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REESE 



inhabit a greater variety of shells. Four other species of 

 Coenobita are present, but they are small and less colorful. 

 The legendary coconut crab, Birgus latro, the largest living 

 terrestrial invertebrate known, is nocturnal. It prefers dense 

 vegetation and is common only on the southwest islands of 

 the atoll from Ikuren to Biken. The brachyuran land crab, 

 Geograpsus crinipes, occurs but is secretive, preferring a 

 habitat of decaying vegetation in the forest. In contrast, 

 their relatives, especially the grapsid crab, Grapsus tenui- 

 crustatus, and the ocypodid ghost crab, Oc\jpode ceratop- 

 thalma, are active and conspicuous scurrying over the 

 intertidal beachrock or digging their burrows in the beach 

 respectively. These species are semiterrestrial only and are 

 not considered here. 



At Enewetak, the behavioral ecology and life history of 

 the coconut crab, Birgus latro, were studied extensively by 

 Helfman (1973, 1977a, b), Reese (1965, 1968), and 

 Reese and Kinzie (1968); and the behavioral ecology of 

 Coer^obita spp. was studied by Held (1960), Page and Wil- 

 lason (1982, 1983) and Willason and Page (1983). 

 Osmoregulation, an important aspect of the physiological 

 ecology of land crabs, was studied by Gross (1964) and 

 aerial respiration by Cameron and Mecklenburg (1973). 

 Elsewhere land crabs were studied recently at Aladabra 

 Atoll, Indian Ocean, by Alexander (1979) and in the Mari- 

 ana Islands by Amesbury (1980). There is an excellent 

 account of the role of land crabs in the atoll ecosystem in 

 Wiens (1962). The discussion which follows is based on 

 these publications, literature citation therein, and my own 

 observations between 1960 and 1979. 



Land crabs are tied to the sea for two reasons. First, 

 they release their fertilized eggs into the sea where they 

 go through typical crustacean larval stages in the plankton. 

 Second, their blood is isosmotic with seawater, and period- 

 ically they must have access to seawater to maintain this 

 condition. They are, nevertheless, surprisingly euryhaline 

 (Gross, 1964). Land crabs are scavengers on terrestrial 

 organisms, so most of their food presumably is less salty, 

 that is hypoosmotic, to their body fluids. 



Birgus is extremely secretive and must be observed at 

 night with infrared viewing equipment if its behavior is to 

 be studied. Helfman (1977b) observed copulation in Birgus 

 on land, and at this time the spermatophore is transferred 

 to the female. It is not clear when fertilization actually 

 occurs. The eggs are carried by the female on her 

 picopods for about 3 weeks. In the case of Coenobita per- 

 latus, males tend to cluster around females on the beach at 

 night. They tumble and fight with one another trying to 

 gain access to the female. Presumably they are attracted 

 to her through chemosensory channels, but vision plays a 

 role too as rocks on the beach are also approached and 

 explored by males. Eventually one male wins access to the 

 female, both crabs partially emerge from their shells, ven- 

 tral sides together, and the spermatophore is transferred to 

 the female. She then proceeds into the water. It appears 

 likely that a ripe batch of eggs are hatched at this time, 

 the larvae are released, a new batch of eggs are extruded 

 to the pleopods, and fertilization occurs. Verification of 



these events is necessary. Matthews (1956) was unable to 

 find adaptations for terrestrial fertilization in either Birgus 

 or Coenobita. 



At Enewetak, Birgus latro females carry eggs from 

 about April through August. Initially the eggs are deep 

 purple red. As they develop on the pleopods of the female 

 they gradually turn lighter until just before hatching they 

 are light, translucent brown. The dark eyespots of the 

 embryos are visible at this time. With remarkable timing, 

 the female crab goes to the shore, walks out into the 

 water, flexes her abdomen repeatedly, and the thousands 

 of eggs hatch into free-swimming, first-stage zoeae larvae. 



The larvae go through three more free-swimming 

 stages, four zoeal stages in all, before metamorphosing into 

 a postlarval stage called a glaucothoe. Reese and Kinzie 

 (1968) provide diagnostic features to distinguish the glau- 

 cothoe of Birgus from those of other Coenobita species. 



The glaucothoe is a critically important life history 

 stage for these crabs. It is at this time that they carry out 

 a unique behavioral program. The glaucothoe, about 4-mm 

 long, settles to the bottom and begins to look for a small, 

 empty gastropod shell. It explores the shell using typically 

 hermit crab patterns of shell exploratory behavior (Reese, 

 1962, 1963). Then, with its newly acquired shell, the 

 Birgus or Coenobita glaucothoe crawls out onto the land 

 (Reese, 1968; Fig. 4). Shortly thereafter it undergoes a 

 second metamorphosis to a miniature crab. The abdomen 

 becomes asymmetrical in typical hermit crab fashion. 

 These tiny creatures are found in the high beach zone usu- 

 ally under rocks or debris. The wrack of the high-tide line 

 is a good place to look for them. 



As they grow, they move inland. Coenobita crabs never 

 give up the behavioral characteristic of living in empty gas- 

 tropod shells, and, indeed, the availability of sufficiently 

 large shells may limit the population of large adult crabs. 

 At Enewetak, Turbo arg^/rostomus is the shell most used 

 by large coenobitid crabs. Many of the shells are broken 

 and worn and appear to have been in use for a long time. 

 The shell must be able to hold a small reservoir of water 

 apparently to keep the reduced gills and vascularized sur- 

 face of the gill cavity (sometimes called a pseudolung in 

 land crabs) moist. Close examination of empty shells found 

 in the jungle reveals a smooth, round hole in the ventral 

 whorl of the shell making it unsuitable for holding water. 

 The hole seems to be caused by solution from within 

 rather than wear from without. 



The coconut crab Birgus lives in shells only when it is 

 small. Crabs reared in the laboratory abandoned shells 

 after about 2 to 3 years when their carapace measured 1 

 to 2 cm in length. During this time they were nocturnal 

 and secretive. Small crabs of this size are extremely diffi- 

 cult to find in the forest on islands such as Ikuren. What is 

 important to note is that Birgus stop living in gastropod 

 shells at a very small size when suitable shells are still 

 available to them. Reports of large coconut crabs living in 

 shells or even in coconuts are misidentifications. In most 

 cases that I am familiar with, the crab is Coenobita breui- 

 manus which, like Birgus, is often bluish in color. 



