean crab density...... 44

Figure 1.8 Mean crab density across sites within the four regions ...... 45

Figure 1.9 Mean crab biomass across sites within the four regions ...... 45

Figure 1.10 Mean scarring frequency against mean shell thickness at the aperture 46

Figure 1.11 Mean scarring frequency against latitude ...... 46

Figure 1.12 Aperture thickness for unscarred and scarred snails ...... 47

Figure 1.13 Aperture thickness of snails with scars close (0.5-1.5 mm) to the aperture edge, with scars far (1.5-3.5 mm) from the aperture edge, and without scars are compared ...... 48

Figure 2.1 Map of the field sites: Wilbur Neck in northeastern Maine (WN) and Odiomes Point, New Hampshire (OP) ...... 62

xiii

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Figure 2.2 Snail dimension measurements diagram ...... 63

Figure 2.3 Dry tissue weight against shell height...... 64

Figure 2.4 Dry shell weight against shell height...... 64

Figure 2.5 Shell width against shell height...... 65

Figure 2.6 Shell thickness at the aperture against shell height...... 65

Figure 2.7 Spire height against total shell height...... 66

Figure 2.8 Photo of snails matched for length (6.85 mm ±0.05) from Odiomes Point, New Hampshire (OP) and Wilbur Neck, Maine (WN) ...... 66

Figure 3.1 Carcinus maenas and Hemigrapsus sanguineus handling times for scarred and unscarred L. saxatilis...... 75

Figure A.1 Average monthly temperatures for four sites closest to the four regions examined ...... 95

Figure B.l Comparison of sites within the Southernmost region (shell thickness at the aperture against shell height)...... 96

Figure B.2 Comparison of sites within the Southern region (shell thickness at the aperture against shell height)...... 97

Figure B.3 Comparison of sites within the Northern region (shell thickness at the aperture against shell height)...... 98

Figure B.4 Comparison of sites within the Northernmost region (shell thickness at the aperture against shell height)...... 99

Figure D.l Comparison of snail movement from Northeastern Maine and New Hampshire...... 101

Figure E.l Diagram of claw measured to calculate claw area ...... 102

Figure E.2 Comparing claw sizes between Hemigrapsus sanguineus and Carcinus maenas...... 102

Figure F.l Carcinus maenas and Hemigrapsus sanguineus snail handling time against crab to snail size ratio ...... 105

Figure F.2 Snail heights of consumed snails against the carapace widths of the Carcinus maenas and Hemigrapsus sanguineus that ate the snails...... 106

xiv

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. ABSTRACT

NATIVE GASTROPODS AND INTRODUCED CRABS: SHELL MORPHOLOGY AND RESISTANCE TO PREDATION IN THE NEW ENGLAND ROCKY INTERTIDAL ZONE

Sarah J. Teck University of New Hampshire, December, 2006

The impact of non-native species is one of the most critical issues facing management

and conservation today. When these invaders are generalist predators, their impacts on

native communities can be a major restructuring force for ecosystems. A new voracious

predator the Asian crab, Hemigrapsus sanguineus, joins another non-native established

species, Carcinus maenas, along the majority of the New England coastline. What

remains poorly understood is how the two introduced predators may modify local

communities, especially considering their impact on native prey, such as the rough

periwinkle snail, Littorina saxatilis. The goal of this research is to investigate the

vulnerability of these snails to shell-breaking predators by examining their clinal

variation in shell morphology and crab-induced scarring history. Focused studies on the

variable morphology of this native gastropod coupled with predation studies provide a

greater understanding of the ecological and evolutionary consequences of the arrival of

novel predators to an ecosystem.

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. GENERAL INTRODUCTION

Introduced species are the second greatest cause of human induced ecological change

(second only to alteration and destruction of habitat) (Park 2004). When these introduced

species are omnivorous predators, understanding their impact on native organisms

becomes fundamental to understanding how introduced species restructure ecosystems

(Elton 1958, Vitousek et al. 1996). Around the world, introduced crabs have impacted

recipient environments by altering community structure and decimating native prey

populations (Glude 1955, Dare et al. 1983, Carlton and Geller 1993, Lodge 1993,

Grosholz and Ruiz 1996, Ruiz et al. 1998, Lohrer and Whitlatch 2002a). It is critical to

consider the role of individual species within a predator-prey network and especially how

an exotic species influences native prey (Lodge 1993). Understanding the impact of

specific introduced predatory species that may cause subtle shifts in trophic interactions

is crucial as communities become increasingly rich with non-native species.

Predator-Prev Interactions

Predators must contend with a prey item’s defenses in order to successfully and

efficiently consume a prey. Prey develop these defenses through optimal physical or

behavioral adaptive mechanisms to escape predation encounters (Vermeij 1987); prey

avoid predators to decrease the likelihood of encountering them and/or develop armor to

reduce predator efficiency and success in handling them (Seitz et al. 2001). Adaptation

occurs through selection for constitutive defenses (which are always present) (Vermeij

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. and Currey 1980, Vermeij 1982b, Seeley 1986), inducible defenses (which are activated

with a stimulus) (Appleton and Palmer 1988, Etter 1988, Harvell 1990, Trussell 1996,

Leonard et al. 1999, Dalziel and Boulding 2005) or a combination of the two

(Johannesson and Johannesson 1996). Prey items must not only contend with the risk of

predation but also other biotic factors, such as competition, and abiotic stresses, such as

wave exposure, in their environment. Species must shift in morphology and behavior as a

result of a spectrum of stresses each individual faces in its lifetime or in one or several

generations of a population. Since predation success is dependent on morphological and

behavioral traits of prey, investigation into these traits will be indicative of an organism’s

potential vulnerability to predation.

Predators seek to consume the most energy while expending as little energy as

possible in pursuing, handling, and consuming prey, as optimal foraging theory states

(MacArthur and Pianka 1966, Bowen et al. 2002). However, prey specifically develop

characteristics to interfere with the process of predators finding, subduing/handling, and

consuming prey (Hughes and Elner 1979) through avoidance, armor, and chemical

deterrents, respectively, and any reduced predator efficiency is associated with an

increased risk of unsuccessful predation (Nilsson and Bronmark 2000, Greenfield et al.

2002). Prey that are large (Floyd and Williams 2004), exhibit plastic phenotypic traits, or

characteristics that otherwise protect the prey may be selected for due to the low

efficiency in predator handling time (Ebling et al. 1964, Vermeij 1982d, Palmer 1985,

Seeley 1986, Appleton and Palmer 1988, Schindler et al. 1994, Kolar and Wahl 1998) or

high risk of predator injury (i.e. claw damage to crabs) (Smallegange and Van der Meer

2003), competition, and predation on the predator itself (Brante and Hughes 2001). Here I

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. will examine morphological features of shelled prey that have developed under the

pressure of shell-breaking predators.

Unsuccessful Predation

Unsuccessful predation can be recorded historically in the form of shell scars on

many gastropod species (Vermeij 1982d) and is an indicator of the strength of selection

present in the system (Vermeij 1982b). Failed predation attempts suggest that prey is

evolving to reduce shell-breaking predator success. The rate of phenotypic evolution

increases with decreasing latitude (Vermeij 1982d), and is evidenced by the increased

frequency of repairs in lower latitudes (Vermeij et al. 1980). Shells tend to be thicker in

lower latitudes in general due to an increased ability to accrete shell in warmer waters

(Vermeij and Currey 1980) and to predator chemical cues inducing phenotypic defenses

(Trussell and Etter 2001). Thus, scarring also increases both with increased temperatures

and predator abundance and diversity (Vermeij 1978).

These non-lethal encounters resulting in chipped shells heal to form scars, which tend

to thicken the shell decreasing the likelihood for fixture successful predation (Greenfield

et al. 2002). Although there is apparently no difference in the overall strength of the

shells (Blundon and Vermeij 1983, Greenfield et al. 2002), scarred shells are thicker than

unscarred snails at the aperture. Snails do not thicken their entire shell when they are

healing from a previous crab attack because the energy required to repair a shell is likely

associated with a reduction in energy allocated for growth and reproduction since

accreting new shell is energetically expensive (Geller 1990b, Greenfield et al. 2002).

However, energy allocation for shell repair is not consistent across species. In

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. experimental trials, the freshwater snail,Helisoma trivolvis, increased growth when

damaged with no increase in fecundity and mortality (Stahl and Lodge 1990). Shell-

damaged Nucella emarginata in California also had increased shell growth, but they had

increased mortality and egg production when compared to uninjured snails (Geller

1990b).

Variability in Shell Form

Shell forming mollusks often have been examined because they reflect ecological and

evolutionary changes over time and space (Vermeij 1987). Species that have limited

genetic mixing based on their reproductive cycle and low rate of widespread adult

dispersal often have highly variable phenotypes among separate populations as the result

of local adaptation. The poorly dispersing snail, Nucella lapillus, displayed rapid

evolution by increasing shell thickness in response to the arrival of a new predator,

Carcinus maenas, in comparison to the widespread-spawning snail, Littorina littorea,

which did not change in shell thickness (Vermeij 1982c). Nucella lapillus also has

displayed clear differences in shell thickness across a landscape; individuals found on

sheltered shores have thicker shells than those on more exposed shores in North Wales

(Ebling et al. 1964, Hughes and Elner 1979) and in Spain (Rolan et al. 2004). Individuals

at protected sites have more time to feed and thus can spend more energy thickening their

shells in order to resist the predators that are more abundant here than at exposed sites.

Additionally, N. lapillus have a larger aperture and pedal surface area to resist high wave

exposure in such sites as opposed to those on more protected shores in North Wales

(Ebling et al. 1964) and in New England (Etter 1988). A smaller aperture for snails in

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. sheltered areas is also more beneficial to reduce the success of predators peeling the shell,

and such predators are more abundant in sheltered areas. The closely related Nucella

lamellosa on the west coast of Canada has been shown to develop larger teeth along the

aperture when exposed to chemical cues of the predatory crab Cancer productus

(Appleton and Palmer 1988). Chemical cues from the predator Hemigrapsus nudus (also

on the west coast of Canada) induces Littorina subrotundata to produce a more massive

shell for greater protection (Dalziel and Boulding 2005). Another predatory crab,

Carcinus maenas, has influenced shell shape inLittorina obtusata in New England;

before 1900 snails were thinner with higher spires than those collected between 1982 and

1984 when the non-native crab was more abundant (Seeley 1986). However, Trussell

(1996) attributes this morphological difference to a combination of rapid morphological

evolution explained by Seeley (1986) and phenotypic plasticity induced by chemical cues

from C. maenas. Additionally, Trussell and Etter (2001) discovered that L. obtusata from

the northern Gulf of Maine displayed a greater capacity for phenotypic plasticity than

conspecifics in the southern Gulf of Maine when transplanted into warmer water,

suggesting that these two populations are genetically disparate (northern snails increased

in shell thickness by 43% in southern waters versus local northern snails, while southern

snails in northern waters had thinner shells by 18% versus local southern snails). There

might be a greater selection for rapid growth and efficient shell accretion in the north,

where water temperatures are much colder. Across littorinid species in the Northeastern

Pacific, there is also selection for thicker shells, and the thinner-shelled species, Littorina

subrotundata, suffers from significantly greater predation by crabs than the thicker-

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. shelled species, L. sitkana and L. scutulata (Boulding and Van Alstyne 1993, Boulding et

al. 1999).

Littorina saxatilis Variability in the Northeast Atlantic

A species which exhibits a high level of variability in morphology is the rough

periwinkle, Littorina saxatilis, which broods its young. Littorina saxatilis occurs from

New Jersey to Hudson Bay, Baffin Island, the MacKenzie Delta, Greenland and the

Barents Sea and in Europe from Gibraltar to Novaya Zemlya (a Russian island in the

Arctic Ocean). This wide range may have resulted from the passive transport of L.

saxatilis on drifting seaweed over evolutionary time (Carlton, J.T., personal

communication, Johannesson 1988), since this species typically has low dispersal of their

crawl-away young.

Littorina saxatilis in Europe are well recognized as displaying high variability in shell

shape across tidal ranges, habitats, and latitudes due to differing levels of predation

pressure, wave exposure, food availability and temperature (which in turn affects the rate

of shell accretion) (Grahame et al. 1990, Clarke et al. 1999, Leonard et al. 1999).

Individuals of L. saxatilis have been shown to differ genetically from conspecifics only

several meters away (Janson and Sundberg 1983, Johannesson 2003, Grahame et al.

2006). Foot size in L. saxatilis enlarges with increasing exposure for snails found in the

United Kingdom (Grahame and Mill 1986), and this also occurs as a plastic phenotypic

response in congener L. obtusata (Trussell 1997) and Nucella lapillus in New England

(Etter 1988). Also in the U.K., two ecotypes, found in the high (H) and mid (M) intertidal

zones (Reid 1996, Wilding et al. 2002), have been shown to exhibit assortative mating

(Pickles and Grahame 1999) and genetic differentiation (Wilding et al. 1998, Grahame et

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. al. 2006). Littorina saxatilis ‘H’ (once described as L. patula (Wilding et al. 2002)) has a

thin shell and a large aperture and is found in the high shore among cliffs and boulders,

and L. saxatilis ‘M’ (once described as L. rudis) has a thick shell and a small aperture and

is found in the mid-shore among small boulders (Wilding et al. 1998, Hull et al. 1999).

Similar patterns in shell shape and strength are found in N. lapillus in Ireland (Ebling et

al. 1964). In addition to small scale variation in shell shape, L. saxatilis exhibits clinal

variation across the U.K.; for example, snails increase in aperture size from Northern

England to the South and then to the East from Devon and increase in jugosity (ratio of

aperture length to columella length) from Cornwall to the North to the Isle of Man and to

the East to Kent (Mill and Grahame 1995).

In Sweden two additional ecotypes are described; small snails with thin shells and

large apertures are found in exposed cliff sites (the E morph), and larger snails with

thicker shells and smaller apertures are found in sheltered boulder areas (the S morph)

with intermediate forms found in between (the I morph) (Janson and Sundberg 1983,

Johannesson 1986, Johannesson and Johannesson 1996, Hollander 2001). Differences in

shell form and growth rates between these ecotypes are likely influenced both by genetic

and environmental variation (Janson 1982). Snails found on sheltered shores grow faster

than those from exposed areas (Janson 1982) likely due to predation pressure by crabs

(Raffaelli and Hughes 1978, Elner and Raffaelli 1980) and the increased feeding time,

and females often grow at a faster rate than males in several other congeners

(Johannesson and Johannesson 1996). Littorina obtusata in New England also differ with

varying exposure regimes; snails found in exposed sites with fewer predators have

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. thinner shells (Trussell 1996) and larger feet (Trussell 1997) than snails found in

protected areas.

In Galicia, Spain, two morphs of L. saxatilis are found in different intertidal habitats,

the RB-morph is ridged and banded and found in the upper tidal heights among barnacles

and the SU-morph is smooth and unbanded and found in the lower tidal heights among

mussels (Kostylev et al. 1997, Carballo et al. 2001). The RB morph is also comparable to

the sheltered morph in Sweden, as its shell is globular and robust with a smaller aperture

than the SU morph. These traits offer protection against crabs which are also more

common in the upper portion of the intertidal with the RB morph (Cruz et al. 2004,

Carvajal-Rodriguez et al. 2005). The genetic and plastic morphological variation in this

species has been closely examined in its European range, but there are limited studies of

this species in its Northwest Atlantic range (Bertness 1999).

