134 



KAY AND JOHNSON 



The families with the greatest number of species are the 

 Turridae (78), Conidae (65), Mitridae (58), Cypraeidae 

 (55), and the Costellariidae (51). 



The bivalve record includes 30 families and 115 

 species. Among the bivalves, 51% of the species are infau- 

 nal, 41% are epifaunal byssate forms, and 6% are 

 free-living species. The family Tellinidae has the largest 

 number of species (17), followed by the Veneridae (10), 

 Arcidae (9), and Cardiidae (8). 



Composition 



There are some noticeable anomalies in the composi- 

 tion of the Enewetak fauna in that several well-known 

 Indo-West Pacific mollusks are conspicuous by their 

 absence. We do not record the turbinid Leptothi/ra picta, 

 the neritid Smaragdia, the rissoid Merelina pisinna, the ver- 

 metid Dendropoma maxima, the mesogastropod limpet 

 Hipponix, the thaid Vexilla, the buccinid Cantharus 

 fumosus, or the heterogastropod Architectonica. Several 

 species such as Giirineum g\irineum and Morum denisoni 

 which arc recorded from Kwajalein are absent at 

 Enewetak. 



Some of the absences are explicable. If a habitat is 

 absent, then the species occurring in that habitat arc also 

 absent. Thus Smaragdia associated with sea grass beds 

 and gastropods associated with volcanic substrates such as 

 Hipponix and the muricid Vexilla (Vermeij et al., 1984) are 

 not expected to occur at Enewetak. Other more sub- 

 tle ecological restrictions have been suggested by Waller 

 (1972) who has shown that several species of Enewetak 

 Pectinidae are extremely restricted in their habitats. These 

 species include Comptopallium uexillum which lives in asso- 

 ciation with terrigenous sediments and marine grasses 

 around elevated islands and is found only as juveniles in 

 the deeper portions of the lagoon of the atoll; and 

 Juxtamusium maidivense, which also lives in the deeper 

 portions of the lagoon, in association with abundant 

 Halimeda on which it may depend for attachment and con- 

 cealment. 



The absence of gastropods such as the reef-associated 

 Leptothi/ra picta, Merelina pisinna, Dendropoma maxima, 

 and Architectonica is not easily explained and, for the 

 present, is attributed to chance as has been suggested for 

 certain aspects of the marine fauna of the Line Islands 

 (Kay, 1971), the Hawaiian Islands (Kay, 1979, 1980), the 

 northern Marianas (Vermeij et al., 1984), and among the 

 islands of the Indian Ocean (Taylor, 1971). 



Biogeography 



With one exception, the subspecies Assiminea nitida 

 marshallensis Abbott, 1958, all the Recent mollusks identi- 

 fied to species have been recorded elsewhere in the Indo- 

 West Pacific. As Waller (1972) has noted, the significance 

 of the latter observation lies in the demonstration that, for 

 Pacific island mollusks, miles of open water without strong 

 current systems have not resulted in much differentiation. 



The marine mollusks of Enewetak are clearly of Indo- 

 West Pacific derivation, and many species have ranges 

 which span the Indo-West Pacific from the Hawaiian 

 Islands on the east to the coast of east Africa on the west. 

 A component of this fauna is restricted to the Pacific Plate 

 (sensii Springer, 1982): 14 of the 56 cowries and five of 

 the 19 strombids are Pacific Plate endemics. 



Four species require special mention. Shells of two 

 species of Nautilus and specimens of M\^tilus edulis are 

 reported from windward beaches, but there is no evidence 

 that either Nautilus or Myti/us lives at Enewetak. One 

 species, Trochus niloticus, was introduced by the Japanese 

 about 1935, and these mollusks are very common. 



FOSSIL MOLLUSKS 

 Fossil Record 



The Enewetak molluscan fossil record was documented 

 by Ladd (1966, 1972, 1977, 1982) who described and/or 

 recorded 274 species primarily from two drill cores on the 

 atoll. Ladd dealt only with chitons and gastropods and, 

 unfortunately, was unable to complete his study of the 

 Enewetak fossil material. Of the gastropods recorded by 

 Ladd (1966 et seq.), archaeogastropods comprise 25%; 

 mesogastropods (including Heterogastropoda), 56%; and 

 neogastropods, 17% of the fossil record. 



Although relatively numerous in terms of species, the 

 Enewetak fossil record is far from complete. Apart from 

 the euthyneurans, bivalves, cephalopods, and scaphopods 

 which have not been considered, the neogastropods are 

 underrepresented, and there are biases in other respects. 

 There are proportionately more lagoonal forms such as ris- 

 soids and cerithiids than there are seaward reef flat species 

 such as cones and muricids. High intertidal littorinids and 

 neritids are not well represented. Four of the six fossil ne- 

 ritids are subtidal forms and only one littorinid (Tectarius) 

 is recorded. Still another bias is in size — 82% of the mea- 

 surements for shell sizes are less than 10 mm in greatest 

 dimension and the largest fossil shell is only 34 mm in 

 length. Despite these deficiencies, the fossil record, 

 together with the current checklist, allows at least three 

 conclusions: 



1. Species composition at Enewetak has changed 

 with time. 



2. Many species were widely distributed in the 

 Pacific in the Miocene. 



3. Previously wide species ranges have become re- 

 stricted with time. 



Geological Record 



The Enewetak geological record comprises a Tertiary 

 sequence extending back into the Eocene and a Quater- 

 nary surface limestone section. Ladd (1966) recognized six 

 subdivisions of the Tertiary, the late Eocene (Tertiary b) to 

 the Pliocene (Tertiary h), based on the system developed 

 for the East Indies by van der Vlerk and Umbgrove (1927) 



