Habitat Selection Among Fishes and Shrimp in the Pelagic Sargassumcommunity: the Role of Habitat Architecture

HABITAT SELECTION AMONG FISHES AND SHRIMP IN THE PELAGIC SARGASSUMCOMMUNITY: THE ROLE OF HABITAT ARCHITECTURE

Chelsea O. Bennice

A Thesis Submitted to the Faculty of

The Charles E. Schmidt College ofScience

in Partial Fulfillment of the Requirements for the Degree of

Master ofScience

Florida Atlantic University

Boca Raton, Florida

December 2012 HABITAT SELECTION AMONG FISHES AND SHRIMP IN THE PELAGIC SARGASSUMCOMMUNITY: THE ROLE OF HABITAT ARCHITECTURE

Chelsea O. Bennice

This thesis was prepared under the direction of the candidate's thesis advisor, Dr. W. Randy Brooks, Department of Biological Sciences, and has been approved by the members ofher supervisory committee. It was submitted to the faculty ofthe Charles E. Schmidt College of Science and was accepted in partial fulfillment of the requirements for the degree ofMaster ofScience.

Gary W. erry, Ph. . Dean, Charles E. Schmidt College of Science ~~z;e_...... 4~J'Z-~)~ Barry T. Ro~n, Ph.D: Date Dean, Graduate College

ii ACKNOWLEDGEMENTS

First and foremost I want to thank my advisor Dr. Randy Brooks for his guidance and supervision throughout my graduate career, as well as his willingness to take me specimen collecting on his boat, even ifthe seas were looking a little ferocious.

I want to thank Dr. Brian Lapointe for his support and guidance as my co-advisor and sampling in the Florida Keys with himself and Captain Carl. Thank you to Dr. Ed

Proffitt for serving on my committee and his guidance for the development ofmy experimental design and statistical analyses. Also, I want to thank numerous funders who have allowed me to pursue my research: Guy Harvey Ocean Foundation and administered by Florida Sea Grant, Manasquan River Marlin and Tuna Club Inc., and the Marsh Scholarship for Marine Biology. Graduate students that deserve a thank you for all their hard work in assisting me in the field and lab: Lorin West, Ed Davis, Derek

Cox, Bethany Augliere, lana Boerner, Bethany Resnick, and Erica Baugh. A special thanks to Lorin West for making long days in the lab interesting and tolerable, Bethany

Augliere for her critiques and edits on my thesis that she has probably read or listened to me talk about more than we both can imagine, and Bethany Resnick for being my personal field assistant. Last but not least, I would like to thank my family for all their love and support.

111 ABSTRACT

Author: Chelsea Bennice

Title: Habitat Selection Among Fishes and Shrimp in the Pelagic Sargassum Community: The Role ofHabitat Architecture

Institution: Florida Atlantic University

Thesis Advisor: Dr. W. Randy Brooks

Degree: Master of Science

Year: 2012

Pelagic Sargassum was used to determine the effects ofhabitat architecture for one species of shrimp (Leander tenuicornis) and two species of fish (Stephanolepis hispidus and Histrio histrio). Inter-thallus spacing (low, medium, and high) and depth

(shallow versus deep) were manipulated independently to test whether the spatial components ofhabitat architecture. Two differing habitats (Sargassum versus seagrass species) were tested for the structural component ofhabitat architecture. There were no significant results for inter-thallus spacing experiments for 1. tenuicornis and S. hispidus. H histrio selected habitats with medium inter-thallus spacing in two treatments. Large individual H histrio contributed mostly to the significant effects. All three species selected habitats with a greater depth aspect. Finally, 1. tenuicornis and H histrio selected habitats with greater structural complexity (i.e., Sargassum). These results demonstrate clearly that habitat architecture ofSargassum influences habitat selection by these shrimp and fishes.

IV HABITAT SELECTION AMONG FISHES AND SHRIMP IN THE PELAGIC

SARGASSUM COMMUNITY: THE ROLE OF HABITAT ARCHITECTURE

FIGURES vii

INTRODUCTION 1

Habitat Architecture 2

Objectives 4

MATERIALS AND METHODS 6

Collection and maintenance ofspecimens 6

General Experimental Procedures 8

Spatial Components ofHabitat Architecture Procedures 9

Differing inter-thallus spacing experiment.. 10

Differing depth experiment "A" 11

Differing depth experiment "B" 12

Structural component ofHabitat Architecture 13

Sargassum versus seagrasses patches experiment.. 13

RESULTS 14

General Habitat Selection 14

Differing inter-thallus spacing experiment 14

High versus Medium 14

Medium versus Low 15

v High versus Low 15

Differing Depth Experiment 16

"A"-Deep (with completely submerged superior surface) versus Shallow 16

"B"- Deep (with floating superior surface) versus Shallow 16

Structural Component for Habitat Selection Experiments 16

Sargassum versus seagrasses patches 16

General Behavioral Observations in Laboratory Experiments and Field 17

DISCUSSION 18

Differing inter-thallus spacing experiments 18

DifferiI renng dept hexperiments . "A"an d "B" .. 20

Structural Component ofHabitat Architecture 21

CONCLUSIONS 24

APPENDIXES 26

REFERENCES 36

VI FIGURES

Figure 1. Experimental aquarium set up 26

Figure 2. Differing inter-thallus spacing experiment set up 27

Figure 3. Image ofinter-thallus space experiment.. 28

Figure 4. Differing depth experiment "A" and "B" set ups 29

Figure 5. Structural component experiment set up 30

Figure 6. Habitat selection based on differing inter-thallus spacing 31

Figure 7. Habitat selection based on differing inter-thallus spacing 32

Figure 8. Habitat selection based on differing inter-thallus spacing 33

Figure 9. Differing depth experiments "A" and "B" 34

Figure 10. Structural component experiment.. 35

VB INTRODUCTION

The importance ofhabitat architecture and structural complexity is often overlooked in studies ofhabitat selection by animals. Some ofthe more well-known systems (e.g., forests, coral reefs, rocky intertidal, mangrove) demonstrate how habitat complexity influences species diversity and abundance (Grinnell 1917, Gause 1934,

