Composition and Distribution of Macrobenthic Invertebrate Fauna 



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BOTTOM SEDIMENTS 



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Figure 217 

 Density and biomass of Echinoida in relation to bottom sediments. 



The proportion of the number of individuals in the 

 total macrobenthos made up by urchins was low to 

 moderate (0.3-5%, Table 22), but in terms of biomass 

 they contributed extraordinarily large amounts (8-47%, 

 Table 24) of the total. 



Echinoids were present in 14 to 44% of the samples 

 in the various temperature range classes. Frequency of 

 occurrence, as for density and biomass, was highest in 

 the mid-range classes and decreased in both the broader 

 and narrower range classes (Table 25). 



Relation to Sediment Organic Carbon 



Echinoids showed a marked preference for low levels of 

 sediment organic carbon and were entirely absent where 

 levels of 2% or more prevailed. Their density was high- 

 est, but only at moderate levels (24 individuals/ m 2 ) , in 

 the lowest carbon content class (0.01-0.49%) and fell off 

 drastically where organic carbon was absent (2.6/m 2 ), as 

 well as in the classes between 0.50 and 1.99% where 

 density ranged from only0.6 to 1/m (Table 26; Fig. 219). 

 Biomass was similarly distributed among the carbon 

 content classes. The lowest content class contained 



nearly 28 g/m- and biomass values fell off markedly in 

 adjacent classes (Table 28; Fig. 219). 



Frequency of occurrence of echinoids in the samples 

 was generally low, ranging between 6 and 34%, and 

 paralleled the trend exhibited by density and biomass 

 measures (Table 30). 



Ophiuroidea — Ophiuroids formed a moderately small 

 but significant component of the New England benthos. 

 On the average they made up about 3% of the number 

 of animals and 2% of the biomass in the total 

 macrobenthos. 



Two orders of ophiuroids (Euryalae — baskets tars, and 

 Ophiurae — brittlestars) inhabit the study area. The 

 basketstars have a limited distribution within the region 

 and are relatively rare. Brittlestars, however, are widely 

 distributed and form the bulk of all ophiuroids in our 

 collections. They have the greatest diversity of species 

 of all echinoderm groups; more than a score of species 

 have been identified within the region. 



The size of brittlestars in our samples ranged from 

 large specimens of Ophiomusium, with disc diameters of 



