PUSHING BOUNDARIES: INVESTIGATING THE DISTURBANCE OF THE PUGNACIOUS HILTONI

A Thesis submitted to the faculty of San Francisco State University In partial fulfillment of the requirements for the Degree

Master of Science

In

Marine Science

by

Emily Elizaoeth Otstott

San Francisco, California

August 2019 Copyright by Emily Elizabeth Otstott 2019 CERTIFICATION OF APPROVAL

I certify that I have read Pushing Boundaries: Investigating the Disturbance of the

Pugnacious Nudibranch by Emily Elizabeth Otstott, and that in my opinion this work meets the criteria for approving a thesis submitted in partial fulfillment of the requirement for the degree Master of Science in Biology: Marine Biology at San

Francisco State University.

Terrence Gosliner, PhD

Research Professor, San Francisco State University

Angel Valdes, PhD

Professor, California State Polytechnic University, Pomona

Katharyn Boyer, PhD

Professor, San Francisco State University Pushing boundaries: Investigating the disturbance of the pugnacious nudibranch Phidiana hiltoni

Emily Elizabeth Otstott San Francisco, California 2019

Warming conditions sparked the poleward range expansion of the pugnacious nudibranch Phidiana hiltoni, whose arrival to northern California in 1992 coincided with significant declines of resident nudibranchs. The nudibranch assemblage of Duxbury Reef was monitored to assess current impacts of P. hiltoni. The density of P. hiltoni at Duxbury

Reef has increased by three-fourths since 2010, now reaching over 18 ind. hr' 1 observer'1. The pooled density of nudibranchs impacted by P. hiltoni at Duxbury Reef (11 individuals h r 1 observer'1) has increased by 40% since 2008-2010 but still falls short of its density before P. hiltoni. Since first being disturbed by P. hiltoni in 1992, the nudibranch assemblage of Duxbury Reef is in recovery. Other aspects of this disturbance are discussed, such as prey overlap and interactions with specific resident nudibranchs.

I certify that the Abstract is a correct representation of the content of this thesis.

OJXjXq 'V - 12/20/2019

Chair, Thesis Committee Date ACKNOWLEDGEMENTS

I would like to thank the Gosliner Lab for helping me develop my project and for supporting my tide pool efforts. I am indebted to my committee members, each of whom offered valuable ideas and steered me on my path. The Steinhart Aquarium at the California

Academy of Sciences graciously made space and time to help me perform feeding trials.

Others helped with this project in various ways and I would like them to know my appreciation: Joren Nisiewicz, Jeff Goddard, and Eric Sanford. TABLE OF CONTENTS

List of Table...... vii

List of Figures...... viii

List of Appendices...... ix

Introduction...... 1

Background...... 1

The Problem...... 2

Project Goals...... 3

Methods...... 5

Timed Counts...... 5

Feeding Overlap...... 8

Feeding Trials—Testing spp. against P. hiltoni...... 9

Results...... 9

Timed Counts...... 11

Feeding Overlap...... 15

Feeding Trials—Testing Hermissenda spp. against P. hiltoni...... 16

Discussion...... 16

Reference...... 27

Appendices...... 34 LIST OF TABLES

Table Page

1. Nudibranch density at Duxbury Reef throughout time...... 10 2. Proportion of nudibranch species that tended to increase or decrease in density at Duxbury Reef from 2008-10 to 2018-19...... 13 3. Reactions of P. hiltoni to each of the Hermissenda spp. present at Duxbury Reef...... 16

vii LIST OF FIGURES

Figures Page

1. Phidiana hiltoni with Northern California nudibranchs and prey...... 3 2. Study sites of reefs in Northern California...... 5 3. Nudibranch densities at Duxbury Reef...... 11 4. Population growth of P. hiltoni, exhibiting exponential growth...... 11 5. Number of individuals and species found at Duxbury Reef...... 12

6 . Regressions of the annual densities of P. hiltoni on Hermissenda spp., F. trilineata, D. amyra, and A. abronia...... 12 7. Regression of the pooled density of impacted species against the density of P. hiltoni from 2007-19. 2007-11, and 2018-19...... 13 8 . Comparison of methods...... 15 9. Hydroid species and the nudibranchs that eat them at Duxbury Reef...... 15 10. Observations of P. hiltoni from online sources reveal range shifts with phases of ENSO...... 32 LIST OF APPENDICES

Appendix Page

1. Range Shift with Phases of ENSO...... 32

ix 1

Introduction

The natural flux of Earth’s climate is being exacerbated by human activities, resulting in modification of the geographic ranges of species 1_4. Climate change is primarily expressed as environmental warming, which is pushing the ranges of species poleward 3,5,6. While even slight changes in temperature can greatly impact species composition and abundance 7, the California coastal ecosystem has been enduring temperature anomalies up to 6 °C 6’8,9. These marine heat waves are becoming more frequent and intense, with species responses matching this intensity 10,1

Since wind and ocean currents distribute heat over the globe, altering the amount of heat in places will impact the patterns of currents. Sea surface temperature in the North Pacific positively correlates with the Pacific decadal oscillation (PDO), which alters the positions of atmospheric circulations and cloud cover 4’5,12. Increased ocean temperatures are also associated with the El Nino Southern Oscillation, which reverses the flow of the California Current from southward and offshore to northward and onshore 4. These modifications directly affect the geographic ranges of marine species, particularly those with planktonic larvae.

Nudibranchs, or sea slugs, have planktonic larvae whose recruitment is affected by climatic phases that alter larval advection 4_6>13. For example, upwelling relaxation events, such as during El Nino, allow high densities of larvae to potentially be delivered to shore as surface flow reverses 4’5,14. During positive or warm phases of El Nino and the PDO, the abundances of “warm water” species increase and the geographic ranges of some species shift poleward 4’5,15. With warm PDO phases increasing the flow of coastal waters northward and El Nino causing the California Current to flow northward and onshore, the overlap of these events would greatly promote northward dispersal of nudibranch larvae 4,5. Such an overlap did occur a few years ago. with a warm PDO phase occurring during the 2014-16 El Nino, resulting in the largest marine heat wave ever recorded 16. Aside from producing a destructive warm-water “blob” off the coast of 2

California that extended 300 m deep, this heat wave resulted in the poleward movement of at least 26 nudibranch species, 11 of which dispersed to new northernmost sites 6,1?. Since these are cycles, the population would likely move back south as the warming period ends; however, with the additional rising temperatures of long-term climate change, northernmost populations may potentially sustain themselves

It has frequently been noted that large and conspicuous nudibranchs can be used as “brilliant indicators of climate change” 18. Recurrent monitoring of specific reefs would allow for observations of changes in the nudibranch fauna that may imply slight changes to climate 18. Studying nudibranch population shifts additionally helps to predict shifts of other marine species with planktonic larvae that are affected by oceanic conditions, such as the economically-important Pacific oyster 19.

As the range of a species is changed, the species must now potentially interact with previously unencountered species with unknown consequences. While these natural range modifications are not considered biological invasions, both events are similar in their potential to alter the ecosystem 5,2°. When the range of a species shifts or expands into new territory, the newcomer may impact the community already present at that site through predation, increasing competition for food, or reducing available space21,22. For example, since the PDO shifted from a cool to warm phase in 1976-77, the nudibranch Phidiana hiltoni23 ( Figure la) has been moving north along the California coast, predating on resident nudibranchs and potentially increasing competition for hydroid p rey 5 (Figure Id). At Duxbury Reef (Figure 2), a state-protected northern reef, the nudibranch assemblage has been hit especially hard by P. hiltoni. As of a decade ago when Duxbury Reef was last studied, 9 out of 15 species classified as vulnerable to P. hiltoni declined in abundance and of those 9 species, 5 disappeared from the reef after 1 hiltoni arrived 5. The current condition of the slug assemblage of Duxbury Reef is unknown and consistent monitoring is necessary to anticipate and prevent any further loss Figure 1. Phidiana hiltoni with representative northern California nudibranchs and prey, (a) The newcomer P. hiltoni, (b) impacted species , and (c) non-impacted dorid species Triopha catalinae. (d) Hydroid prey (Obelia sp.j growing epiphytically on kelp; each string of hydroid represents one hydrocaulus. of . Additionally, the northern range limit of P. hiltoni, which is currently Bodega Bay, must be monitored to anticipate further ecological impacts.