Crisp and Barnes 1954, Huffaker 1958, Connell 1961, Emson and Faller-Fritsch 1976,

Keough & Downes 1982, Fletcher and Underwood 1987, Walters and Wethey 1996,

Beck 1998). Pelagic Sargassum represents a community with excellent potential to study effects ofhabitat architecture on biotic interactions with its associated inhabitants.

Floating, macrophytic algal mats (either permanently floating forms or from detached benthic forms) can serve as nursery areas for juvenile fishes, as they provide prey resources and protection from predators (Lenanton et al. 1982, Lenanton and

Caputi 1989, Wells and Rooker 2004). Ofthese nursery habitats, pelagic Sargassum has received the least amount ofresearch inquiry. Pelagic Sargassum is commonly labeled

"floating reefs" because ofthe plethora oforganisms, such as juvenile fishes, invertebrates, and endemic organisms (e.g., Sargassum fish Histrio histrio) that use the patches for food, shelter, or substrate attachment in the open ocean. The complex

Pelagic Sargassum mats are also used for shelter by organisms that have been swept away by strong currents from their previous habitats and would otherwise be left stranded and vulnerable in the open ocean (Lapointe 1986).

Habitat Architecture

In general, objects floating in the ocean, including Sargassum spp., attract and concentrate fauna (Hunter and Mitchell 1968, Lapointe 1995, Ing6lfsson 1998, Robert and Poore, 2005, Casazza & Ross 2008) by creating habitat substrate and thereby increasing the complexity ofthe pelagic environment (Kingsford 1995, Ingolfsson

1998). Dean and Connell (1987) studied specific mechanisms ofecological succession in such habitats. They concluded that the "ecological time" (i.e., inhabitant diversity increases with time as more algae accumulates) and "algal toxicity" (i.e., inhabitant diversity is low initially because early algal mat species are more toxic than subsequent algae) hypotheses were less likely valid than the "habitat complexity" hypothesis (i.e., based on biomass and surface area characteristics ofalgal thalli) in explaining succession and species diversity in pelagic algal mats. Algal biomass alone can be deceptive, as similar shaped thalli can be rearranged into different densities by natural effects (e.g., currents, wind) in the open ocean. This, in turn, could cause a change in habitat complexity by changing the habitat architecture (i.e., relative spatial arrangement ofalgal thalli). Additionally, surface area characteristics alone might be manipulated naturally (e.g., through grazing or biodegradation) to confound interpretation ofeffects on colonizing inhabitants. Thus, a comprehensive approach would involve looking at a system in which both algal biomass and surface characteristics are studied simultaneously.

2 Hacker and Steneck (1990) and Hacker and Madin (1991) define algal habitat architecture as the number, size, shape and arrangement ofhabitable spaces and structures for organisms. This definition takes a comprehensive approach in that both spatial (i.e., size, shape and arrangement ofspaces between fronds) and structural (i.e., number, length, and the width offronds, branches, and vesicles) components are measured. Hacker and Steneck (1990) examined the influence ofdiffering spatial components by using different benthic species and algal mimic counterparts that ranged from filamentous, foliose and leathery macrophyte morphologies. They found that the more densely branched algal mimic attracted greater numbers ofamphipods.

Subsequently, Hacker and Madin (1991) focused specifically on structural components ofthe Sargassum habitat, because the two species ofshrimp used in their experiments

(Latreutes fucorum and Hippolyte coerulescens) both have mimicry resembling pelagic

Sargassum (cf., Gurney 1936, Brown 1939, Sisson 1976, Williams 1984). Hacker and

Madin (1991) manipulated structural components ofSargassum thalli to produce two variants: 1) a "frond only" form by removing vesicles; and 2) a "vesicle only" form by removing fronds. The shrimp 1. fucorum selected Sargassum with "fronds only" over the "vesicles only" form, while the reverse was true for the shrimp H coerulescens.

Chemello and Milazzo (2002) found that molluscan assemblages in the southern

Mediterranean Sea were correlated to the different attributes ofalgal architecture.

Specifically, high species diversity ofmolluscs was associated with algal types that displayed complex architecture.

Druce and Kingsford (1995) concluded that the main factor attracting fishes in the pelagic environment is the presence ofobjects themselves, regardless ofform or

3 color (Ingolfsson 1998). While this broad analysis may be true, clearly characteristics ofhabitats can further affect species richness and diversity offauna (Hicks 1985, Gee and Warwick 1994). Generally, fish communities are more diverse within and below floating communities of seaweed compared to other floating items (i.e., flotsam)

(Dooley 1972, Fedoryako 1989, Lapointe 1995, Vandendriessche et al. 2007). However, there are still conflicting results between habitat architecture studies. For example, structural complexity oftwo different brown algae (Sargassum globulariaefolium and

Hormosira banksii) generally did not influence abundances ofepifaunal amphipods

(Schreider et al. 2003). Such discrepancies regarding the importance ofarchitecture among different habitats illustrate the need for further investigations, as little is known about the ultimate factors for patch-seeking behavior by animals (Ingolfsson 1998).