Predators of Littorina saxatilis in the Northwest Atlantic

A likely selective force in the Northwest Atlantic on L. saxatilis is the introduced

European green crab, Carcinus maenas, which has overlapped withL. saxatilis, since its

arrival on the New England shores in the early 1800s; however, these two species have a

long evolutionary history in Europe. Carcinus maenas was introduced to New England

prior to 1817, and by the mid-1900s its abundance increased along the shores of northern

New England and Canada (Glude 1955). A combination of warming trends (Scattergood

1952, Welch 1968, Welch and Churchill 1983, Audet et al. 2003) and pulses of new

invasion events (Roman 2006) explain the range expansion of C. maenas in New England

and Eastern Canada. In high numbers C. maenas can have significant impacts upon prey

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. populations, particularly mo Husks (Glude 1955, Dare et al. 1983, Lohrer and Whitlatch

2002b, Floyd and Williams 2004). Carcinus maenas also resides among juvenile lobsters

and has the potential to compete with and prey upon this commercially valuable species

(Rossong et al. 2006, Williams et al. 2006).

The more recent introduction of Hemigrapsus sanguineus to the Atlantic coast may

have an even greater inpact upon invertebrate prey populations than C. maenas (Tyrrell

and Harris 1999, Lohrer and Whitlatch 2002b, Brousseau and Baglivo 2005). However,

interspecific crab predation and competition may influence the impact on their shared

prey items. Some prey items may actually have a reduced risk of predation when these

two predators interact (Griffen and Byers 2006a), and results likely are influenced by

habitat type (i.e. rock versus macroalgae) (Griffen and Byers 2006b).

About five years after the discovery of H. sanguineus in North America, it arrived in

southern New England, where populations dramatically increased, supplanting the long-

established European green crab invader, Carcinus maenas (McDermott 1998, Ahl and

Moss 1999, Tyrrell and Harris 1999, Lohrer and Whitlatch 2002a). Hemigrapsus

sanguineus was first found in New Jersey in 1988 (Williams and McDermott 1990), and

since its discovery, it has been spreading north, extending its range into Maine

(McDermott 1998, Tyrrell and Harris 1999). Hemigrapsus sanguineus now surpasses C.

maenas in density in many intertidal locations (McDermott 1998, Ahl and Moss 1999,

Lohrer and Whitlatch 2002a). The multiple genetic lineages of C. maenas in New

England have allowed this species to pervade in locations once thought to be too cold for

C. maenas (Roman 2006), so H. sanguineus is likely to expand its range further north and

increase in dominance, especially if new invasion events occur and as waters warm.

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. However, Byers and Pringle (2006) predict that populations of H. sanguineus in mid-

Maine and further north will remain ephemeral unless water temperatures do in fact

increase.

In the Western Atlantic, there may be different predator-prey dynamics on the

northern (eastern) versus southern (western) sides of Cape Cod due to temperature and

predation history. Temperature differences influence growth, feeding, and healing rates

of snails (Vermeij and Currey 1980), and increased temperatures south of Cape Cod

increase crab residency in the intertidal resulting in greater exposure of prey to predators.

Historically, Cape Cod may have been a barrier to the rapid spread of C. maenas—south

of Cape Cod C. maenas has been present for -200 years, whereas north of Cape Cod it

has been present for -100 years (since 1905) (Roman 2006). In contrast, H. sanguineus

very quickly spread from south to north and within two decades reached areas well north

of Cape Cod (perhaps the opening of the Cape Cod Canal in 1914 increased the speed of

range expansion of this later invasion).

Littorina saxatilis Variability in the Northwest Atlantic

Snail characteristics vary with latitude due to differing crab abundance, wave

exposure, and temperature regimes (Boulding and Van Alstyne 1993, Boulding et al.

1999). Additionally, Littorina obtusata have been shown to exhibit phenotypic plasticity

in shell shape likely due to a combination of differences in crab abundance and water

temperature. Northern New England snails are much thinner and weaker than southern

New England snails (Trussell 2000), which is a trend across many species of shell

bearing mollusks (Vermeij 1978, Vermeij and Currey 1980, Irie and Iwasa 2003).

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Carcinus maenas also have smaller crusher claws in the northern versus the southern

Gulf of Maine, likely due to a reduced need for large claws to crush the generally thinner

snails (Smith 2004). In the short time period H. sanguineus has been present in southern

New England, the mussel Mytilus edulis showed rapid evolution of inducible defenses

when exposed to chemical cues from this new predator, while mussels further north did

not (where H. sanguineus is not established yet); both populations responded to the older

invader, C. maenas (Freeman and Byers 2006). Thus, other prey are likely to exhibit

similar species-specific responses to predators, and these responses can rapidly evolve

based on the intensity and composition of predators present in the system.

Both prey and predator characteristics have varied in the past and likely will vary in

the future with different predator species regimes, ranging from the 1800-1995 single

invasive crab species regime (C. maenas), to today with two non-native crab species

present (C. maenas and H. sanguineus). WhetherH. sanguineus causes C. maenas

populations to increase, decrease, or remain unchanged, there is the potential in all

scenarios for total crab abundances in the intertidal areas to increase. While this new

invader may not in fact cause total crab abundance in the intertidal rocky shores to

increase, it may redistribute crab abundances in areas that impact a different tidal height

more heavily and thus a different assemblage of species. Researchers have a poor

understanding of how the presence of these two invasive species together will affect

native coastal species and how they may each have a unique influence on their prey.

Recent studies have focused on the habitat and prey selection of these invasive crabs

(Tyrrell 1999, Lohrer et al. 2000a, Lohrer et al. 2000b, Brousseau et al. 2001, Ledesma

and O'Connor 2001, Tyrrell and Harris 1999, Tyrrell 2002, Bourdeau and O'Connor

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 2003), but there is a paucity of research that focuses specifically on the morphology of

native prey species in New England and their specific vulnerability to these predators

across their range (but see Vermeij 1982c, Seeley 1986, Trussell 1996, Trussell and

Smith 2000, Freeman and Byers 2006). No current research specifically addresses the

consequences of both crab invaders, H. sanguineus and C. maenas, on herbivores, such as

L. saxatilis, which have an vital role in regulating algal populations in intertidal

communities. Additionally, L. saxatilis has not been studied extensively in its North

American range (Bertness 1999), so the following studies will provide essential baseline

information on the morphology and vulnerability of this important and abundant native

intertidal species.

In Chapter I, I examined the clinal variation of Littorina saxatilis across its New

England range. I hypothesized that snails found in the northern portion of New England

exhibit thinner shells than those found further south. Unscarred snails are also thinner

than crab-scarred snails as expected, and there is a greater frequency of scarred snails

when their shells are thicker. I also examined the frequency of crab-scarred snails and

crab abundance across New England. As was documented in Chapter II, in general

thinner-shelled snails in the north will likely be more vulnerable to predation by crabs

than thicker shelled conspecifics in the south. In Chapter III, I tested my prediction that

scarred snails have a decreased risk of future predation due to their likely thicker shell

where the previous chip has healed.

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. CHAPTER I

VARIATION IN LITTORINA SAXATILIS MORPHOLOGY ACROSS NEW ENGLAND: SHELL THICKNESS AND EVIDENCE OF CRAB PREDATION ATTEMPTS

Abstract

In the Northwestern Atlantic, the morphology of the rough periwinkle, Littorina

saxatilis, has not been studied in such detail as its European populations. Clinal variation

in shell thickness exists across its New England range; snails found in the northern

portion of New England exhibit thinner shells than those found further south. I also

examined the frequency of crab-scarred snails and crab density in New England. Scarring

thickens the shell, and thus leaves a snail less vulnerable to future predation. Examining

scarring frequency may indicate the degree of selection for thick shells present at a

particular site. Previous studies indicated that crabs were rare in northern New England

and Canada and thus have not presented a substantial threat to local snail populations.

More recent studies have shown that crabs are becoming more common in the northern

reaches of their ranges, and thus northern snails may now be more vulnerable to crab

predation.

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Introduction

The morphology of the common rocky shore rough periwinkle, Littorina saxatilis,

has been studied extensively in Europe, but in less detail elsewhere across its North

Atlantic range in Iceland, Greenland, and North America. Populations of L. saxatilis

show high variation in shell morphology across spatial scales. Two ecotypes ofZ,.

saxatilis have been examined in British populations: L. saxatilis ‘H’ (with thin shells and

large apertures found in high shore among cliffs and boulders) and L. saxatilis ‘M’ (with

thick shells and small apertures found in mid-shores among smaller boulders) (Wilding et

al. 1998, Hull et al. 1999). L. saxatilis ‘M’ probably corresponds to what was once

described as L. rudis (Wilding et al. 1998), and L. saxatilis ‘H’ was often referred to as L.

patula (Wilding et al. 2002). Both are now regarded as ecotypes of L. saxatilis (Reid

1996, Wilding et al. 2002). In Swedish populations of L. saxatilis, two ecotypes have

been identified as the E-morph, which have thin shells and a large foot/aperture (found in

exposed areas), and the thicker-shelled, small apertured S-morph (found in sheltered

areas) (Janson and Sundberg 1983, Johannesson 1986, Johannesson and Johannesson

1996, Hollander 2001).

Since there are fewer studies on L. saxatilis in its northwest Atlantic range, I focused

on large scale clinal variations in L. saxatilis across latitude rather than small scale

variations across exposure and tidal height/substrate. Since it is easier to accrete calcium

carbonate shell at higher temperatures (Vermeij and Currey 1980), snails in lower

latitudes are typically thicker than those in higher latitudes. Often there is a greater

predation pressure in these lower latitudes as well (a greater abundance and species

diversity of crabs), so there is a greater selective advantage for thicker shells (Vermeij

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. and Currey 1980, Vermeij 1982d, Vermeij 1987). Thicker shelled snails are not as

reproductively productive as thinner shelled snails, however snails will upkeep their

shells in terms of growth and repair equally (Geller 1990a).

Snail shell thickness will indicate a species’ potential vulnerability to shell-breaking

predators. Crabs often “peel” the shell by chipping away at the aperture until the tissue of

the snail is reachable (Bertness and Cunningham 1981, Vermeij 1982b, Lindstrom 2005);

crabs usually only crush the snail if there is very large crab to snail size ratio (Bertness

and Cunningham 1981). Crabs prefer to break thinner versus thicker shelled snails (Elner

and Raffaelli 1980, Palmer 1985, Geller 1990a). A snail may be left chipped and uneaten

if a crab cannot quickly access the snail tissue, so there is typically a greater proportion of

scarred snails that are thick-shelled than thin-shelled (Elner and Raffaelli 1980, Vermeij

1982a). This chipped snail will form a scar as the shell re-grows. New shell grows

beneath the outer edge of the break (Lindstrom 2005), thus it is usually thicker than the

surrounding unscarred shell and leaves a noticeable ridge revealing the shape of the

original chip. General thickening of the shell may occur, based on the amount of

exposure the snail has to crab chemical cues. However, this ridge is likely the

physiological byproduct of the shell repairing process and not likely induced as a

phenotypic response to the crab encounter.

Crab-induced scars can be distinguished from those caused by abiotic forces because

they are repeated in a distinctly regular pattern (easily observed both in field collections

and lab predation experiments). Abiotic scars, caused by wave action knocking shells off

rocks or rocks being turned over, result in chips and cracks that are random and highly

variable in nature, and these scars are not as common as those caused by lip-peeling

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. predators (Cadee et al. 1997, Lindstrom 2005). Additionally, Lindstrom (2005) noted that

if abiotic forces were a major cause of shell injury there would be a higher frequency of

scarring in highly wave-exposed areas, which is not the case. Snails tend to have bigger

feet in more exposed areas, which also reduces the likelihood of abiotic-induced scarring

in these areas. Scarring generally increases in littorinids as latitude decreases (Vermeij

1982d), so scarring is likely positively related to shell thickness or higher predation.

Additionally, the accumulation of scars will increase as the snail gets older, and since L.

saxatilis can live up to 9 to 11 years, a snail can reflect the historical presence of shell-

breaking predators in a particular location (Gorbushin and Levakin 1999).

The frequency of scarred snails in a population is an indication of the degree to which

selection for shell strength is present in the system (Vermeij 1982b), or how important

predators are in shaping the adaptations of their shelled prey. A high frequency of

scarring means that predators are likely a strong selective force in the system; snails have

likely evolved thicker shells due to predator’s presence, and this results in a greater

number of unsuccessful predation events, or scarred snail shells. A low number of scarred

snails in a population may be the result of at least two processes: shell-breaking predators

are not a substantial threat to the snails or shell-breaking predator attacks are primarily

lethal (Vermeij 1982a, Lindstrom 2005). Thus, it is difficult to gauge predator intensity

from solely examining crab-induced scars within a population. Additionally, some crabs

may leave no evidence of an attack, if they simply pull out the tissue of the snail from the

aperture without cracking the shell (Johannesson 1986). Although C. maenas is known to

be able to extract most of the animal of Littorina littorea out of its shell in the laboratory

(J. T. Carlton, personal communication), the species and size classes of crabs capable of

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. preying upon L. saxatilis primarily use the peeling technique to access snail tissue

(personal observations in the lab). Since scarred snails are typically thicker than

unscarred snails, crabs will likely take longer to handle these snails resulting in reduced

predation success. Additionally, snails that are simply thinner due to clinal differences in

shell thickness are likely more vulnerable to predation by shell-breaking crabs.

History of Non-native Crabs in New England

The history of the European green crab Carcinus maenas on the North American

Atlantic coast was reviewed by Carlton and Cohen (2003). From its appearance in the

early 1800s in Long Island Sound, it spread in the late 19th century to waters north of

Cape Cod, and by the mid-20th century had reached Nova Scotia. Surveys conducted in

1984, indicated that C. maenas was well-established in some areas of Northeastern Maine

and rare to absent in others (Seeley 1986). In this same area, there was a peak in crab

abundance in the late 1990s, which has now faded, so current populations are

inconsistently distributed among sites once again (Harris, L. G. and Robin H. Seeley,

personal communication, Matthews-Cascon 1997).

Fishermen in southern Nova Scotia (east of Northeastern Maine across the Bay of

Fundy) considered, C. maenas, to be abundant in 1964, but there seemed to be a

reduction in density over the subsequent years coinciding with reported colder than

average winters (Audet et al. 2003). Although the UK experienced a similar drop in crab

densities during this cooler period, populations of C. maenas rebounded faster than those

in its US exotic range. Not until the mid-1990s did Gulf of St. Lawrence, Canadian

fishermen begin to see that green crab abundance had increased once again and spread

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. into new areas (Audet et al. 2003). By 2000, shellfish farmers on Prince Edward Island

(even further North than Northeastern Maine) identified C. maenas as a major concern to

local aquaculture (Miron et al. 2005); on the north coast of Nova Scotia fishing gear was

reported to catch hundreds of green crabs within a 24-hour time period in 2003 (Audet et

al. 2003).

The 1990s range extension of C. maenas into the Gulf of St. Lawrence was initially

interpreted as a continued northward movement of the green crab from southern waters,

enhanced by warming coastal ocean temperatures (Audet et al., 2003). However, Roman

(2006) has shown, based upon genetic evidence, that these northernmost colder-water

populations represent a separate invasion of C. maenas from northern Europe. Although

there are limited data on present day population sizes and long-term decadal trends of C.

maenas in New England, as coastal ocean temperatures continue to rise, it is predicted

that green crabs will increase in density and range (Audet et al. 2003, Roman 2006). With

this likely expansion of green crabs, the spread of the newer non-native crab,

Hemigrapsus sanguineus is highly probable as well. However, this will likely happen

only with an increase in water temperatures (Byers and Pringle 2006).