The aim of this study was to further assess the potential impacts of P. hiltoni on resident nudibranchs by monitoring 3 intertidal reefs n Northern California, with Duxbury Reef being the focal point (Figure 2). Considering the extreme abundance (>10 individuals hr"1 oberserver'1) of P hiltoni at Duxbury Reef from 2007-09 5,1 expected P. hiltoni to continue to dominate the nudibranch assemblage. I hypothesized that the populations of species impacted by P. hiltoni would continue to decline and fewer species would be present since P. hiltoni readily consumed and likely shared hydroid prey with the impacted species 5.1 also monitored Asilomar State Beach, where P. hiltoni has historically been found. Although P. hiltoni has been known to predate on other nudibranchs in its southern range 24, there have not been reports of P. hiltoni reducing 4

populations until P. hiltoni began shifting poleward. Since individuals evolve defenses against familiar predators 25,26, 1 hypothesized that P. hiltoni would be coexisting with nudibranchs at Asilomar State Beach and would be as abundant as other species at this site. The last site I visited was Coleman Beach, which already hosts a nudibranch assemblage and would likely be the next northern location for dispersal of P. hiltoni. Considering the warm conditions that have been occurring over the California coast the last several years, it seemed likely that P. hiltoni would be found at Coleman Beach. If this were the case, I hypothesized that species vulnerable to P. hiltoni, based on feeding trials and diet overlap 5, would decline as they had at Duxbury Reef.

To determine how the nudibranch assemblage of Duxbury Reef has changed with the addition of P. hiltoni, I conducted timed counts and compared these to historical counts. The observed changes provide insights into which nudibranch species have been impacted, likely attributed to the presence of P. hiltoni, and how quickly the population of P. hiltoni is growing. Additionally and importantly, these results will provide a third snapshot of the invasion history of P. hiltoni at Duxbury Reef.

The robustness of counts using the traditional method was verified by conducting a second suite of timed counts using a transect method 27_29. It has been argued that, compared to a transect method, timed counts are more practical and statistically better suited to the study of opisthobranchs because of their random distributions 27“29.1 aimed to test this by comparing both methods.

I investigated the nudibranch feeding ecology at Duxbury Reef by identifying which hydroid a nudibranch was feeding on and quantifying where it was feeding on the hydroid30. To more clearly discern the predation patterns of P. hiltoni on resident nudibranchs, I redid feeding tiials from a decade ago between P. hiltoni and two Hermissenda spp. that were previously believed to be a single species 5,3*. 5

_ * Vacaville 'Coleman The significance of this study is far-reaching. Beach Aside from evaluating the relative abundance and

Concoid composition of the nudibranch assemblage of Duxbury Reef, this study adds to a few others that are beginning San Franciscoo / to build a framework to draw generalizations about Duxbury invasions over time 32,33. These generalizations can be Reef Palo Aho ran San Jos6 used to better predict and mitigate invasions, inform w o policy to reduce the spread of invasive species, and

anticipate the trajectory of observed impacts '4 35. Asilomar Generalizations gathered from this study will be State especially beneficial in forecast and management, of . “V Beach nt«rey marine species with larvae that are susceptible to

Figure 2. Study sites of reefs changing ocean conditions. in Northern California. Duxbury Reef and Asilomar This study was conducted to answer (1) State Beach are marine protected areas. whether populations of nudibranchs impacted by P. hiltoni were still decreasing or in recovery, (2) if P. hiltoni had expanded further north, and (3) what mechanisms can account for the impacts of P. hiltoni on resident nudibranchs.

Methods Timed Counts

To determine the current abundances of nudibranchs, especially P. hiltoni, monthly to bimonthly timed counts were conducted at Coleman Beach, Duxbury Reef, and Asilomar State Beach (Figure 2).

Duxbury Reef (37.8897, -122.6997) was the focus location of this study since this reef had been significantly impacted, likely from P hiltoni5. With the largest shale reef on the West Coast of North America, Duxbury Reef is home to a diverse community 6

spanning birds, mammals, fish, invertebrates, and algae. This reef has been a marine protected area (MPA) since 1971. Duxbury Reef marks the western limit of Bolinas Bay, which has cyclonic circulation5,36. The study area consists of wide, low segments of shale and limestone that create pools, channels, and ledges. The water has large amounts of settled and suspended sediment from erosion of cliffs on the shore. Duxbury Reef was sampled 14 times from January 2018 to July 2019.

Coleman Beach (38.3632, -123.0708) is less than 8 km north of the new northern limit of P. hiltoni, thus this location was chosen to determine whether P. hiltoni would travel further north. This reef consisted of large boulders, ledges, and overhanging rocks. Coleman was sampled 11 times from January 2018 to June 2019.

Asilomar State Beach (36.6282, -121.9421) was the previous northern limit for P. hiltoni before its poleward trek in 1976. This reef was chosen to represent a community tnat has evolved along with P. hiltoni. Asilomar consisted of variously sized tide pools with rock boulders and ledges reaching 2 m high 28,29. Asilomar was sampled 6 times from February-July 2018 since P. hiltoni was not found there.

Timed counts were done during the lowest tides of the month, i.e. below -0.5 ft. I aimed for tide pool areas with suitable slug habitat, e.g. areas with abundant nudibranch prey like hydroids and bryozoans. Pools and rocks were thoroughly searched for one hour and each nudibranch was identified and counted.

While this method of estimating abundance has been used in many studies 5’17,27_

29,37-39 and considered the best way to sample nudibranchs, this method has not been explicitly tested against a transect method. To test whether these methods produce similar results, I established a transect through the reef and searched along it (±1 m) for one hour. Transects were consistently placed in the same spot. An additional 16 timed counts were done using this transect method: 6 at Duxbury Reef, 7 at Coleman Beach, and 3 at Asilomar State Beach. Thirteen of these additional counts were done either immediately 7

before or after conducting a traditional count, with one survey done an hour before low tide and the next survey beginning at low tide. Differences in average species density and hourly richness between the traditional and transect methods were detected using t-tests, with all statistics conducted using R 1.1.463.

Duxbury Reef was the only reef included in the subsequent analyses since this was the only place where P. hiltoni was observed in the current study. Current species density and hourly species richness (using counts only from the traditional method) at

Duxbury Reef were compared to previous surveys (n=16) between 2007-11 5 and unpublished surveys (n=7) done by Terry Gosliner and Jeff Goddard between 1969-75.

All nudibranch counts were converted to density by standardizing to individuals h' 1 observer' 1 or species h' 1 observer'1. All observers from the previous studies and myself were familiar with nudibranchs found in this region of California, thus it is unlikely that there was a significant effect of observer on nudibranchs found.

To reduce effects of autocorrelation, surveys done within 2 months of each other were averaged and separated by a minimum of 4 months. The Mul yariate ENSO Index (MEI; https://ggweather.com/enso/oni.htm) was then determined for each time period and made similar (excluded Dec. ’75, Dec. ’07. Apr & June ’08, July ’11, Nov. ’11, Apr.- June ’ 18 and Mar. & May ’ 19). The exception was October 2009, which was only separated by about 3 months, otherwise the results for 2008-10 would be biased towards La Nina. The average/sum MEI was 0.06/0.25 for the current study, 0.01/0.05 from 2008- 10, and 0.02/0.1 from 1969-75.

T-tests and ANOVAs were used to test for differences in densities from different time periods. Using classification of species “vulnerable” to P. hiltoni5, a one-tailed t-test was used for species impacted by P. hiltoni since I hypothesized that their densities would continue to decline. A two-tailed test was used for unimpacted species 5. 8

To test the alternate hypothesis that the change in abundance of nudibranchs is not because of P. hiltoni but is an artifact of species geographic ranges (i.e., that other southern species like P. hiltoni might be also moving north), I classified species as having a northern, southern, or widespread range. Southern species were those that occur in Baja California or southern California, northern species were said to occur in northern California or southern Oregon, and widespread species occurred north of southern species and south of northern species 4’5. Using counts from 2008-10 and 2018-19,1 compared ihe number of species that tended to increase or decrease clustered by geographic range and vulnerability to P. hiltoni5. The observed changes in population densities from 2008-

10 to 2018-19 were similar for southern and widespread species, so these were grouped together and compared with northern species 5.

Population growth of P. hiltoni was calculated as annual percent change n density. Regressions were conducted to determine if the density of P. hiltoni correlated with densities of impacted species. Pooled dens s of impacted species were regressed on the density of P. hiltoni using a cubic polynomial model to reveal whether a certain density of P. hiltoni will predict declines of other nudibranchs 40. Thresholds were first identified visually. Linear regressions were be done before and after the threshold to test for a significant impact threshold that once reached, densities of impacted species would decline.

Feeding Overlap

Trophic interactions of nudibranchs at Duxbury Reef were studied to determine the extent of competition between P. hiltoni and resident nudibranchs. I searched the reef for slugs feeding on hydroids (Figure Id) and recorded all that I observed. Since multiple nudibranch species can coexist on one hydroid species if the hydroid is partitioned, a closer picture was required of the situation. Adapted from a lab protocol30,1 measured

( 1) the body length of the nudibranch, (2 ) the distance up the hydroid where the slug was feeding, (3) the mean height of five surrounding (2.5cm2 area around nudibranch) 9

hydrocauli (Figure Id) which are simple of branched stems of hydroids, and (4) the density of hydrocauli surrounding the nudibranch (number/2 .5cm2) in the field.

