80 



GRANT 



forms that are tightly attached, T. congregata and 

 Arg\/rotheca arguta, axe clearly adapted to avoid encrusta- 

 tion by the ubiquitous sponges and algae; they grow away 

 from the substrate and thus keep the commissure elevated 

 from the encrusters. Frenulina sanguinolenta attaches by a 

 pedicle that holds the shell well above the substratum 

 (actually below it, on these undersides) and affords some 

 rotational mobility that can help to ward off encrustation. 



Frenulina sanguinolenta is not as negative to light as is 

 T. congregata. Several samples with F. sanguinolenta also 

 contain abundant Halimeda algae, but other pieces in the 

 collection have F sanguinolenta and T. congregreta in 

 association. The brachiopod found in the shallowest habitat 

 was F. sanguinolenta in 5 m off Kidrenen (loc. 32013a) 

 where a single tiny individual was taken from a shady 

 place in a niche under an overhang. Apparently the some- 

 what warmer water at this shallow depth was more a fac- 

 tor than shelter from light in limiting the presence of 

 brachiopods. 



Two representatives of the Class Inarticulata also prob- 

 ably occur at Enewetak. The larva of Lingula sp. is 

 reported in plankton hauls made in transects across the 

 southern part of the lagoon (Gilmartin, 1958). Species of 

 Lingula normally burrow in unconsolidated sediment in the 

 intertidal' and shallow subtidal zones. This habitat was not 

 explored thoroughly, but the presence of the larva indi- 

 cates that such a search would produce specimens of a 

 Lingula. Another inarticulate genus that can be expected 

 to be found at Enewetak is Crania. An undetermined 

 species in this genus was reported from Bikini where the 

 fauna is essentially the same as at Enewetak (Cooper, 

 1954). Its small size and tightly cemented habit make it 

 difficult to find (it looks like a small, very flat limpet); the 

 largest specimen reported by Cooper is 3 mm wide, a pale 

 salmon color that blends into the surroundings, and a 

 shape that tends to conform to the contours of the 

 substrate. 



ARTICULATA 



Arg\irotheca arguta is characterized by its small size 

 and white, clear, or translucent shell (Figs. 2 and 3) with 

 weak to absent costae. It had been recognized from 

 Enewetak as not belonging to any described species 

 (Cooper, 1954) but was not described for fear that it was 

 represented only by juveniles. The present Enewetak sam- 

 ple is adequate, however, and contains small shells along 

 with the majority that seem to cease growth between 2 

 and 3 mm in width (Table 3). The narrow outline, lack of 

 costae (other sjjecies are typically strongly costate), and 

 lack of pigmentation make it unique among species of 

 Arg\;rotheca. It was described recently (Grant, 1983), is 

 known only from Enewetak and Bikini so far, and may 

 prove to be endemic to the Marshall Islands. 



The two Pacific species described by Blochmann 

 (1910, 1914) from Australia and Tasmania, A. australis 

 and A. ma\ii, are much longer than A. arguta and have the 

 strong costae that characterize most species of the genus. 



Fig. 2 Argyrotheca arguta from underside of coral in lagoon 

 off Ikurin Island at depth 15 m, locality 32014; juvenile in 

 small depression (4X). 



■M 



:>^.'.'^*' 



^ i 



■3>* 



Fig. 3 Argi/rotheca arguta on same slab from 32014 as in 

 Fig. 2; adult and juvenile shells in close proximity (4X). 



The colorless shell of A. arguta is another distinguishing 

 feature. Argyrotheca arguta differs from the other p>edi- 

 culate species of Enewetak, Frenulina sanguinolenta, 

 in its smallness, its flatter profile, its wide hinge, its tri- 

 angular rather than round foramen, and, especially, its pel- 

 lucid shell material. 



Argyrotheca arguta attaches to the substrate by means 

 of a short pedicle. It attaches somewhat loosely, i.e., the 

 pedicle is longer than is normal in other members of the 

 genus. Arg]^rotbeca arguta favors cryptic habitats under 

 coral fronds and in recesses, as would be expected from its 

 colorless and translucent shell. At Enewetak it was found 