The Asian shore crab, H. sanguineus, was first discovered in 1988 in New Jersey, and

has since spread both south to North Carolina and north to mid-Maine (McDermott

1998), reaching New Hampshire by 1998 (McDermott 1999). In southern portions of its

New England intertidal range, it reaches high densities and has largely replaced C.

maenas (Lohrer and Whitlatch 2002a). At the northern end of the range of H. sanguineus

in Maine, populations are still not as abundant as those of C. maenas. However, H.

sanguineus populations have increased considerably since 2001 when they were largely

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. absent from sites as far north as New Hampshire (Tyrrell 1999, 2002). Both crab species

overlap with L. saxatilis in New England’s intertidal rocky shores and are the most

frequent predators on these snails. Previous studies have assumed than northern New

England has a very low predation pressure from crabs due to low populations of C.

maenas coinciding with harsh northern New England winters. I propose here that crabs,

as reported most recently by Audet et al. in 2003, may begin to increase in number in the

northern reaches of its range and pose a threat to this snail species even in locations far

north which previously may have had only rare crab peaks of abundance (Seeley 1986,

Trussell and Smith 2000, Trussell and Etter 2001).

Objectives

Since shell thickness is one of the most important features determining a mo Husk’s

susceptibility to predation by shell-breaking predators (Vermeij 1987) and scarring may

change shell thickness (Greenfield et al. 2002), I examined these features in Littorina

saxatilis at sites across New England. The measurements serve as baseline data on the

clinal variability in morphology of L. saxatilis in New England specifically in regard to

their vulnerability to crab predation. I hypothesize that snails in the North will have

thinner shells, indicating a greater degree of vulnerability to predation, than snails in the

south. Additionally, shell scarring will increase shell thickness, suggesting that scars

provide snails with a reduced risk of successful predation by shell-breaking predators.

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Methods

Field Sites

To examine variation in the morphology of Littorina saxatilis, I clustered sampling

sites within limited regions to look at small scale differences and spaced these sampling

regions across a broad range to see larger scale differences. I chose four sampling regions

(with 2-5 sites within each region) ranging from northeastern Maine to Rhode Island

(Figure 1.1, Appendix A). These four regions will be referred to (from north to south) as

Northernmost (the area of Eastport, Maine), Northern (the area of Winter Harbor,

Maine), Southern (coastal New Hampshire/southern Maine), and Southernmost (the

Rhode Island/Connecticut border area). The Southernmost and Southern region are 213

km apart; the Southern and Northern regions are 259 km apart; and the Northern and

Northernmost regions are 99 km apart. There are 571 km separating the Southernmost

and Northernmost regions. I spaced each site within each region with 7 +/-1.2 km SE.

The two northern regions have established populations of Carcinus maenas but not of

Hemigrapsus sanguineus. Both species of crabs inhabit rocky intertidal areas within the

two southern regions; however, there is a greater abundance of H. sanguineus than C.

maenas in the Southernmost region than in the Southern region as of the sample dates. I

sampled snails haphazardly from the upper intertidal zone both on and under rocks at low

tide. Regions vary in coastal temperatures (Appendix A, Figure A.1); from May to

September temperatures are on average 4 degrees Celsius colder in the Northernmost

region than in the Northern region. These summer temperatures are only one-third of a

degree Celsius cooler in the Northern region versus the Southern region, and

temperatures are on average 5 degrees Celsius warmer in the Southernmost region in

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. comparison to the Southern region. The Southernmost region in the summer is on average

9.8°C warmer than the Northernmost region. These temperatures provide relative

estimates for regional differences, as intertidal temperatures will range more extremely

(much colder in the winter and warmer in the summer) than even near-shore buoys. All

sites are protected from intense wave exposure to support populations of crabs among

Ascophyllum nodosum and under small boulders at low tide.

Three sites were excluded from the analyses presented here due to extreme physical

differences from the other sites: in the Northern region the Inlet site in Winter Harbor

(few rocks and muddy substrate provided little structure for snail and crab shelter) and

Schoodic Point (steep cliff site), Winter Harbor (primarily cliff and steep ledges) were

eliminated. Avery Point from the Southernmost region was eliminated because the site

was comprised primarily of ledges rather than boulders. Wilbur Neck was kept in the

analysis although it was much more protected than the other sites because shell thickness

was statistically similar to a much more exposed site within the region, West Quoddy

Head (See Appendix B, Figure B.4).

Sample Sizes and Height Ranges for Examining Thickness of Unscarred Snails

No snails were collected that were less than 5 mm in shell height from the Southern

region nor greater than 14 mm in height from the Northernmost region. I focused my

analyses on unscarred snails that were between 5 and 13 mm in shell height, as the

number of snails found within each region was fairly even across regions (mean of

N=471+/- 59 SE) compared to several other possible size ranges (including 5 to 9 mm, 5

to 10 mm, and 5 to 14 mm), and L. saxatilis are most commonly less that 12 mm (Gosner

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 1978). I focused analyses on snails of similar heights, thus averages in shell height

calculated reflect the average heights of my cropped samples rather than aiming to

capture average heights in the field.

Snail Shell Thickness and Scarring Frequency

Samples were collected between 19 May 2005 and 6 June 2005 from 14 sites (Figure

1.1). I sorted all collected snails into three categories: unscarred (shell clear of scars,

chips, and blemishes), scarred (shell shows clear evidence of a crab predation attempt

either healed or unhealed (Figure 1.2)), and blemished (shell is marred or chipped in

some fashion but the origin of the damage is unclear). I scored shells with a distinct “U”

or “V” shaped scar as evidence of a previous crab attack.

Snails that were questionable or only faintly scarred were not considered to be crab

scarred and were categorized as blemished. As some shells scored as blemished may

have been attacked by crabs, the frequency of crab-scarred snails in each sample may

thus be underestimated. In examining snail morphology, I excluded blemished snails.

When calculating scarring frequency per site I counted blemished snails as a part of the

total unscarred portion of the sample.

As previously discussed, shell thickness is an important shell feature in determining a

snail’s vulnerability to predation by crabs. Shell thickness coupled with height was thus

used to compare shell morphology within and among regions. All shell thickness

measurements were made at the outermost point of the aperture using digital calipers; in

order to measure the aperture lip thickness consistently without breaking the curved edge

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. of the lip, I measured 1.0 mm inward from the edge of the outer lip of the aperture, by

marking the calipers appropriately.

Shell thickness measurements were log transformed for normality. As aperture

thickness changes as the snail grows, I looked at the relationship between shell thickness

and shell height. Since within-site and within-region differences were minimal, I focused

primarily on the differences among the four regions in this chapter (see Appendix B for

within region differences).

Crab Density and Biomass

Ten to twenty 0.5 m quadrats were placed haphazardly across a 25 m transect in the

Ascophyllum nodosum mid-to-upper intertidal zone during low tide to conduct crab

density measures (transects were placed parallel to the low tide edge). Crabs were found

among the macroalgae and under rocks; they were identified, sexed, measured, and

ovigerous females were noted. Quadrat data were collected between 19 May 2005 and 6

June 2005 from 12 of the 14 sites where snails were sampled. The 13th site was collected

on 20 July 2006 at Odiomes Point, NH (OP), and the Evergreen site in Winter Harbor,

ME (EV) was not sampled because the large boulders present there made it impossible to

sample for crabs. The biomass of each crab was calculated based on power curve

equations (H. sanguineus: y=0.000362x3'071420, R2 = 0.99; C. maenas: y =

0.000239x2 984721, R 2 = 0.99; y=weight (g); x=carapace width (mm)) generated from

measuring and weighing crabs collected from Odiomes Point (20 January 2005) starved

for at least 72 hours.

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Shell Thickness of Unscarred versus Scarred Snails

To understand how scarring in snail shells may influence future predation attempts, I

compared the shell thicknesses of unscarred and scarred snails at five of the most heavily

scarred sites (within the three most southern regions). To elucidate how a snail shell

changes as a result of healing from a scar, I measured the thickness of the shells at the

aperture and the distance from the aperture edge to the scars. To investigate whether scars

thicken the shell only at the scar or also thicken any subsequent shell growth, I compared

shell thickness in snails that were recently scarred (the scar is close to the aperture edge)

to snails that had been scarred earlier (the scar is further from the aperture edge) to

unscarred snails. Since I measured snails 1.0 mm into the aperture, I considered recent

scars to be 0.5-1.5 mm from the edge and old scars to be greater than 1.5 mm from the

aperture edge. I eliminated snails with scars less than 0.5 mm from the aperture edge

since they were too recently chipped and may not have had ample time to heal over to

form a scar.

Analyses

I examined relationships among mean shell thickness, mean shell height, mean crab

density, mean crab biomass, region, latitude, and temperature. Mean height and mean

shell thickness at the aperture were both normally distributed, and crab density and

biomass were log transformed (log(x+l)). I also compared the frequency of scarred snails

among regions to examine if this was related to mean shell thickness and mean crab

density. Mean scarring frequencies were normally distributed. Finally, I compared shell

thickness between scarred and unscarred snails and among scarred snails that had varying

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. history of crab encounters; some had been recently scarred (scar is close to the aperture

edge) and some had been scarred long ago (scar is far from the aperture edge). All

statistical analyses were performed with the statistical software JMP 5.1.

Results

Shell Thickness and Height Differences across Regions

The Northernmost region (Figure 1.3a) has significantly thinner unscarred snails

(0.15 +/- 0.003 mm SE, n=572) than the other three regions (ANCOVA, REstricted or

REsidual Maximum Likelihood (REML) Effect Tests: P=0.0010; LS Mean Differences

Tukey HSD: a=0.05). The three southern regions have similar shell thicknesses (Northern

region=0.29 +/- 0.006 mm SE, n=570 N; the Southern region=0.41 +/- 0.008 SE mm,

n=402; the Southernmost region=0.35 +/- 0.007 mm SE, n=339). I included height as a

covariate, region as a fixed effect, and site nested within region as a random effect (see

Table 1.1 for number of snails measured per size class per region.) Aperture thickness is

shown for snails of particular size classes of shell height (mm) for the four regions

(Figure 1.3b), and levels not connected by the same letter are significantly different from

one another (ANOVA: F5; 1882=235.8, PO.OOOOl, LS Means Differences Tukey HSD,

a=0.05). Shell thickness decreases with increasing latitude (Figure 1.4a, R =0.50,

ANOVA: Flji3=l 1.8, P=0.0050), and increases with increasing temperature (Figure 1.4b,

R2 = 0.52, ANOVA: FU3=12.9, /MJ.0037).

The three southerly regions have similar shell heights (8.17 +/-0.262 mm SE, 9.77 +/-

0.439 mm SE, 8.52 +/- 0.474 mm SE from south to north respectively), and the

Southernmost, Northern, and Northernmost (7.45 +/-0.321 mm SE) regions also have

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. similar shell heights (Table 1.2, LS Means Differences Tukey HSD, a=0.05). While size

ranges were cropped to examine only shell features of snails 5 to 13 mm in shell height,

regions still reflect some differences in average heights. Additionally, as shell height

increases, aperture thickness also increases (Figure 1.5, R2=0.6745, ANOVA: Fiji3=24.8,

P=0.0003), and the southerly region individual sites show a trend of having thicker and

taller shells than those from the northerly regions.

Crab Density and Biomass across Regions

Neither snail scarring frequency (Figure 1.6) nor snail shell thickness (Figure 1.7) is

significantly associated with crab density at a particular site (scarring frequency

ANOVA: FU2=0.90, P=0.3643 and shell thickness ANOVA: F 1,i2=1.5, P=0.2439). There

is a greater density of crabs in the southernmost region than in the other three regions

with site as a nested random variable (2-5 sites per region) (Region effect: P=0.0090; LS

Means Differences Tukey HSD, a=0.05). These density figures include all intertidal

crabs which were comprised of only two species: H. sanguineus and C. maenas.

Hemigrapsus sanguineus was not found in either of the two northern regions, whereas I

found 95% H. sanguineus in the Southernmost region and 17% H. sanguineus in the

Southern region (Figure 1.8, Table 1.3). There is a greater biomass of crabs in the

Southernmost, Northern, and Northernmost regions than in the Southern, Northern, and

Northernmost regions with site as a nested random variable (2-5 sites per region) (Region

effect: P=0.0401; LS Means Differences Tukey HSD, a=0.05) (Figure 1.9, Table 1.4).

In the Southernmost region the average carapace width (CW) of H. sanguineus was

11.0 mm (+/- 0.26 mm SE, N=574) and the average CW for C. maenas was 30.2 mm (+/-

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 5.11 mm SE, N=10). In the Southern region the average carapace width (CW) of H.

sanguineus was 12.4 mm (+/- 1.89 mm SE, N=17) and the average CW for C. maenas

was 8.9 mm (+/- 0.33 mm SE, N=123). The average CW for C. maenas was 30.8 mm (+/-

1.02 mm SE, N=116) in the Northern region and 32.4 mm (+/- 0.86 mm SE, N=124) in

the Northernmost region. (See Tables 1.3 and 1.4 for details on crab density, biomass and

species within sites and regions.)

Shell Thickness and Scarring Frequency

Scarring frequency is highly dependant on the thickness of the shell (Figure 1.10, y=-

0.9396x2 + 0.7826x - 0.0663, R 2 = 0.4793, ANOVA: F 2,i3=6.4, M .0147); scarring

frequency positively increases with shell thickness at the aperture in a curvilinear fashion

(R2 = 0.4793). There are no significant differences in mean scarring frequency across the

four regions with site as a nested random variable (2-5 sites per region; 14 sites total) (LS

Means Differences Tukey HSD, a=0.05), but there appears to be a trend that the

Northernmost region has a lower number of scars per site than the other three regions

when mean scarring frequency is plotted against latitude (Figure 1.11; y=-0.0177x +

1.5115x - 32.152, R2 = 0.42, ANOVA: F2,i3=4.0, P=0.050). The percentage ofblemished

snails ranged from 3.6 to 17.6 % of total snails collected per region (Table 1.5b). (See

Table 1.5 for details on scarring frequency within sites and regions.)

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Shell Thickness of Unscarred versus Scarred Snails

To compare unscarred shell thickness to scarred shell thickness, I selected the five

sites which had scarring frequencies greater than 0.08 (Table 1.6). Scarred snails within

these sites are significantly greater in shell thickness at the aperture (0.42 +/-0.011 mm

SE, N=234) than unscarred snails (0.34 +/-0.006 mm SE, N=658) (Figure 1.12, Table

1.7; ANCOVA with height as a covariate, region as a fixed effect, and site nested within

region as a random effect: F io,89i=107.2, P=0.0284).

Since scarred snails heal as they grow, their shells are thick at the scar and then thin

out to the thickness of the rest of the shell (Figure 1.13, ANOVA: F2, 1053=8.1, P=0.0003,

LS Means Differences Tukey HSD, a=0.05). Shell thickness (log transformed) is greater

when the scar is close to the aperture than when it is farther away, but this relationship is

not significant. Snails that had been scarred further from the edge are statistically similar

in shell thickness to unscarred snails, although there appears to be a trend for thicker

shells in the old-scarred snails. Recently scarred snails are significantly thicker than

unscarred snails. Thickness of the shell decreases as the distance from the scar to the

aperture increases until the scar becomes greater than 1.5 mm from the aperture edge.

