FISHERY BULLETIN: VOL. 71, NO. I 



Figure 10. — Bathymetric range and mean depth of oc- 

 currence of the common pelecypods. (Observed values 

 are listed in Table 7.) 



500/m^; the maximum density for most of these 

 species was less than 100/m-. 



Exceptions to the direct relation between high 

 density and wide distribution were two types: 

 (1) species widely distributed, but present in low 

 density, such as Cerastodetma pinnulatum, 

 Crenella glandula, Placopecten magellanicus, 

 and Yoldia sapotilla; and (2) geographically 



restricted species of relatively high local den- 

 sities, such as Bathyarca pectunculoides and 

 Thyasira ovata (shells of these two species oc- 

 curred at only seven and five stations each, but 

 densities were as high as 300 and 430/m-). 



Four patterns of geographic distribution re- 

 vealed by these samples are: (1) Narrow band 

 extending east-west across the area, such as: 

 Bathyarca pectunculoides, Crenella decussata, 

 Cuspidaria perrostrata, Cyclopecten thallassin- 

 us, Limatula subauriculata, Mesodesma arctat- 

 um, and Nuculana acuta. (2) Broad east-west 

 band exemplified by: Arctica islandica, Peri- 

 ploma papyracea, Placopecten magellanicus, and 

 Thyasira trisinuata. (3) Encircling distribu- 

 tion surrounding the center of the area, illus- 

 trated by Astarte undata. (4) Wide inshore-off- 

 shore distribution, as typified by: Anomia 

 aculeata, Cerastoderma pinnulatum, Crenella 

 glandula, Nucula proxima, Venericardia boreal- 

 is, and Yoldia sapotilla. 



Hydrographic conditions and the type of bot- 

 tom sediments appear to have a substantial in- 

 fluence on the suitability of a habitat for some 

 species of bivalves in this region. Unfortunately 

 the common co-occurrence of fine-grained sedi- 

 ments in areas of low energy and relatively 

 stable water temperature, as opposed to coarse 

 sediments in high-energy and changeable water 

 temperature does not lend itself to an evaluation 

 of the specific conditions that limit the occur- 

 rence of the various species. Additionally, the 

 presence of fossil shells invalidates a detailed 

 evaluation of inferred habitat based on the pres- 

 ence of shell remains. For example, the shells 

 of Mesodesma arctatum from depths of 91 to 

 113 m probably are remains of populations that 

 inhabited nearshore areas during the rapid rise 

 in sea level of the post-Pleistocene period. Ra- 

 diocarbon age determinations for shells collected 

 in this region at depths between 86 to 130 m, 

 studied by Emery and Garrison (1967), range 

 from 10,850 ± 150 to 14,850 ± 250 years be- 

 fore present. 



Species that occurred in moderately deep wa- 

 ters and appeared to require a stenothermic ha- 

 bitat were: Arctica islandica, Nuculana acuta, 

 Thyasira plana, and Thyasira trisinuata (Fig- 

 ure 9). Species that inhabited stenothermic 



16 



