146 



KOHN 



Ward, 1980)^ However, most syllids are among the small- 

 est polychaetes present. Their contribution to biomass is 

 small in proportion to their numbers; however, their pro- 

 ductivity is unknown and may well be very high. 

 Tubicolous members of the families Spionidae, Sabellidae, 

 Capitellidae, and Dodecaceha (family Cirratulidae), and the 

 errant Syllidae predominated in the samples within 30 m 

 of shore where algae are sparse. In the central portion of 

 the platform with algal turf, the latter four taxa, Chaetop- 

 teridae, and the errant Amphinomidae are the most abun- 

 dant polychaetes. The thick layers of smooth, encrusting 

 coralline algae of the ridge support primarily Nereidae, Syl- 

 lidae, Vermi/iopsis (Serpulidae), Spionidae, and Eunicidae. 



Polychaetes are also the most common invertebrates 

 associated with Porites lutea heads on the interisland plat- 

 form (Highsmith, 1981: App. II, Part C, Nos. CI, C3, C4, 

 C5, C6, C9, 39, 41, 84, 87). Syllids dominated numeri- 

 cally (121 individuals), followed by eunicids (67, but with 

 the greatest biomass), and cirratulids (60) in a sample total- 

 ling 264 polychaetes from nine heads. Highsmith (1981) 

 considered most of the polychaetes to be nestlers, mainly 

 in empty chambers that had been excavated by sponges in 

 the coral skeleton. 



Sipuncula 



The only information on the distribution of inter- 

 tidal sipunculans at Enewetak appears to be Highsmith's 

 (1981: App. II, Part C) report of limestone-boring species 

 in the skeletons of Porites lutea microatolls on the inter- 

 island platform. Seven corals examined contained 58 sipun- 

 culans of at least seven species. Aspidosipfion muelleri 

 comprised 60% of the sample. Other identified species 

 were Cloeosiphon aspergillus. Lithacrosiphon gurjanovae, 

 and Paraspidosiphon gigas. Sipunculans probably occur 

 densely in the reef rock substrate of the platform; as noted 

 below they are the sole food of a common predatory gas- 

 tropod, Mitra Htterata. 



Echinodermata 



The sea cucumber Hohthuria atra is the most conspicu- 

 ous invertebrate of the interisland windward reef platforms. 

 Bakus (1973) estimated 0.1 m~ north of Enewetak 

 Island. Maximal densities of from 3 m~ (Webb, DuPaul, 

 and D'Elia, 1977) to 5 to 35 m"^ (Bakus, 1973) occur in 

 areas protected from high water velocities and surf. The 

 highest densities occur in depressions and gutters that 

 retain water at low tide (Ebert, 1978). 



The sea urchins Echinometra mathaei and 

 Echinostrephus aciculatus are commonly associated with 

 Porites lutea microatolls and dead coral heads. They 

 shelter adjacent to these and weaken them by eroding 

 skeletal material, presumably by mechanical action of both 

 teeth and spines. The densest populations of these echi- 

 noids also occur on slightly subtidal portions of the 

 Enewetak windward reef platform. Here densities of 

 Echinometra mathaei reach 6.5 m~ and Echinostrephus 



aciculatus, 1.1 m~ ; the two species have similar environ- 

 mental requirements and are significantly positively associ- 

 ated (Russo, 1980). Although probably second in impor- 

 tance to sponges as bioeroders, the roles of these urchins 

 in removing CaCOa has been estimated more quantita- 

 tively. Russo (1980) calculated erosion rate by the two 

 species together as 325 g m^ yr~^ on the mid-portion of 

 the reef platform and at 108 g m~^ yr~' on the outer 

 platform, where urchin densities are lower. This represents 

 removal of about 2 to 8% of annual CaCOs deposition 

 (Smith, 1973). 



At the seaward edge of some windward platforms, 

 especially on Japtan Island, Heterocentrotus trigonarius 

 occurs commonly, wedged in cavities on the outer portion 

 of the coral-algal ridge. It is the only sea urchin species in 

 this surf-swept zone, and its massive body wall and thick, 

 heavy spines adapt it to this harsh environment. In addi- 

 tion, H trigonarius appears to have a physiological require- 

 ment for considerable water movement (Ebert, 1982). 



Ebert (1982) also estimated growth and mortality rates 

 of Heterocentrotus trigonarius and Echinometra mathaei at 

 Enewetak. The former is a long-lived species (probability 

 of annual survival is 0.97) that grows slowly and has a low 

 instantaneous annual mortality rate per individual (0.006). 

 Echinometra mathaei grows an order of magnitude more 

 rapidly but has a shorter life span (probability of annual 

 survival is 0.42) and a higher and quite variable mortality 

 rate (1.26 in 1978; 0.48 in 1979). 



Mollusca 



Small prosobranch gastropods are particularly charac- 

 teristic benthic invertebrates of windward reef platforms. 

 Detritus-feeding vermetids, mainly Dendropoma psaro- 

 cephala. occur at densities of 150 to 1100 m^ (Miller, 

 1983). Detritus-feeding and herbivorous members of the 

 family Cerithiidae attain densities of 800 m~ ; Cerithium 

 alveolus is the most common species (Miller, personal com- 

 munication). The cowry C\;praea moneta occurs centrally 

 on the reef platform, typically in male-female pairs, at den- 

 sities of 0.2 to 0.7 m~ and is more abundant in tidal 

 pools on the platform (2.0 to 6.7 m^ ) and subtidally in 

 the quarry (Renaud, 1976). Carnivorous neogastropods, 

 represented most abundantly by the families Conidae and 

 Muricidae and secondarily by the Mitridae, Buccinidae, and 

 Vasidae, have been more intensively studied. The first two 

 families represent about 50% and 40%, respectively, of the 

 predatory gastropods present (Kohn and Leviten, 1976; 

 Kohn, 1980). Substrate topography is the most important 

 factor controlling population density and species diversity 

 of these gastropods. Depressions in the bench surface that 

 collect coral rubble and flatter areas with thick algal turf 

 that binds sand provide refuge from the harsh physical 

 stresses of desiccation and rain at low tide and heavy wave 

 action at high tide, and probably from prcdation (Ayal and 

 Safriel, 1982). 



Population densities of Conus species and of other 

 predatory gastropods are significantly higher on portions of 