Discussion

Crab abundance and species diversity in New England has been ever-increasing

northward in recent years. Although in my 2005 survey, the Northern region had only

19% H. sanguineus versus C. maenas, this is a great increase since 2001, when/7.

sanguineus was reportedly not established yet in this region (Tyrrell 1999, 2002). There

is a greater density of crabs, regardless of species, from south to north, and there is

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. overlap in crab density among the northern three regions. There is high overlap in total

crab biomass among all four regions although species composition varies from south to

north. Previous studies specify that northern New England had substantially lower crab

abundances than in more southerly regions (Seeley 1986, Trussell and Smith 2000,

Trussell and Etter 2001), however more recent studies have indicated that C. maenas has

substantially increased in abundance in Canada, north of my Northernmost region (Audet

et al. 2003). While current populations of C. maenas in Northeastern Maine remain

inconsistent (Seeley, R. H., personal communication), with an increase in temperature,

populations will likely become more consistently abundant. Additionally, sporadic pulses

in crab abundances may be associated with the evolution of thicker shells. For instance,

in 2001 there was a considerable increase in C. maenas abundance in Winter Harbor, ME

(Harris, L. G, personal communication), which may have influenced local snail

populations. Even if crab populations subsided in subsequent years, since L. saxatilis live

on average six years, populations may still have remained thicker.

It is not surprising that my 2005 crab density measures do not relate to snail scarring

frequency or shell thickness at a site, since my data are only a snapshot in time of crab

density at a site. More importantly, my surveys indicate the presence of intertidal crabs

and the relative proportions of these particular crab species within the range of L.

saxatilis. Long-term decadal trends would be important to consider in order to understand

more fully how crab abundances are specifically influencing shell thickness which will

then affect scarring frequency. Thinner shells in the Northernmost region suggest that

differences are likely primarily due to temperature as well as historical crab abundance

differences between the regions (see Figure 1.4), as Trussell and Etter (2001) reported for

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. congener L. obtusata. The Northernmost region has summer temperatures that are on

average four degrees Celsius colder than in the Northern region. Temperatures are similar

in the Northern and Southern regions corresponding with similar shell thicknesses, but

one would have expected shell thickness differences between the Southern and

Southernmost region, as these regions have an on average five degree Celsius

temperature difference in the summer. However, it is important to acknowledge that

temperature and crab abundance alone do not influence shell features; other abiotic and

biotic forces, including life history and genetic traits, together influence shell shape and

thickness.

Although L. saxatilis in the northern Gulf of Maine are thinner than populations

further south, perhaps northern snails are more efficient at depositing calcium carbonate

and thus grow at a fester rate. Trussell and Etter (2001) report that L. obtusata from the

northern Gulf of Maine are genetically disparate from southern Gulf of Maine

populations; they suggest that there may be a greater selection for rapid growth and

efficient shell accretion in the north, where water temperatures are much colder. Perhaps

the same is true for L. saxatilis, however, my investigations do indicate that even taller

snails from the north are significantly thinner than those matched for height from the

three southerly regions; even if it takes a shorter amount of time to reach this length, they

are still more vulnerable to predation due to their thin shells. In general, faster growth

generates a thinner shell, and thickly-shelled snails tend to grow at a slower rate (Harvell

1990).

Snails from the Northernmost region in the 11-13 mm size class are statistically

similar in shell thickness to snails from the Northern region in the 9-11 mm size class and

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. snails from all other regions in the 5-7 mm and 7-9 mm size classes (Figure 1.3b). Even

though tall snails from the Northernmost region are likely just as vulnerable as these

snails from other regions that are statistically similar in shell thickness, there are much

fewer snails in the Northernmost region that fall into this tall size class (see Table 1.1:

11-13 mm, N=11). If we consider one size class smaller in the Northernmost region (9-11

mm), these snails are statistically similar to snails from the Northern region in the 5-7

mm size class but thinner than all other snails from the smaller size classes from the other

three regions. Even if snails from the Northernmost region get to a greater height faster

than those from the south, they will still likely be more vulnerable than most snails from

further south.

As supported previously, scarring frequency is highly dependent upon shell thickness,

(Elner and Raffaelli 1980, Vermeij 1982a); crabs will have more difficulty in chipping a

thicker shell, so thicker shells are more likely to be left uneaten than thinner shells.

Similarly, my studies show that mean scarring frequency is positively related to mean

shell thickness at the aperture (Figure 1.10). The thinner the shell, the more likely a crab

will be successful in preying upon a snail, and thus the snail (and its shell) will be

destroyed. Similarly, with a thicker shell, a crab may have a more difficult time cracking

the snail, and thus may be more likely to leave a chipped and uneaten snail. Additionally,

as snails get older, they become thicker, and there is a greater likelihood for scars to be

present in an older snail that has had time to accumulate them over one or more years.

However, above 0.30 mm in shell thickness, scarring frequency is highly variable, and

these thick shells are largely found in the southern three regions. As shell thickness

increases even further, a crab may have such a difficult time in cracking the snail that it

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. will not be able to damage the shell at all, so scarring frequency may decrease slightly for

these very thick shells.

Although there are no significant differences in mean scarring frequency across the

four regions, there is a trend showing that snails from the northerly sites have fewer scars

and thinner shells than those from more southerly sites (Figure 1.11). Scarring frequency

tapers off in areas with very thick shells in the south because it may be difficult for some

crabs to leave any evidence of a predation attempt, and the negative polynomial

relationship is largely driven by the low rate of scarring frequency in the northernmost

region. This trend for lower scarring frequency in the Northernmost region may be

occurring for several possible reasons; ( 1) there are likely fewer crabs, (2) the crabs that

are there may have a lower metabolism, slowed by the lower temperature, and thus the

predation pressure is lower, and (3) predation encounters that do occur are likely always

lethal, due to the thinner shelled snails.

Recently scarred snails have significantly thicker shells than unscarred snails (Figure

1.13), so these scarred snails are likely to be less vulnerable to predation than unscarred

snails. Protection to the snail from the scar would likely only last for one or two growing

seasons, as the scar will grow further from the aperture over time and then thin out to the

thickness of an unscarred snail. Snails with sub-lethal injuries to their shell may be less

vulnerable to future predation attempts by crabs, due to their increased thickness at the

site of the scar. This is contrary to studies conducted where sublethal predation has been

associated with an increased risk of lethal predation (Meyer and Byers 2005 and

references therein). Greenfield et al. (2002) found that scarred Littoraria irrorata had

thicker shells than unscarred conspecifics, which provided scarred snails with a reduced

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. risk of predation by Callinectes sapidus, the blue crab. Future studies (Chapter III)

presenting scarred versus unscarred Littorina saxatilis to its crab predators will reveal

important information on whether scarred L. saxatilis are in fact less vulnerable to

predation.

Since crabs are increasing in abundance and species diversity in northern New

England, it is imperative to look at the vulnerability of their prey items. Here I show that

one of the most important features in determining a shelled prey item’s susceptibility to a

shell-breaking predator, shell thickness, varies among populations of L. saxatilis within

New England. Additionally, there is evidence that scarred snails may have a decreased

risk of future predation due to their likely thicker shell where the previous chip has

healed (Chapter III, Greenfield et al. 2002). In general thinner shelled snails in the north

will likely be more vulnerable to predation by crabs than thicker shelled conspecifics in

the south (Chapter II). As coastal temperatures warm, crabs are likely to increase in the

north, which may have a serious impact on local populations of L. saxatilis. Even if thin-

shelled northern snails are able to increase in shell thickness as a response to increasing

crab exposure and warmer waters, crabs may peak in abundance before snails have the

time or the physiological capacity to response phenotypically. It is of utmost importance

to document patterns of morphological variation among native prey that are likely

vulnerable to increasing and expanding populations of exotic predators.

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Table 1.1 Number of snails measured per size class per region.

5-7 7-9 9-11 11-13 mm mm mm mm Southernmost 111 117 75 36 Southern 18 126 159 99 Northern 177 173 124 96 Northernmost 282 202 77 11

Table 1.2a Summary of each site: unscarred snails 5-13 mm in height: shell thickness (mm) +/-SE, shell height (mm) +/-SE, and N (number of snails measured).

Shell Shell Latitude Region Site Thickness SE Height SE N (°N) (mm) (mm) SP 41.33 0.32 0.011 7.91 0.171 152 Southernmost WP 41.33 0.38 0.010 8.44 0.132 187 RH 43.00 0.47 0.018 10.64 0.149 105 Southern OP 43.04 0.46 0.008 9.28 0.125 159 KP 43.10 0.32 0.011 9.38 0.132 138 EV 44.34 0.31 0.017 9.55 0.255 89 WO 44.36 0.32 0.012 9.10 0.164 173 Northern FR 44.38 0.24 0.009 7.52 0.170 91 MP 44.40 0.27 0.008 7.92 0.135 217 QU 44.82 0.23 0.012 8.54 0.177 94 CP 44.89 0.13 0.006 7.16 0.147 91 Northernmost EA 44.90 0.11 0.004 7.70 0.132 132 WN 44.90 0.16 0.003 6.62 0.064 168 PA 44.95 0.09 0.005 7.22 0.140 87 Mlean N= 135+/-11.4SE

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Table 1.2b Summary of each region: unscarred snails 5-13 mm in height: mean shell thickness (mm) +/-SE, mean shell height (mm) +/-SE, and number of sites in each region (N).

Mean US Mean US Latitude Shell Shell Region SE SE N (°N) Thickness Height (mm) (mm) Southernmost 41.33 0.35 0.028 8.17 0.262 2 Southern 43.05 0.42 0.049 9.77 0.439 3 Northern 44.37 0.28 0.025 8.52 0.474 4 Northernmost 44.89 0.15 0.023 7.45 0.321 5

Table 1.3a Summary of each site: mean crab density per 0.5 m 2 at each site +/-SE, % C. maenas (CM), % H. sanguineus and(HS), number of quadrats.

Lati #of Mean Crab % % Region Site tude SE quad Density CM HS (°N) rats SP 41.33 10.70 2.450 9.3% 90.7% 10 Southernmost WP 41.33 47.70 9.185 0% 100% 10 RH 43.00 0.82 0.536 77.8% 22.2% 11 Southern OP 43.04 4.96 0.562 89.1% 10.9% 12 KP 43.10 1.50 0.886 83.3% 16.7% 8 WO 44.36 1.63 0.483 100% 0% 30 Northern FR 44.38 2.87 1.320 100% 0% 15 MP 44.40 0.88 0.328 100% 0% 25 QU 44.82 0.13 0.091 100% 0% 15 CP 44.89 0.40 0.190 100% 0% 15 Northernmost EA 44.90 2.40 0.729 100% 0% 15 WN 44.90 0.20 0.092 100% 0% 20 PA 44.95 3.90 0.827 100% 0% 20

Table 1.3b Summary of each region: mean crab density per 0.5 m 2 at each region +/-SE, % C. maenas (CM), % H. sanguineus and(HS), number of quadrats.

Lati # of Mean Crab Region tude SE % CM % HS quad Density (°N) rats Southernmost 41.33 29.20 6.278 4.7% 95.3% 20 Southern 43.03 2.60 0.490 83.4% 16.6% 31 Northern 44.38 1.63 0.373 100% 0% 70 Northernmost 44.89 1.48 0.288 100% 0% 85

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Table 1.4a Summary of each site: mean total crab biomass (g) per 0.5 m 2 at each site +/- SE, mean biomass (g) of C. maenas (CM), mean biomass (g) oiH. sanguineus (HS), and number of quadrats.

Mean Mean Lati Mean Total Bio Bio # of Region Site tude Crab SE mass mass quad (°N) Biomass (g) (g)of (g)of rats CM HS SP 41.33 25.07 8.674 11.20 13.88 10 Southernmost WP 41.33 59.36 13.09 0 59.36 10 RH 43.00 1.39 1.145 0.44 0.95 11 Southern OP 43.04 3.02 0.803 1.75 1.28 12 KP 43.10 1.04 0.439 0.74 0 8 WO 44.36 13.13 4.79 13.13 0 30 Northern FR 44.38 25.81 14.79 25.81 0 15 MP 44.40 9.82 5.195 9.82 0 25 QU 44.82 1.83 1.483 1.83 0 15 CP 44.89 2.34 1.395 2.34 0 15 Northernmost EA 44.90 20.35 7.345 20.35 0 15 WN 44.90 2.97 2.973 2.97 0 20 PA 44.95 39.45 9.965 39.45 0 20

Table 1.4b Summary of each region: mean total crab biomass per 0.5 m 2 at each region +/-SE, mean biomass of C. maenas (CM), mean biomass oiH. sanguineus (HS), and number of quadrats.

Mean Mean Lati Mean Total # o f Biomass Biomass Region tude Crab SE quad (g)of (g)of (°N) Biomass (g) rats CM HS Southernmost 41.33 42.21 17.143 5.60 36.62 20 Southern 43.03 1.82 0.612 0.97 0.84 31 Northern 44.38 16.25 4.876 16.25 0 70 Northernmost 44.89 13.39 7.388 13.39 0 85

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Table 1.5a Summary of each site: number of snails collected, number of scarred snails collected, and mean scarring frequency +/-SE, number of samples collected per site (N), and percent of total snails collected blemished per site.

Mean Total #of % Latitude Scarring Region Site # o f Scarred N SE Blem (°N) Frequ Snails Snails ished ency Southern SP 41.33 555 68 0.126 3 0.015 17.8 most WP 41.33 10936 510 0.050 20 0.004 11.9 RH 43.00 647 87 0.134 1 0 24.6 Southern OP 43.04 2300 118 0.045 5 0.010 14.9 KP 43.10 656 69 0.105 1 0 13.4 EV 44.34 208 34 0.163 1 0 26.0 WO 44.36 727 64 0.089 0.006 16.0 Northern FR 44.38 203 13 0.064 1 0 12.8 MP 44.40 877 43 0.048 0.008 7.8 QU 44.82 312 10 0.032 1 0 5.1 CP 44.89 149 3 0.020 1 0 1.3 Northern EA 44.90 214 5 0.021 0.004 0.5 most WN 44.90 201 0 0.000 1 0 8.0 PA 44.95 322 5 0.016 1 0 3.1

Table 1.5b Summary of each region: number of unscarred snails collected, number of scarred snails collected, mean scarring frequency +/-SE, number of sites in each region (N), and average percent of total snails collected blemished per region.

Total Total Mean % Latitude Region # o f it of Scarring SEN Blem (°N) US S Frequency ished Southernmost 41.33 11491 578 0.088 0.0277 2 14.9 Southern 43.05 3603 274 0.095 0.0226 3 17.6 Northern 44.37 2015 154 0.091 0.0196 4 15.6 Northernmost 44.89 1198 23 0.018 0.0175 5 3.6

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Table 1.6 Mean scarring frequencies at the five most heavily scarred sites within the three southern regions.

Latitude Mean Scarring Region Site (°N) Frequency Southernmost SP 41.33 0.126 Southern RH 43.00 0.134 Southern KP 43.10 0.105 Northern EV 44.34 0.163 Northern WO 44.36 0.089

Table 1.7 Number of snails measured per size class per region for scarred and unscarred snails. Snail measurements were treated as continuous variables for statistical analyses and separated into size classes only for graphical purposes.

5-7 7-9 9-11 11-13 mm mm mm mm Scarred 8 32 98 96 Unscarred 133 166 194 165

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. O C. maenas only o Wilbur Neck (WN) o West Quoddy Head Northernmost (QU) C. maenas and o Passamaquoddy (PA) H. sanguineus o Eastport Harbor (EA) o Comstock Point (CP)

o Mermaid s Purse (MP) o Wonsqueak (WO) o Frazer (FR) o Evergreen (EV)

Kittery Point, ME (KP) • Rye Harbor, NH (RH) Odiomes Point, NH (OP)

Southernmost • Weekapaug Point, RI (WP) Stonington Point, CT (SP)

Figure 1.1 Map of four regions where sampling occurred: Northern Maine (Northernmost) 44.89 °N, Mid-coast Maine (Northern) 44.38 °N, New Hampshire and Southern Maine (Southern) 43.05 °N, and Rhode Island/Connecticut border (Southernmost) 41.33 °N. Sites with C. maenas only (O); sites with both C. maenas and H. sanguineus (•). Individual sites (and site codes) are listed from North to South in the boxes to the right of the map.