Feeding Trials—Testing Hermissenda spp. against P. hiltoni

Feeding trials between P. hiltoni and resident nudibranchs of Duxbury Reef were conducted nearly a decade ago 5 when H. crassicornis and H. opalescens, which both occur at Duxbury, were thought to be the same species. Therefore, the results for these species may not accurately depict each of the species alone. To clarify this, I redid feeding trials between P. hiltoni and both Hermissenda spp. found at Duxbury.

Individuals of P. hiltoni, H. crassicornis, and H. opalescens were collected from Duxbury Reef and Coleman Beach under permit. Some trials were done in the field without collecting the specimens, but other times specimens were collected and tested in the lab. Setting of the trials did not affect results (x2(6)=8 , p=0.24). Since nematocysts, i.e., stinging cells that aeolid nudibranchs acquire from prey and retain in their dorsal appendages for their own defense, are only retained for 3 days, all lab trials were conducted within 3 days of collection. A trial was conducted by first placing an individual of P. hiltoni in a bowl with about 300 mL of fresh seawater and allowing P. hiltoni to crawl on the bottom of the bowl. An individual of either H. crassicornis or H. opalescens was then introduced into the bowl and gently nudged with blunt forceps to crawl straight towards the head of P. hiltoni. Once the two slugs made contact, the reaction of P. hiltoni to Hermissenda spp. was categorized as (1) withdrawal of head potentially followed by a change in crawling direction, (2 ) neutral or no distinct reaction, (3) repeated contact with the test nudibranch without attack, and (4) attempt to attack and ingest the test nudibranch5. Escape behaviors of Hermissenda spp. were noted. To test for differences in reactions of P. hiltoni to the two Hermissenda spp., Pearson’s chi- squared test was used.

Results 10

Table 1. Nudibranch species density at Duxbury Reef throughout time. Probability (P) is the output of a t-test (assuming unequal variance) comparing log-transformed densities between time periods, with 1 tail for impacted species and 2 tails for non­ impacted species.

Geog. Species 1969-1975 2008-2010 2018-2019 Since 197S p Since 2010 p Range Phidiana hiltoni 0.00 10.43 18.34 + 0.0003 + 0.05 S Impacted species _ Hermissenda spp. 8.02 2.75 4.02 0.24 + 0.35 w Do to amyra 3.46 0.29 4.60 + 0.39 + 0.04 N Flabellina triiineata 1.31 3.2S 0.31 - 0.04 - 0.19 N Diaphoreolis flavavulta 0.74 0.76 0.70 - 0.47 - 0.46 S Cuthana divae 0.41 0.17 0.00 - 0.07 - 0.20 N Do to kyo 0.34 0.00 0.30 - 0.46 + 0.10 N Tenellia aiboausta 0.21 0.00 0.00 - 0.13 0 W Aeolidio loui 0.13 0.20 0.00 - 0.14 - 0.22 N Anteaeaiidiella oliviae 0.13 0.00 0.20 + 0.36 + 0.10 S Do to columbiano 0.04 0.00 0.00 - 0.22 0 N venustus 0.03 0.00 0.00 - 0.22 0 N Abronica obrania 0.03 0.29 0.69 + 0.04 + 0.18 N Non-im pacted spedes Codlino madesta 2.10 0.05 0.13 m m m 0.23 + 0.50 W Triopha cotalinoe 1.98 9.30 0.07 - 0.002 - 0.08 N Rostanga pulchro 1.95 0.10 0.20 - 0.33 + 0.54 W Aegires oibopunctatus 1.83 0.38 0.00 - 0.31 - 0.34 W Dioululo sandiegensis 1.07 0.96 0.63 - 0.50 - 0.61 W Triopha maculata 0.81 3.69 2.28 + 0.17 - 0.43 S Dirono picta 0.63 0.00 0.19 - 0.60 + 0.20 S Diaphoreolis lagunae 0.56 1.05 1.18 + 0.33 + 0.92 S Diapharodoris lirulatocoudo 0.39 2.29 0.30 - 0.82 - 0.25 w montereyensis 0.35 1.31 0.31 - 0.89 - 0.24 N Codlino luteomarginata 0.32 0.00 0.00 - 0.07 0 N Acanthodoris lutea 0.28 0.14 0.00 - 0.10 - 0.31 S Anculo gibboso 0.24 0.29 0.36 + 0.64 + 0.82 N oibopunctatus 0.24 0.00 0.00 - 0.35 0 S Umacio cockerelli 0.17 0.00 0.00 - 0.15 0 w Antiopella fusca 0.09 0.10 0.00 - 0.29 - 0.48 N Acanthodoris rhodoceras 0.08 0.00 0.05 - 0.73 + 0.20 S Acanthodoris nanolmoensis 0.07 0.17 0.00 - 0.25 - 0.29 N Geitodoris heathi 0.07 0.37 0.06 - 0.93 - 0.17 N Dendronotus albus 0.04 0.00 0.00 - 0.45 0 N Dendronotus subramosus 0.04 0.14 0.00 - 0.45 - 0.48 N Haiiaxo dtanl 0.04 0.24 0.00 - 0.45 - 0.29 N Peltodoris nabiiis 0.04 0.38 0.00 - 0.45 - 0.30 N Onchidoris muricato 0.03 0.00 0.00 - 0.45 0 N Dirono oibolineota 0.00 0.10 0.00 0 - 0.48 N Okenta rosacea 0.00 0.10 0.00 0 - 0.48 s . 11

40 ■ Hermissenda spp. ■ Doto omyra Cadlina modesta ■ Triopha catallnoe L. ■ Rostanga pulchra Aeglres oibopunctatus 0) £ ■ Flabelllna trllineata spp. ■ Dtoulula sondlegensls ■ Trlapha maculoto m Cuthana fiavavulta i/i0) -O m Dirono picta ■ lagunae O I ■ Cuthona divoe m Dlapharodoris llrulatocauda rH L. m Doris montereyensis ■ Doto kyo J= ■ Cadlino luteomorginoto ■ Aconthadorls lutea ■ Ancula gibbasa ■ Dorlapsllla oibopunctatus C » Tenelki albacrusta m Limacla cockerelll m Aeafidia papillose ■ Anteaeolldiella ollviae a Janalus fuscus Acanthodoris rhadoceras OJ Acanthodoris nanaimoensis ■ Geitodorls heothi Dendronotus albus ■ Dendronotus subramasus ■ Hallaxa chanl ■ Peltodorls nobills ■ Dato calumblana m Dendronotus frondasus - Onchidarls murlcata ■ Abronica abronla Dirana albollneata ■ Okenio rosacea 1969-1975 2008-2010 2018-2019 Phidiona hiltoni

Figure 3. Nudibranch densities at Duxbury Reef. Phidiana hiltoni continues to increase and Hermissenda spp. is recovering.

Timed Counts

Phidiana hiltoni was found at Duxbury Reef but not at Coleman Beach or Asilomar State Beach. At Duxbury Reef, P. hiltoni continued to be the most abundant nudibranch present, not only overall but also in each timed count ("Table 1 and I.0J 0£J Figure 3). At Duxbury Reef, the ul -GO rH density of P. hiltoni increased by three- L. -C c fourths since 2 0 1 0 , with a mean of 18 I £ individuals hr' 1 (Table 1 and Figure 4). CL M— o The annual population growth rate of P. k_ JQ

Figure 5. Number of (a) individuals and (b) species found at Duxbury Reef. Light gray bars represent impacted species and dark gray bars represent non-impacted species. Error bars represent standard deviation.

y = -0.0758X + 2.5735 0 R1 - 0.1262 4 ■

3 ■

2 - 1 •

10 15 20 25 30 y = 0.0512x-0.1199 0 R1 = 0.4454

0 5 10 15 20 25 u => 10 15 20 25 30

Number of P. hiltoni hr' 1 observer-1

Figure 6 . Regressions of the annual densities of P. hiltoni on (a) Hermissenda spp., (b) F. trilineata, (c) D. amyra, and (d) A. abronia. increased by 1.18 individuals each year since from 2007 to 2019 (ti4.97=303.57, p«0.001). Over 63% of individuals found in each count from 2018-19 were P. hiltoni, 13

Table 2. Proportion of nudibranch species that tended to increase or decrease in density at Duxbury Reef from 2008-10 to 2018-19.

