SHELL DEFORMITY OF MOLLUSKS ATTRIBUTABLE TO 

 THE HYDROID, HYDRACTINIA ECHINATA 



By Arthur S. Merrill, Fishery Biologist 



Bureau of Commercial Fisheries Biological Laboratory 



Oxford, Maryland 21654 



ABSTRACT 



The colonial hydroid, a common epizoon on the external 

 surface of the shell of the sea scallop, Placopecten magel- 

 lunictis, sometimes becomes established on the internal shell 

 surface. This intrusion interferes with the normal ac- 

 tivities of the scallop's mantle, often causing shell de- 

 formity. The scallop reacts by producing a new shell 

 edge within the existing perimeter of the shell and by- 

 passing the hydroid colony. This relation is amensal — one 



organism is inhibited, and the other is not a£Fected. No 

 proof was found that the hydroid may ultimately cause 

 the death of the scallop. 



These same hydroids, in symbiotic association with 

 pagurid crabs, deform and enlarge the apertures of empty 

 gastropod shells. Enlargement of the "house" is to their 

 mutual advantage. 



The availability of surface on which to at- 

 tach and grow is vital for sessile fouling or- 

 ganisms. The larval forms of these species 

 must eventually settle on firm substrate or 

 perish. The organisms are often able to aug- 

 ment available substrate by settling on the 

 surface of other organisms. The upper valve 

 of the sea scallop, Placopecten magellanicus 

 (Gmelin), provides such a surface. 



Organisms found on shells of sea scallops are 

 those which are found on any suitable sub- 

 strate in the vicinity. They are commensals, 

 typical epizoa competing for space. They may 

 be pelagic and settle by chance in a particular 

 area, or benthic and possess mobility during 

 larval and early postlarval stages. 



Postlarval commensal species found on the 

 sea scallop include representatives of most 

 marine phyla (Merrill, 1961). Most abundant 

 are the boring sponges, sea anemones, branch- 

 ing and encrusting hydroids and bryozoans, 

 pelecypods, barnacles, tubeworms, and simple 

 and colonial ascidians. Wells and Wells (1964), 

 reporting on a calico scallop, Aeqrdpecten gihhus 

 (Linnaeus), community study, made similar 

 observations. They found that of the major 

 taxonomic groups only Asteroidea and Ophi- 

 noidea were lacking on calico shells. 



FISHERY bulletin: VOLUME 66 NO. 2 



Published April 1967. 



Most of these animals have a casual associ- 

 ation with the sea scallop. They may gain to 

 some extent in having a shell substrate on 

 which to live and possibly by having particles 

 of food brought to them by water currents 

 produced by the scallop. Although the com- 

 mensal may cause little inconvenience to the 

 host, the scallop certainly does not appear to 

 benefit from the association. In fact, the asso- 

 ciation may be detrimental to the scallop if 

 extensive fouling of the shell hinders swim- 

 ming, or if marine borers excavate excessive 

 quantities of the shell ; but usually the associa- 

 tion provides neither advantage nor harm to 

 the participants. 



An association in which the sea scallop is 

 placed at a distinct disadvantage was observed 

 during routine sea scallop studies. The colonial 

 hydroid, Hydractinia echinata (Fleming), 

 which grows frequently on the external shell 

 surface of the sea scallop, was observed occa- 

 sionally to expand around the shell periphery 

 and invade the internal shell surface. The for- 

 ward elements of the colony, coming in contact 

 with the mantle of the scallop, caused certain 

 inhibitory reactions by the scallop. This paper 

 describes the association, empnasizing the 

 means by which the scallop reacts to internal 



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