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Figure 1.2 Scars as evidence of previous crab predation attempts onZ. saxatilis shells: (A) smooth “U”-shaped chips healed, (B) “V”-shaped chip healed, and most common (C), (D), (E), and (F) one smooth “U”-shaped chip healed.

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. ♦ Southernmost ■ Southern a Northern (0» 0.8 o Northernmost

® 0.2

7 9 11 13 Height (mm)

Figure 1.3a Aperture thickness (mm +/-SE) was plotted against shell height (mm) for the four regions. Model incorporates region as a fixed effect, height as a covariate, and site as a random effect nested within region. The Southernmost region is shown in black diamonds, Southern region is shown in dark grey squares, the Northern region is shown in light gray triangles, and the Northernmost region is shown in open circles. The Northernmost region (dotted line) has significantly thinner shells than the other three regions (ANCOVA, REstricted or REsidual Maximum Likelihood (REML) Effect Tests: P=0.0010; LS Mean Differences Tukey HSD: a=0.05; solid lines: Northern, Southern, Southernmost in order from thinnest to thickest). Thickness data were log transformed to normalize distribution but presented in the figure with non-transformed measurements.

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. I Southernmost I Southern AB I Northern 8 0.5 □ Northernmost

3 0.2 - 9- 0.1

5-7 mm 7-9 mm 9-11 mm 11-13 mm

Figure 1.3b Aperture thickness (mm +/-SE) is shown for snails of particular size classes of shell height (mm) for the four regions. The Southernmost region is shown in black bars, Southern region is shown in dark grey bars, the Northern region is shown in light gray bars, and the Northernmost region is shown in open bars (see Table 1.1 for number of snails measured per size class per region). Aperture thickness was log transformed to normalize its distribution but shown in the figure as non-transformed data (ANOVA: F5j 1882=235.8, P<0.00001). Levels not connected by same letter are significantly different (LS Means Differences Tukey HSD, a=0.05).

CO 0.60 <0a) c 0.50 a 0.40 2 3 0.30 t: 0) Q. 0.20 < C (0 0.10 a> 0.00 41 42 43 44 45 Latitude (°N)

Figure 1.4a Mean snail shell thickness at the aperture (mm +/-SE) decreases across latitude (y=-0.0663x + 3.18, R2=0.50, ANOVA: Fi,i 3=l 1.8, P=0.0050). Each point represents an average shell thickness per site.

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 0.6 v> 4) c 0.5

0.4

£ £ 0.3 ! £ Q.4) 0.2 < re 0.1 o> 0 8 12 16 20 Temperature (°C)

Figure 1.4b Mean snail shell thickness at the aperture (mm +/-SE) increases across temperature (°C) (y=0.0258x - 0.0584, R 2 = 0.5162, ANOVA: FU3=12.9, P=0.0037). Each point represents an average shell thickness per site. NOAA temperature data were used on a regional basis.

re 0.50 (A re 0.45 c 0.40 O 0.35 0.30 2 E 0.25 A Southernmost 4) 0.20 O. □ Southern < 0.15 0.10 A Northern re 4> 0.05 ■ Northernmost 0.00 6 7 8 9 10 11 Mean Height (mm)

Figure 1.5 Mean shell thickness at the aperture (mm +/-SE) is positively related with mean height (mm+/-SE) (y=0.0879x - 0.4619, R2=0.6745, ANOVA: FU3=24.8, P=0.0003). Each point represents an average per site, and points are grouped by regions: Southernmost region (open triangles); Southern region (open squares); Northern region (filled triangles); Northernmost region (filled squares). The three southerly regions have similar shell heights, and the Southernmost, Northern, and Northernmost regions also have similar shell heights (LS Means Differences Tukey HSD, a=0.05).

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. A Southernmost >* - m J-. o □ Southern A Northern 0.10 - CT ■ Northernmost I- 9)

U i HJT (0 O (0 c (0 —r“ 0 -3 -1

-0.05 Log Mean Crab Density per 0.5 m2

Figure 1.6 Mean scarring frequency (+/-SE) is not significantly related to log transformed mean crab density (+/-SE) per 0.5 m 2 (ANOVA: Fi,i2=0.90, P=0.3643). Each point represents an average per site, and points are grouped by region: Southernmost region (open triangles); Southern region (open squares); Northern region (filled triangles); Northernmost region (filled squares).

0 0 H3 0 A Southernmost hCN c □ Southern 0.4 o A Northern 2 E ■ Northernmost

0.2 - Q.0 < C 0 0 -& e - -3 -1 1 Log Mean Crab Density per 0.5 m

Figure 1.7 Mean shell thickness at the aperture (mm +/-SE) is not significantly related to log mean crab density (+/-SE) per 0.5 m 2 (ANOVA: Fiti2=l-5 , P=0.2439). Each point represents an average per site, and points are grouped by region: Southernmost region (open triangles); Southern region (open squares); Northern region (filled triangles); Northernmost region (filled squares).

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. E I H. sanguineus in □ C. maenas 0) Q.

+ & SP WP RH OP KP FR WO MP CP EA PA QU WN o> o Southern Southern Northern Northernmost most

Figure 1.8 Log transformed mean crab density per 0.5 m2 across sites within the four regions. The proportion of each crab species is shown for each site; open bars for C. maenas and grey bars for H. sanguineus.

E 2.5 in ■ H. sanguineus □ C. maenas 0) 2 o.

+ 1.5 (0 w m E 1 o m a 0.5 2 o c 0 h (0 w n © SP WP RH OP KP FR WO MP CP EA PA QU WN o> Southern Southern Northern Northernmost o most

Figure 1.9 Log transformed mean crab biomass per 0.5 m2 across sites within the four regions. The proportion of each crab species is shown for each site; open bars for C. maenas and grey bars for H. sanguineus.

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 0.16 o A Southernmost □ Southern i-Dh a- 0.12 H o ▲ Northern i ■ Northernmost O) c 0.08 ■ck_ <0 o (0 0.04 c (00>

0 0.1 0.2 0.3 0.4 0.5 Mean Aperture Thickness (mm)

Figure 1.10 Mean scarring frequency (+/-SE) is positively related to mean shell thickness at the aperture (mm +/-SE) (y=-0.9396x2 + 0.7826x - 0.0663, R2 = 0.4793, ANOVA: F2,13=6.4, P=0.0147). Each point represents an average per site, and points are grouped by region: Southernmost region (open triangles); Southern region (open squares); Northern region (filled triangles); Northernmost region (filled squares).

>% o c 0.16 o> cr3 2 0.12 LL o> I 0.08 IB

wc 0.04 a>(0 2 41 42 43 44 45 Latitude (° N)

Figure 1.11 Mean scarring frequency against latitude has a negative curvilinear trend from south to north (y=-0.0177x2 + 1.5115x - 32.152, R2 = 0.42, ANOVA: F2,i3=4.0, P=0.050).

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 5-7 mm 7-9 mm 9-11 mm 11-13 mm

to -0.15 (0

2 -0.45

□ Unscarred -0.75

Figure 1.12 Unscarred snails had thinner shells at the aperture (mm +/-SE, log transformed) than scarred snails (ANOVA: F 3>89i=282.3, P=0.0290). Data from the five most heavily scarred sites were grouped into size classes for graphical purposes, however data were left as continuous variables for statistical analyses.

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. E E. B (0 a> c o 868 !E I- 2 3 r o a. <

Recent Scar Old Scar Unscarred (close) (far) (0.5-1.5 mm) (1.5-3.5 mm)

Figure 1.13 Shell thickness of snails with scars close (0.5-1.5 mm) to the aperture edge (filled bar), with scars far (1.5-3.5 mm) from the aperture edge (grey bar), and without scars are compared (open bar). Shell thickness (mm +/-SE, log transformed) is greater when scar is close to the aperture than when a shell is unscarred. (ANOVA: F2, 1053=8.1, P=0.0003). Levels not connected by same letter are significantly different (LS Means Differences Tukey HSD, a=0.05), and number of snails measured is shown at the top of each bar.

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. CHAPTER II

VARIATION IN LITTORINA SAXATILIS MORPHOLOGY ACROSS THE GULF OF MAINE: SNAIL VULNERABILITY TO CRAB PREDATION

Abstract

The introduced predatory Asian shore crab Hemigrapsus sanguineus has the potential

to impact the native snail Littorina saxatilis along the Atlantic coast of North America,

overlapping as it does in both the habitat and range of the snail. Snail populations located

500 km apart in New Hampshire and Maine are significantly different in shell shape, and

northern individuals have significantly thinner shells and are more vulnerable to crab

predation. As H. sanguineus continues to spread and increase in abundance,

understanding the vulnerability of native prey will be critical to elucidating potential

impacts on intertidal community structure.

Introduction

Prey employ chemical, morphological and behavioral defenses to contend with

predation. The expenditure of energy on these mechanisms must be balanced with energy

required for metabolism, growth, and reproduction. Defenses may already be in place

because they are always present (constitutive) or defenses may be plastic and produced

when exposed to a predator (inducible) (Harvell 1986). When predators are introduced to

an area, their prey may not have developed appropriate defense mechanisms, or the prey

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. may have had prior experience with a similar predator which may have required defense

responses (Freeman and Byers 2006).

Shelled gastropods provide clear evidence for evolution influencing morphological

changes over time and space (Vermeij 1987). Snails in the presence of shell-breaking

predators often will develop thicker shells through adaptation or phenotypic plasticity

(Ebling et al. 1964, Hughes and Elner 1979, Johannesson 1986, Appleton and Palmer

1988, Trussell 2000), since thicker shells are often stronger shells (Vermeij 1987).

Variation occurs through the influence of not only predation but also other biotic

influences, such as competition. Abiotic factors also play an important role in

development and adaptation, and may interact with biotic factors: for example, calcium-

carbonate shells are formed more efficiently (requiring less energy) in warmer

environments (Vermeij and Currey 1980a), but the need to produce thicker shells in the

face of predation may cost more energy in colder environments. Wave-energy may also

influence morphology: snails in wave-exposed, high energy environments spend more

energy on the development of a large foot to grip the substrate than snails in low-energy

systems (Etter 1988, Trussell 1997).

The common intertidal periwinkle Littorina saxatilis displays a latitudinal difference

in shell thickness in the Gulf of Maine (Chapter I). Shells in the northern portion of the

Gulf of Maine are much thinner than those in the southern portion. Crab predators occur

throughout this range, and include a mixture of native species, an older crab invader, and

a newer crab invader. Potential native crab predators include xanthid and cancrid crabs

(Gosner 1978), however the two newest exotic species now dominate the intertidal rocky

shores of most of New England. The portunid, European shore crab, Carcinus maenas,

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. has been present along the Gulf of Maine coast since the 1890’s. The grapsid, Asian

shore crab, Hemigrapsus sanguineus, which was first discovered on the New Jersey shore

in 1988, moved north to southern Cape Cod by 1992, and crossed north and east into the

Gulf of Maine by 1998 (McDermott 1998, Tyrrell and Harris 1999), arriving in 2005 on

the central Maine coast in Acadia. South of Cape Cod, Hemigrapsus has apparently

replaced Carcinus in many intertidal locations (McDermott 1998, Ahl and Moss 1999,

Lohrer and Whitlatch 2002a). In the southern Gulf of Maine, both crab species now co

occur in rocky intertidal areas, but Carcinus remains as of 2006 the most abundant

intertidal crab.

Objectives

I tested whetherLittorina saxatilis in the northern Gulf of Maine differ in

vulnerability to predation by H. sanguineus from snails in the southern Gulf of Maine by

examining their variability in shell structure and by measuring crab preference when

given a choice between snails from the two locations. I hypothesize that northern Gulf of

Maine snails will be both thinner-shelled and more vulnerable to predation than

conspecifics from the South.

Methods

Snail Morphology

As there appears to be variation in morphology across latitude for Littorina saxatilis, I

chose two sites (Figure 2.1) separated by 1.86 degrees in latitude to examine geographic

differences in snail morphology and vulnerability to crab predation. The northeastem-

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. most site in Maine is Wilbur Neck (WN), located within Cobscook Bay (44.90°N -

67.15°W) and is protected from intense wave exposure. The southern site is in New

Hampshire at Odiomes Point (OP) (43.04°N -70.71 °W) and is semi-exposed. The sites

differ in temperature regime; coastal water in WN is about 5 °C colder in the summer

(May to September) temperatures than in OP (Appendix A, Figure A. 1). However both

sites have similar salinities largely ranging from 30 to 33 ppt

(http://www.ndbc.noaa.gov/).

From March to April 2005,1 collected snails from the upper-intertidal zone of each

site and measured shell height, spire height, shell width, shell thickness at the aperture

(Figure 2.2), dry shell weight, and dry tissue weight. Only snails with intact shells (shells

without blemishes, chips, and scars) were used for these measurements. Shell dimensions

were measured using digital calipers. In order to measure the aperture lip thickness

consistently without breaking the curved edge of the lip, I measured 1.0 mm inward from

the edge of the outer lip of the aperture, by marking the calipers appropriately.

All measurements were checked for normal distributions. Dry tissue weight and dry

shell weight were normally distributed, and for these two measurements, I restricted the

pool of studied shells to shell heights between 5.5 and 7.5 mm because most of the

dissected snails (haphazardly selected from the sample) fell between these shell heights.

For shell thickness and width site-to-site comparisons, snails used were 5.0 to 8.0 mm in

height because most of the snails collected were within these heights. The mean shell

height was 6.8 mm (+/-0.101 mm SE; n=62) for OP snails and 6.5 mm (+/-0.059 mm SE;

n=126) for WN snails, but all analyses used height as a covariate. Shell thickness, width,

height and spire height measurements were all normally distributed. For all snail

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. morphological measurements, I compared the two sites (WN and OP) using analysis of

covariance (ANCOVA), with site as a fixed effect and height as a covariate. If the

interaction between site and height was significant, I performed a least squares (LS)

means (adjusted means) contrast between the sites. All statistical analyses were

performed with the statistical software JMP 5.1.

Snail Vulnerability to Predation

To test whether snails of differing thickness are differentially vulnerable to predation,

I performed a series of predation trials using male H. sanguineus. Male crabs were used

to keep claw sizes more consistent, as especially for this species, there is a sexual

dimorphism in claw size (female crabs have much smaller claws than males). I limited

the present studies to one species, the species that currently occurs at the southerly site

(OP) but not yet at the northerly site (WN). Crabs were starved for 48 hours prior to the

trials. Then crabs 22.6 to 31.0 mm in carapace width (mean 26.8 +/- 0.34 mm SE; all

crabs were statistically similar in carapace width) were placed in microcosm feeding

chambers (500 ml of aerated 35 ppt seawater in plastic 750 ml Ziploc® containers). Five

to ten chambers at a time were placed in a large covered opaque chamber (39-gallon

plastic Sterilite® storage bin) which did not permit the entry of light or other visual

stimuli. One OP and one WN snail (matched for height +/- 0.04 mm SE) were placed in

each chamber. Snails ranged from 5.8 to 7.8 mm in height (mean 6.5 +/-0.06 mm SE) and

were sized proportionally to the carapace width of the crab used per trial in order to

minimize variability in crab to snail size ratio. I wanted to examine crabs’ behavior when

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. presented with a large snail close to the maximum size the crab could successfully prey

upon.

