150 



NORTH AMERICAN FAUNA 74 



Fig. 98. Location in life of some of the mollusks eaten b>' walruses: (a) Mya truncata: 

 (b) Clinocardium ciliaium: (c) Biiccinum sp.; (d) Spi.sula polijmjma: (e) Octopus cali- 

 fornicus: (f) Tellina lutca: and (g) Macoma calcarea. Stippled area is vertical section of 

 the bottom sediments to about 25 cm. Hea\y shading indicates the parts of each 

 organism usually found in the walrus' stomach, (b and g after Savilov 1961; e after 

 PaVlovskii 1955)' 



sediments, where it feeds mainh' on detritus. Large adults may exceed 15 cm in 

 length and 2 cm in diameter (Fig. 97c). This organism occurs often in the feces 

 and stomach contents of walruses, but usually as only one or two specimens per 

 stomach. 



A/o//?wca.— Bivalve mollusks (clams and mussels) have been found more often 

 and in greater quantities than any other class of benthic invertebrates in the 

 stomachs examined thus far. These comprised at least 15 genera, most of which 

 are relatively large, long-lived organisms that make up about a third to a half of 

 the total biomass of benthos on the intercontinental shelf of the Bering and 

 Chukchi seas (Zenkevitch 1963; Stoker 1973, 1978). Gastropods (snails) were 

 represented by at least eight genera, some of which are predaceous and feed on 

 other mollusks. Cephalopods (two genera) rarely were represented. Nearly all of 

 these mollusks are known to live on or in the upper few centimeters of bottom 

 sediments, rather than deep beneath the surface (Fig. 98). Only two genera {Mya 

 and Spisula) ma\' burrow to considerable depths (20-30 cm), although Mya 

 seems to be most often nearer the surface (Petersen 1978). 



Very few of the mollusks represented in the stomachs examined thus far have 

 been certainh' identifiable to species, since the shell usually was absent. It is 



