Displays of Defense : Behavioral Differences in Antagonist
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DISPLAYS OF DEFENSE: BEHAVIORAL DIFFERENCES IN ANTAGONIST AVOIDANCE IN FOUR OPISTHOBRANCH MOLLUSKS Sameen R. Ghazali Department of Environmental Science, Policy, and Management, University of California, Berkeley, California 94720 USA Abstract. The defensive behaviors of four opisthobranchs (Glossodoris cincta, Risbecia imperials, Stylochelius striatus, and Dolabrifera dolabrifera) were observed and categorized. The displays studied were mantle flexation, mucus production, mantle secretion, inking, and rearing. Members of each species were placed in two laboratory situations containing two different antagonists. The antagonists (Dardanus lagopodes and Lutjanus fulvus) were chosen because they were carnivorous, abundant, and found in the same ecology as the opisthobranchs studied. Additionally, they were chosen because they differed phylogenetically, physiologically, and behaviorally and, therefore, represented two very different predators. In some cases, individuals exhibited different defensive behaviors in the presence of different antagonists. Differential responses could reflect physiological, biological, or phylogenetic differences between the four observed opisthobranch species. In some instances, defensive displays were observed across lineages. Key words: defense; inking; mantle flexations; rearing; opisthobranch; nudibranch; sea hare; Chromodorididae; Aplysiidae; Moorea; French Polynesia INTRODUCTION 2000, Behrens 2005). Aside from the present theories surrounding chemical defense and aposematic Although nudibranchs lack the protective coloration, the multiple behaviors associated with shell characteristic of numerous gastropods, many defensive displays are still poorly understood. many possess chemical signals or toxins to deter predators (Mallet & Joron 1999, Penney 2004). This study is aimed at examining some of the Evidence suggests that many opisthobranchs behavioral responses exhibited by opisthobranchs use sequestered chemical compounds from food when in the presence of an antagonist. This paper sources as a means for defense (Faulkner & centers on two major questions. The first and Ghiselin 1983, Avila 1995). In some cases, the preliminary question being, what are the different compounds involved in defense are known to be predator defense mechanisms exhibited by four toxic, and in other cases they are not, but still opisthobranch species found in Moorea? This study appear to act as effective deterrents for identifies and compares the behaviors exhibited by predators (Gimenez‐Casalduero et al. 1999, various opisthobranchs in the presence of an Long & Hay 2006). Additionally, the bright antagonist. Many behaviors such as the release of colors of many opisthobranch species may have mucus or ink should be observed only in the evolved to warn predators against attacking presence of an antagonist, since the likely cost of these soft‐bodied marine invertebrates producing the compounds for these compounds is (Rosenberg 1989, Tullrot 1994, Lindstrom 2001). great. Since opisthobranchs cannot see color, evidence suggests that it is unlikely that bright markings My second major question is how do these and bold patterns would have evolved from opisthobranchs behave when presented with sexual selection or species recognition (Servedio different antagonists? That is, does defensive behavior change in the presence of antagonists Two nudibranch species (Chromodorididae) and that differ phylogenetically, physiologically, and two species of sea hares (Aplysiidae) were observed behaviorally? I expect prey subjects to display a in this study (FIG. 2). variety of defensive behaviors, some of which will only be apparent in the presence of a particular antagonist. If this is the case, I expect to observe differential response to antagonists because of phylogenetic, developmental, or physiological differences between the opisthobranch species. I expect defensive displays to be lineage‐specific and to observe the same behaviors in organisms that are more closely related. METHODS Study site FIG. 2. Phylogeny of opisthobranch species (adapted from Johnson 1994). All opisthobranch species were collected while snorkeling at Cook’s Bay (FIG. 1) in Glossodoris cincta (Bergh, 1888) (FIG. 3) is the first Moorea, French Polynesia. Opisthobranchs of two conspicuously colored opisthobranch species were found in close proximity to the Gump used in this study. This nudibranch is characterized Research Station, primarily between the as having three colored bands around the mantle waterfront bungalows and the small grove of border: an outermost pale blue, middle black, and trees just north of the station. All subjects were innermost yellow or khaki band (Rudman 2000). collected