Change in Impacted species Geographic Range i Impacted Geographic Range aensity yes no N Sand W species N Sand W increase 0.42 0.23 0.20 0.44 yes 0.67 0.33 decrease 0.33 0.58 0.55 0.39 no 0.50 0.50 0.25 0.19 0.25 0.17 compared to 33% in 2008-10 (t6.9= -2.33, p=0.05).

A total of 20 nudibranch spe< es were observed at Duxbury Reef in the current study, compared to 27 from 2008-10 and 36 from 1969-75 (Figure 3). Over these years, the pooled density of all individuals has remained constant (F3.131~O.2 6 , p=0.85) while an increasing trend exists for species found per hour (F2,66=3.70, p=0.09).

The pooled density of impacted individuals increased by more than one-third since 2010 (Figure 5a; t4.8=-0.40, p=0.70) and the average hourly richness of impacted species has increased by nearly half (Figure 5b; t3.7--0.52, p=0.63). Similar numbers of impacted species increased (5 spp.), decreased (4 spp.), or had no change (3 spp.) in density since 2010. Hermissenda spp.. which was the most abundant species prior to the arrival of P. hiltoni, has increased its density by nearly half since 2010 (Table 1). The density of Doto amyra has increased 15 times since 2010 and this slug is now the second most abundant species after P. hiltoni (Table 1).

In contrast, the pooled density of non-impacted individuals significantly decreased by nearly three-fourths since 2010 (Figure 5a; t7.8=2 .4 5 , p=0.04) while the average hourly richness increased by half (Figure 5b; t3.9=-0 .88 , p=0.43). Most of the non-impacted species declined since 1975 and 2010 (Table 1). The most abundant non­ impacted species prior to the arrival of P. hiltoni was Triopha catalinae, which has now decreased by nearly 100% (Table 1). 14

As the density of P. hiltoni increased, increases were also observed in

Hermissenda spp. (Figure 6 a; R2=0.23, p=0.28), D. amyra (Figure 6 c; R2=0.86, p=0.002),

y = -0.0006k3 + 0.0419xJ - 0.286x + 5.4333 and A. abronia (Figure 6 d;

R2=0.44, p=0.10). In contrast, the density of F. trilineata decreased as

P. hiltoni increased (Figure 6 b; R2=0.13, p=0.43).

More impacted species had northern ranges compared to southern and widespread ranges (Table 2). Comparing population densities from 2008-10 to 2018-19, southern and widespread species were equally likely to increase or decrease. Northern species were

more likely decrease (Table 2 ). o y = -0.0007x3 + 0.O451X2 - 0.0731X + 1.9031 S-H

40 -| each threshold, the next step of linear rH 35 - regression could not be conducted and aj £ QJ 30 - in significant impact thresholds were not -Q 25 - O found. 20 - v_ 15 - OJ _Q 10 - More individuals are found using £ D 5 - z the traditional method compared to the 0 - transect (Figure 8 ; t29.9=3.52, p=0.001) Individuals Species but hourly species richness is not Figure 8. Comparison of methods. The traditional method (dark blue) is better dependent on the method used ( Figure 8 ; suited for finding nudibranchs than the t3S.3=0.76, p=0.45). transect method (light blue). Error bars represent standard deviation. Feeding Overlap

A total of 12 observations were made on 4 nudibranch species feeding on 5 distinct hydroids at Duxbury Reef (Figure 9). Ten of these observ at ions were of P. hiltoni and D. lagunae. Conspecific nudibranchs ate the same hydroid, or hydroids in the case of Figure 9. Hydroid species and nudibranchs that eat them at P. hiltoni (Figure 9). Duxbury Reef, (a) Obeli a sp. and (b) Abie t inaria abietina There was no observed with P. hiltoni. (c) Aglaophcnia struthionides with A. abronia. (d) Ectopleura marina with F. trilineata. (e) sharing of hydroid prey Sertularella sp. with D. lagunae. 16

Table 3. Reactions of P. hiltoni to each of the Hermissenda spp. present at Duxbury Reef Withdraw Repeat Attack and Ingest Species Withdraw, Turn Withdraw Neutral Contact Whole Partis! Fail Total H. opalescens 5 8 2 i 1 2 1 20 H. crassicornis 8 4 4 l 0 1 0 18 between nudibranch species (Figure 9).

Comparison of hydroid colony metrics only makes sense within the same hydroid species. With Obelia sp., there was an increasing trend between the length of P. hiltoni and the average height of surrounding hydrocauli (R2=0.87, p=0.24) and a decreasing trend with slug length and the density of surrounding hydrocauli (R2=0.84, p=0.26). Individuals of P. hiltoni found feeding on A. abietina were slightly and non-significantly larger than individuals feeding on Obelia sp. (ts=-0.27, p=0.80). With A. abietina, longer individuals of P. hiltoni ate higher upon the hydroid (R2=0.91, p=0.04). Diaphoreolis lagunae was always (n=3) found at the base of the non-branching hydroid Sertularella sp.

Feeding Trials—Testing Hermissenda spp. against P. hiltoni

While P. hiltoni does attack and consume both H. opalescens and H. crassicornis,

P. hiltoni did not attack one species more than the other (Table 3; %2(3)=0.54, p=0.91). The most common reaction was withdraw (Table 3, 65% of time with H. opalescens and 62% with H. crassicornis) followed by attack (Table 3, 20% with H. opalescens and 15% with H. crassicornis). When P. hiltoni made contact, H. opalescens would often violently wriggle to escape while H. crassicornis would only crawl away.

Discussion

Phidiana hiltoni has not yet surpassed its current northern range limit of Bodega Bay, as indicated by the absence of P. hiltoni at Coleman Beach. North of Bodega Bay, there are few distinct bays, which have long been studied for their retention of larvae and 17

protection from upwelling events that would otherwise disperse larvae away from nearby suitable habitats41^ 8. Organisms with short-lived larvae, such as P. hiltoni with lecithotrophic development, have a unique advantage in this scenario because the entrapment of their larvae in the bay, although decreasing the dispersal range of the organism, would increase the probability of the larvae finding suitable habitat before the larvae expire 5>44>48>49. This explains why large populations of P. hiltoni are restricted to northern ends of bays, i.e. upwelling shadows, such as Soquel Point, Shell Beach, and Cayucos 5,4S. However, in Bodega Bay, which is small (<10 km), retention of surface larvae occurs to a lesser extent than in larger bays (> 10 km) because the recircula on of waters in the bay occurs at depth 4 Phidiana hiltoni has been found at Pinnacle Gulch, which is located at the northern end of Bodega Bay and experiences events of upwelling waters and equatorward surface flow, contrary to the protection and retention of upwelling shadows6,44. While Bodega Bay does not possess the ideal characteristics to grow a population of P. hiltoni, a total of 5 individuals were observed in 2 days of November 2018 and some of the individuals were small, suggesting continued recruitment to this area (Eric Sanford, pers. comm.). Pinnacle Gulch must be monitored to anticipate negative effects of P. hiltoni to the resident community.

Adjacent to Duxbury Reef is Bolinas Bay, measuring 6.5 km wide at the mouth. Although this is a small embayment, the surface waters of Bolinas Bay exhibit cyclonic circulation which may locally trap larvae5’36. Larvae can also become trapped in the upwelling shadow below Point Reyes, where a cyclonic eddy retains larvae for a few days until upwelling relaxes5,4 While most of this water would then begin flowing around Point Reyes and poleward, it is possible that some could travel alongshore to Duxbury Reef ,47. In either case, the larvae of P. hiltoni in the southern retention area of Point Reyes would likely not be viable by the time they reached suitable habitat5. Therefore, local recruitment and retention of larvae in Bolinas Bay is most likely the main driver of recruitment in the population of P. hiltoni at Duxbury Reef. Further study of water transport in Bolinas Bay could confirm this. 18

Since the arrival of P. hiltoni to Duxbury Reef in 1992, the population of this pugnacious aeolid has been increasing. The observed exponential growth reveals a time lag from the time of first detection to significant population expansion. The time lag was about 20 years, wfi ;h is comparable to nonfish in the Western Atlantic 50,51. Time lags are observed at the onset of new popula ons when the population needs time to adapt to the biotic and abiotic aspects of the new environment or reach a critical density 52,53. The time lag of P. hiltoni was likely an artifact of slow or sporadic larval recruitment to Duxbury Reef, which gradually increased the population until individuals were relatively abundant on the reef and could find a mate 53,54. As mates became more common, P. hiltoni became more abundant, creating a positive feedback loop. Further surveys are required to determine whether the population of P. hiltoni will continue increasing or whether it is approaching some limit to population growth, i.e. carrying capacity. The annual population growth rate of 22.8% yr' 1 for P. hiltoni was higher than that of lionfish

(12% ) 55 and an invasive sea snail (6 % )56. This high population growth rate is reflective of the population dynamics of P. hiltoni and of the community at Duxbury Reef, although it is likely that P. hiltoni has a high population growth rate whenever it is located in an upwelling shadow 5,51.

