126 



NOAA Technical Report NMFS 140 



UJ 



2 I 





I 5 20.0 



!9 



NUMBER 

 WEI8HT 



0.01 0.8 



1.0 1.9 2.0 3.0 

 PERCENT ORGANIC CARBON 



=»= — • i 



12.0 



10.0 



z 

 o 



8.0 



or u. 



o o 

 z a 



6.0 E uj 



2* 



UJ UJ 



*5 



4.0 



a. 

 uj 

 o. 



Figure 116 

 Density and biomass of Gastropoda in relation to sediment organic carbon. 



Bivalvia were distributed throughout the entire area 

 sampled in all water depths, sediment ivpes, and tem- 

 perature range classes. They were especially plentiful 

 on the continental shelf where their average density 

 commonly ranged from about 50 to 500/m. Biomass 

 of bivalves averaged 100g/m 2 or more over a large 

 portion of the continental shelf. 



A rather wide variety of bivalve species occurs in this 

 region, and it is estimated that more than 125 species 

 were present in the samples (Theroux and Wigley, 1983) . 

 Some of the more common families represented were 

 Astartidae, Veneridae, Mytilidae, and Nuculanidae. 



Size of specimens ranged from roughly 15 cm for 

 Modiolus modiolus and Placopecten magellanicus to about 3 

 mm for Gemma gemma, Thyasira gouldi, and other small 

 forms. Large specimens occurred only in shallow and 

 moderately shallow water, and in medium to coarse 

 sediments, whereas small specimens were taken at all 

 depths but mostly in fine-grained sediments. 



The color of bivalves in these collections ranged from 

 white to blackish-brown. The most common colors were 



white, light gray, and various shades of brown or olive. 

 No bright or vividly colored species were represented. 

 Some of the more colorful forms were Tellina, Thyasira 

 ovata, and some specimens of Placopecten magellanicus. 



In addition to the importance of oysters, soft-shell 

 clams, quahogs, surf clams, and scallops as food for 

 man, the bivalves as a group are a major source of 

 nourishment for many marine animals. Mammals, bit els, 

 fishes, and invertebrates all have members that prey 

 heavily upon bivalves. 



Bivalves occurred in 893 samples (83% of total). Their 

 density averaged 163/m 2 and biomass averaged 81 g/ 

 m- (Table 5). 



C Geographic Distribution 



Bivalves were distributed over the entire New England 

 region (Fig. 117). Average densities per 20-minute unit 

 area ranged from 1 to nearly 10,000 individuals/m. 

 High densities (>500/m 2 ) were most common in 

 nearshore areas. Moderate densities (50 to 500 indi- 

 viduals/m 2 ) occurred over extensive areas in the Gulf 