Pelagic Sargassum offers excellent systems for testing both spatial and structural effects in the field and laboratory. Studies on colonization offragmented (i.e., differing spatial arrangements) algal habitats should consider the multi-dimensional aspects of such patches, which would include horizontal (i.e., diameter ofpatches), vertical (i.e., depth the alga descends into the water column) and density (i.e., spaces created between algal fronds).

Objectives

In this laboratory and field study, using one species ofgulfweed shrimp

(Leander tenuicornis) and two species offish (Stephanolepis hispidus and Histrio histrio), the following questions were asked:

4 1. Do spatial components within Sargassum patches influence selection by these

animals?

2. Do structural components ofdifferent habitats influence habitat selection by

these animals?

5 MATERIALS AND METHODS

Sargassum habitat is extremely difficult to sample consistently and quantitatively, and thus no single method ofsampling provides a complete survey ofthe

Sargassum community. Moser et al. (1998) recommended using multiple methods, including visual surveys (Casazza & Ross 2008). My study includes laboratory experiments supplemented with field observations, which allowed for the: 1) proper execution oflaboratory experiments by becoming familiar with calm or natural behaviors ofshrimp and fishes, and 2) gathering ofinformation on the behavior and location of shrimp and fishes within and under Sargassum patches.

Collection and maintenance of specimens

Floating clumps ofSargassum mats containing both S. natans and S.fluitans were collected via boat 1.5-3.5 km offthe southeast coast ofFlorida from May to

August, 2010-2012. Sargassum was collected using a fine-mesh dip net. Small clumps ofSargassum and all associated organisms were placed into a cooler aerated with a portable air pump until we reached the laboratory at Florida Atlantic University, Boca

Raton, Florida, USA.

Once in the laboratory, the two species ofSargassum were sorted to remove most organisms (except for hydroids, bryozoans, and cyanobacteria). Both species of

6 Sargassum were maintained in separate aquaria from animals prior to use in trials.

However, Sargassum species were separated before use in trials. Because most ofthe

Sargassum collected was S.jluitans, all experiments used this species only. This allowed for consistency when weighing the Sargassum for trials; S.jluitans has larger fronds and tends to be denser than S. natans.

Two species of seagrass (Thalassia testudinum and Syringodium jiliforme) were also observed in the field within the floating "Sargassum" clumps. These seagrass species were collected for use in the habitat structure complexity experiment.

The shrimp and two species offish were separated by species and maintained in

38 land 76 llaboratory aquaria containing natural seawater (32-35 ppt). Aeration systems and water filters were used in the tanks to assist in maintaining appropriate living conditions for the organisms. Aquaria were exposed to a 12L: 12D photoperiod.

Shrimp and fishes were fed commercial flake food and brine shrimp, respectively, 5-7 times per week. Sargassum was not used in trials after 72 h from collection time because the alga deteriorated rapidly in aquaria (lobe and Brooks 2009). Shrimp and fishes were not used in experimental trials until 24h after collection, and were not used after a week in captivity. Animals were only used once for each specific experimental treatment. Lengths for all animals were measured (for shrimp, from rostrum tip to end oftelson; for fishes, from snout tip to caudal fin tip). Additionally, experimental data from inter-thallus space trials for each animal type were partitioned into two groups based on size for post hoc analysis for potential size effects (after Brooks et al., 2007, lobe and Brooks, 2009). The size range for L. tenuicornis was 15-33 mm, with small and large groups of< 25 mm and ~ 25 mm, respectively. S. hispidus ranged from 10-90

7 mm, with small and large groups of< 40 mm and 2:40 mm, respectively. H histrio ranged from 12-87 mm, with small and large groups of< 40 mm and 2: 40 mm, respectively.

General Experimental Procedures

Habitat selection experiments were conducted in 38 I aquaria. Preliminary observations showed that organisms typically clung to or positioned themselves next to the filter or air stone; therefore, these items were removed before starting a trial. When trials were in progress, dividers between aquaria were used to minimize distractions and observer interference. Water changes (20%) were done before and after trials, to ensure fresh seawater was used.

Three different habitat selection experiments were conducted in this study (two for the spatial component ofhabitat architecture and one for the structural component of habitat architecture). Each experiment had two different habitat options for habitat selection. The experimental aquaria were divided into three equal zones (Figure 1).

Zones 1 and 3 both had a habitat patch. Zone 2 in the aquaria was always designated as an "open water/no habitat" zone.

Habitats (e.g., algal or seagrass patches) were placed in Zones 1 and 3 prior to the experiment. Either a shrimp or a fish (one ofthe two species) was placed initially in

Zone 2 "open water area/no habitat" (Figure 1). In preliminary observations, shrimp showed increased nocturnal activity (swimming). Thus, to ensure that shrimp had adequate opportunities for selection, shrimp experiments ranged from 15- 24h, so that both diurnal and nocturnal activity periods were included. Once the shrimp was in the

8 patch at the end ofthe trial, the patch selected was recorded. If the shrimp was still in

Zone 2, "no selection" was recorded.

For the fish experiments, individual fish were allowed to acclimate for 1 h (i.e., show behaviors similar to that observed in the field) in the experimental aquaria. Ifthe fish did not acclimate during this time (i.e., showed erratic swimming or other aberrant behaviors), it was placed back in the holding tank and used at another time. As soon as the fish acclimated, but not sooner than 5 min, the trial began. Trials ran for 30 min with continuous observation. Total time spent in each zone/habitat, and behavioral observations (i.e., location in or around patch and whether fish was swimming or stationary) were recorded. All experiments in this study consisted of dichotomous data, with habitat selection ofone patch over the other patch. Data for fishes were analyzed by using the averaged proportion oftime each species offish spent with each habitat; these data were also analyzed using the binomial (Z) test. As stated previously,posf hoc analysis ofdata based on size differences ofanimals was also done using the binomial (Z) test.

