72 



NOAA Technical Report NMFS 140 



the benthic biomass (Table 3). Members of 

 this group were most abundant in shallow wa- 

 ters and were attached to firm substrates, fre- 

 quently where water currents were moderate to 

 strong. 



Hydrozoans in our samples were small in size 

 and delicately tinted in white, pink, violet, tan, 

 and brown. Thev were exclusively carnivores 

 preying on planktonic crustaceans and other 

 small animals carried to them by water cur- 

 rents. In turn, hydrozoans are preyed upon by 

 nudibranchs and, probably, other omnivores 

 and carnivores. 



The most common forms encountered were 

 Leptomedusae, represented by the genera 

 Campanularia, Sertulaiia, Obelia, and others. Less 

 common were representatives of Antho- 

 medusae, of which Hydractinia is a member. 

 This hydrozoan occurred on live mollusks and 

 on dead shell. Some encrusted gastropod shells 

 were inhabited by hermit crabs. 



Hydrozoans occurred in 126 samples (12% 

 of total); their average density was 6.4/m and 

 biomass averaged 0.5 g/m L (Table 5). 



Geographic Distribution 



Hydrozoans were common in coastal areas and 

 on offshore banks (Fig. 33). Thev were absent, 

 or present in only small quantities, in much of 

 the central sections of the Gulf of Maine, in 

 large areas of the continental shelf south of 

 Rhode Island, and on the continental slope 

 and rise. High densities, 100 to 500 colonies/ 

 irr, and high biomass, 10 to 45 g/m 2 , occurred 

 in only a few scattered localities. Low densities, 

 to 49 colonies/m 2 , and average weights less 

 than 1 g/m 2 were much more common and 

 widely distributed. 



Three geographic areas contained moderate 

 to large quantities of hydrozoans: Nova Scotia, 

 Georges Bank, and the Southern New England 

 Shelf (Tables 6, 8; Fig. 34). Average densities in 

 these areas ranged from 7 to 12 colonies/m 2 ; 

 biomass averaged from 0.4 to 1.6 g/m 2 . Georges Bank 

 ranked first in terms of weight, and Nova Scotia ranked 

 first in number of specimens. Small quantities of hydro- 

 zoans were found in the Gulf of Maine, Georges Slope, 

 and Southern New England Slope. Quantities in these 

 three areas averaged between 3.3 and 0.1 colonies/m 2 , 

 and 0.12 and <0.1 g/m 2 . 



The frequency of occurrence of hydrozoans in the 

 samples (Table 10) indicates the same general trend of 

 abundance as the average number and weight of speci- 

 mens. Percentages of samples containing hydrozoans 

 ranged from a high of 29% in the Nova Scotia area (8 to 



OS 



UJ Q; 

 OD < 



2 to 



2 — 



□ NUMBER 

 ■ WEIGHT 



I 

 O 



o si 



< 



<* u. 

 o o 



? a. 



IL_ 



SHELL SAN0 



I 



L 



SAND- SILT- 



SILT CLAY 



BOTTOM SEDIMENTS 



Figure 30 

 Density and biomass of Porifera in relation to bottom sediments. 



15% in the other shelf areas) to a low frequency (1 to 

 4%) in the Georges Slope and Southern New England 

 Slope area. 



Bathymetric Distribution 



Hvdrozoans were most abundant in shallow water (0-24 

 m) where their densitv averaged 37 colonies/m 2 and thev 

 diminished in quantity more or less directly with increased 

 depth (Tables 11, 12; Fig. 35). In deeper waters (25-200 

 m) that cover the offshore continental shelf, their average 

 density was between 2 and 6 colonies/m 2 . They were 

 uncommon to rare on the continental slope and rise. 



