One Is the Lonliest Number: the Effects of Isolation on the Behavioral Interactions of Juvelnille Land Hermit Crabs (Coenobiti
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UC Berkeley Student Research Papers, Fall 2008 Title One is the lonliest number: The effects of isolation on the behavioral interactions of juvenile land hermit crabs (Coenobitidae) from the motus of Mo'orea, French Polynesia Permalink https://escholarship.org/uc/item/3f4961kq Author Van Zerr, Vanessa E. Publication Date 2008-12-01 eScholarship.org Powered by the California Digital Library University of California ONE IS THE LONLIEST NUMBER: THE EFFECTS OF ISOLATION ON THE BEHAVIORAL INTERACTIONS OF JUVELNILLE LAND HERMIT CRABS (COENOBITIDAE) FROM THE MOTUS OF MO’OREA, FRENCH POLYNESIA *WITH AN APPENDIX SURVEYING THE HERMIT CRAB SPECIES PRESENT ON SELECT MO’OREAN MOTUS. VANESSA E. VAN ZERR Integrative Biology, University of California, Berkeley, California 94720 USA, [email protected] Abstract. Hermit crabs interact with each other in a variety of ways involving spatial use (aggregations, migrations), housing (shells), mating, recognition of conspecifics, and food. To test if isolation from conspecifics affects the behavioral interactions of hermit crabs, crabs of the species Coenobita rugosus (Milne‐Edwards 1837) of Mo’orea, French Polynesia were isolated from each other for two days, four days, six days, fifteen days, and twenty‐two days. They were kept in individual opaque containers with separate running seawater systems to prevent them from seeing or smelling each other. Afterwards, the hermit crabs were put into a tank two at a time and their behavior was recorded and compared to the behaviors of non‐isolated crabs. Behaviors looked at fell into two categories: 1) “social” interactions, meaning that the crabs reacted to each other’s presence, and 2) “nonsocial” interactions, meaning that the crabs either ignored each other’s presence or actively avoided behavioral interactions with other crabs. Results indicated that although “social” behavior showed a slight decreasing trend over time, it was not significant; however, the amount of “nonsocial” avoidance behavior seen increased significantly the longer crabs were isolated. Key words: hermit crab, Coenobita, Calcinus, Dardanus, isolation, behavior, motu. INTRODUCTION: sex ratios are uneven (Wada S. 1999), and fights over the gastropod shells that they Hermit crabs, as crustaceans depend on to house their uncalcified (Decapoda: Anomura), reveal many vulnerable abdomens (Briffa M, Williams R. behaviors that provide insight into their 2006). Interestingly, aggressive behaviors interactions with conspecifics (Hazlett 1966, over food resources (detritus) are not Hazlett 1981). Depending on the species normally observed, thereby leading to the involved and nature of the contact, they will tentative conclusion that food is not a exhibit a variety of behaviors including: driving force behind hermit crab social aggregation (Tura 2000), stereotyped behavior (Hazlett 1981). displays of chelipeds or pereopods to In order to interact with each other communicate intraspecifically during socially, hermit crabs need to be able to aggressive interactions (Hazlett 1966), perceive each other and recognize chirping in certain Coenobitids (land hermit conspecifics in their environments. They crabs) during battles over resources (Hazlett can detect each other through seawater via 1966), mate guarding and defending when chemoreception (Diaz H, Forward Jr RB, Orihuela B, Rittschof D. 1994; Gherardi, F., Austral archipelago (Raevavae); the Society Tricarico, E., Atema, J. 2005) and crustaceans Islands (Tahiti, Mo’orea); and the Tuamotu are known to have eyes capable of detecting archipelago (Hikueru, Kaukura, Raraka, shapes and a range of electromagnetic Raroia, Takume) (Poupin J. 1996). Hermit frequencies on land and in water (Johnson et crabs in the family Coenobitidae are al 2002, Barry et al 1974, Chiou T. et al. terrestrial and have developed many 2008). Hermit crabs are also capable of adaptations to help them live on land distinguishing familiar social partners by a including having reduced gills, a planktonic binary recognition system that classifies larval stage, and a unique respiratory organ crabs into familiar and unfamiliar categories on their dorsal abdominal wall (the (Gherardi F. and Tiedemann J. 2004a; constraints of living in a shell limit the Gherardi F. and Tiedemann J. 2004b); growth of branchiostegal lungs) (Greenaway however, a hermit crab’s ability to recognize 2003). Furthermore, many are known to individual crabs has not yet been studied drink seawater, fully submerge themselves (Gherardi, F. and Tricarico, E. 2007). Hermit in seawater, or sit in damp sand after rains crabs, in particular Pagurus longicarpus, are (C. rugosus) in order to control their salt and known to remember familiar social partners water balance (Greenaway 2003). When for 4 to 6 days (Gherardi F and Atema J. threatened, Coenobitids are able to block the 2005). Furthermore, studies have shown opening of their shell like an operculum due that some related decapods, such as the to adaptations of the left, dominant cheliped crayfish Cherax