82 



GRANT 



TABLE 4 



Shell Measurements of 

 FrenuUna sanguinolenta in mm 



and the ventral lower as the shells hang from the ceilings 

 of their cryptic recesses. The red color looks greenish 

 under water, and the stripes render the shells nearly invisi- 

 ble to the human eye in the dim and shimmering light. 



Thecidellina congregata was described originally from 

 Bikini Atoll where it occurred as deep as 480 m (Cooper, 

 1954). It is the most abundant brachiopod at Enewetak, 

 although not the most widespread. Thecidellina congregata 

 can be identified by its cemented habit, with low cup- 

 shaped ventral valve and lid-like dorsal valve (Figs. 6 

 through 9). Its color ranges from white (mostly dead shells) 

 to bluish gray in the majority of living shells. It is not 

 immediately distinguishable from T. maxilla, known from 

 Bikini (Coojjer, 1964), although close inspection reveals 

 several differences. Thecidellina congregata is smaller 

 (Table 5), has the blue-gray color, and the dorsal median 

 septum is wider at the anterior than at the jjosterior, 

 where that of T. maxilla maintains nearly a uniform width 

 from back to front. 



Zumwalt (1978) conducted a detailed anatomical and 

 functional study of this species, based upon material he 

 collected at Enewetak. He found that both the diductor 

 and the adductor muscles insert in the hemispondylium, 

 which is a muscle platform that changes length and shajie 

 with growth and general configuration of the valve, to pro- 

 duce the most advantageous angles for muscle action. 

 Hence the hemispondylium varies from one individual to 

 the next. 



As implied by the name, T. congregata can occur in 

 abundant patches where extrapolated densities run to 

 several thousand per square meter. Judging from the sam- 

 ples taken at Enewetak and reports of other species of 

 Thecidellina from elsewhere (e.g., Jackson et al., 1971), 

 Thecidellina survives best in cryptic habitats. It feeds by 

 opening the valves and exposing the lophophore to the 

 seawater, which is then pumped across the filaments of 

 the lophophore. The lophophore is considered a modified 



.♦jT-'-^V-lU* 





Fig. 6 Thecidellina congregata clustered on underside of 

 coral frond from sea side of Biken Island at USNM locality 

 32022 (2X). These are the specimens whose measurements in 

 the species description indicate a width range from 0.5 to 5.2 

 mm. 



Fig. 7 Thecidellina congregata from shallow cave at depth 

 about 33 m on sea side of Biken Island. USNM locality 32022; 

 combination of living shells and dead empty ventral veilves 

 with narrow variation in size. 



trocholophe, with a deep median indentation along the 

 sides of the dorsal median septum. The norma! position for 

 feeding seems to be with the dorsal valve open to a posi- 

 tion nearly perpendicular to the margin of the ventral valve | 



