Composition and Distribution of Macrobenthic Invertebrate Fauna 



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Although hydroids did not occur in a 

 large proportion of the samples, there was 

 a general trend of increasing occurrence 

 rate from 4% in the low-range temperature 

 classes to 26 and 18% in the high-range 

 temperature classes (Table 25). 



Relation to Sediment Organic Carbon 

 Hvdrozoan abundance was generally nega- 

 tively correlated to the quantity of organic 

 carbon in the sediments (Fig. 38). Density 

 of hydroids was greatest (9 colonies/m 2 ) 

 where sediment organic carbon content 

 was low (<1%), declining to moderate lev- 

 els where carbon content was between 1 

 and 3% (Tables 26, 27). They were absent 

 in sediments containing the greatest 

 amounts of organic carbon (>3%). 



Biomass of hydrozoans paralleled density, 

 diminishing with increasing organic carbon 

 content (Tables 28, 29). Biomass ranged from 

 slightly over 1 g/m 2 to <0.01 g/m 2 . 



Frequency of occurrence of hydroids in 

 samples ranged from 4 to 19% (Table 30). 

 The trend differed from density and bio- 

 mass, however, in being parabolic with low- 

 est occurrence in the middle ranges and 

 increasing at each extreme. 



Anthozoa — 



Alcyonaria — AJcyonarian coelenterates in 

 our samples were composed of soft corals, 

 orders Alcyonacea and Gorgonacea, and 

 sea pens, order Pennatulacea. Because of 

 the limited occurrence of both groups 

 (<0.1% of the number of all organisms), 

 and despite their large size, they also con- 

 stituted <0.1% of the total benthic biomass 

 (Table 3). None of the alcyonarians in our 

 samples were taken from depths less than 

 50 m. They were most abundant between 

 200 and 500 m. 



Soft corals are typically bush- or treelike 

 in shape and they attach to hard substrates, 

 usually rock outcrops or gravel. Soft corals range in 

 height from a few millimeters to several meters. Trunk 

 diameters are proportional in size, and in large speci- 

 mens occasionally exceed 10 cm. Thus, some species of 

 this group may rank as the largest sessile invertebrates 

 in this region. Colors are light tan, pink, or various dark 

 shades of red. 



Pennatulacea are feather-shaped animals commonly 

 10 to 25 cm in length. They characteristically dwell in 

 soft bottom sediments anchored by a peduncle. Color 

 of the majority of specimens in our samples was tan or a 



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Figure 34 



Density and biomass of Hydrozoa in each of the six geographic areas. 



combination of light cream (rachis) and deep bur- 

 gundy (pinnae). 



Alcyonarians occurred in 63 samples (6% of total). 

 Their density averaged 0.8/m and biomass averaged 

 0.2 g/m 2 (Table 5). 



Geographic Distribution 



Alcyonarians were present along the outer margin of 

 the continental shelf and on the continental slope and 

 rise. They were sparse in all sections and their occur- 

 rence was patchy, especially in the northern section 



