ANNUAL MARKS ON SHELL AND LIGAMENT OF SEA SCALLOP 

 (PLACOPECTEN MAGELLANICUS) 



By Arthur S. Merrill, Julius A. Posgay, and Fred E. Nichy, Fishery Biologists (Research), Bureau of 

 Commercial Fisheries Biological Laboratory, Woods Hole, Mass. 



ABSTRACT 



The annual rings on the shell of the sea scallop are 

 frequently weak or obscured by other rings caused by 

 some nonannual stress. Methods are presented by 

 which it is possible to locate the true annual rings by 

 reference to marks on the resilium, changes in shell 

 curvature, changes in color pattern, weight of the shell, 

 and areas of attack by boring organisms. The results 



obtained are validated by comparing a growth rate 

 derived through location of the annual rings with one 

 derived from the growth increments of a large sample 

 which had been tagged and released and recaptured 

 after a year at large. The objectivity of the criteria 

 used to identify the annual rings was tested. 



ANNUAL MARKS 



Many investigators have been able to determine 

 the growth rate of various species of mollusks by 

 identifying those rings on the shell that are caused 

 by some consistent annual phenomenon. Measur- 

 ing the amount of shell between the rings provides 

 data from which it is possible to estimate growth 

 rates for various areas and year classes. Cana- 

 dian investigators (Stevenson and Dickie, 1954; 

 Dickie, 1955) succeeded in applying this technique 

 to the sea scallop, Placopecten magellanicus 

 (Gmelin), of the Bay of Fundy. Dickie (per- 

 sonal communication) also succeeded, with some 

 difficulty, in locating annual rings on the sea 

 scallops of Georges Bank. His difficulty was 

 caused by the fact that the annual rings are 

 usually rather weak and frequently masked by 

 the presence of strong shock rings. Sea scallops 

 are rather sensitive creatures, and any serious 

 disturbance causes them to mark the event with 

 a shock ring on the shell (Posgay, 1950). The 

 grounds that interest us most are those on Georges 

 Bank, which sustain the heaviest fishing pressure 

 and where concentrated dredging operations 



Note.— Approved for publication August 28, 1961. 



may cause uncaught scallops to form many 

 shock rings (fig. 1). 



Because of the difficulties of interpreting the 

 rings on the shell, we have examined some of the 

 other hard parts of the sea scallop for charac- 

 teristics that might offer clues to age. The 

 most useful of these structures proved to be the 

 calcareous part of the resilium. Areas attacked 

 by boring organisms, weight of the shell, color 

 changes, and changes in the curvature of the 

 shell also provided valuable clues to age. 



This paper describes the methods used to 

 determine the position of the annual rings, gives 

 the result of applying them to a sample of shells, 

 compares the derived growth rate with the growth 

 rate determined from tagged and recaptured 

 animals, and gives the result of an experiment 

 testing the objectivity of the criteria. Each 

 phase of the work was carried out by a different 

 investigator to avoid subjective bias. 



RINGS ON THE SHELL 



The surface of the shell of the sea scallop is 

 covered with a series of fine concentric lines 

 (circuli) as a result of the addition of new shell 

 along the margin during growth. At intervals, 



FISHERY BULLETIN: VOLUME 65, NO. 2 



299 