Compared to 1969-75, more impacted species are found in surveys today, but each species is represented by fewer individuals. This indicates that impacted species were dominated by one or two nudibranchs before the arrival of P. hiltoni, but now dominance is shared more evenly among impacted species. Instead of the previously dominant impacted species such as Hermissenda spp. or F. trilineata, P. hiltoni now dominates Duxbury Reef. Its abundance corresponded with reduced populations of previously dominant species, which has led to increased abundances of less common species like A. abronia and D. amyra. This explains the positive correlation between densities of those two species and P. hiltoni and the negative correlation between F. trilineata and P. hiltoni, but does not clarify why the density of Hermissenda spp. was positively correlated. 19

Since a decade ago, populations of species impacted by P. hiltoni at Duxbury Reef are now recovering, with higher densities of both individuals and species occurring today. The simultaneous increase in populations of P. hiltoni and impacted species suggests that P. hiltoni must not be exerting the same pressure as before, or other external factors are at plav. As a species establishes itself in a new habitat, not only will the newcomer evolve to fit in with its new environment, the pre-existing community will also evolve in response to the newcomer 58. Therefore, predation by P. hiltoni on impacted species may have selected for individuals that had better defenses against P. hiltoni, which would lead to lower mortality and higher abundance of those impacted species 59. It is also possible that P. hiltoni is eating less of the impacted nudibranchs and filling its diet with more hydroids. This could be studied in a series of experiments. An interaction web could be formed by combining field surveys of hydroid and nudibranch species densities with preference experiments of nudibranchs to hydroids and additional tests to see whether P. hiltoni prefers hydroids or nudibranchs 38,6°. These preference experiments would require significant effort with the added difficulty of keeping hydroids alive once taken from their environment. The approach of the current study, i.e. observing nudibranch-hydroid feeding interactions in the field, is a simpler approach with less of an impact on the reef but is only practical at sites with many sightings of nudibranchs on hydroids. For example, few observations were made at Duxbury Reef, yet sightings were more frequent at Coleman Beach (data not shown). Compared to Coleman Beach, Duxbury Reef had low water visibility, which made it harder to find nudibranchs and hydroids. Furthermore, nudibranchs feeding subtidally, such as in deep pools at Duxbury Reef, could not be included in the study since disturbing the would change the measurements. Therefore, the lab preference experiments would likely be better suited to Duxbury Reef. Another interesting strategy would be to perform additional fecal analyses on P. hiltoni to determine what proportion of individuals contained unidentifiable soft- tissue (likely from nudibranchs) in their feces and compare to previous analyses 5. Genetic analyses could also be used on the feces to more accurately identify the contents. 20

While no nudibranchs were seen eating the same hydroid species at Duxbury Reef, there is likely some overlap in prey that was not detected in this study. Feeding observations are scarce and, even with fifty years of literature, there are large gaps of knowledge in nudibranch feeding ecology 5,61_65. Furthermore, diet can vary among locations and species thought to eat multiple prey could be different populations that specialize on one prey species 66,67. Therefore, the literature that does exist may not apply to individuals of a specific location. The feeding observations of this study provide a foundation for future study of the nudibranch feeding habits at Duxbury Reef.

Since Hermissenda spp. were never observed on hydroids at Duxbury Reef, it is unclear whether they share prey with P. hiltoni at this site, although these species in general have been found on the hydroids Plumularia sp., Obelia spp., and A. abietina 5,62,63 Specific feeding habits of H. opalescens and H. crassicornis have not yet been studied since being classified as different species68, so the reported prey of these species vary widely 62,63. If Hermissenda spp. truly are generalists, competition among Hermissenda spp. and P. hiltoni would be reduced. This could help explain how Hermissenda spp. is regaining its abundance at Duxbury Reef.

The species with the most dramatic decline after P. hiltoni arrived was Hermissenda spp., represented in the present study by mostly (n=73) H. opalescens and only 5 individuals of H. crassicornis. Given that H. crassicornis did not have the escape tactics of H. opalescens, it is possible that H. crassicornis was more abundant prior to the arrival of P. hiltoni but its lack of defensive responses caused the population of H. crassicornis to decline as it fell prey to P. hiltoni. While H. opalescens recognizes P. hiltoni by contact chemoreception 5, H. crassicornis has not encountered P. hiltoni evolutionary long enough to recognize P. hiltoni. However, if H. crassicornis was more susceptible to predation by P. hiltoni than H. opalescens, then it would have been attacked more often than H. opalescens, yet both Hermissenda spp. were attacked at similar rates. This suggests that H. crassicornis is repelling D. hiltoni equally as often as 21

H. opalescens and that H. crassicornis may have other defenses that H. opalescens does not possess. If this is the case and no other forces are significantly decreasing the population of H. crassicornis at Duxbury Reef, the abundance of this species should start increasing along with the population of H. opalescens.

Aside from Hermissenda spp.. D. amyra, was the next most abundant species before P. hiltoni arrived. Doto amyra was observed often and in respectable numbers, now being the second most abundant of all nudibranchs at Duxbury Reef. Feeding trials 5 found that the body of D. amyra and other Doto, though small (<10 mm), were difficult for P. hiltoni to ingest, with P. hiltoni only partially consuming some individuals. Since the population of D. amyra initially declined when P. hiltoni arrived yet is now significantly more abundant (Table 1), it is possible that P. hiltoni preyed upon D. amyra when it first arrived at Duxbury Reef but eventually acquired a distaste for this species. This relates to the need of an organism to balance the cost of acquiring a food source by the caloric gain of that food 69,70. Perhaps D. amyra was not worth the effort of ingestion for P. hiltoni and this provided a refuge for D. amyra. Additional support for this hypothesis is that P. hiltoni and D. amyra were often found nearby each other (within 1 m) and their densities were significantly, positively correlated. This hypothesis may also apply to D. kya, which disappeared from Duxbury Reef when P. hiltoni arrived but is now present at densities similar to those in 1969-75. The same may be true for D. columbiana, although this species was rare to begin with and has not been recorded since 1970, before the presence of P. hiltoni.

Like D. amyra and P. hiltoni, A. abronia has been growing in abundance over the years. This suggests A. abronia may repel attack by P. hiltoni, which has previously been suggested5. Diaphoreolis lagunae is another nudibranch that repels attack by P. hiltoni. At Asilomar State Beach, I observed D. lagunae feeding on the hydroid that A. abronia was observed on at Duxbury Reef, A. struthionides. Further study of this may find an interesting relationship between hydroid prey and reaction of P. hiltoni to a species 5. 22

Flabellina trilineata was the only impacted species of reasonable density (>0.5 hr' 1 observer'1) to have a negative correlation with the density of P. hiltoni (Figure 6 ). Since F. trilineata eats athecate hydroids and P. hiltoni eats thecate hydroids, there is likely little overlap of prey between these species 5 (present study). Thus, if P. hiltoni was not influencing the density of F. trilineata through competition for resources, it may be that F. trilineata was a preferred prey of P. hiltoni. However, this contradicts the previous feeding trials 5 in which P. hiltoni was often repelled by F. trilineata. If F. trilineata is a preferred prey of P. hiltoni, it could be that this species only remains at Duxbury Reef because it sometimes has episodes of high recruitment5. Once again, further study of hydroid abundance at Duxbury Reef would likely help answer these questions. Since F. trilineata is at its lowest recorded density at Duxbury Reef, this population should be monitored to ensure F. trilineata does not decline any further. If the abundance of F. trilineata does decline further, surrounding populations should be assessed to determine if the population genetics of this species would be in jeopardy. The disappearance of a single, moderately abundant species at Duxbury Reef does not, in my opinion, warrant the extraction of P. hiltoni.

Species that were always attacked by P. hiltoni in previous feeding trials 5 (i.e., A. loui, C. divae, T. albocrusta, and D. columbiana) are now gone from Duxbury Reef. These species were present at low densities before P. hiltoni arrived and apparently had no defense against the new predator, as indicated by the 100% attack rate 5 and absence of these species at Duxbury Reef (present study).

As is typically the case when assessing ecology, many outside factors are likely to influence the observed patterns. For the case of nudibranchs at Duxbury Reef, it is possible that the abundance changes were reflecting the geographic ranges of the species. For example, maybe the southern species A. oliviae increased in abundance because it, like P. hiltoni, was moving poleward with warming environmental conditions. While this may be true for some of the southern species, comparisons of geographic range with 23

change in abundance demonstrate that, overall, southern species were not more likely to increase or decrease in abundance. Changes in abundance could also be from species- specific sensitivities or limitations. For example, perhaps the decline in abundance of C. divae was due to increased prevalence of a fish predator that prefers C. divae, a disease or parasite that C. divae was especially susceptible to, or recruitment limitations. While it cannot concretely be said that P. hiltoni caused all the initial declines or the prolonged declines, this is a likely reasoning.