in the late morning or early afternoon. Most of its body is mottled dark brown and light pink in color. Its rhinophores and gills appear to be dark brown speckled with light pink. Glossodoris nudibranchs excrete some secondary metabolites from their diets which are thought to deter some though not all potential predators (Rogers 1991). Subjects were found feeding on algae on rock rubble. This species was the most abundant at the collection sites. FIG. 1 Site map of Cook’s Bay (grove and FIG. 3. Glossodoris cincta bungalows noted by z and field site noted by ). Risbecia imperialis (Pease, 1860) (FIG. 4) is the Description of species second brightly colored opisthobranch species used in this study. This species was also found feeding on algae growing on rocks and on the sandy substrate near coral heads. These nudibranchs of the organism is rounded and narrows closer to were often found occurring in pairs, and were the head. Its parapodia are fused except for a observed tailing—a behavior in which one portion in the posterior midline where two flaps opisthobranch follows the mucus trail of form a respiratory opening (Rudman 2003). This another, using the sensory cells in its oral species is known to have glands incapable of tentacles (Behrens 2005). This nudibranch is producing ink or its associated anti‐predator characterized as having a primarily white proteins (Prince 2006). The species can be found mantle and foot with yellow dots. Its gills are attached to rock rubble. white edged with dark blue, and its rhinophores are dark blue with white flecks. The zigzagged mantle border of R. imperialis is dark blue and encompasses small yellow dots. FIG. 6. Dolabrifera dolabrifera To address my primary question, does defensive behavior in opisthobranchs change when different antagonists are present, I used two antagonists in FIG. 4. Risbecia imperialis this study. I chose hermit crabs (Dardanus lagopodes) (Forskål, 1775) because they represent a predator Stylochelius striatus (Quoy and Gaimard, that crushes and chews its prey. It is likely that an 1832) (FIG. 5) is the first of two inconspicuous opisthobranch would have a lower probability of opisthobranch species examined in this study. surviving an encounter with a hermit crab than with This sea hare can be characterized as having a a predator that swallows its prey whole (Penney mottled green and brown body. Also 2004). This is also why I chose the blacktail snapper characteristic of S. striatus, are dark longitudinal (Lutjanus fulvus) (Scneider, 1801) for my second lines and scattered royal blue dots along its antagonist. This snapper is representative of fish mantle. This species was found feeding on blue‐ predators that mouth and swallow their prey. While green algae that forms film over rocks or muddy neither of these species may represent natural surfaces. opisthobranch predators, they still posed a potential threat and were expected to trigger a defense mechanism in the prey subject. Furthermore, both antagonists are carnivorous, abundant, and share the same habitat as the four opisthobranch species used in this study. There is also very little information available on natural opisthobranch predators. For the purposes of this study, six hermit crabs FIG. 5. Stylochelius striatus were collected while snorkeling in Cook’s Bay. Eight blacktail snappers were also caught off of the Dolabrifera dolabrifera (Cuvier, 1817) (FIG. 5) Gump Station dock using lunchmeat for bait and a is the second cryptic opisthobranch observed in bamboo fishing pole. Antagonists were not injured this study. The body of D. dolabrifera is flattened during collection and were maintained in tanks with and a mottled green in color. The posterior half continuously flowing seawater. They were returned to the site of capture at the conclusion of the removed. The tanks used in the fish trials had fresh experiment. salt water flowing though them constantly to ensure that the water contained enough oxygen to sustain Experiment design the fish. Six subjects of each opisthobranch species were exposed to antagonists in three‐hour intervals. This experiment was conducted between I observed the actions of the prey subjects for the 10/1/06 and 11/12/06. While in captivity, all first and last half hour of each trial. Chi‐square opisthobranchs were kept in plastic containers. analyses were used to test for significant differences The sides of each container were punctured between crab and fish trials. using a piece of heated metal to allow for adequate water flow through the container. The The opisthobranchs were first exposed to the container lid was replaced with a rubber‐banded hermit crabs. The hermit crab was placed in an mesh sheet to generate additional flow between observation tank and allowed to acclimate for a half the water in the container and the