Once snails were placed in each feeding chamber, I removed the cover to the large

chamber to examine the snails every 5 minutes for damage or mortality. Each trial was

terminated when the first snail was eaten. If both snails were eaten within any five minute

period, the trial was eliminated from the analysis. If neither snail was eaten after a one

hour period, the trial was eliminated. A total of 41 crab trials (using unique crabs that

chose one snail first over another snail) were used in the analysis (Chi Square Goodness

of Fit).

Results

SnaU Morphology

Southern snails from Odiomes Point, New Hampshire (OP) have significantly greater

dry tissue weight than snails from Wilbur Neck, Maine (WN) (LS means contrast:

PO.OOOl; Figure 2.3, Table 2.1). As snails increase in height their tissue weight also

increases, and this relationship is significantly different between the sites (F=0.0108,

Table 2.1). Snails from OP have significantly greater dry shell weight than snails from

WN (LS means contrast: PO.OOOl; Figure 2.4, Table 2.1). As snails increase in height

their shell weight also increases, and this relationship is significantly different between

the sites (P=0.0004, Table 2.1)

Snails from OP are significantly wider than snails from WN (LS means contrast:

P<0.0001; Figure 2.5, Table 2.2). As snails increase in height their shell width also

increases, and this relationship is significantly different between the sites (P=0.0062,

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Table 2.2). Snails from OP have significantly thicker shells than those from WN while

controlling for height (ANCOVA: PO.OOOl; Figure 2.6, Table 2.2). The mean aperture

shell thickness is 0.36 mm (+/-0.010 mm SE; n=62) for OP snails and 0.17 mm (+/-0.003

mm SE; n=126) for WN snails. As snails increase in height their shell thickness at the

aperture also increases, and this increasing relationship is not statistically different

between the sites for snails 5-8 mm in shell height; there is no interaction between height

and site (ANCOVA: P=0.2913, Table 2.2a). Snails from WN have significantly taller

spires than snails from OP (LS means contrast: PO.OOOl; Figure 2.7, Table 2.2). As

snails increase in height their spire height also increases, and this relationship is

significantly different between the sites (P=0.0049, Table 2.2). (See Figure 2.8 for a

visual comparison of snails from the two sites matched for height.)

Snail Vulnerability to Predation

Out of 41 trials during which Hemigrapsus sanguineus were observed eating one

snail before another snail, the crabs chose the northeastern Maine (WN) snail first over

the New Hampshire (OP) snail during 33 (80.5%) of the trials. Thus, crabs chose the

thinner-shelled WN snail over the thicker-shelled OP snail in a significantly greater

number of trials (x2—8.4, P<0.005, DF=1) (Zar 1999). (See Appendix C for results of 36

additional trials performed.)

Discussion

Phenotypic characteristics may be highly variable among and within populations, as

they are potentially influenced by many local biotic and abiotic forces. Species that have

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. low larval dispersal often exhibit high variability in morphology as the result of local

shaping forces. The rough periwinkle, Littorina saxatilis, broods its young and has been

shown to exhibit high variability across its range and even within a site based on its

habitat (Janson and Sundberg 1983, Johannesson 2003, Grahame et al. 2006). I studied

the morphology of this prey species, L. saxatilis, at two sites separated by about 400 km.

Since L. saxatilis are ovoviviparous, little gene flow is expected to occur between

populations many kilometers apart. Populations of this snail in northeastern Maine and

New Hampshire are significantly different in shell morphology (Figure 2.8). Although

the morphological variables that describe shell shape cannot be divorced from one

another (Vermeij 1987), the individual variables: dry tissue weight, dry shell weight,

width, shell thickness at the aperture, and spire height are all significantly different

between the sites. Since snails from WN are narrower, so there is less room for tissue

growth, thus snails from WN have less tissue weight than OP snails, which are wider.

Snail behavior reflects this difference in shell morphology as well: snails from WN,

which have lighter, narrower, and thinner shells, move faster than those from OP with

heavier, wider, and thicker shells (Appendix D). Fast crawling behavior may be

beneficial to snails vulnerable to Nucella lapillus, the predatory dogwhelk (Harris, L.G.

personal communication, Matthews-Cascon 1997); however, crawling fast would not

likely benefit a snail matched with a crab predator.

In addition to these populations being separated by a great distance, which limits or

largely prevents gene flow, local abiotic and biotic factors at each site likely explain their

morphological differences. The sites chosen in this study differ in temperature regime,

wave exposure, and crab species assemblage. Although the locations have comparable

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. crab abundances in the region as a whole (Chapter I, Figure 1.8), the individual sites

differ in crab abundance (Chapter I, Table 1.3). Seeley (1986) examined L. obtusata in

the Gulf of Maine and found that previous to the invasion of C. maenas snails had thinner

shells with a higher spire, suggesting that the crabs’ presence stimulated rapid selection

for thicker shells with a lower spire.Littorina saxatilis shells in the north have both a

higher spire and thinner shells (Figures 2.6, 2.7 and 2.8), which makes them more

vulnerable to crab predation, as shown in my predation trials. Perhaps differences in crab

abundance accounts for differences in shell morphology, as selection pressures may be

reduced in the north due to lower crab abundance.

The colder temperatures (by as much as 5°C during prime growth season) in

northeastern Maine (Appendix A, Figure A.l) may also explain the thinner snail shells in

comparison to New Hampshire snails, as less calcium carbonate is accreted in colder

temperatures (Vermeij and Currey 1980b). As noted earlier, snails exposed to greater

wave action allocate more energy to body size in order to more firmly grip to the

substrate (Etter 1988), and less energy to shell thickness (Ebling et al. 1964, Hughes and

Elner 1979, Johannesson 1986) because there are typically fewer predators at more

exposed sites. However, the study sites I chose have wave exposure/predator regimes

contrary to expected scenarios. Odiomes Point is a semi-exposed site with predators

present, and Wilbur Neck is a protected site with a low number of predators present, and I

expected a more equal number of predators at the two sites (see Chapter I, Figure 1.8).

The reduced number of predators in Wilbur Neck is likely due to the fact that this site is

more liable to freeze in the shallow waters than a more exposed site in the region, such as

Eastport (EA) (Chapter I, Figure 1.1) (Harris, L.G., personal communication). During

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. wanning trends in water temperature, C. maenas is likely to increase in number in these

shallow bay sites, which occurred in 1995 when Matthews-Cascon (1997) sampled in this

particular area of the bay. In order to more precisely understand differences in shell

thickness between these two areas, examining snails in sites of high wave exposure (with

potentially lower crab abundance) and sites of low wave exposure (with potentially

higher crab abundance) in both regions will be of value. Even at Eastport (EA), a site

with similar wave exposure and predator abundance to that of Odiomes Point (Appendix

A, Table A.4), shells are even thinner than those from WN (Appendix A, Table A.3; EA:

0.11 +/-0.004 mm SE; WN: 0.16 +/- 0.003 mm SE). Snails in the north in general are

thinner likely due to both temperature differences and historical crab abundance

differences (see General Introduction and Chapter I).

Shell thickness is one of the most important features in determining vulnerability to

predation (Vermeij 1987); thinner shelled individuals from WN are more vulnerable to

predation by crabs than those from OP. Of interest is that the newly introduced crab H.

sanguineus has larger claws than a C. maenas of equal carapace width (Appendix E),

which indicates thatH. sanguineus may have an equal or greater effect on prey items than

C. maenas. A fairer comparison of H. sanguineus and C. maenas compares the per capita

effect of crabs that are likely to influence L. saxatilis of the common size range.

Comparisons of these sized crabs (~22-30 mm for H. sanguineus and ~44-56 mm for C.

maenas) show that both species have similar sized claws (Figure E.2). In practice, H.

sanguineus is suspected to have a larger effect because in the intertidal zone where L.

saxatilis is common, the expected crabs to overlap here would be small C. maenas that

would not likely have the ability to crack L. saxatilis and H. sanguineus that would

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. become large enough here to successfully prey upon the snails (larger C. maenas would

move to the lower intertidal and subtidal zone and overlap with L. saxatilis for a shorter

period of time during high tide). However more detailed analyses of internal musculature

would be necessary to compare crabs of different genera more precisely (Vermeij 1977,

Taylor 2000, 2001). In areas where H. sanguineus has replaced C. maenas in the

intertidal zone, impacts on invertebrate populations are comparable or even greater than

when C. maenas was the only crab invader on the shores, likely due to the greater

densities of the newer invader (Lohrer and Whitlatch 2002b). As H. sanguineus move

further northward with warming ocean temperatures, thinner shelled L. saxatilis in the

north may be more vulnerable to crab predation than conspecifics from the south. In turn,

however, there is the potential that, as the oceans warm, northern snails may be able to

increase calcium carbonate deposition, and thus have a modicum of hope as a new

predator arrives.

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Table 2.1a ANCOVAs for snail dry tissue and shell weight with shell height as the covariate between snails from Odiomes Point and Wilbur Neck.

Dry1 "issue Dry Shell Weig ht Weight F P F P Site 39.3 <0.0001 293.1 <0.0001 Height 21.3 <0.0001 67.9 <0.0001 Site*Height 7.3 0.0108 15.7 0.0004 DF: Whole Model, Corrected 3, 37 3, 37 Total LS means F,,34=39.3, F,,34=293.1, contrast for P<0.0001 P0.0001 Site

Table 2.1b Differences in L. saxatilis dry tissue and shell weight between Odiomes Point and Wilbur Neck (N=the number of observations).

Dry Tissue Dry SheU Weight N Weight (g) (g) mean +/-SE mean +/-SE Odiornes 14 0.0038 0.00049 0.0701 0.00470 Point, NH Wilbur 24 0.0015 0.00011 0.0258 0.00125 Neck, ME

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Table 2.2a ANCOVAs for shell width, thickness and spire height with shell height as the covariate between snails from Odiomes Point and Wilbur Neck.

Shell Shell Width Thickness at Spire Height the Aperture F p F P F P Site 62.5 <0.0001 560.7 <0.0001 666.1 <0.0001 Height 170 <0.0001 18.8 <0.0001 308.9 <0.0001 6.5 Site*Height 7.7 0.0062 1.1 0.2913 8.1 0.0049 DF: Whole Model, 3, 187 3, 187 3, 187 Corrected Total LS means Fi,184=62.5, Fi,184=666.1, contrast for N/A PO.OOOl PO.OOOl Site

Table 2.2b Differences in L. saxatilis shell width, thickness and spire height between Odiomes Point and Wilbur Neck (N=the number of observations).

Shell Thickness Shell Width Spire Height at the Aperture N (mm) (mm) (mm) mean +/-SE mean +/-SE mean +/-SE Odiornes 62 6.12 0.094 0.36 0.010 1.60 0.041 Point, NH Wilbur 126 5.64 0.049 0.17 0.003 2.26 0.026 Neck, ME

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Figure 2.1 Field sites: Wilbur Neck in northeastern Maine (WN) and Odiomes Point, New Hampshire (OP).

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. B

Figure 2.2 Snail dimension measurements diagram: (A) spire height, (B) width, (C) shell thickness, and (D) height (drawing adapted from Janson and Sundberg 1983).

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 0.008 0.007 3 o WN .c 0.006 u> 0.005 I 0 0.004 3 (0 (0 0.003 i— 0.002 Q 0.001 0 5.5 6.5 7 7.5 Height (mm)

Figure 2.3 Dry tissue weight (g) against height (mm) for L. saxatilis specimens from Odiomes Point, NH (OP) (filled diamonds, solid line) y=0.0023x - 0.0111, R2=0.413, P=0.0132 and from Wilbur Neck, Northeastern Maine (WN) (open diamonds, dashed line) y=0.0006x - 0.0023, R2=0.362, P=0.0019. Snails from OP have significantly greater dry tissue weight than snails from WN (LS means contrast: P>0.0001, Table 2.1).

0.12

3 0.1 o> 0.08

%il 0.06

5.5 6 6.5 7 7.5 Height (mm)

Figure 2.4 Dry shell weight (g) against height (mm) for L. saxatilis specimens from Odiomes Point, NH (OP) (filled diamonds, solid line) y=0.0279x -0.1131, R2=0.669, P=0.0004 and from Wilbur Neck, Northeastern Maine (WN) (open diamonds, dashed line) y=0.0098x - 0.0365, R2=0.703, P<0.0001. Snails from OP have significantly greater dry shell weight than snails from WN (LS means contrast: ,P>0.0001, Table 2.1).

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 7.5 7 6.5 6 5.5 5 4.5 4 5 6 7 8 Height (mm)

Figure 2.5 Shell width (mm) against height (mm) for L. saxatilis specimens from Odiomes Point, NH (OP) (filled diamonds, solid line) y=0.8967x + 0.0482, R2=0.918, P<0.0001 and from Wilbur Neck, ME (WN) (open diamonds, dashed line) y=0.784x + 0.5493, R2=0.891, PO.OOOl. Snails from OP are significantly wider than snails from WN (LS means contrast: PO.OOOl, Table 2.2).

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® 0.1 a < 0 “i “I 6 8 Height (mm)

Figure 2.6 Shell thickness at the aperture(mm) against height (mm) for L. saxatilis specimens from Odiomes Point, NH (OP) (filled diamonds, solid line) y=0.0271x + 0.1734, R2=0.0808, P=0.0252 and from Wilbur Neck, Northeastern Maine (WN) (open diamonds, dashed line) y=0.0165x + 0.0649, R2=0.132, PO.OOOl. Snails from OP have significantly greater shell thickness at the aperture than snails from WN (ANCOVA: PO.OOOl, Table 2.2).

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. ♦ OP oWN Oo £ £ ♦ O) ♦ ♦ '5 x 'E. ♦♦ (0

0.5 5 6 7 8 Total Shell Height (mm)

Figure 2.7 Spire height (mm) against total height (mm) for L. saxatilis specimens from Odiomes Point, NH (OP) (filled diamonds, solid line) y=0.281 lx - 0.2998, R2=0.4849, .PO.OOOl and from Wilbur Neck, Northeastern Maine (WN) (open diamonds, dashed line) y=0.3899x - 0.2764, R2=0.7462, PO.OOOl. Snails from WN have significantly taller spires than snails from OP (LS means contrast: PO.OOOl, Table 2.2).

Figure 2.8 Snails matched for length (6.85 mm ±0.05) from (A) Odiomes Point, New Hampshire (OP) and (B) Wilbur Neck, Maine (WN).

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. CHAPTER IH

DOES SUB-LETHAL INJURY PROVIDE PREY WITH A REDUCED RISK OF LETHAL PREDATION?

Abstract

Although scars are the physiological repairs of chipped shells, they also may confer

subsequent advantage to snails against future attack. Crabs spend a longer time handling

scarred Littorina saxatilis over unscarred conspecifics; however, this trend is only

significant when considering the most recent intertidal crab invader, Hemigrapsus

sanguineus. Carcinus maenas did not differ significantly in the amount of time to handle

L. saxatilis of differing scarring history, which may be explained by their species-specific

handling behavior, claw morphology, and evolutionary history. While sub-lethal injuries

in the form of shell scars can provide snails with a reduced risk of future predation, it is

important to investigate crabs’ species-specific handling behavior.

