Composition and Distribution of Macrobenthic Invertebrate Fauna 



121 



Frequency of occurrence of gastropods was 

 moderately high in all geographic areas. They 

 were present in 35 to 58% of the samples (Table 

 10). The high frequency of occurrence in the 

 Georges Slope and Southern New England 

 Slope areas, in comparison to the low density 

 and small average biomass in these areas, is 

 indicative of small-size specimens and of rather 

 widespread and uniform distribution. 



Bathymetric Distribution 



Gastropods occurred at water depths ranging 

 from 3 to 3,310 m. They were far more abun- 

 dant (64/m 2 ) in shallow water (0 to 24 in) than 

 at other depths. Their average density (Table 

 11: Fig. 113) generally diminished with increas- 

 ing water depth, except for a slight reversal oi 

 this trend on the upper and middle sections 

 of the continental slope. Density was roughly 

 uniform (11 to 24 individuals/m 2 ) between 25 

 and 1,000 m. In the two deepwater classes the 

 densities were considerably lower (4 and 

 1/m 2 ). 



Average biomass also was largest in shallow 

 water and smallest in deep water (Table 13; 

 Fig. 113). The average biomass in all depth 

 classes on the continental shelf was moderate 

 (1.1 to 4.8 g/m 2 ), whereas at all depths greater 

 than 200 m the average biomass was small (0.15 

 to 0.29 g/m 2 ). 



Frequency of occurrence of gastropods was 

 moderately high (37 to 52%) on the continen- 

 tal shelf and upper slope (Table 15). At mid- 

 and lower-slope depths occurrence was high 

 (74 to 77%). On the continental rise their rate 

 of occurrence diminished to only 30%. 



Relation to Sediments 



Although there was no consistent trend in den- 

 sity of gastropods in relation to sediment par- 

 ticle size, there were several correlative points 

 of interest (Table 16; Fig. 114). Gastropods 

 occurred in particularly high densities in shelly 

 sediments, where their average concentration 

 was 83 individuals/m 2 . Presumably these gas- 

 tropods were predators on the bivalves whose 

 shells formed the substrate. Densities were also 

 high (40/m 2 ) on gravel bottoms. In all other 

 sediment types they occurred in only moderate 

 densities (9 to 22 individuals/m 2 ). The unex- 

 pectedly low density of 9/m 2 in till substrates 

 indicates that till is more closely allied to silt- 

 clay as a gastropod habitat than it is to gravel. 

 Just the reverse is true for other molluscan 

 groups. 



o a 



a uj 

 UJ (T 



§5 



r~] NUMBER 

 ■ WEIGHT 



1- D 

 uj O 



Xt_ 



GEORGES SOUTHERN GEORGES SOUTHERN 



BANK NEW ENGLAND SLOPE NEW ENGLAND 

 SHELF SLOPE 



GEOGRAPHIC AREA 



Figure 106 



Density and biomass of Polyplacophora in each of the six 

 geographic areas. 



o o NUMBER 



• • WEIGHT 



a 



o 



H 06 ? ' 



uj or 



— 02 5 



10 25 50 100 200 500 1000 2000 4000 



WATER DEPTH IN METERS 



Figure 107 

 Density and biomass of Polyplacophora in relation to water depth. 