Spatial Components ofHabitat Architecture Procedures

In nature, Sargassum patches can vary not only in abundance, but also in their width (or diameter) and shape at the surface ofthe water and their vertical depth into the water column. Additionally, natural patches have varying densities ofalgal biomass, as measured by distances between neighboring stipes and fronds (i.e., two patches might have the same diameter and depth values, but one patch could have much more or less

9 algal material per unit area than the other). Therefore, in this study I attempted to look at the effect on host selection by shrimp and fishes ofsome ofthese varying characteristics by manipulating spatial components ofpatches in two specific ways: 1) varying distances within given patches between algal thalli (i.e., inter-thallus distances); and 2) varying patch depth aspects in the water column.

In the first experiment, the degree ofinter-thallus spacing was manipulated while keeping algal biomass, dimensions at surface (i.e., diameter, as most mats are circular/oval in shape when viewed from above), and depth constant. For this experiment, inter-thallus space was defined as the empty space or space occupied by water between Sargassum thalli. For the second set ofexperiments, the depth at which the Sargassum thalli were placed was manipulated, while keeping the algal biomass and diameter ofeach patch constant. The specific experiments were as follows:

Differing inter-thallus spacing experiment. There were 3 treatment levels for inter-thallus spacing experiments (hereafter referred to as high, medium and low)

(Figure 2). Patches had the same algal biomass of20g ± 0.5g, and were alternated consecutively between Zones 1 and 3 between trials. The "open water area/no habitat choice" always occurred in Zone 2. Pieces ofSargassum thalli used to construct patches that ranged from 6-10 em. Sargassum thalli were tied together using string to reach an average distance of 12.2 em across the width ofthe tank (Figure 3). To ensure patches did not deviate significantly in density (determined by visual comparison), and thalli did not shift positions, strings were used to affix thalli ofSargassum to the sides ofthe aquarium. Twenty photographic images ofeach treatment set-up were chosen at random to calculate the inter-thallus spacing using Image J software. Specifically,

10 average distances between thalli ofSargassum within patch treatments were as follows:

1) 4.2 em ± 0.5 em for "high;" 2.1 em ± 0.2 cm for "medium," and 1.2 em ± 0.2 em for

"low." Significant inter-thallus spacing between the 3 treatment levels was tested using a Kruskal Wallis test and a Tukey test for pairwise comparisons. Each treatment was significantly different from the other (Kruskal Wallis p < 0.001, Tukey p < 0.05).

Sample sizes for this experiment were as follows: L'tenuicornis, n = 28; s. hispidus, n

= 20; and H histrio, n = 28. When separated into size classes the sample sizes for small and large 1. tenuicornis and H histrio were n = 14 for both size classes. Sample size for size classes for S. hispidus varied between each inter-thallus treatment: high versus medium small and large S. hispidus: n = 15, n = 5, respectively, medium versus low: n =

12, n = 8, respectively, and high versus low: n = 13, n= 7, respectively.

Differing depth experiment "A": The objective ofthis experiment was to vary the vertical/depth spatial component ofhabitat architecture to determine the effects of habitat selection among shrimp and fishes. Specifically, one patch was left to float at the water surface while the superior surface ofthe other patch was completely submerged

(Figure 4). Depths ofSargassum in the field, as measured by the deepest point ofthe patch, typically ranged from 3-4 em minimally to a depth of 10-12 ern, with some patches occasionally observed descending to more than 30 em into the water column

(Lapointe, Pers. Observ.) Thus, I chose a shallow depth of3-4 ern and deep-depth of

1O-12cm. Both patch choices consisted ofequal algal biomass of30g ± 0.5g. Strings affixed to pieces ofSargassum within each patch and brown, weighted netting (roughly the same color as the Sargassum collected) were used to ensure both patches did not shift positions vertically or horizontally in the aquaria. Shallow- and deep-depth patches

11 were alternated consecutively between zones 1 and 3 between trials. The "open water area/no habitat choice" always occurred in zone 2. Sample sizes for this experiment were as follows: L. tenuicornis, n = 20; S. hispidus, n = 20; and H histrio, n = 20.

Differing depth experiment "B": The objective ofthis experiment was to construct a more natural scenario ofpatches varying in depth, while the superior portions ofboth patches remained floating at the surface (Figure 4). However, because ofthe need to place some Sargssum thalli vertically in these trials to achieve a deeper depth aspect, and simultaneously keep biomass constant between the two patch choices, the confounding factor ofdiffering inter-thallus distances (i.e., density) was introduced.

This factor is addressed in the inter-thallus trials in the first set ofexperiments

(described above). Thus, this version of"differing depth" experiments should be compared directly to the Differing Depth Experiment "A" (directly above) in which

Sargassum patches were used with only depth aspect as the varying factor. The same biomass and depths as the previous depth experiment were used for these trials. The deep depth patch was structurally supplemented by the use ofwires, which were bent to manipulate and maintain vertical positions ofthalli in the aquaria. To control for a wire effect, both Sargassum patch types within each trial were wrapped with the same amount ofwire. The shallow depth Sargassum patches had wire applied that was unbent, which allowed these patches to float so that the primary axis ofthe stipe ofeach

Sargassum strand maintained a horizontal aspect in the water column. Shallow- and deep-depth patches were alternated consecutively between Zones 1 and 3 between trials.

