236 



GARTH, HAIG, AND KNUDSEN 



family Coenobitidac, genus Coenobita, and the coconut 

 crab, Birgus latro. Brachyuran crabs found in the terrestrial 

 environment are grapsoid crabs of the families Gecarcini- 

 dae, Ocypodidae, and a few Grapsidae. 



The spray zone on rocky shore is inhabited by the 

 Sally Lightfoot crab, here Grapsus tenuicrustatus rather 

 than Grapsus grapsus Linnaeus, 1758. The highest eleva- 

 tions of sandy beaches are inhabited by the grapsid crabs 

 Ciiclograpsus and Pseudograpsus; the intermediate eleva- 

 tions (with burrows extending below water level) are in- 

 habited by the ghost crab, Ocypode ceratophthalma. The 

 mole crab, Hippa pacifica, an anomuran, lives in the surf 

 zone, burying itself quickly and emerging suddenly to grasp 

 its food with its first pair of legs. Analogous situations on 

 muddy beaches (rare at Enewetak) are inhabited by the 

 ocypodid crabs Macrophthalwus and Uca. Where soil accu- 

 mulates, as at the bases of coconut palms, the land crab 

 Gecarcinus, family Gecarcinidae, burrows. The collector 

 who turns inland will encounter the grapsids Geograpsus 

 crinipes and G. grayi among leaf litter and the 

 Metasesarma and Sesarwa among roots and low branches. 



It was no accident that the collector responsible for 

 most of the records of land crabs in the Pacific Science 

 Board's Coral Atoll Survey (Holthuis, 1953) was F. R. Fos- 

 berg, a terrestrial botanist, or that the first crabs to reach 

 the senior author soon after arriving at Enewetak were 

 brought to him by Edward Held and Ralph Palumbo, 

 members of the University of Washington group studying 

 the effects of radiation on terrestrial organisms. Thus every 

 habitat available to them, both terrestrial and marine, has 

 been colonized by these ubiquitous arthropods, the 

 decapod crustaceans. 



CRABS AS SYMBIONTS 



The relationships between crabs and the corals in 

 which they may be found vary widely, from parasitism and 

 commensalism (or mutualism) to facultative symbiosis. In 

 parasitism, the crab burrows into the coral or the coral 

 grows around the crab, in both cases enclosing it. In com- 

 mensalism, the crab, although apparently free-living, 

 invariably selects a living coral of a particular kind as its 

 host. Facultative symbiosis is a relationship in which the 

 crab, while often found in living coral, is also found in dead 

 coral, in coral rubble, or even on a noncoral, rubbly 

 substrate. 



The true parasites, long thought to include only the 

 coral gall crabs and coral-burrowing crabs of the family 

 Hajjalocarcinidac, arc now known to include some of the 

 apparently free-living crabs of the family Xanthidac as 

 well — in particular, the genera Trapezia and Tetralia, the 

 former found on pocilloporid, the latter on acroporid 

 corals. These were shown by Knudsen (1967) to feed on 

 coral polyps, which they macerate with their specially 

 adapted dactyls before ingesting. The genera Domecia and 

 Ci/mo, different species of which occur on pocilloporid and 

 acrop)orid corals, apparently have similar feeding habits, 



although only those of Domecia have been investigated 

 (Patton, 1967). 



Other crabs found in coral apparently use it only for 

 shelter and protection from predators. These include not 

 only xanthid crabs like Liomera { = Carpilodes) and 

 Pseudoliomera — some species of which rarely, if ever, 

 occur elsewhere — but also a host of genera such as 

 Chlorodiella, Ph\;modius. and Pilodius, which are found 

 abundantly in dead coral and coral rubble. These are 

 joined by crab genera of other families: the smaller swim- 

 ming crabs of the genus Thalamita, family Portunidae; a 

 number of spider crabs, including Perinea and 

 Tijlocarcinus, family Majidae; hermit crabs of the genus 

 Calcinus, family Diogenidae; Coralliogalathea and Galathea, 

 family Galatheidae; and porcelain crabs of the genera 

 Pachi/cheles and Petrolisthes, family Porcellanidae. 



Small xanthid crabs of the genus Actumr)us have been 

 observed to carry a small piece of live coral as they move 

 about the ocean bottom, much as a hermit crab carries a 

 gastropod shell. Actumnus antelmi Ward, the species origi- 

 nally described as having this habit, occurs at Enewetak, 

 but its coral-carrying propensity was not observed (Lam- 

 berts and Garth, 1977). 



Crabs that use not a coral fKi'VP but an actinian 

 coelenterate, which they carry in each claw to fend off 

 attackers, are members of the genera Lybia and 

 Pol\jdectus. Several species of diogenid hermit crabs of the 

 genus Dardanus carry anemones on their shells. In both 

 cases, the crab benefits from the stinging nematocysts of 

 the coelenterate, and the anemone achieves mobility and 

 probably food as well. The porcelain crab Neopetrolisthes, 

 family Porcellanidae, inhabits several species of large 

 anemones. 



Crabs associated with living mollusks include the pin- 

 notherid Xanthasia murigera, found in the mantle cavity of 

 the giant clam, Tridacna gigas. Crabs associated with living 

 echinoderms include the parthenopid Harrovia elegans, 

 found on the crinoid Comanthus, and the galatheids 

 Allogalathea elegans and Galathea amboinensis, also found 

 on crinoids. The portunids Lissocarcinus orbicularis and 

 L. hohthuricola found in the respiratory tree or cloaca of 

 the sea cucumbers Holothuria atra Jaeger and Actinopiiga 

 mauritiana (Quoy and Gaimard) proved impossible to parti- 

 tion between their holothurian hosts. 



CRABS AS SUBJECTS FOR RESEARCH 



The Anomura and Brachyura of Enewetak Atoll have 

 proven valuable as subjects for research. The terrestrial 

 hermit crabs, Coenobita, were used by University of Wash- 

 ington School of Fisheries personnel in determining re- 

 sidual activity from tests conducted by the Atomic Energy 

 Commission in the late 1940s and early 1950s (Held, 

 1960). Reese (1968a) used the coconut crab, Birgus latro, 

 in life history studies showing use of a shell by the glau- 

 cothoe larva. Hermit crabs, Coenobita, and the ghost crab, 

 Ocvpode ceratophthalma, were used by Reese in motion 

 picture studies of locomotion. Experiments conducted by 