Introduction

Shell forming mollusks offer accessible traits that often have been examined as

evidence reflecting ecological and evolutionary changes over space or time (Vermeij

1987). Shell thickness is one of the most important features determining a mo Husk’s

susceptibility to predation (Vermeij 1987) and scarring may change shell thickness

(Greenfield et al. 2002). Evidence of sublethal predation in prey populations is often

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. associated with an increased risk of lethal predation (Geller 1990b, Meyer and Byers

2005), which is a generally accepted notion among researchers. However, this

phenomenon does not always occur in nature. Greenfield et al. (2002) found that shell

scars, from previous crab predation events, provide marsh snails, Littoraria irrorata, with

a reduced risk of predation from blue crabs, Callinectes sapidus. While a scar may

weaken a shell by compromising its structural integrity, scar repair may protect the snails

from predation by creating a thicker shell at the site of the wound (see Chapter I; Figure

1.13). That such prolonged handling time does decrease predator success has been shown

for the predator, C. sapidus, with its prey the snail,Littoraria irrorata in marshes on the

southern Atlantic coast of the U.S. (Greenfield et al. 2002). I test this observation for a

northern rocky intertidal system with the snail Littorina saxatilis and the crab predators,

Hemigrapsus sanguineus and Carcinus maenas. While the southern system Littoraria(

irrorata— Callinectes sapidus) consists of a pair of native prey and native predator, the

northern system consists of a native prey and two non-native predators, one of which

(Carcinus maenas), however, has had a long evolutionary history with the prey (Littorina

saxatilis) in Europe.

Blundon and Vermeij (1983) found that scarred and unscarred Littoraria irrorata, the

marsh periwinkle, were equally resistant to crushing forces, however the method crabs

typically use to eat Littorina saxatilis is peeling rather than crushing, unless the snail is

very small in comparison to the crab size (personal observation, Bertness and

Cunningham 1981). Thus, if snails had thicker shells at the scar, as scarred snails

typically do (see Chapter I, Figure 1.12 and 1.13), then as the crab peels away the shell to

the point of the scar, the crab may spend more time attempting to crack this portion and

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. thus potentially give up the attack. The longer it takes a predator to eat a prey item, the

less likely the predator will be successful (Rilov et al. 2004). Here, I investigate whether

sublethal predation may in fact provide prey with a reduced risk of lethal predation.

Objectives

I investigated the morphology of the native rocky shore snail, Littorina saxatilis, and

the snail’s differential vulnerability to predation based on its scarring history. To

understand how scarring in snails influences the success of predation by crabs,

observations of crabs handling both scarred and unscarred snails were made. I

hypothesized that crabs take longer to handle scarred snails and that as a result of this,

scarring provides these snails with a reduced risk of successful predation encounters.

Methods

To understand how scarring in snails influences the success of predation by crabs,

observations of crabs handling both scarred and unscarred snails were made. All

specimens were collected from southern New England, where Carcinus maenas and

Hemigrapsus sanguineus have overlapped for the longest period of time. Collections

were made from a rocky-intertidal site where Littorina saxatilis and the crab predators C.

maenas and H. sanguineus are all consistently abundant (Weekapaug Point, RI). Since H.

sanguineus are in such greater abundance than C. maenas in the intertidal zone at

Weekapaug Point, H. sanguineus were collected by hand, and C. maenas were collected

using minnow traps baited with canned cat food.

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Morphometric analyses of snail height and crab carapace width were performed in

order to properly match crabs with snails, based on preliminary predation studies offering

snails of different sizes to crabs (Teck, unpublished data). On average, H. sanguineus

were 27.6 +/-0.60 mm SE in CW matched with snails 7.7 +/-0.41 mm SE in height, and

C. maenas were 49.3 +/-1.31 mm SE in CW matched with snails 8.4 +/-0.35 mm SE in

height. While the experimental animals may have differed in average CW, both species

were successful in eating snails of similar sizes (Appendix F, Figure F.2).

After 48 hours of acclimation to room temperature and laboratory conditions (30 ppt

seawater in tanks with aeration and pump filters) experimental trials began in a laboratory

dark room under a red light. Trials were performed in aquaria filled with 2.25 L of 30-35

ppt aerated seawater. Individual crabs (starved for at least 48 hours) were observed when

presented simultaneously with two snails matched for height, one unscarred snail and one

scarred snail. During each trial the crab’s handling of each snail was timed until both

snails were entirely consumed or for one hour (whichever came first). Trials were

discontinued if crabs did not touch the snails within the first 10 minutes. Only handling

times for 27 out of 94 snails were analyzed because crabs left many snails untouched,

unchipped, or uneaten (see Appendix F, Table F.l for more details on the trial outcomes).

Handling times for each snail that was consumed first were compared across species and

scarring history. Handling times were log transformed for normality. All analyses were

performed with the statistical software JMP 5.1.

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Results

Hemigrapsus sanguineus on average took longer to handle and consume scarred over

unscarred L. saxatilis (Figure 3.1, Table 3.1; ANOVA: Fi,i4=6.0, iM).0297). However,

Carcinus maenas did not differ significantly in the amount of time to handle and

consume scarred and unscarred L. saxatilis (Figure 3.1, Table 3.1; ANOVA: Fi,h=1.9,

P=0.1993). Although there is a trend for C. maenas to take longer to handle scarred over

unscarred snails, there is high variance in handling times, especially for scarred snails

(Table 3.1). Also, I expected a decreasing relationship between handling time and crab to

snail size ratio. I did not see this trend most likely because I did not include trials with

very small crab to snail size ratios (Appendix F: Table F.2, Figure F.l).

Discussion

Scarring provides historical evidence of failed predation events, revealing a level of

resistance by the prey to shell-breaking predators. Greenfield et al. (2002) suggest that

scarred marsh snails initially may be thicker than unscarred snails, and the scar is

verification that natural selection is in fact taking place in the system (Vermeij 1982b).

General snail shell thickening could occur as an inducible defense either from crabs

directly handling snails or from crab cues present in the water (Appleton and Palmer

1988, Etter 1988, Trussell 1996). However, the scar itself is thicker than the surrounding

shell, so the mechanism of healing from a crab’s failed attack results in a portion of the

shell that is more difficult to crack than the rest of the shell (see Chapter I). Overall

thickening of a snail shell provides snails with a reduced risk of predation (Chapter II),

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. and here, I show that localized thickening of the shell at a scar also may provide

protection for a snail.

Hemigrapsus sanguineus take longer to handle L. saxatilis with shell scars than those

without scars. The thicker portion of the shell at the scar lengthens the handling time for

H. sanguineus; as predicted, it is more difficult for these crabs to handle and consume

scarred snails over unscarred snails. Thus, these scarred snails are in fact less vulnerable

to predation by this species of crab, as increased handling time reduces the success of

predation (Rilov et al. 2004). Additionally, in Southern New England, where this study

was conducted, H. sanguineus is likely the principal predator that this snail will come

into contact with (personal observation, Lohrer and Whitlatch 2002a). The fortification of

the shell provided by the scar, however, does not always decrease the vulnerability of the

snail, since each crab species likely handles the snails in a different manner.

Carcinus maenas did not spend more time handling and consuming scarred snails

over unscarred snails, so the thickening of the shell at the scar did not prove to prolong

the act of predation consistently. The scar likely protects the snail best when a crab uses

the peeling technique of chipping away at the aperture over simply crushing the shell.

Crabs typically use the crushing technique when the crab to snail size ratio is high; as the

crab to snail size ratio decreases, a crab is more likely to have trouble crushing the shell

and will switch to peeling the snail shell at the aperture (Bertness and Cunningham

1981). However, there is a trend for C. maenas to take longer to handle the scarred snails

over the unscarred snails, but there is high variability in these handling times. Since I

have quite low power for this study (0.24), it is likely that with a higher sample size there

may be differences in C. maenas handling times for the scarred versus unscarred snails.

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Both crab species in my experimental trials appeared to use the peeling technique

more often than the crushing technique. When the peeling technique is applied with one

claw, the other claw often holds the major whorl with a firm grip. As a result of this grip,

sometimes crabs of either species will puncture a hole into the whorl, even if they are not

successful at chipping away at the aperture. A few trials thus ended with a snail shell

intact apart from a hole in the body whorl caused by this behavior. Perhaps, C. maenas

despite its similar sized claws to H. sanguineus did use the crushing technique more often

than the peeling technique; larger crabs preying on relatively smaller snails will, as noted,

likely crush the entire shell, and the potential role of a thickened scar would thus be

eliminated or reduced substantially.

Hemigrapsus sanguineus has occurred on the southern New England coast only since

1993, whereas Carcinus maenas has been in the same region since around 1800. The

differences observed in handling scarred versus unscarred shells may be due to the much

longer experience C. maenas has had withL. saxatilis (both in North America and

Europe). In Georgia, U.S., scarred marsh snails Littoraria irrorata were thicker at the

aperture and were chosen less frequently over unscarred conspecifics by the predator

Callinectes sapidus (Greenfield et al. 2002). In shores further north, in Rhode Island,

Littorina saxatilis also had thicker scarred individuals, however, contrary to the

Littoraria irrorata-Callinectes sapidus pattern, the crab with the longest co-occurring

history withLittorina saxatilis, Carcinus maenas, did not take longer to consume scarred

versus unscarred snails. Perhaps the difference in thickness between scarred and

unscarred marsh snails is much greater than differences between scarred and unscarred L.

saxatilis in rocky shores further north. The healing process may produce a thicker scar

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. further south due to a greater ability to accrete calcium carbonate in the warmer waters

(Vermeij and Currey 1980); and thus Callinectes sapidus in the south may not have

developed the ability to overcome such great irregularities in shell thickness. Differences

in scarred and unscarred predation success may also be due to differences in claw

anatomy of the predators.

While these studies highlight the importance of recognizing the intricate variability

both within prey species and within predator species, conclusions can only be tentative as

trends would need to be examined with a higher sample size. The behavior of shell-

breaking predators may appear similar (i.e. they may appear to use the same techniques),

but behavior cannot be generalized across a crab guild—individual species must be

examined to see if the application of a particular behavior yields similar results across

species. Finally, investigating interactions between non-native predators and native prey

elucidates the role of time in mediating interactions between these guilds.

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Table 3.1 Scarred (S) versus unscarred (US) snails’ handling times for C. maenas (CM) and H. sanguineus (HS), all crabs were only used once.

Mean Log SE Log N ANOVA Handling (Mean (Mean (num Time Handling Handling ber of F ratio P (sec) Time) Time) crabs) CM US 129.0 2.00 0.122 7 Fi,ii=1.9 0.1993 S 343.8 2.33 0.224 5 HS US 102.9 1.95 0.075 11 Fi,i4 6.0 0.0297 s 200.0 2.28 0.080 4

I Scarred 500 □ Unscarred g 400 42- 0) E 300 H O) _ _ _ .E 200 11 ■ o « 100 - t -

0 Unscarred Scarred Unscarred Scarred C. maenas H. sanguineus

Figure 3.1 Carcinus maenas did not differ significantly in the amount of time (sec +/- SE) to handle and consume scarred and unscarred L. saxatilis (ANOVA: Fi;n=1.9, P=0.1993, power=0.24). Hemigrapsus sanguineus on average took longer to handle and consume scarred over unscarred L. saxatilis (ANOVA: Fi,i4=6.0, P=0.0297). Both species took the same amount of time to handle unscarred snails (ANOVA: F i ,17=0.16, .P=0.6911) and scarred snails (ANOVA: Fi,g=0.03, P=0.8583). The sample size (number of crabs) for each average is shown above each bar, and crabs were only used once. Handling times were log transformed for normality but presented in the figure with untransformed handling times.

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. SUMMARY

In biogeographical studies phenotypic variation is often examined across a latitudinal

gradient. This is an example of a cline, or a gradual variation of a phenotype in a species

across a landscape. Clines can occur over large geographic scales, such as latitude, or

they can occur on smaller scales, such as across a tidal gradient. Through selection,

species shift their morphology and behavior as a result of a spectrum of abiotic and biotic

stresses that vary over time and space. Shell forming mollusks serve as a good model

because they often express ecological and evolutionary changes over time and space in

conspicuous ways such as shell morphology. Additionally, species that have limited

genetic mixing based on their reproductive cycle and low rate of dispersal often have

highly variable phenotypes among separate populations as the result of local adaptation.

One variation commonly seen across shelled mollusks is that shells tend to get thinner

in higher latitudes. Specifically for shell thickness, this negative relationship with latitude

is largely influenced by temperature and predation intensity. It is more difficult to accrete

calcium carbonate (shell) in colder waters, and shell-breaking predators can influence the

adaptation of shell form in a variety of ways. In the example of shell thickness, when

there are fewer predators there is a reduced pressure for the selection of thick shells.

Additionally, there tends to be fewer predators in these colder waters, so in this case,

temperature is also closely influencing predation intensity.

In general, many gastropods such as Nucella and littorinids express some similar

phenotypic patterns in several species and in various locations. When there is low wave

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. exposure, there tends to be a higher predation pressure because it is easier for crabs to

inhabit a wave protected site. Snails in these locations tend to have thicker shells, with

lower spire heights, and smaller apertures in response to this higher predation pressure,

while snails at sites with high wave exposure and thus lower predation pressure, likely

will have thinner shells, higher spires and larger apertures. Also, snails would need to

have a larger foot in these wave exposed sites in order to more firmly grasp to the

substrate. However, this regime may change at different temperatures and habitats. A site

may have high predation pressure and low wave exposure, but at cold temperatures it

may be difficult for snails to accrete calcium carbonate to thicken their shells.

The objective of my research was to explore a potential cline in a shell-forming

mollusk, Littorina saxatilis, across a dynamic landscape. The model I used is unique

because I examined a native mollusk species overlapping with two non-native predatory

crabs, whereas earlier studies have investigated native predation pressure influencing

shell shape. I found that clinal variation in shell morphology exists across the New

England range of Littorina saxatilis, and this variation is likely influenced largely by

historical differences in crab abundance and temperature. Snails in the north have shell

characteristics that make them more vulnerable to shell-breaking predators than those

from the south. The frequency of scarred snails at a site decreases as shell thickness

decreases largely due to the fact that shell-breaking predators are more often successful in

cracking thinner shells. Furthermore, those snails that are left scarred from failed

predation attempts may have protection from future predation events, due to localized

shell thickening at the healed over scar.

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. In the future, it would be interesting to test whether these differences in morphology

across New England are the result of genetic differences or phenotypic plasticity or a

combination of the two. Carcinus maenas has influenced shell shape inLittorina

obtusata in New England; before 1900, snails were thinner with higher spires than those

collected more recently among greater crab populations (Seeley 1986). Additionally, the

shells of northern New England snails are much thinner and weaker than southern New

England snails because predators tend to be more abundant further south, and Littorina

obtusata have been shown to exhibit phenotypic plasticity in shell shape in the presence

of chemical cues from predators (Trussell 1996). Freeman and Byers (2006) showed that

in the short time period H. sanguineus has been present in southern New England, the

mussel Mytilus edulis showed rapid evolution of inducible defenses when exposed to

chemical cues from this new predator, while mussels in areas further north did not show

inducible defenses when exposed to H. sanguineus (and H. sanguineus is not established

yet in these northern areas). However, both northern and southern populations responded

to the older invader, C. maenas which is present in both places. Thus, other prey, such as

Littorina saxatilis, are likely to exhibit similar species-specific responses to predators,

and these responses can rapidly evolve based on the intensity and composition of

predators present in the system

Currently, C. maenas populations are patchy over time and space among sites in

Downeast Maine. However, as temperatures rise, C. maenas will likely become more

abundant in areas further north, posing a threat to local thin-shelled L. saxatilis

populations. Additionally, it is likely that with these warmer waters, H. sanguineus will

begin to extend its range into these northern areas, and it would be interesting to see how

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. both predators might influence shell shape. Although in my 2005 survey, the Southern

region (NH and southern ME coast) had only 19% H. sanguineus versus C. maenas, this

is a great increase since 2001, when H. sanguineus was reportedly not established yet in

this region (Tyrrell 1999, 2002).

