116 



NOAA Technical Report NMFS 140 



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NOV* SULFOF GEORGES SOUTHERN 8C0R8ES SOUTHERN 



SCOTI* MAINE BANK NEW ENOIANO SLOPE NEW ENGLAND 



SHELF SLOPE 



GEOGRAPHIC AREA 



Figure 100 



Densitv and biomass of Mollusca in each of the six geographic areas. 



m ranged from 0.4 to 4.0/m L . Below 300 m the average 

 density in the various depth classes was 0.2 to 0.5/m 2 . 



Average biomass of chitons was greater (0.3 to 0.8 g/ 

 m 2 ) at depths less than 500 m than in the deeper water 

 where their biomass averaged 0.01 g/irr or less (Table 

 13; Fig. 107). 



Frequency of occurrence was low (1 to 18%) and no 

 clear relations with depth were evident. The trend indi- 

 cated a slightly higher rate of occurrence in moderate 

 (50 to 200 m) and deep (>200 m) water (Table 15). 



These results indicate that polyplacophorans are less 

 abundant but more uniformly distributed in deep wa- 

 ter (>500 m) than on the continental shelf and upper 

 portion of the continental slope. 



Relation to Sediments 



Polyplacophorans occurred in all sediments except shell 

 (Table 16; Fig. 108). Highest density (3.8/m 2 ) was en- 

 countered in sand-silt sediment. Till, gravel, and 

 silt-clay ranked second to fourth, respectively, and only 

 very small quantities (0.3/m 2 ) were found in sand 

 sediments. 



The relationship of average biomass to various types 

 of bottom sediments was quite different from that of 

 density (Table 18; Fig. 108). Gravel and till yielded the 

 largest biomasses (0.7 and 0.3 g/m 2 , respectively), 

 whereas in all other sediments it averaged <0.07 g/m 2 . 



Frequency of occurrence was highest (11 to 27% of 

 the samples) in those sediments where chitons were 



