INTERTIDAL ECOLOGY 



145 



in an overhang of tissue-covered, growing skeleton, and (2) 

 they are weakened by the skeletal boring activity of clionid 

 sponges and sea urchins. Detached, living pieces of 

 P lutea microatolls tend to move lagoonward across the 

 interisland flat in the unidirectional water flow. They con- 

 solidate in a zone along the lagoon edge, where they con- 

 tribute importantly to active reef growth (Highsmith, 

 1980). Odum and Odum (1955) noted the presence of less 

 abundant corals; these are listed above in the description 

 of the coral-algal ridge taken from their paper. They 

 referred the anemones they noted to the genus 

 Actiniogeton. Miller (1983) noted a common, undetermined 

 high and mid-intertidal anemone. 



The most abundant intertidal hydroid is the inconspicu- 

 ous tropical and subtropical cosmopolitan species 

 Di^namena chsioides. Its irregular branching stems extend 

 5 to 15 cm from a thick hydrorhiza, but it is often 

 obscured by heavy encrustation of detritus and algae. It 

 occurs intertidally in beachrock crevices at the north end 

 of Enewetak Island and commonly on the outer portion of 

 the island reef platform (Cooke, 1975). 



Annelida 



As on reef platforms elsewhere dominated by algal 

 turfs (Kohn and Lloyd, 1973; Kohn and White, 1977), 



polychaete annelids arc a numerically dominant component 

 of the benthic invertebrate community at Enewetak. Blocks 

 of reef limestone chiseled from the central portion of the 

 windward Enewetak Island platform, including the overly- 

 ing algal turf, support a mean density of about 90,000 

 polychaetes m"^ (range 82,000 to 100,000; 27 to 39 

 species; biomass 7.4 to 9.3 g m~ dry weight). About 

 10% of the polychaetes (mean 8000; range 400 to 32,700 

 m~^; 2 to 34 species) were associated with the algal turf 

 alone, indicated by samples of turf scraped from the reef 

 rock surface (Bailey-Brock, White, and Ward, 1980). 



Species richness, population density, and biomass of 

 polychaetes increased from shore toward seaward edge, 

 with maxima at 56 to 66 m, closely paralleling the 

 biomass of algal turf (Table 4, Fig. 3). Nearshore 

 tidepools that hold more water than the surrounding plat- 

 form support higher densities of polychaetes but not 

 greater algal biomass. Near the coralline-dominated algal 

 ridge (85 m from shore; Table 4), limestone blocks con- 

 tained an estimated 60,000 polychaetes of 25 species 

 m"2 (Bailey-Brock, White, and Ward, 1980). 



Also typical of similar habitats elsewhere (Kohn and 

 Lloyd, 1973; Kohn and White, 1977), the family Syllidae 

 are both the most diverse (31 species) and the most abun- 

 dant (up to 48,500 m~ ) polychaete family on the 

 Enewetak windward platform (Bailey-Brock, White, and 



20 30 40 50 60 

 Distance from Shore, m 



70 



Fig. 3 Algal biomass and species richness, abundance, and biomass of polychaetes along a 

 shoreward-to-seaward transect on the windward island platform at Enewetak Island. From Bailey- 

 Brock, White, and Ward (1982). 