The impact of an introduced, either naturally or anthropogenically, species can vary over time. Significant impacts may initially be observed on sensitive species that are weeded out, such as those mentioned above. This results in a more robust community consisting of species that are resilient to the newcomer and future, functionally similar invaders33. These resilient species would have the opportunity to gain resources left by the sensitive species, which would grow their populations. However, the newcomer can also gain the newly available resources and grow its population, like with P. hiltoni, which could remove more species from the community71. Additionally, these open niches could be filled by previously unsuccessful species that can coexist with the new community. This did not seem to occur in the case of P. hiltoni at Duxbury Reef, although there was one sighting of a dorid species (Cadlina flavomaculata in the excluded survey of June 2018) that had not been observed at Duxbury Reef before.

Much work has been done to identify impact thresholds of disturbance or species density in an ecosystem, after which rapid change occurs in that ecosystem with slight increases in disturbance or density 40,72_74. The present dataset does not hold enough statistical power to identify an impact threshold, thus these results should be taken cautiously. Until further data are collected, it is unclear whether the pooled density of impacted species will follow a linear or polynomic trajectory. If the true relationship is best described as a polynomial, results of this study predict that the density of impacted species will decline when the density of P. hiltoni is 1.5-2 times the pooled density of 24

impacted species. Below this density of P. hiltoni, impacted species would be unaffected or affected little by P. hiltoni but once this density is approached, impacted species could decline in abundance. However, an issue with pooling the densities of impacted species is that the individual responses of species are lost and could run in opposition of each other, which would confound the results. For example, the population responses of F. trilineata and D. amyra to P. hiltoni are opposite (Figure 6) and pooling their densities would not glean any useful information.

The nudibranch assemblage of Duxbury Reef was disturbed when P. hiltoni arrived, underwent an adaptive period as they learned how to coexist (or not) with P. hiltoni, and is now returning to stability. The resilience of Duxbury Reef is likely an artifact of the reef being structurally and ecologically diverse, allowing space for a wide variety of niches to be filled 75,76. Strong ecological interactions between species can also increase community resilience and reduces the probability that a newcomer will find a niche to settle in 77. However, at Duxbury Reef, nudibranchs were strongly associated with their prey 56>78, yet predation from P. hiltoni overwhelmed the pre-existing interactions. Duxbury Reef is also an MPA, which are theoretically exposed to fewer anthropological disturbances that would further devastate low population densities 79,8°.

It is unclear why P. hiltoni was not found at Asilomar State Beach. Observations on iNaturalist show that P. hiltoni was present around the study site yet I never saw it during any trips (more than the 8 of the present study) to this site. Although the traditional and transect surveys totaling 2 hours of search should have been ample time to find nearly all species present81, it is possible that P. hiltoni was present at such low densities that they were never detected in my survey. Species with variable abundances can have variable detectability, thus the abundance of P. hiltoni at Asilomar is likely variable 82.

As P. hiltoni continues with its poleward expansion, these same impacts will likely be observed wherever P. hiltoni has an exceptionally large density in comparison 25

to resident species. The lack of protected bays with upwelling shadows north of Bodega Bay decreases the likelihood that P. hiltoni will be significantly abundant at reefs further north. I believe that because of this, many years will pass before P. hiltoni is again observed at densities that would lead to impacts.

With increasing threats to biodiversity occurring around the world, the value of long-term datasets cannot be emphasized enough. These datasets are uncommon, yet copious amounts of valuable information are embedded within them. Returning to disturbed sites and documenting the gradual shift from disturbance to recovery is incredibly important not only for management of the disturbed community, but also for building a framework of general theories of biological invasions. Additionally, conservation practices rely on historic data to put present populations into context and validate the need for conservation 37,83. With extreme weather events occurring more frequently, there is a greater need for baseline studies that will likely be useful in the determining future impacts.

The avenues for future study are numerous. As suggested before, the composition and density of sessile plants and in the rocky intertidal at Duxbury Reef combined with prey-preference experiments would be exceptionally useful for determining interaction webs, understanding how prey density changes through time, and correlating prey density with nudibranch density. Functional diversity of nudibranchs, which would have to be measured at minute scales, would provide a better understanding of how species coexist in a reef and improve predictions of how assemblages react to the addition or removal of a species 84_87. Feeding observations of the current study, claiming that nudibranchs do not overlap in hydroid prey, begs the question of how selective or specialized are nudibranch populations and how does this compare among sites 61. The significant decline of pooled dorid populations is a striking finding, yet beyond the scope of this study. Future efforts should compare the abundance of dor ids at Duxbury Reef to other reefs to see if this is only occurring at this site or if a whole region is being affected. 26

Instances such as these are the reality of this changing climate. The disturbance at Duxbury Reef that was likely caused by P. hiltoni is one of countless records of range shifts that have caused ecological disarray, and records are mounting. However, the story of this reef provides hope that even if the future looks grim, it always has the potential to brighten back up. 27

References

1. Leadley, C. et al. Biodiversity Scenarios: projections o f 21st century change in biodiversity and associated ecosystem services. (2 0 1 0 ).

2. Lluch-Belda, D., Lluch-Cota, D. B. & Lluch-Cota, S. E. Changes in marine faunal distributions and ENSO events in the California Current. Fish. Oceanogr 14,458- 467 (2005).

3. King, C. J., Ellingson, R., Goddard, J. H. R. & Johnson, R. F. Bulletin of the Southern California Academy of Sciences Range expansion or range shift ? Population genetics and historic range data analyses of the predatory benthic sea slug Phidiana hiltoni. 118, (2019).

4. Schultz, S. T. et al. Climate-index response profiling indicates larval transport is driving population fluctuations in nudibranch gastropods from the northeast Pacific Ocean. Limnol. Oceanogr. 56, 749-763 (2011).

5. Goddard, J. H. R., Gosliner, T. M. & Pearse, J. S. Impacts associated with the recent range shift of the aeolid nudibranch Phidiana hiltoni (, Opisthobranchia) in California. Mar. Biol. 158, 1095-1109 (2011).

6 . Sanford, E., Sones, J. L., Garcia-Reyes, M., Goddard, J. H. R. & Largier, J. L. Widespread shifts in the coastal biota of northern California during the 2014-2016 marine heatwaves. Sci. Rep. 9, (2019).

7. Barry, J. P., Baxter, C. H., Sagarin, R. D. & Gilman, S. E. Climate-Related, Long- Term Faunal Changes in a California Rocky Intertidal Community. Science (80-.). 267,672-675(1995).

8 . Gentemann, C. L., Fewings, M. R. & Garcia-Reyes, M. Satellite sea surface temperatures along the West Coast of the United States during the 2014-2016 northeast Pacific marine heat wave. Geophys. Res. Lett. 44, 312-319 (2017).

9. Welch, C. Warming Pacific Makes for Increasingly Weird Ocean Life. National Geographic Socety (2015).

10. Pansch, C. et al. Heat waves and their significance for a temperate benthic community: A near-natural experimental approach. Glob. Chang. Biol. 24,4357- 4367 (2018).

11. Frolicher, T. L. & Laufkotter, C. Emerging risks from marine heat waves. Nature Communications 9, (2018).

12. Spencer, R. Global Warming as a Natural Response to Cloud Changes Associated with the Pacific Decadal Oscillation(PDO). (2008). Available at: 28

http://www.drroyspencer.com/research-articles/global-warming-as-a-natural- response/. (Accessed: 22nd April 2018)

13. Goddard, J. H., Treneman, N., Pence Douglas E Mason, W. E. & Dobry, P. M. Nudibranch Range Shifts associated with the 2014 Warm Anomaly in the Northeast Pacific. Bull. South. Calif. Acad. Sci. 115, 15-40 (2016).

14. Lonhart, S. & Tupen, J. New range records of 12 marine invertebrates: the role of El Nino and other mechanisms in southern and central California. Bull. South. Calif. Acad. Sci. 100, 238 (2001).

15. Keister, J. E., Di Lorenzo, C., Combes, V. & Peterson, W. T. Zooplankton species composition is linked to ocean transport in the Northern California Current. Glob. Chang. Biol. 17, 2498-2511 (2011).

