34 D. A. HANCOCK AND A. C. SIMPSON 



periostracum (Hancock & Urquhart, 1959). Oysters are frequently trans- 

 planted to fresh plots and their age is often known from their previous 

 history. Growth increments may also be determined from individuals 

 marked with paint. 



The Crustacea present greater difficulties. Here growth can occur only by 

 shedding the rigid exoskeleton, or 'moulting'. This is followed by an 

 immediate swelling of the newly emerged soft-shelled form, after which the 

 shell hardens once again. Growth estimates, therefore, involve a knowledge 

 not only of the moult increment, but also of the frequency of moulting. The 

 loss of all hard structures during moulting has made it impossible to determine 

 the age of individual Crustacea. The usual tags attached by wire to a cheliped 

 are of course lost with the exoskeleton during moulting. Holes punched 

 through the telson of a lobster are, however, found to be identifiable after 

 moulting, so that by using a code system for punching holes in one or more 

 flaps of the tail fans of different sizes of lobsters, growth increments can be 

 obtained (Wilder, 1953). This method is not suitable for marking crabs, but 

 tags attached along the line of separation of the carapace of the edible crab 

 have been found to survive the moult (Mistakidis, 1959). It has been shown 

 experimentally (Williamson, 1904) that crabs may moult several times 

 during each of the first few years of life, reducing to about once a year in 

 later years, but a satisfactory method of determining the moult frequency 

 under natural conditions has yet to be found. Shrimps and prawns are too 

 small for the attachment of tags, and they are handled in such vast numbers 

 that marked individuals would be too easily overlooked. The size frequency 

 of populations of pink shrimps (Mistakidis, 1957) and prawns (Cole, 1958; 

 Forster, 1959) have however been analysed, and age/length determinations 

 made. 



(2) RESULTS 



Growth is usually confmed to defmite seasons. In cockles (Fig. 6), oysters 

 (Walne, 1958), scallops (Mason, 1957) and mussels (Baird, personal com- 

 munication), there is usually little or no shell growth during the winter 

 months. In whelks, the growth rate is reduced during the summer. In 

 commercial molluscs the period of rapid growth is restricted to the first few 

 years of life, and is followed by a slowing down in the growth rate in subse- 

 quent years (Fig. 3). The slower growth rate follows the onset of first 

 maturity. This has been found to occur after the deposition of the second 

 growth ring in scallops (Mason, 1957), and may commence even before that 

 in cockles. Fig. 3 shows the differences which may arise when calculating 

 the growth rate of cockles. The measurement of growth rings on seven- 

 year-old cockles gave a slower growth rate than that obtained by measuring 