The "open water area/no habitat choice" always occurred in Zone 2. Sample sizes for

12 this experiment were as follows: 1. tenuicornis, n = 20; S. hispidus, n = 20; and H histrio, n = 20.

Structural Component ofHabitat Architecture Procedures

Seagrass species, such as Thalassia testudinum and Syringodium filiforme, are also found occasionally in these patches, which add to the structural components.

Additionally, some weedlines have been observed consisting entirely ofseagrasses (i.e., no Sargassum present) (Brooks, Bennice, Pers. Observ.). Structural complexity and species composition ofpatches can be important factors affecting habitat selection.

Clearly, the three-dimensional aspects ofthe Sargassum thallus (with its highly frilled fronds, stipes and vesicles) are much more structurally complex than the relatively flat blades of T. testudinum and needle-like blades S.filiforme, respectively. Thus, I tested for these effects by having the shrimp and fishes select between patches ofSargassum only and two species of seagrass intermingled. The general experimental methodologies were similar to those described above.

Sargassum versus seagrass patches experiment. The objective ofthis experiment was to test whether differences in structural complexity between patches ofSargassum versus patches oftwo seagrass species influenced habitat selection ofshrimp and fishes.

Patches consisted ofthe same biomass (20g ± 0.5g), and were ofsimilar surface dimensions, depth, and density (figure 5). Sample sizes for this experiment were as follows: 1. tenuicornis, n = 20; S. hispidus, n = 20; and H histrio, n = 20.

13 RESULTS

General Habitat Selection

There were significant results when pooling data for the average proportion of time spent in both habitats versus open area for experiments. For the first spatial component experiment, each treatment for inter-thallus space showed that L. tenuicornis, S. hispidus, and H histrio spent the entire proportion (100%) oftheir time with a habitat rather than in the open water area. This trend also occurred for all three test animals in the differing depth experiments and structural components for habitat selection experiment.

Differing Inter-thallus Spacing Experiment

High versus Medium: Figure 6 summarizes these results. Specifically, L. tenuicornis spent 46% and 54% in the high versus medium patches, respectively (z =

0.54; P = 0.74). S. hispidus spent 50% and 50% in the high versus medium patches, respectively (z = 0.32; P = 0.752). However, H histrio spent 24% and 76% in the high versus medium patches, respectively (z = 3.62; p:S 0.001). Post hoc analyses for size effects showed no significant differences for small and large size classes for L. tenucornis (z = 0.39; p = 0.71 and z = 0.36; P = 0.72, respectively), for small and large

14 S. hispidus (z = 0.37; p = 0.72 and z = 0.51; p = 0.61, respectively) and for small H histrio (z = 1.10; P = 0.27). However, large H histrio did significantly select the medium inter-thallus patch more often than the high inter-thallus patch (z = 3.70, p:S

0.001).

Medium versus Low: Figure 7 summarizes these results. Specifically, 1. tenuicornis spent 57% and 43% in medium versus low patches, respectively (z = 0.78; p

= 0.44). S. hispidus spent 52% and 48% in medium versus low patches, respectively (z

= 0.63; p = 0.95). Again, H histrio showed a significant difference with 70% and 30% spent in medium versus low patches, respectively (z = 2.73, p:S 0.05). Post hoc analyses for size effects showed no significant differences for small and large size classes for 1. tenuicornis (z = 0.36; p = 0.72 and z = 1.84; p = 0.07, respectively), for small and large

S. hispidus (z = 0.21; p = 0.83 and z = 0.22; p = 0.41, respectively) and for small H histrio (z = 1.42; p = 0.16). Large H histrio did significantly select the medium inter thallus patch more often than the low inter-thallus patch (z = 1.95, p :S 0.05).

High versus Low: Figure 8 summarizes these results, none ofwhich were significant. Specifically, for high versus low, 1. tenucornis spent 46% and 54% (z =

0.33, p = 0.33); S. hispidus spent 49% and 51% (z = 0.19, P = 0.85); and H histrio spent

54% and 64% (z = 0.33, p = 0.74), respectively. Post hoc analyses for size effects showed no significant differences for small and large size classes for 1. tenucornis (z =

0.38; p = 0.71 and z = 0.36; p = 0.71, respectively), for small and large S. hispidus (z =

0.39; p = 0.70 and z = 0.46; p = 0.65, respectively) and for small H histrio (z = 1.74; p

= 0.08). Large H histrio did significantly select the medium inter-thallus patch more often than the low inter-thallus patch (z = 2.69, p :S 0.05).

15 Differing Depth Experiment

"A"- Deep (with Completely Submerged Superior Surface) versus Shallow.

Figure 9 summarizes these results. 1. tenuicornis spent 60% and 40% oftheir time in deep versus shallow patches, respectively (z = 0.95, P = 0.34). S. hispidus spent 55% and 45% in deep versus shallow patches, respectively (z = 0.32, P = 0.75). However, H histrio spent 95% and 5% in deep versus shallow patches, respectively (z = 5.38, P ~

0.001).

"B"- Deep (With Floating Superior Surface) versus Shallow. Figure 9 summarizes these results. 1. tenuicornis, S. hispidus, and H histrio spent significantly more time in the deep patches, which maintained a floating aspect. Specifically, 1. tenuicornis spent 70% and 30% oftheir time in deep versus shallow patches, respectively (z = 2.21, P ~ 0.05). S. hispidus spent 85% and 15% in deep versus shallow patches, respectively (z = 4.11, P ~ 0.001). H histrio spent 88% and 12% in deep versus shallow patches, respectively (z = 4.49, P ~ 0.001). Post hoc analyses for size effects in all ofthe trials above showed no significant differences.