Abiotic and biotic factors, which influence the cline in shell morphology, are not only

changing across the landscape and shifting with seasonal patterns, but also they are

changing over time as the result of two major issues currently affecting the study of

biogeography. With global climate change, temperature may be modified causing a shift

in many aspects of the community including the abundance of predators in the system.

Additionally, taking into account that two of these predators are non-native species also

influences the intensity of their presence in the system. Understanding the impact of

specific introduced species that may cause shifts in predator-prey dynamics and

community structure is crucial as communities become increasingly rich with non-native

species.

Future Directions

To help explain why clinal variation exists in L. saxatilis, I will outline six studies

that could be applied to L. saxatilis (or any species). (1) Lab experiments: one could

expose individuals from the same broods to varying regimes of temperature, predator

presence, and composition, and then measure shell shape and growth. (2) Observational

data: one could examine both phenotypic and genetic clines, examine the gene flow and

dispersal of L. saxatilis, look at detailed abiotic data (such as oceanographic patterns and

microclimate temperature (air and water temperature) in the intertidal zone), and examine

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. long term predator intensity and composition. (3) Transplant experiments: transplant

individuals from the same broods to a new location and compare their morphology to

individuals left in original location (this study could be performed in the lab to avoid

causing any genetic mixing in the field). (4) Tethering experiments: tether snails in the

intertidal zone at various tidal heights in order to compare predation rates and evidence of

unsuccessful predation attempts on snails of different origins in various locations. (5)

Biogeography: looking at both invasive and native ranges of a species, one could

examine how the species differs across varying environments of multiple ranges.

Consider how long the species has occurred in each area. For example, how do L.

saxatilis in New England differ from L. saxatilis in San Francisco Bay? Although L.

saxatilis in San Francisco Bay have been traced back to New England genetically

(Carlton and Cohen 1998), have their exposure to differing temperature and predator

regimes caused differences in shell shape between the two regions? One could also look

at congeners (such as considering the guild of Littorinids in an area) and compare guilds

across a variety of ranges. (6) Historical data: one could compare shells from museum

collections during different time periods for shell shape and shell damage and consider

how the predator and temperature regimes have changed over time. For example,

littorinids in New England were first exposed to the predator combination of native crabs

and lobsters prior to 1800, then C. maenas was added to the system in around 1817, and

then in 1988, H. sanguineus joined the predator guild. In the future, an additional non

native species may become established in the region, for example, H. penicillatus could

arrive from Europe. How will this new predator regime within a likely altered future

temperature regime influence shell form in L. saxatilis?

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Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. APPENDICES

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. APPENDIX A

Field Site Details

Table A.1 Site names and codes.

Site Region Site Name Code Stonington Point, CT SP Southernmost Weekapaug Point, RI WP Rye Harbor, NH RH Southern Odiomes Point, NH OP Kittery Point, ME KP Evergreen site in Winter Harbor, ME EV Frazer site in Winter Harbor, ME FR Northern Wonsqueak site in Winter Harbor, ME WO Mermaid's Purse site in Prospect Harbor, ME MP Comstock Point, ME CP Eastport Harbor, ME EA Northernmost Passamaquoddy site near Pleasant Point, ME PA West Quoddy Head, ME QU Wilbur Neck, ME (near Pembroke, ME) WN

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Table A.2 Individual sites’ coordinates.

Site Latitude Longitude Region Code (°N) (°W)

SP 41.33 -71.91 Southernmost WP 41.33 -71.75 RH 43.00 -70.74 Southern OP 43.04 -70.71 KP 43.10 -70.66 EV 44.34 -68.05 WO 44.36 -68.05 Northern FR 44.38 -68.07 MP 44.40 -68.02 CP 44.82 -66.95 EA 44.89 -67.02 Northernmost PA 44.90 -66.99 QU 44.90 -67.15 WN 44.95 -67.04

Table A.3 Individual sites’ mean shell height and thickness (mm) (+/- SE) across sampled snails (N=number of observations).

Shell Height Shell Thickness Site Region N (mm) (mm) Code mean +/-SE mean +/-SE SP 182 7.41 0.180 0.30 0.010 Southernmost WP 200 8.76 0.167 0.39 0.011 RH 121 11.08 0.171 0.50 0.018 Southern OP 167 9.54 0.149 0.47 0.008 KP 140 9.45 0.138 0.32 0.011 EV 115 9.62 0.296 0.32 0.016 FR 123 6.63 0.186 0.22 0.008 Northern WO 228 8.75 0.205 0.31 0.012 MP 263 7.67 0.162 0.26 0.008 CP 107 6.68 0.169 0.12 0.005 EA 159 7.11 0.151 0.11 0.004 Northernmost PA 101 6.75 0.170 0.09 0.004 QU 100 8.64 0.202 0.23 0.011 WN 172 6.57 0.067 0.16 0.003

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Table A.4 Individual sites differences in log transformed mean crab density (per 0.5 m quadrat), comparison for all pairs using Tukey-Kramer HSD. Levels not connected by same letter are significantly different (a=0.05).

Log Region Site (Mean Crabs per 0.5m2) Southernmost WP 1.63 A Southernmost SP 1.00 B Southern OP 0.75 B C Northernmost PA 0.57 C D Northernmost EA 0.40 C D E Northern FR 0.37 D EF Southern KP 0.29 D EF Northern WO 0.29 EF Northern MP 0.19 EF Southern RH 0.16 EF Northernmost CP 0.10 EF Northernmost WN 0.06 F Northernmost QU 0.04 F

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Eastport, ME Bar Harbor, ME Portsmouth Harbor, NH

25 New London, CT

o o £ 3 2 o Q. E a> l-

Jan Apr May Jul Aug Oct Dec

Figure A.1 Average monthly temperatures for four sites closest to the four regions examined. I compiled data obtained from the National Oceanographic Data Center: NOAA/National Ocean Service (NOS) tide stations and NOAA/National Data Buoy Center (NDBC) buoys; “average water temperatures were computed from long-period records ranging from several years to several decades depending on how long observations have been taken at a given station” (http://www.nodc.noaa.gov/dsdt/cwtg/aboutCWTG.html). Thus, these temperatures are conservative averages bearing in mind more recent increasing seawater temperatures. During the months of April through October, bi-monthly averages are shown.

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. APPENDIX B

Shell Thickness Comparisons between Field Sites within each Region

Figure B.l Comparison of sites within the Southernmost region: Weekapaug Point (WP) has significantly thicker shells (0.39 +/- 0.011 mm SE) than Stonington Point (SP) (0.30 +/- 0.010 mm SE) (ANCOVA: F3>274=55.3, P=0.0091), and there is no interaction between the covariate, height, and site (thickness measurements were log transformed for statistical analyses but left untransformed for graphical purposes). Only snails between 5 and 10 mm in height were included in the above analyses.

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 0 -I------1------1------1------1 0 5 10 15 20 Height (mm)

Figure B.2 Comparison of sites within the Southern region: Rye Harbor (RH) has significantly thicker shells (0.50 +/- 0.018 mm SE) than Odiomes Point (OP), which has significantly thicker shells (0.47 +/- 0.008 mm SE) than Kittery Point (KP) (0.32 +/- 0.011 mm SE) (ANCOVA: F5i238=65.1, P<0.0001; LS Means Differences Tukey HSD: a=0.05). All three sites have significantly different thicknesses from one another, and there is no interaction between the covariate, height, and site (thickness measurements were log transformed for statistical analyses but left untransformed for graphical purposes). Only snails between 5 and 10 mm in height were included in the above analyses.

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 1 1 E 0.9 - O EV E 0.8 - • FR (0 (0 4) 0.7 - -WO C 0.6 A MP o 0.5 - 0.4 - £ 3 0.3 - t 0) 0.2 - Q. < 0.1 - o - 0 10 15 20 Height (mm)

Figure B.3 There are no significant differences among the sites within the Northern region: Evergreen (EV) (0.32 +/- 0.016 mm SE), Frazer (FR) (0.22 +/- 0.008 mm SE), Wonsqueak (WO) (0.31 +/- 0.012 mm SE), and Mermaid's Purse (MP) (0.26 +/- 0.008 mm SE) (ANCOVA: F7,397=26.2, P=0.0664). There is no interaction between the covariate, height, and site (thickness measurements were log transformed for statistical analyses but left untransformed for graphical purposes). Only snails between 5 and 10 mm in height were included in the above analyses.

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 0.8 0 CP E 0.7 H E • EA » 0.6 - © PA c 0.5 i A QU o 0.4 X WN 0.3 * © 0.2 0 2a . 0.1 <

o 10 15 20 Height (mm)

Figure B.4 All sites within the Northernmost site are significantly different from one another in shell thickness except for the two thickest-shelled sites: West Quoddy Head (QU) (0.23 +/- 0.011 mm SE) and Wilbur Neck (WN) (0.16 +/- 0.003 mm SE). The other three sites are significantly different from one another and from the two thickest-shelled sites: Passamaquoddy (PA) (0.09 +/- 0.004 mm SE), Eastport Harbor (EA) (0.11 +/- 0.004 mm SE), and Comstock Point (CP) (0.12 +/- 0.005 mm SE) (ANCOVA: F9,535 =47.4; LS Means Differences Tukey HSD, a=0.05). There is an interaction between the covariate, height, and site (thickness measurements were log transformed for statistical analyses but left untransformed for graphical purposes). Only snails between 5 and 10 mm in height were included in the above analyses.

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. APPENDIX C

Additional Results Comparing Crab Predation of Snails Differing in Thickness

Results for 36 additional trials are as follows (these data were not used in analyses): (A) Eight crabs ate neither snail within an hour (during 14 trials, 1-3 trials for each crab) (B) Three crabs ate both snails between observations (during three trials) (C) Four crabs ate both snails between observations (during five trials) after eating one in a previous trial (previous trials included in data analysis) (D) Three crabs ate neither snail within an hour (during four trials) and in a subsequent trial were observed to choose one snail first (subsequent trials included in data analysis) (E) During 10 trials, crabs were observed eating one snail, but these trials were eliminated because these crabs were already observed eating one snail in a previous trial (previous trials included in data analysis). During 9 of these trials, the WN snail was eaten first over the OP snail.

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Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. APPENDIX D

Comparison of Odiomes Point versus Wilbur Neck Snail Movement

♦ OP 200 - E oWN E 150 - o c +(0 ■ • (0 100 - TJ c (0 0) 50 - 2

0 1 2 3 4 5 6 7 8 9 10 11 Time (minutes)

Figure D.l Comparison of snail movement from Northeastern Maine and New Hampshire. Snails from Wilbur Neck, Northeastern Maine (WN) (open diamonds) moved significantly further (276 mm) than those from Odiomes Point, New Hampshire (OP) (filled diamonds) (112.5 mm) (repeated measures ANOVA: P=0.004). Movement was observed (mm +/-SE) after snails (6 mm ±0.5) were placed in the center of a Plexiglas arena in 5 cm of seawater. For each site the mean distance 10 snails moved dining five 10 minute trials is shown. All trials were performed in August 2004 at the UNH Coastal Lab in Newcastle, NH after all snails had been acclimated to ambient NH seawater temperatures for 7 weeks.

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Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. APPENDIX E

Comparing Claw Sizes between Hemieransus saneuineus and Carcinus maenas

Figure E .l Diagram of claw measured to calculate claw area: (A) length and (B) height.

600 • H. sanguineus ^ 500 o C. maenas E 400 (0 22 300 200 %ra O 100

20 30 40 50 Carapace Width (mm)

Figure E.2 Comparing claw sizes between two crab species: Hemigrapsus sanguineus (filled circles, solid line) and Carcinus maenas (open circles, dashed line) with similar claw areas (t=0.518, DF=29, P=0.6087), differ significantly in carapace width (t=14.490, DF=20.3, P<0.0001); small H. sanguineus (28.6 +/- 0.79 mm CW) have similar claw areas (264.6 +/- 20.58 and 280.3 +/- 16.1, respectively) to larger C. maenas (48.1 +/- 1.09 mm CW). Crabs measured here were used in predation trials with Littorina saxatilis (Chapter III). Claw areas were calculated by multiplying average claw length by average claw height per crab as shown in Figure E.l; claw areas were normally distributed.

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Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. APPENDIX F

Handling Time Details for Trials with Carcinus maenas and Hemieravsus saneuineus Preying noon Scarred and Unscarred Snails

Table F.la Outcomes of 124 handling time trials for C. maenas (CM) and H. sanguineus (HS) preying upon scarred (S) and unscarred (US) snails matched for height (some of these trials used crabs more than once).

Number of Outcome trials Handled Chipped Eaten HSCM none none none 29 3 both US only none 1 3 both S only none 3 1 both both none 1 2 both none none 4 10 both S only US only 6 2 both US only S only 0 0 both none US only 0 0 both none S only 1 2 both none both 21 19 S only none none 1 1 S only S only none 3 1 S only none S only 3 0 US only none none 1 1 US only US only none 1 0 US only none US only 3 1 Total number of trials: 124 78 46

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Table Fl.b Choice results for trials with C. maenas (CM) and H. sanguineus (HS) during which both unscarred (US) or scarred (S) snails were handled (some of these trials used crabs more than once).

CM HS

Ate only US 2 6 Ate US 1st over S 10 13

Total US eaten 12 19

Ate only S 2 1 Ate S 1st over US 9 8

Total S eaten 11 9

Table F.2 There were no significant relationships between log transformed handling time and crab to snail size ratio for unscarred and scarred snails handled by both C. maenas and H. sanguineus (some of these trials used crabs more than once, so analyses were blocked by crab identity) (see Figure F.l).

ANOVA Linear Fit R2 F ratio P CM US y=-0.0188x + 2.24 0.00079 Fi,12=0.0087 0.9274 S y=-0.1895x + 3.43 0.09 Fi,io=0.89 0.3711 HS US y=-0.0227x + 2.16 0.00045 Fi^i=0.0090 0.9253 s y=-0.0245x + 2.46 0.0017 FU1=0.0168 0.8993

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 1000 □ CM US m CMS 3 800 CO oHSUS o> E 600 H ♦ HS S H O) 400 TJ C 10 o o □ 200 ■ □ 0 2.5 3.5 4.5 5.5 6.5 7.5 Crab to Snail Size Ratio

Figure F.l Snail handling time is plotted against crab to snail size ratio. Carcinus maenas (CM) is plotted with squares; Hemigrapsus sanguineus (HS) is plotted with diamonds; unscarred snails (US) are open shapes, and scarred (S) snails are filled shapes (some of these trials used crabs more than once) (see Table A.2). Analyses were blocked by crab identity and handling times were long transformed (but left untransformed for graphical purposes).

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Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 10. o CM o 4* o ♦ o i 9 0 o $ Ui ♦ ♦♦ © S o $ ns ♦♦♦• ♦ o o c ♦ o w 6 ♦ ♦

5.5 22 32 42 52 Crab Carapace Width (mm)

Figure F.2 Snails were presented to crabs and successfully consumed. Snail heights of consumed snails are plotted against the carapace widths of the crabs that ate the snails. Carcinus maenas (CM) is plotted with open diamonds; Hemigrapsus sanguineus is (HS) plotted with filled diamonds. Since I chose which snails would be presented to which crabs unsystematically and individual crabs appear more than once in the figure, it is not appropriate to compare the average snail heights successfully eaten by each species. The figure simply shows that the two species of crabs overlap in the size of snails they can eat, despite their difference in carapace width.

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