16. Di Lorenzo, E. & Mantua, N. Multi-year persistence of the 2014/15 North Pacific marine heatwave. 11 (2016). doi:10.1038/NCLIMATE3082

17. Goddard, J H. R., Treneman, N., Pence, W. E. & Mason, D. E. Nudibranch Range Shifts associated with the 2014 Warm Anomaly in the Northeast Pacific Nudibranch Range Shifts associated with the 2014 Warm Anomaly in the. 115, (2016).

18. Goddard, J. H. R., Pearse, J. S. & Gosliner T. M. Sea slugs as brilliant indicators of climate change in central California. UC San Diego Res. Summ. (2011). doi: 10.1007/s00227-011 -1633-7

19. Pauley, G. B., Van Der Raay, B. & Troutt, D. Species Profiles: Life Histories and Environmental Requirements of Coastal Fishes and Invertebrates (Pacific Northwest), Pacific Oyster. Biol. Rep. 1-28 (1988).

20. Sorte, C. J. B., Williams, S. L. & Carlton, J. T. Marine range shifts and species introductions: comparative spread rates and community impactsg eb_519 303..316. Glob. Ecol. Biogeogr. Ecol. Biogeogr.) 19, 303-316 (2010).

21. Grosholz, E. D. & Ruiz, G. M. Predicting the impact of introduced marine species: lessons from the multiple invasions of the European green crab Carcinus maenas. Biol. Conserv. 78, 59-66 (1996).

22. Osland, M. J. et al. Mangrove expansion and contraction at a poleward range limit: climate extremes and land-ocean temperature gradients. Ecology 98, 125-137 (2017).

23. O’Donoghue, C. H. Notes on a collection of nudibranchs from Laguna Beach. J. Entomol. Zool. 19, 77-119 (1927). 29

24. Lance, J. R. Two new opisthobranch mollusks from southern California. Veliger 4, 155-159(1962).

25. Cotton, P. A., Rundle, S. D. & Smith, K. E. Trait compensation in marine gastropods: shell shape, avoidance behavior, and susceptibility to predation. Reports Ecology 85, (2004).

26. Van Alstyne, K. L. Herbivore grazing increases polyphenolic defenses in the intertidal brown alga Fucus distichus. Ecology 69, 655-663 (1988).

27. Bertsch, H. Ten-Year Baseline Study o f Annual Variation in the Opisthobranch (Mollusca: ) Populations at Bahia de los Angeles, Baja California, Mexico. (2008).

28. Nybakken, J. Abundance, diversity and temporal variability in a California intertidal nudibranch assemblage. Mar. Biol. 45, 129-146 (1978).

29. Nybakken, J. A phenology of the smaller dendronotacean, arminacean and aeolidacean nudibranchs at Asilomar State Beach over a twenty-seven month period. Veliger 16, 373 (1974).

30. Lambert, W. J. Coexistence of Hydroid Eating Nudibranchs: Do Feeding Biology and Habitat Use Matter? Biol. Bull. 181, 248-260 (1991).

31. Hoover, C., Lindsay, T., Goddard, J. H. R. & Valdes, A. Seeing double: Pseudocryptic diversity in the Doriopsilla albopunctata-Doriopsilla gemela species complex of the north-eastern Pacific. Zool. Scr. 44, 1-20 (2015).

32. Strayer, D. L., Eviner, V. T., Jeschke, J. M. & Pace, M. L. Understanding the long­ term effects of species invasions. 2 1 , (2006).

33. Ricciardi, A., Hoopes, M. F., Marchetti, M. P. & Lockwood, J. L. Progress toward understanding the ecological impacts of normative species. Ecol. Monogr. 83,263- 282 (2013).

34. Vaclavik, T. & Meentemeyer, R. K. Equilibrium or not? Modelling potential distribution of invasive species in different stages of invasion. Divers. Distrib. 18, 73-83 (2012).

35. Capiat, P. et al. Movement, impacts and management of plant distributions in response to climate change: insights from invasions. Oikos 122, 1265-1274 (2013).

36. Wilde, P. & Yancey, T. Sediment Distribution and its Relations to Circulation Patterns in Bolinas Bay, California, in Coastal Engineering 1970 1397-1415 (American Society of Civil Engineers, 1970). doi: 10.1061/9780872620285.086 30

37. Bertsch, H. Biogeography o f northeast Pacific opisthobranchs: Comparative faunal province studies betwee. (2 0 1 0 ).

38. Bertsch, H. Nudibranch feeding biogeography: ecological network analysis of inter-and intra-provincial variations. Thalassas 27, 155-168 (2011).

39. Goddard, J. H. R. et al. Heterobranch Sea Slug Range Shifts in the Northeast Pacific Ocean associated with the 2015-16 El Nino Heterobranch Sea Slug Range Shifts in the Northeast Pacific Ocean associated with the 2015-16 El Nino. (2018).

40. Gooden, B., French, K., Turner, P. J. & Downey, P. O. Impact threshold for an alien plant invader Lantana camara L ., on native plant communities. 142,2631- 2641 (2009).

41. Vander Woude, A. J., Largier, J. L. & Kudela, R. M. Nearshore retention of upwelled waters north and south of Point Reyes (northern Califomia)-Pattems of surface temperature and chlorophyll observed in CoOP WEST. Deep. Res. Part II Top. Stud. Oceanogr. 53,2985-2998 (2006).

42. Wing, S. R., Louis, W. B., Largier, J. L. & Morgan, L. E. Spatial structure o f relaxation events and crab settlement in the northern California upwelling system. Marine Ecology Progress Series 128, (1995).

43. Lagos, N. A., Barria, I. D. & Paolini, P. Upwelling ecosystem of northern Chile: integrating benthic ecology and coastal oceanography through remote sensing. Oceanogr. Ecol. nearshore bays Chile 117-141 (2002).

44. Roughan, M. et al. Subsurface recirculation and larval retention in the lee of a small headland: A variation on the upwelling shadow theme. J. Geophys. Res. C Ocean. 110, 1-18 (2005).

45. Archambault, P ., Bourget", E., Bourget", B., Archambault, P. & Bourget, E. Influence o f shoreline configuration on spatial variation o f meroplanktonic larvae, recruitment and diversity o f benthic subtidal communities. Journal of Experimental Marine Biology and Ecology 241, (1999).

46. Gaines, S. D. & Bertness, M. D. Dispersal o f juveniles and variable recruitment in sessile marine species.

47. Kaplan, D. M. & Largier, J. HF radar-derived origin and destination of surface waters off Bodega Bay , California. Deep. Res. 7/53,2906-2930 (2006).

48. Graham, W. M. & Largiert, J. L. Upwelling shadows as nearshore retention sites: the example o f northern Monterey Bay. Continental Shelf Research 17, (1997).

49. Goddard, J. H. Developmental mode in benthic opisthobranch molluscs from the 31

northeast Pacific Ocean: feeding in a sea of plenty. Can. J. Zool. (2004). doi:10.1139/Z05-008

50. Whitfield, P. E., Hare, J.. David, A. & Harter, S. L. Abundance estimates of the Indo-Pacific lionfish Pterois volitans/miles complex in the Western North Atlantic. (2006). doi: 10.1007/s 10530-006-9005-9

51 Cote, I. M., Green, S. J. & Hixon, M. A. Predatory fish invaders : Insights from Indo-Pacific lionfish in the western Atlantic and Caribbean. 164, 50-61 (2013).

52. Kowarik, I. Time lags in biological invasions with regard to the success and failure of alien species. Plant invasions Gen. Asp. Spec. Probl. 15-38 (1995).

53. Crooks, J. A. Lag times and exotic species: The ecology and management of biological invasions in slow-motionl. Ecoscience 12, 316-329 (2005).

54. Parker, I. M. Mating patterns and rates of biological invasion. Proc. Natl. Acad. Sci. U. S. A. 101, 13695-13696 (2004).

55. Morris, J., Rice, J. A., Morris Jr, J. A. & Shertzer, K. W. A stage-based matrix population model of invasive lionfish with implications for control. (2 0 1 1 ). doi: 10.1007/s 10530-010-9786-8

56. Buhle, E. R., Margolis, M. & Ruesink, J. L. Bang for buck: Cost-effective control of invasive species with different life histories. Ecol. Econ. 52, 355-366 (2005).

57. Parker, I. M. et al. Impact: toward a framework for understanding the ecological effects of invaders. 3-19 (1999).

58. Mealor, B. A. & Hild, A. L. Post-invasion evolution of native plant populations: a test of biological resilience. Oikos 116, 1493-1500 (2007).

59. Nunes, A. L., Orizaola, G. Laurila, A. & Rebelo, R. Rapid evolution of constitutive and inducible defenses against an invasive predator. Ecology 95, 1520-1530(2014).

60. Trowbridge, C. D. Northeastern Pacific sacoglossan opisthobranchs: natural history review, bibliography, and prospectus. Veliger-Berkeley 45, 1-24 (2002).

