colony of Hydractinia echinata seen expanding 

 over another colonial species. 



Because of competition with other epizoons, 

 the hydroid often can occupy only a relatively 

 small area of scallop shell surface, hence can 

 inhibit scallop growth over only a relatively 

 small area of the mantle periphery. Limited 

 invasion along the anterior or posterior edge 

 of the shell results frequently in a change in 

 symmetry of the growing scallop; the shell 

 tends to grow more rapidly in a direction away 

 from the disturbed region. Figure 3 shows the 

 interior and exterior view of a scallop shell on 

 which a hydroid that overgrew a short segment 

 of the edge produced a shift in the growth axis. 

 (Note the excavations and deterioration of the 

 shell caused by boring annelids.) ■ 



In several instances, expanding colonies of 

 hydroids extended up the plates of barnacles 

 which were also attached to scallop shells. 

 Were they simply seeking additional substrate 

 or did the water currents, created by the feed- 

 ing barnacles, attract them? Schijfsma (1939) 

 concluded that water currents influence the 

 direction of growth of the colony. Certainly 

 water currents created by a scallop effectively 

 direct growth of a colony toward the shell 

 periphery. Numerous scallops were seen on 

 which the hydroid colony, with equal oppor- 

 tunity to expand in any direction, grew towards 

 the periphery, even obliquely crossing growth 

 lines on the shell in the process. 



EFFECTS OF THE ASSOCIATION 



Natural mortality of sea scallops, as deter- 

 mined by the ratio of clapper^ shells to live 

 shells, was uncommonly high in 1960-62 on 

 parts of the important commercial grounds of 

 Georges Bank, off Cape Cod, Mass. (Merrill 

 and Posgay, 1964). We were naturally con- 

 cerned with the possibility that the hydroid 

 might be responsible for part of this mortality ; 

 therefore, during research cruises to Georges 

 Bank in August and September 1961, frequency 

 estimates of Placopecten magellanicus and 

 Hydractinia echinata were made at all stations 

 where the two species occurred together. Also 



samples of quick-frozen material from areas of 

 high natural mortality and from areas of high 

 hydroid occurrence were taken to the labora- 

 tory for further analyses. 



A summary of the results of the analyses is 

 shown in table 1. Samples 1 and 2 are from 

 areas of high hydroid-scallop frequency as re- 

 flected by the numbers of hydroids on live and 

 clapper scallops. Samples 3 and 4 are from 

 areas of high clapper-live shell ratios and show 

 a much lower incidence of hydroids. These 

 data make it obvious that the incidence of hy- 

 droids and clappers in the same population of 

 sea scallops is not necessarily correlated. 



T.\Bi.E 1. — hicidcnce of occurrence of Hydractinia echinata on 

 live and clapper shells nf Placopecten magellanicus 



[Samples taken during M/V Delaware cruises 61-13 and 61-16] 



Still more apparent is the lack of correlation 

 in the distribution of Hydractinia echinata and 

 clappers (fig. 4). As indicated by the dark- 

 ened area on the chart in Figure 4, areas of 

 high clapper concentration (clapper ratios 

 over 10 percent) in 1961 generally rimmed the 



-The ligament iresilium) holds together the upper and lower 

 valves of a scallop for a period of time after the scallop dies. In 

 this state the shell is referred to as a "clapper." 



70 W. 68 66 



Figure 4. — Chart of Georges Bank showing the distri- 

 bution and density of Hydractinia echinata on Placo- 

 pecten magellanicus (symbols) at Georges Bank in 

 relation to the area encompassing high natural mor- 

 tality in the sea scallop in 1961 (darkened area). 

 Percentage of hydroids on sea scallops indicated. 



U.S. FISH AND WILDLIFE SERVICE 



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