148 



KOHN 



Cook, 1981). After foraging, S. normalis returns to its 

 home scar over its outbound route (Cook and Cook, 

 1978) Activity ceases for long periods of emersion during 

 neap tide periods. Cook and Cook (1981) found no rela- 

 tion between distance moved and either body size or graz- 

 ing interval in S. normalis. 



In general, gastropods inhabiting the windward reef 

 platforms are small in comparison with congeners occupy- 

 ing subtidal reef habitats. Few Conus exceed 25 mm in 

 shell length in the former habitats (Kohn, 1971, 1980), 

 and extensive field observations suggest the same to be 

 true for other gastropod taxa. 



Arthropoda 



Xanthid crabs are the most prominent intertidal Crusta- 

 cea at Encwetak. Havens (1974) studied their distribution 

 and comparative ecology and presented detailed informa- 

 tion on their habitats and habits. He listed the most 

 abundant species by zones as follows. (Population densities 

 are given in parentheses.) 



Windward Buttress Zone and Coral-Algal Ridge. 



Paraxanthias notatus most common, then Liocarpilodes 

 integerhmus and Circloxanthops cauatus. associated with 

 both dead coral and coralline algae; Globopilumnus glo- 

 bosus and Dacr[;opilumnus emerita in algal rock on the 

 highest parts of the algal ridge; Chlorodiella laevissima on 

 dead coral, and Trapezia and Tetralia spp. in living corals. 

 Crabs other than xanthids in this zone include species of 

 Pachygrapsus, Percnon, and Plagusia (Grapsidae) and 

 Thahmita (Portunidae). 



Inner Portion of Algal Ridge. Eriphia scabricuh on coral- 

 line algal mounds; this species, Dacri/opilumnus rathbunae 

 (14 m~^), Liocarpilodes biunguis and L. pumilis (5 m^^) in 

 the rims of rubble-filled former surge channels; L^dia annu- 

 lipes (10 m^ ) on inner algal mounds. 



Smooth Reef Platform. Liocarpilodes biunguis (33 to 

 104 m~^; this is the most common intertidal xanthid in 

 the Marshall Islands), Xanthias lamarcki. Etisus bifrontalis, 

 and Pilodius areolatus in broad, shallow tidal pools such as 

 the swale at our Sta. F7 described above (Leviten and 

 Kohn, 1980: Fig. 1). The most diverse crab fauna of all 

 intertidal environments Havens studied at Enewetak occurs 

 here, including representatives of the families Atelecyclidae 

 (Kraussiaj, Portunidae (Thalamita. Portunus), Grapsidae 

 (Pach^igrapsus, Percnon), Ocypodidae (Macrophthalmus), 

 Majidae (Micippa), and Parthenopidae. On the northern 

 part of the Enewetak Island windward platform the algal 

 turf is reduced to a film of blue-green algae, possibly 

 because of grazing by herbivorous fishes at high tide as 

 noted earlier. Here Liocarpi/odes biunguis and Leptodius 

 davaoensis (29 m~^) are the most common xanthids, and 

 Leptodius sanguineus, Eriphia scabricula (9 m ), and 

 E. sebana also occur. At night, the last species is the most 

 prominent large crustacean. The grapsid species mentioned 

 above also occur here. The xanthid fauna of the innermost 

 part of the platform is restricted to small Liocarpilodes 



biunguis and Pachi/grapsus minutus that occur uncommonly 

 in small holes. 



Scalloped and Pitted Reef Rock. The dominant xanthid 

 here is L^/dia annulipes (10 m"^), found in holes and crev- 

 ices. 



Beachrock. Ridges of exposed, intertidal beachrock occur 

 adjacent to both seaward and lagoonward sides of islets 

 (Kohn, 1981: Fig. 1) at Enewetak. The predominant 

 xanthid crabs in seaward beachrock areas are Pseudozius 

 catjstrus. Lydia annulipes, and Liocarpilodes biunguis. Holes 

 and cracks in intertidal beachrock on the lagoon side are 

 an important habitat of Eriphia scabricula (15 m ). Eriphia 

 sebana and L annulipes (2 m~ ) also occur here. 



Rubble-Covered Beaches. Loose coral rubble often covers 

 sand and gravel of beaches at Enewetak; this is particularly 

 apparent on the windward side after storms (Kohn, 1980: 

 Fig. la). This habitat supports a characteristic crab fauna 

 of the xanthids Pseudozius caysfrus. Leptodius davaoensis. 

 L. gracilis, and L. sanguineus (2 m~ ) and the grapsid 

 Pach\^grapsus planifrons. Leptodius davaoensis is more 

 common than L. gracilis in the low intertidal (16 vs. 4 

 m ), but the latter tends to replace the former in the high 

 intertidal (15 vs. 5 m~ ). Pseudozius capstrus attains a 

 density of 21 m~ even higher on the beach. Species of 

 Pseudograpsus and Ci^clograpsus occur higher, on sand 

 under rocks at the supra tidal fringe. 



Porites Microatolls. The highest diversity of Xanthidae at 

 Enewetak occurs on interisland reef platforms in 

 association with the Porites microatolls discussed previ- 

 ously Among the 35 species of xanthids are Pilodius areo- 

 latus. P pilumnoides. Xanthias lamarcki, Liomera bella, 

 L. pallida. Chlorodiella cytherea. C. laevissima. Para- 

 medaeus simplex, Medaeus elegans. Etisus bifrontalis. E. 

 demani. Lachnopodus subacutus, and Carpilius convexus. 

 and the portunid Thalamita picta. Highsmith (1981: App. 

 II, Part C) noted a somewhat different set of crabs in this 

 habitat. He also reported acrothoracican barnacles in the 

 coral heads. 



Havens (1974) considered the five species listed in 

 Table 6 as the characteristic low intertidal xanthids of 

 Enewetak. As the Table indicates, they tend to partition 

 microhabitat resources. Table 6 also lists the three charac- 

 teristic high intertidal species; the vertical distributions of 

 the two sets of species do not overlap in the localities 

 studied. These species also occupy quite distinct micro- 

 habitats from each other. Because Eriphia scabricula and 

 Lydia annulipes occur at the same localities and occupy 

 similar microhabitats but at different heights, they are 

 good indicators of tidal level. 



A large crustacean not seen on windward reef plat- 

 forms during the day but active there at night is the 

 macruran Panulirus penicillatus During the day, it remains 

 in subtidal dens on the reef front. Its major peak of activity 

 follows dusk, when it crosses the reef crest to forage; a 

 minor peak occurs at dawn. At Enewetak Island, it was 

 observed to use primarily depressions covered with a thin 