61. Gosliner, T. M. & Williams, G. C. The Opisthobranch Mollusks of Marin County, California NOAA Pacific Coast Deep Sea Corals View project Cryptic Diversity and Feeding Ecology of Dermatobranchus van Hasselt, 1824 (Nudibranchia: Arminidae) View project. Veliger 13,175-180 (1970).

62. Mcdonald, G. & Nybakken, J. List o f the Worldwide Food Habits o f Nudibranchs. (1997). 32

63. McDonald, G. & Nybakken, J. Additional notes on the food of some California nudibranchs with a summary of known food habits of California species. Veliger 21, 110-119(1978).

64. Bertsch, H. & Gosliner, T. M. Natural history and occurrence of opisthobranch gastropods from the open coast of San Mateo County, California Molecular phylogeny and evolution of marine gastropods in the Indo-Pacific and Antarctica View project. Veliger 14, 302-314 (1972).

65. Behrens, D. W. Pacific Coast Nudibranchs, Supplement II New Species to the Pacific Coast and New Information on the Oldies. Proceedings o f the California Academy o f Sciences 55, (2004).

6 6 . Todd, C. D., Walter, J. & Daviee, J. Some perspectives on the biology and ecology of nudibranch molluscs: generalisations and variations on the theme that prove the rule. Boll. Malacol. 105-120 (2001).

67. Penney, B. K. How specialized are the diets of Northeastern Pacific -eating dorid nudibranchs? J. Molluscan Stud. 79, 64—73 (2013).

6 8 . Lindsay, T. & Valdes, A. The Model Organism Hermissenda crassicornis (Gastropoda: ) Is a Species Complex. PLoS One 11, e0154265 (2016).

69 Griffiths, D. Foraging Costs and Relative Prey Size. Am. Nat. 116, 743-752 (1980).

70. MacArthur, R. H. & Pianka, E. R. On Optimal Use of a Patchy Environment. Am. Nat. 100, 603-609 (1966).

71. Lyons, K. G. & Schwartz, M. W. Rare species loss alters ecosystem function - invasion resistance. Ecol. Lett. 4, 358-365 (2001).

72. Panetta, F. & James, R. Weed control thresholds: a useful concept in natural ecosystems? Plant Prot. Q. 14,68-76(1999).

73. Drinnan, I. N. The search for fragmentation thresholds in a Southern Sydney Suburb. Biol. Conserv. 124, 339-349 (2005).

74. MONAMY, V. & FOX, B. J. Responses of two species of heathland rodents to habitat manipulation: Vegetation density thresholds and the habitat accommodation model. Austral Ecol. 35, 334—347 (2010).

75. Halpem, C. B. Early Successional Pathways and the Resistance and Resilience of Forest Communities. Ecology 69, 1703-1715 (1988). 33

76. Elton, C. S. The ecology of invasions by plants and animal. Methuen (1958).

77. Case, T. E. D. J. Invasion resistance arises in strongly interacting species-rich model competition communities. 87, 9610-9614 (1990).

78. Goodheart, J. A., Bazinet, A. L., Valdes, A., Collins, A. G. & Cummings, M. P. Prey preference follows phylogeny: evolutionary dietary patterns within the marine gastropod group (Gastropoda: Heterobranchia: Nudibranchia). BMC Evol. Biol. 17, 221 (2017).

79. Casu, D., Ceccherelli, G., Curini-Galletti, M. & Castelli, A. Human exclusion from rocky shores in a mediterranean marine protected area (MPA): An opportunity to investigate the effects of trampling. 15-32 (2005). doi: 10.1016/j .marenvres.2006.02.004

80. Mellin, C., Aaron MacNeil, M., Cheal, A. J.> Emslie, M. J. & Julian Caley, M. Marine protected areas increase resilience among coral reef communities. Ecol. Lett. 19, 629-637 (2016).

81. Smith, S. D. A. & Nimbs, M. J. Quantifying temporal variation in heterobranch (Mollusca: Gastropoda) sea slug assemblages: tests of alternate models. Molluscan Res. 37, 140-147 (2017).

82. Bates, A. E. et al. Distinguishing geographical range shifts from artefacts of detectability and sampling effort. Divers. Distrib. 21, 13-22 (2015).

83. Dayton, P. The Importance of the Natural Sciences to Conservation. 1-13 (2002). doi: 10.1086/376572

84. Villeger, S., Novack-Gottshall, P. M., Bastien Ville Ger, S. & Mouillot, D. The multidimensionality of the niche reveals functional diversity changes in benthic marine biotas across geological time. 561-568 (2011). doi: 10.1111 /j. 1461 - 0248.2011.01618.x

85. Hooper, D. U. et al. Species diversity, functional diversity, and ecosystem functioning. (2002).

8 6. Halpem, B. S. & Floeter, S. R. Functional diversity responses to changing species richness in reef fish communities. Mar. Ecol. Prog. Ser. Mar Ecol Prog Ser 364, 147-156 (2008).

87. Mayfield, M. M. et al. What does species richness tell us about functional trait diversity? Predictions and evidence for responses of species and functional trait diversity to land-use change. Glob. Ecol. Biogeogr. (2010). doi:10.1111/j.l466- 8238.2010.00532.x 34

Appendix I: Supplementary Materials

Range Shift with ENSO Phases

Geographic ranges of species are shifting in concordance w-th climatic regimes such as ENSO. To determine if the range of P. hiltoni shifts with ENSO, I searched iNaturalist, Flikr and Facebook for pictures of P. hiltoni with dates and locations. Search phrases included Phidiana. Phidiana hiltoni, Phidianapugnax, and Phidiana nigra.

I- SI

m

SL ME SL, ML r>~18 n=5 n=5 7/07-7/08 7/09-3/10 6/10-3/12 ' j & x ^I * ■ r ' #>•_:> X MM -I* ft

IB

WE, VSE WL WL n= 2 5 9 :«r ^ " 3 0 ^ n=80 *58 11/14-5/16 8/16-12/16 10/17-3/18

Figure 10. Observations of P. hiltoni from online sources reveal a range shift with ENSO phases. ME moderate El Nino, VSE very strong El Nino. WL weak La Nina, ML moderate La Nina, SL strong La Nina. 35

El Nino/ La Nina phases were determined from https://ggweather.com/enso/oni.htm. With an exception of the weak La Nina from 11/08- 3/09, which had less than 5 observations, all ENSO phases occurring from 2007 to 2017 were used: the La Nina phases of 7/07-7/08, 6/10-3/12, 8/16-12/16, and 10/17-3/18 and the El Nino phases from 7/09-3/10 and 11/14-5/16. Observations from each phase were mapped to see the distribution of P. hiltoni.

A total of 397 acceptable observations of P. hiltoni were found on the internet during phases from 2007 to 2018. Over these years, the range of P. hiltoni both shifts with phases of ENSO and climbs the California coast; the latter must be taken into consideration when looking for the former.

Prior to the warm “blob” that persisted over the Pacific Ocean from 2013 to 2016, P. hiltoni did not occur north of the San Francisco (SF) Bay and the northern limit remained Pillar Point for the three El Nino/ La Nina phases from 2007-2012. However, the southern limit of P. hiltoni reached as far south as San Nicolas Island during La Nina but did not go past Santa Barbara during El Nino.

During and after the years of the “blob,” P. hiltoni was consistently found above the SF Bay. The northern limit of P. hiltoni went as far north as Bodega Bay during the El Nino phase yet remained near Duxbury Reef during the La Nina phases. Ever since the “blob,” P. hiltoni does not occur more south than Pismo Beach during El Nino yet it reaches the Channel Islands during La Nina.

While the shift is not immediately apparent, a poleward shift with El Nino and an equatorward shift with La Nina can be discerned at the southern range limit. The northern limit does not fluctuate as much, likely since P. hiltoni persists at these northernmost locations even after a switch to a La Nifia phase. Northward expansions occur during El Nino. These correspond to the direction of water flow off the coast of California for each ENSO phase. The normally upwelled waters travel equatorward during La Nina and carry 36

the larvae of P. hiltoni south (also offshore, but this would be observed as potentially fewer observations during La Nina). As the upwelling relaxes and waters begin to flow poleward (and onshore, potentially increasing observations), the larvae of P. hiltoni will be carried north, perhaps further north than they had been taken before.

This study can be improved as additional ENSO phases occur and the involvement of citizen scientists increase. While mited in scope, citizen science opens floodgates of potentially useful data. These estimations of range are not complete records of species occurrences, as indicated by the lack of observations of P. hiltoni at Duxbury Reef before the 2014-16 El Nino. Furthermore, these can only represent presence and cannot translate to abundance.