MOLLUSCA 



133 



(Berg, 1974); Vermetidae (Hopper, 1975, 1978); Morula 

 (Sinclair, 1975); Stylocheilus (Sarver, 1978); Phestilla 

 (Switzer, 1971); Siphonaria (Menge, 1973); and soft bot- 

 tom communities (Ekdale et al., 1979). 



Enewetak has also been the site of research on the 

 genetic variability and the physiology of the Tridacnidae 

 (Valentine and Ayaia, 1974; Ayala et al., 1973; Valentine 

 et al., 1973; Fankboner, 1971a, 1971b, 1974), of physio- 

 logical studies of the Strombidae (Gillary, 1974; Gillary 

 and Gillary, 1979), and of studies of growth and mortality 

 rates in marine snails (Turk, 1979a, 1979b). 



MODERN MOLLUSCAN FAUNA 

 Nonmarine Mollusks 



The nonmarine molluscan fauna, comprised of eight 

 species, conforms to the description of C. M. Cooke, Jr. 

 (in f-larry, 1966) of a typical atoll land snail fauna: ". . . 

 very restricted. . . made up mostly of species that are 

 accidentally distributed by man." Thus, of the eight species 

 we record, five (both Gastrocopta, both Subulinidae, and 

 Lamellidea pusilla) are probably adventive and widely dis- 

 tributed among the islands of the Pacific as a result of the 

 activities of man (see Christensen and Kirch, 1981; Harry, 

 1966; and Solem, 1964 for discussion of Pacific basin land 

 snails and their dispersal by humans). The remaining three 

 nonmarine molluscan species are strandline species: 

 Melampus flavus and Truncatella guernii are widely distrib- 

 uted on Pacific islands; Assiminea nitida marshallensis is 

 endemic to the Marshall Islands, according to Abbott 

 (1958). The list of Enewetak nonmarine mollusks is similar 



to that compiled for neighboring Rongelap in the Marshall 

 Islands (Reigle, 1964). Of the 11 species recorded from 

 Rongelap, three of the five adventive species also occur at 

 Enewetak, and the two atolls also share two of the strand- 

 line species. 



Marine Mollusks 



At least three factors affect the records of the presence 

 or absence of marine mollusks at Enewetak and, indeed, 

 on any atoll. The three factors are the collecting effort, 

 kinds of habitat available, and the chance occurrence or 

 recruitment of young to the island. 



Given 40 years of collection activity and the numerous 

 collectors who have visited Enewetak, the approximately 

 1000 species of marine mollusks that are recorded as 

 occurring at Enewetak today are probably fairly representa- 

 tive of the molluscan fauna that occurs between the inter- 

 tidal zone and depths of about 30 m. In terms of numbers 

 of species, the Enewetak molluscan fauna is about ¥5 that 

 of the island of Okinawa, 2500 miles northwest; just 

 slightly larger than that recorded for the Hawaiian Islands, 

 2000 miles northeast; and two and a half to three times 

 larger than those reported for two Pacific atolls. Fanning 

 and Funafuti (Table 2). 



The fauna is dominated by gastropods, and the 

 gastropod:bivalve ratio of 90:10 is the highest of all Pacific 

 faunas for which there are records (Table 2). The gastro- 

 pod record includes 947 species in 109 families. Neogas- 

 tropods comprise 44% of the gastropod species, mesogas- 

 tropods (including Heterogastropoda), 32%; euthyneurans, 

 17%; and archaeogastropods, about 8% of the mollusks. 



TABLE 2 

 Characteristics of Pacific Island Marine Molluscan Faunas 



'Kuroda (1960). 

 tKay (1979). 



tHedley (1899). 



UKay and Switzer (1974). 



