OOc: 



d 





Figure 1. — Interior and exterior views of a group of upper (left) valves of 

 Placopecten mageUanicus showing shell malformation resulting from inner shell 

 invasion by Hydractinia echinata. Interior views (a-d) illustrate the persistence of the 

 invading hydroid which causes the scallop to retire further within the shell. External 

 views (e-h) show several scallops that have managed to pass over and grow beyond 

 the hydroids. 



thin, slightly irregular, concentric margins 

 secreted by the mantle. Each margin repre- 

 sents successive retreats of the mantle edge. 



To resume a normal existence the scallop 

 must successfully overgrow the impinging 

 hydroid colony (fig. 1, eh). Each time the 

 mantle extends towards the margin it secretes 

 conchiolin, and over this foundation the mantle 

 attempts to produce a new edge of shell over 

 the fringe of the hydroid colony. If it succeeds, 

 shell growth resumes once more. Figure lb 

 shows an edge of new shell which indicates 

 a successful bridging by the scallop. 



A pigment is usually produced in the outer 

 shell layer of the left (upper) valve of the 

 sea scallop. Under some conditions, such as 

 a serious break at the edge of the shell, a 

 scallop secretes new shell material quickly, 

 omitting pigment production until a repair is 

 made and growth becomes normal. The reduc- 

 tion or lack of pigment in scallops as they grow 

 a new lip (fig. 1, e-h) indicates faster rate 

 of shell deposition than usual. 



Sometimes a scallop is forced to produce a 



U.S. FISH AND WILDLIFE SERVICE 



new edge over the hydroid more than once. 

 Figure 2 shows a scallop that had grown 

 several new shell margins. This is an extreme 

 example but it does illustrate the result of 

 difficulties that sometimes confront scallops. 

 A young scallop increases the periphery of its 

 shell faster than an older one (fig. 1, e) and 

 thus has a better chance to stay ahead of 

 the advancing hydroid. 



Hydractinia echinata does not always occupy 

 the entire external shell surface ; other epi- 

 zoons compete for this space as well. Most 

 colonial epizoons live to the severe exclusion 

 of others ; it is unusual to see one colony over- 

 growing another. Usually a distinct zone of 

 demarcation is formed. This also occurs when 

 two colonies of Hydractinia echinata meet on 

 the same surface (Schijfsma, 1939). I have 

 examined hundreds of colonies of Hydractinia 

 echinata but only once did I find another 

 colonial species growing over the hydroid. This 

 was a granular, encrusting type of unidentified 

 bryozoan spread over the older part of a well- 

 established colony of hydroids. Never was a 



275 



