perimeter from the northern to the lower 

 eastern part of Georges Bank, mostly in depths 

 of 40 to 50 fathoms (73 to 92 m.). Hydroids 

 on scallops in this area, however, were dis- 

 tributed in a more or less straight line from 

 the northern to the eastern part. Furthermore, 

 a high incidence of scallop-hydroid association 

 was found in the western part of Georges Bank 

 where natural mortality was relatively low 

 (fig. 4). 



In summary, it can be stated that Hydrac- 

 tinia echinata inhibits the sea scallop, but no 

 proof has been found that the hydroid causes 

 the death of scallops. 



ASSOCIATION OF HYDRACTINIA ECHINATA 

 AND OTHER MOLLUSKS 



One colony of hydroids was found partially 

 covering the shell of a 10 mm. pelecypod, 

 Anomia aculcata Gmelin, that was attached to 

 the shell of the sea scallop. Careful dissection 

 of the perisarc in this area revealed the shell 

 remains of two other A. aculcata, both less- than 

 2 mm. in diameter, completely covered by the 

 colony. Anomia attaches to a surface by a 

 partly calcified byssal plug that passes through 

 the bottom shell valve. After the animal dies 

 and the soft parts decompose, the valves soon 

 separate and wash away. As the two valves 

 of both animals were still intact, it appears that 

 the young Anomia may have been completely 

 enveloped and smothered by the invader. This 

 circumstantial evidence suggests that mortality 

 of a pelecypod can be attributed to Hydractinia 

 echinata. 



Gastropod shells also may be occupied by 

 Hydractinia echinata. Dead shells of Nassarius 

 trivittatus (Say), Lunatia hcros (Say), Bucci- 

 num undatum Linnaeus, Acirsa costidata 

 (Mighels and Adams), Cobis pygmaca (Gould) , 

 and Epitoninm greenlandicus (Perry) dredged 

 from the Northern Edge of Georges Bank were 

 covered with Hydractinia echinata. The shell 

 apertures were badly deformed, greatly en- 

 larged, and globose. A survey of a large assort- 

 ment of Nassarius trivittatus in the mollusk 

 collection of the Museum of Comparative 

 Zoology at Harvard University revealed several 

 other specimens almost identical to the dis- 

 figured ones described above. Apparently this 



phenomenon is not particularly unusual in 

 nature. Until recently I believed that these 

 deformities of gastropod shell apertures were 

 due to adverse relation between gastropods and 

 hydroids (Merrill, 1964). Further field obser- 

 vations and literature research show, however, 

 that I was wrong. The hydroid grows out on 

 the mouth of the shell only after the snail is 

 dead and only when a pagurid crab inhabits the 

 shell. The deformed and enlarged portion of 

 the shell is made of two layers, the lower by 

 glands of the pagurid, the upper by the hydroid 

 (Aurivillius, 1891). This, then, is a symbiotic 

 association. Enlarging the domain is advan- 

 tageous to both animals. 



One other interesting observation was made 

 regarding the association of Nas.'^ariiitf tririfta- 

 tiis and Hydractinia echinata. In one dredge 

 haul during R/V DELAWARE Cruise 62-7 

 (station 27, south of Nantucket (lat. 41°11' 

 N.; long. 70 T6' W.) on June 16, 1962, in 15 

 fathoms (27 m.)) many live specimens of 

 Buccinum undatum were taken. Most of the 

 specimens had colonies of Hydractinia echinata. 

 On top of some of the colonies, A^ tririftatus 

 had deposited masses of egg ca.ses. Evidently 

 the organs necessary for locomotion and egg 

 laying in gastropods are not as sensitive as the 

 mantle in pelecypods to the defensive elements 

 of the hydroid. 



CONCLUSIONS AND SUMMARY 



Hydractinia echinata frequently lives as an 

 epizoon on the shell of the sea scallop, 

 Placopccten magcllanicm^. It often expands 

 over and around the margin of the shell and 

 interferes with the normal mantle activity of 

 the host. This interference in turn affects 

 noi'mal metabolism and can cause shell mal- 

 formation. Mortality possibly attributable to 

 this hydroid was noted in a pelecypod, Anomia 

 aculcata, as was deformity of many gastropod 

 shells. Thus, Hydractinia echinata, which 

 normally uses a shell only as a substrate, is 

 capable of becoming a harmful epizoon. 



Geographic distribution of .scallops in areas 

 of known high natural mortality and areas of 

 high hydroid occurrence were analyzed to de- 

 termine the possibility that the hydroid is an 

 epizootic agent. A lack of correlation indicated 



278 



HYDRACTINA ECHINATA AND SHELL DEFORMITIES 



