Figure 2. — Closeup view of a portion of the upper valve 

 of a sea scallop, showing a band of narrowly spaced 

 circuli between two areas of widely spaced circuli. 



phenomena. In shallow water, a severe storm 

 may cause sufficient disturbance on the bottom 

 to cause the scallops to form shock rings. Off- 

 shore, shock rings are usually caused by dredging 

 activities of the fishing fleet. Lightly fished areas 

 yield unmarked scallops, while heavily fished 

 areas yield scallops whose shells are a confusion 

 of shock rings. These injuries are sometimes so 

 severe as to distort the shape of the shell (fig. 4). 

 On Georges Bank, which provides more than 75 

 percent of the sea scallop catch, both phenomena, 

 weak annual rings and many shock rings, combine 

 to make the location of the annual rings difficult 

 (fig. 1). Not all scallops, however, react in the 

 same, degree to the stress that causes the annual 

 ring. Any reasonably large sample will contain 

 some sensitive individuals bearing clear annual 

 rings. Likewise, not all scallops are subjected to 

 disturbances sufficient to cause many shock rings. 

 It is usually possible, therefore, to sort out at least 

 a few shells which are fairly easy to interpret. 

 While such a small sample of selected individuals 

 cannot give a valid estimate of the growth rate of 

 the population, it can give an idea of what to 

 expect from the rest of the sample. The more 

 deeply curved valve, which is uppermost when the 

 scallop is at rest, usually bears the clearest annual 

 rings; but in some individuals, the lower valve will 

 be more easily interpreted. 



MARKS ON THE LIGAMENT 



The structure and the function of the ligament 

 in Pecten have been fully described by Trueman 

 (1953a, and 1953b). The outer layer (fig. 5) unites 

 the two valves and acts as a flexible hinge. The 

 inner layer, the resilium, is composed of a large, 

 dark-brown, conical, central structure of rubbery, 

 noncalcareous material and two small lateral 

 calcareous plates, which cement the resilium into 

 a shallow socket, the resilifer, on each valve. The 

 resilium acts as a sort of compressible spring work- 

 ing in opposition to the adductor muscle. When 

 the muscle is relaxed, the resilium forces the 

 margins of the valves apart. 



As the scallop grows and adds new shell along 

 the margins, it also adds new material to the liga- 

 ment. When shell growth slows or ceases, liga- 

 ment growth also slows or ceases producing a 

 mark. The resilium, and the epithelial cells which 

 produce it are well protected and less exposed to 

 shock and injury than are the margins of the shell 

 and the cells which produce it. Therefore, marks 

 on the resilium caused by an annual period of slow 

 growth are relatively more prominent compared 

 with shock marks than are the corresponding 

 marks on the shell. The spaces between the bands 

 on the resilium are proportional to the spaces 

 between the bands on the shell; hence, a specific 

 area of the resilium can be referred to a corre- 

 sponding part of the shell. This section of the 

 shell can then be studied closely in search of the 

 annual ring. 



Figure 6 shows the upper valve of a sea scallop, 

 selected because of its distinct annual rings and 

 lack of strong shock rings, which has been sawed 

 down the midline. The numbers indicate annual 

 rings. Figure 7 shows the resulting cross section 

 of the hinge area of the same scallop. The dark 

 area in the center of the cross section is the resil- 

 ium ; the lighter areas immediately to the left and 

 right are the calcareous plates. The numbered, 

 darker bands on the plates and the constrictions 

 of the resilium correspond to the five annual rings 

 on the shell. 



It is not necessary to make a cross section in 

 order to observe the annual marks on the plates 

 of the resilium. When the valves of a sea scallop 

 are separated, the resilium usually splits down the 

 middle. After drying for a few days, the remain- 

 ing half of the resilium, including the attached 



ANNUAL MARKS ON SHELL OF SEA SCALLOP 



301 