Structural Component for Habitat Selection Experiment

Sargassum versus seagrass patches. Figure 10 summarizes these results. There were no significant results for S. hispidus, which spent essentially the same portion of time in the Sargassum patch (51%) as the seagrass patch (49%) (z = 0.19, P = 0.85).

However, both 1. tenuicornis and H histrio did have significant results. Specifically, both 1. tenuicornis and H histrio spent a greater portion oftime in the Sargassum habitat (both 75%) than the seagrass habitat (both 25%) (z = 2.85, P ~ 0.05 for both

16 trials). Post hoc analyses for size effects in all ofthe trials above showed no significant differences.

General Behavioral Observations in Laboratory Experiments and Field

Certain behavioral trends were observed over the course ofthis research for each ofthe three species. In laboratory trials, L. tenuicornis was eventually always found clinging to a habitat. The majority ofthe time L. tenuicornis was observed clinging to

Sargassum thalli. The shrimp's body was positioned in a parallel orientation and direction to the fronds. This positioning was also observed in the field.

S. hispidus was always found swimming directly below the habitat (laboratory and field observation) unless startled, and then it would swim up into the Sargassum habitat. For the deep versus shallow habitat selection in experiment "B," S. hispidus remained swimming between the vertical Sargassum thalli.

Similar to the previous two species, H histrio also selected a habitat. Unlike S. hispidus, H histrio would select a spot in the habitat and remain there for the entire duration ofthe trial or slowly moved short distances across the habitat with its modified pectoral fins clinging to the Sargassum thalli. For the inter-thallus space experiments,

H histrio would wedge itselfbetween two Sargassum thalli. This was also observed for experiment "B" (shallow versus deep habitat selection). H Histrio used its pectoral fins to grasp the vertically dispersed Sargassum thalli. It was difficult to observe H histrio in the field due to its highly adaptive morphology and effective camouflage. However, ifa patch ofSargassum was collected using a dip net and H histrio was left without a habitat; it would quickly swim to the closest area ofSargassum.

17 DISCUSSION

Interactions between organisms, such as predation, parasitism, and mutualism, are often areas of focus when determining the distribution and abundance ofspecies

(Ricklefs 1984, Krebs 1985, Begon et al. 1990, Jones et al. 1994). However, many organisms playa role in creating and modifying habitats. These organisms are termed ecosystem engineers because they either directly or indirectly alter the availability of resources to other species (i.e., biotic or abiotic materials) (Jones et al. 1994). Pelagic

Sargassum can be regarded as an autogenic engineer by altering the otherwise depauperate marine, pelagic environment by providing physical structure for the attraction offauna.

Shrimp and fishes in the pelagic Sargassum community selected habitats that provided resources (e.g., food or shelter). Macrophytes and vegetated areas in general support a greater abundance and diversity than unvegetated areas (Pollard 1984,

Crowder et al. 1998, Heck et al. 2003, Warfe and Barmuta 2004, Gullstrom et al. 2008).

Spatial Component ofHabitat Architecture

Differing inter-thallus spacing experiment. The importance ofspatial components ofhabitat architecture to animals searching for a suitable habitat has been demonstrated in previous studies. Hacker and Steneck (1990) used amphipods as their model organism to study habitat selection because they are mobile organisms and are

18 relatively abundant. However, the generalization that mobile animals would likely seek spatial components for habitat selection is generally contradicted by results in my study.

There were no significant results for differing inter-thallus space for 1. tenuicornis, S hispidus, and "small" H histrio in this study. One reason for different results may be related to the length ofthe animals (body size) used. Hacker and Steneck (1990) found a correlation between body size and use of spatial components ofhabitat architecture. For example, small « 4.0 mm) and medium (4.0 mm- 10.0 mm) amphipods used spatial components for habitat selection, while the large (> 10.0 mm) amphipods did not. The body sizes of1. tenuicornis shrimps used in my study were larger than 10 mm and may be the reason there were no significant results for the inter-thallus experiment. Also,

Hacker and Madin (1991) focused on the habitat structural features, because shrimp morphology and color patterns more closely mimic the structures and color patterns of

Sargassum. This would suggest that spaces between the fronds and vesicles (see structural component ofhabitat architecture below) were less important for the shrimps; my study also supports this premise.

H histrio behaved differently from the shrimp in the Hacker and Steneck (1990) study. Small H histrio did not significantly select between patches with differing inter thallus spacing, but large H histrio did. Unlike the mobile fish species (S. hispidus) used in this study, H histrio typically swam only when separated from a Sargassum patch. Therefore, inter-thallus spacing is more likely to have an important role in habitat selection for H histrio. Large H histrio selected for medium inter-thallus spacing 88% ofthe time when given high versus medium patches, and selected for medium inter thallus spacing 72% ofthe time when given medium versus low patches. When given

19 high versus low patches, large H histrio selected for low inter-thallus spacing 79% of the time. H histrio was observed clinging onto thalli while positioning itself in the inter-thallus spaces (i.e., open spaces between thalli). These results suggest that an optimum inter-thallus spacing exists for H histrio to maximize its camouflage (from both its prey and predators), while allowing enough space to maneuver for its "lie and wait" predatory strategy.