destructor, are also able to and the second ambulatory leg (Grubb recognize the faces of fight opponents, 1971). This behavior is seen frequently indicating the capability for certain displayed by C. rugosus during social crustaceans to have complex social interactions such as shell fights with interactions (Van der Velden, J. 2008). conspecifics (personal observation). These Considering the evidence that many hermit adaptations within Anomura seem to be crabs interact with each other socially, relatively recent in evolution with the studying the effects of the lack of social earliest known Coenobitid fossils dating to stimuli through isolation may provide many the Lower Miocene (Greenaway 2003). previously unrealized insights to their lives C. rugosus is also known to eat detritus, and ecology. including decaying algae, fish, small invertebrates, dead giant tortoise (on Aldabra), and feces (Grubb 1971, MATERIALS AND METHODS Greenaway 2003). They are occasionally cannibalistic (Barnes 1997, personal Coenobita rugosus natural history observation). All work was conducted on juveniles of Study site: the land hermit crab Coenobita rugosus (H. Milne Edwards, 1837) (Coenobitidae) All C. rugosus were collected from the (identified from Poupin, J. Internet, and shoreline of Motu Tiahura (Figure 2) personal communication). C. rugosus is (17°29’14.33”S, 149°54’37.50”W, elevation 7 broadly distributed throughout the Indo‐ meters) on the north side of Mo’orea, French West Pacific including Japan (Bonin, Polynesia (Figure 1) during the months of Amami, and Okinawa), the Territory of September through November 2008. Wallis and Futuna Islands, and French Polynesia. (Poupin, J., Internet). Within French Polynesia, C. rugosus is found in the crab chemical cues. Every isolation chamber had fourteen to sixteen small holes drilled into the sides to allow proper air ventilation. Crabs that were part of the communal crab experiments lived collectively in a large twelve‐liter tank with constant running seawater and coral rubble on which to climb. The setup provided crabs the choice of full submersion in seawater or air. Many were observed to switch back and forth. All crabs were fed a diet of cat food FIG. 1. Mo’orea, French every third day and seemed to thrive on it, Polynesia. Motu Fareone (A) and with many moving quickly to the location of Motu Tiahura (B) can be seen in the food on feeding days. Housing was also northwest corner. Image taken from cleaned every third day. Additionally, crabs Google Earth. were kept at ambient outside temperatures and humidities found in Cook’s Bay on Crab maintenance and preparation: Mo’orea. C. rugosus have been found along the shores of Cook’s Bay suggesting that the Isolation chambers consisted of conditions are equally favorable between individual opaque potting cups fitted with the two locations (personal observation). mesh and solid plastic bottoms with holes to provide water drainage. A piece of coral Experimental design: rubble was placed inside to increase the vertical distance a crab could crawl. Each Isolation times consisted of two days, isolation chamber had its own running four days, six days, fifteen days, and seawater system and was sealed with a lid twenty‐two days. Ten replicates were that had two holes drilled into it to allow the conducted for every experiment. For each seawater to enter. All cups were elevated to one of these intervals, two randomly prevent seawater pooling at the bottom. selected isolated crabs were paired and This was to avoid cross contamination of placed in a two‐liter tank at opposite ends. Crabs were only paired that came from the same isolation times, ie. only two six‐day isolated crabs were paired together, never a six‐day isolated crab and a two day isolated crab. To compare the results of the experiments, ten replicates were conducted in an identical manner with communally living crabs. Again, only communal crabs were paired with each other. The two‐liter experimental tanks were filled with 0.24 liters (half a cup) of seawater, allowing the FIG. 2. Motu Tiahura study crabs to have access to seawater if needed site. All crabs were collected from without being fully submerged. The an 80.5‐meter length of beach on experiment tank was then thoroughly rinsed the east side of the motu. Image with seawater three times and refilled taken from Google Earth. between sequential experiments to remove shell, fleeing from another crab, not moving all previous crab chemical cues. while normally extended from the shell, and Each crab pairing was observed for ten sitting in opposite corners of the tank minutes at a time and behavior was (widthwise, lengthwise, and diagonally). recorded in a notebook. Videos of behavior These behaviors are described in more detail were also taken at the end of each isolation in Appendix B at the end of this paper. period for three to five minutes. This was to record the initial behaviors of the hermit Experimental results crabs upon reintroduction to a crowded (approximately 20 individuals) The time crabs spent behaving socially environment. and nonsocially differed significantly depending on how long they had been Statistical analysis: isolated (Figure 3).