INTERTIDAL ECOLOGY 



149 



TABLE 6 



Vertical Zonation and Microhabitats of the Eight 

 Dominant Intertida! Xanthid Crabs at Enewetak* 



Species 



Primary microhabitat 



Low Intertidal Species (Approximately +0.3 to 0.9 m) 



Liocarpilodes biunguis 

 Eriphia scabricula 



Leptodius datxioensis 



Leptodius sanguineus 



Dacr\;opilumnus 

 rathbunae 



Reef flats, in small holesf 

 Reef flats without algal turf 



binding sand, in large 



holest 

 Reef flats witfi tfiick algal turf; 



under small rocks^:; in 



large holes 

 Reef flats and rubble-covered beaches; 



under large rocks;): and overhangs; 



overhangs in large holes 

 Eroding algal rock, beachrock, 



reef blocks with thin algal film; 



in sipunculan burrows 



(small holes) 



High Intertidal Species (Approximately +0.8 to +1.3 m) 



Pseudozius cavstrus 



Lydia annulipes 

 Leptodius gracilis 



Reef flats and rubble-covered 

 beaches; under large 

 rocks mainly on gravel; less 

 common on sand under rocks; 



Eroding reef rock, beachrock; 

 in large holes 



Reef flats and rubble-covered 

 beaches; on sand or 

 gravelly sand under rocks of all sizes 



•Data from Havens, 1974 



fSmall holes have aperture si7e (height + width) <17 mm; 

 large holes have aperture size >17 mm. 



4:Small rocks are <23 cm long; large rocks are >23 cm long, 



layer of algae; at Ananij it occurred most commonly on a 

 reef with Pontes lutea. Acropora spp., and mats of the 

 brown alga Turbinaria; at Enjebi it occurred in larger 

 expanses of coral, primarily P lutea and some Acropora 

 spp. (McCollum, 1981). 



Of the six species of shallow water stomatopods at 

 Enewetak, Conodact\^lus incipiens is the most abundant, 

 uses the widest range of habitats, and occurs most com- 

 monly in the intertidal zone, it Is often seen in isolated 

 shallow tidepools. Smaller individuals predominate in the 

 higher, inshore portion of the windward platforms. The 

 other species are mainly subtidal, but Haptosquilla gli^pto- 

 cercus occupies coral rubble and holes constructed by 

 other invertebrates on the windward platform, and it is 

 active in tide pools. Gonodact\^lus platiisoma is associated 

 with the bases of microatolls on the intertidal platform 

 northeast of Enewetak Island, and G. micronesica and 

 juveniles of G smithii occur in rubble in the same region 

 (Reaka and Manning, Volume II, Chapter 17, this 

 publication). 



Of the few smaller intertidal crustaceans known from 

 Enewetak, the amphipod Melita celericuh occurs on the 



undersides of rocks from mid-intertidal to subtidal. In a 

 transect on the lagoon side of the north end of Enewetak 

 Island it reached densities of 104 m~^ at tidal levels of 

 0.2 to 0.3 m (Croker, 1971). In this habitat, it was associ- 

 ated with other species of amphipods as well as with iso- 

 pods and tanaids. 



Hermit crabs, primarily Clibanarius corallinus (54% of 

 total censused) and Calcinus laeuimanus (38%), occur on 

 the Enewetak windward platform at densities of 3 to 

 65 m (Miller, personal communication). 



Benthic Fauna: Trophic Roles and Interactions 



Although the standing crop, number, and biomass of 

 several intertidal benthic invertebrate taxa have been 

 assessed at Enewetak, very few quantitative data exist on 

 their rates of resource utilization or their population 

 dynamics. This section is thus mainly limited to summariz- 

 ing available information on the roles of the numerically or 

 biomass-dominant taxa, and others that have Been studied, 

 in the trophic structure of their community. 



Suspension and Deposit Feeders 

 on Small Particles 



Tube-dwelling polychaetes predominate numerically in 

 this category, especially where depressions retain water at 

 low tide near shore on island platforms. Here the family 

 Spionidae, chiefly Pseudopolydora antennata and Microspio 

 microcera, averages 4300 individuals m ^. These animals 

 are selective detritivores. With a pair of long, tentacIe-Iike 

 palps they catch food particles both in susp)ension and 

 deposited on the substrate. The numerically dominant 

 suspension feeder is an unidentified filter-feeding sabellid 

 polychaete (5900 m"^). Next in abundance is the deposit- 

 feeding capitellid polychaete Leiochrides sp. In the 

 encrusting zone, common polychaetes in this category are 

 the chaetopterid Phi;llochaetopterus ramosus and the cirra- 

 tulid Cirriformia semicincta (Bailey-Brock, White, and 

 Ward, 1980). 



Even less conspicuous than the polychaetes but 

 bionomically important are sponges oi the genera Cliona 

 and Aka, the most important boring organisms of coral 

 skeletons at Enewetak (Highsmith, 1980, 1981); they also 

 penetrate reef limestone. Tropical sponges effectively 

 remove a high proportion of particles in the T^m range 

 from the water they pump through their bodies, and unlike 

 other suspension feeders they are able to subsist 

 exclusively on bacteria and smaller particles (Reiswig, 

 1971). 



The most conspicuous deposit-feeding invertebrate on 

 interisland platforms is Holothuria atra. From analysis of its 

 gut contents, fecal pellets, and the surrounding sediment, 

 Webb, DuPaul, and D'Elia (1977) showed this sea 

 cucumber to feed selectively on materials considerably 

 richer in organic content than the adjacent sediment, and 

 they calculated its feeding efficiency at 40%. A median- 

 sized H. atra (60 g) passes about 80 g (dry weight) of sedi- 