Differing depth experiments "A" and "B". Pelagic algal studies have focused on the spatial component ofalgal patches across the surface ofthe water, but have not compared shallow versus deep depth habitat types. Sargassum habitats with greater biomass typically descend deeper into the water column. My study demonstrated that differences in vertical spatial components significantly affected selection for all three species. Many studies have shown that animals use visual cues to select habitats (c.f.,

Myrberg and Fuiman 2002, Montgomery et al. 2006, Huijbers et al. 2008, Simpson et al. 2008, Igulu et al. 2011, Huijbers et al. 2011). Deeper depth habitats in the open ocean will likely present greater opportunities for visual detection ofSargassum by animals seeking refuge.

Experiment "A" was strictly examining patches at two distinct depths, and did not attempt to mimic what animals might see in their natural environment (i.e.,

Sargassum patches will almost always have a surface aspect because the air bladders on fronds make the patches extremely buoyant). This was done to attempt to isolate the variable ofdepth when giving the animals a habitat choice. Experiment "B" was more natural where the deep depth habitat also had an aspect that ascended to the water's surface. Only H histrio significantly selected the deep depth habitat type with the

20 completely submerged superior surface, while all three species significantly selected the deep depth habitat type with the floating superior surface. The surface coverage of

Sargassum with thalli descending vertically in the water column likely creates a more complex habitat in which prey or predators can hide, a benefit which all three species tested could accrue. Interestingly, H histrio also distinguished between patches that were completely at two different depths. Perhaps predators from above (e.g., sea birds), or differences in physical parameters in the water at the surface versus deeper depths triggered H histrio to avoid the superior-most areas ofpatches. H histrio typically resided in the deeper depth portions in patches. Clearly, selection by H histrio involved an added dimension not prevalent in 1. tenuicornis and S. hispidus.

Structural Component ofHabitat Architecture

Mimicry and camouflage are frequently associated with organisms living in complex habitats. Habitat mimicry was defined by Endler (1978, 1981, 1984) and

Robinson (1981) as an organism resembling in size, shape, color and behavior a particular structural component ofthe habitat to avoid recognition as prey. Camouflage has been defined as an organism resembling the mosaic ofpatches or spots ofvarying sizes, shapes, colors, and brightness ofits habitat such that other organisms do not perceive it against the background. Both mimicry and camouflage usually occur simultaneously based on some level ofprotection provided to the animal.

Many organisms have adaptations resembling parts ofor their entire habitat.

Results from my study demonstrated that both 1. tenuicornis and H histrio significantly selected Sargassum over seagrass. These differences are likely related to habitat

21 structural complexity. Seagrass morphology ofthe species used in this study consists of flat or round blades, whereas Sargassum has elongated fronds branching from its stipe and round air bladders. L. tenuicornis was frequently positioned on or around fronds and parallel to the long axes offronds. A similar species ofshrimp;L. fucorum, that also uses Sargassum as its habitat was also shown to have the same algal-part mimicry or camouflage with Sargassum (Hacker and Madin 1991, Brooks et al. 2007). L. tenuicornis may be exhibiting algal-part mimicry (i.e., morphology that resembles

Sargassum fronds and/or air bladders) to avoid predators (Brooks et al. 2007).

The predatory fish H histro is also well camouflaged within Sargassum, and is likely the reason for the selection ofSargassum over seagrass. Most fish predators do not have such highly adapted morphological features to habitats, because they are adept at swimming and can typically chase down prey with relative success. Camouflage in

Sargassum certainly allows H histrio to operate effectively as a "lie in wait" or ambush predator. Specifically, small H histrio may resemble an air bladder or frond, while a large H histrio may resemble an entire thallus. The mimicry ofH histrio is so remarkable that on many collection trips, when sifting through the algal fronds it was not apparent that a fish was present until it slowly started swimming away from the dip net used to remove specimens.

Surprisingly, S. hispidus did not select the structurally more complex

Sargassum. This may be due to this species being relatively more mobile and swimming primarily underneath patches instead ofspaces within the fronds. S. hispidus can be found within the fronds when avoiding predators. Additionally, S. hispidus is commonly found in different habitats, including seagrasses - which it also consumes. For example,

22 Prado and Heck (2011) found that S. hispidus was one ofthe most abundant omnivorous fishes to inhabit seagrass communities and stomach contents showed frequent consumption ofseagrass species. Neither 1. tenuicornis nor H histrio are known to consume their host alga materials (Johnson and Atema, 1986).

23 CONCLUSIONS

This study has added evidence to the broad biological concept that habitat architecture influences habitat selection oforganisms in a community that has recently gained research interest: the pelagic Sargassum community. It is also the first study to examine the role ofboth spatial and structural components ofhabitat architecture in habitat selection by three common animal species within the Sargassum community.

The use ofthree different species allowed for a comparison ofhow each species uses the Sargassum. Highly mobile fishes, such as S. hispidus, typically use the

Sargassum as a temporary refuge when they are juveniles (Wells and Rooker 2004,

Casazza and Ross 2008). Therefore, complex habitat architecture may not be as important as it is to other species that are either endemic or spend a majority oftheir life in Sargassum (i.e., H histrio and 1. tenuicornis, respectively). However, ifthis habitat did not persist in the open ocean, S. hispidus would likely have a lesser chance of survival to its adult stage (Rogers et al. 2001). Those animals that do spend a majority oftheir life in Sargassum are typically highly adapted to the structurally complex habitat by the use ofmimicry and camouflage (involving both color patterns and modified morphology.

Recently, H histrio individuals were observed in three mangrove-fringed bays in the Virgin Islands Coral ReefNational Monument, St. John, US Virgin Islands

24 (Rogers et al. 2010) and in Belize (Littler and Littler 2000). An interesting question would be to study why these fish, which are considered obligate species ofSargassum, are found in this type ofhabitat and ifthis habitat - which differs significantly in architecture from Sargassum - is suitable for H histrio's survival. Clearly, whether the ecosystem engineer is Sargassum or some other species, it is crucial to identify and conserve these essential fish habitats to maintain a healthy environment and sustainable fisheries (Rosenberg et al. 2000, Wells and Rooker 2004).

25 APPENDIXES

fish or shrimp

Zone 1 Zone 2 Zone 3

Dimensions (em): 40.64 x20.32 x25.40

Figure 1: Experimental aquarium set up. Aquarium was divided into three zones. Zones 1 and 2 were the zones used for habitat patches. Zone 2 simulated open water/ open ocean. Fishes and shrimp were placed into zone 2 at the start ofeach experimental trial. Tank Dimensions (em): 40.64 x 20.32 x 25.40

26 A

JHigh Medium Medium Low

~= Sargassum thallus

Figure 2: Differing inter-thallus spacing experimental set up (top view ofaquaria). Treatment A: high inter-thallus space vs. medium inter-thallus space. Treatment B: medium inter-thallus space vs. low inter-thallus space. Treatment C: high inter-thallus space vs. low inter-thallus space. Biomass ofeach habitat =20g ±.5g. Inter-thallus spacing for the three levels: high = 4.2 em ± 0.5 em inter-thallus distance apart, medium = 2.1 em ± 0.2 em inter-thallus distance apart, low = l.2cm ± 0.2 em inter-thallus distance apart.

27 Figure 3: Image ofinter-thallus space experiment. Average distances of inter-thallus space measured using ImageJ software.

28 Shallow Shallow

Figure 4: Differing depth experiments "A" and "B" set ups. Shallow-depth patch 3-4cm and deep-depth patch lO-12cm. Patches have same mass of 30g ± .5g. This is strictly a representation ofexperiments and does not show true biomass.

29 ......

Sargassum Seagrass

~= Sargassum thallus _=- Seagrass mixture Figure 5: Structural component experimental set up. Comparing a patch ofSargassum vs. a patch ofseagrass mixture Patches have same mass of20g ± .5g. This is strictly a representation ofthe experiments and does not show true biomass.

30 A 100 90 l. 80 .. lit ~.... 70 ;! 60 . .,.0 -..: SO 'Ii.. 40 30 fnI. u .-E 20 e-.,. 10 0

B 100 90 ··v·;·'·,.. ~...~ !" I- 80 .. ~ 70 !.. .a 60 :! SO , 40 1ii ;a.0» 30 ~ 0» 20 .ee- 10 ::e.. 0 Snaall Laa-ge Smidl Laa-ge Small Large

Figure 6: Habitat selection based on differing inter-thallus spacing. Animals were given a choice between high (solid) vs. medium (striped) inter-thallus spacing. A: Pooled data for each animal. B: Pooled data were separated into small and large size classes. * = significance ofp S .05 and ** = significance ofp s .001.

31 A 100 90 ' X"' '~' 1(1'.. ~ ~f ". CI. 80 ~.... ;@ 70 * .Cl == 60 i 50 -=5 40 CI. "-l 30 ~ .5 20 E-o ~ 10 0

B 100 90

~ 80 .. CI. ~.... 70 * ;@ .Cl 60 = 50 =oS 'Ii 40

iCI. 30 "-l ~ ....Ei 20 E-o 10 =~ 0 Small Large Small Large Large

Figure 7: Habitat selection based on differing inter-thallus spacing. Animals were given a choice between medium (solid) vs. low (striped) inter-thallus spacing. A: Pooled data for each animal. B: Pooled data were separated into small and large size classes. * = significance ofp:::; .05 and ** = significance ofp:::; .001.

32 AlOO ,..------~-----·. 90 · ·: .. ~ ~ · 160 ·: ~50 ~ ·• ·: ·• · ·: i40 ·:• ·• • ·• 130 · ·• ·: : .t • ·• -20 ·• • ·: ·: ~10 ·: · ·• "# 0 ....>--·•.- . ·l __ · ·• · ·• Bl00 90 G> SO .. P...... S 70 'i 60 =oS SO .~ ... 40 ~ "• 30 ! 20 ,,0 Cli' 10 0

Figure 8: Habitat selection based on differing inter-thallus spacing. Animals were given a choice between high (solid) vs. low (striped) inter-thallus spacing. A: Pooled data for each animal. B: Pooled data were separated into small and large size classes. * = significance ofp ::; .05 and ** = significance ofp::; .001.

33 100 tie 90

41 Clo 80 ?: .. 70 !,Q :c(II 60 t 50 e 40 Clo Vi 41 .5 30 ~ "$. 20

Figure 9: Differing depth experiments. Experiment "A" (blue): animals were given a choice between shallow (solid) vs. deep (completely submerged superior surface) (striped). Experiment "B" (green): animals were given a choice between shallow (solid) vs. deep (with floating superior surface) (striped). * = significance ofp:S .05 and ** = significance ofp:S .001.

34 100

80 ~ ... * * ~=- • .. 70 !.= 60 ::c= t 50 i 40 l:I.l=- ~ .!il 30 E-o

0'::.:'e 20

Figure 10: Structural component experiment. Animals were given a choice between seagrass (solid) vs. Sargassum (striped) habitats. * = significance of p ~ .05 and ** = significance ofp ~ .001.

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