PARAMETERS OF MARINE INVERTEBRATE POPULATIONS 



35 



the average total length of the different year-groups of cockles. This may have 

 been due in part to poor early growth seasons for the seven-year-old cockles. 

 However, the inset figures show the effect of fishing on cockles during their 

 second winter, when only a proportion of the year-group had reached 

 minimum size. This suggests that selection by sieving could have produced 

 the apparent difference in growth rate. 



A method used to estimate the size at first maturity of whelks was to 

 divide the males according to penis size. This was usually either small and 

 narrow or very large, with few intermediate sizes (Fig. 4). The shell length 

 at which the change-over from small to large penis occurred corresponded 



_|20 

 UJ 



X 



I/) 



z 

 < 



UJlO 



5 



10 



LENGTH OF Sh 



BEFORE] 

 AFTER I' 



— r 

 VI 



in AGE 17 



w 



Fig. 3.— Growth of cockles. Growth curve obtained by measuring the lengths of growth rings 

 on cockles of seven years of age from Llanrhidian Sands, April 1958. Points marked (O) 

 show mean sizes of cockles of each year group collected from the same area. (2) shows 

 the length distribution of 2-ring cockles (B) and the position of their first rings (A), before 

 and after (blocked in) fishing. (3) Growth curves from cockles shown in (2). 



with that of female whelks at the time of first depositing spawn in the 

 laboratory, and this was concluded to be the shell length at maturity. Above 

 this size the growth increments reduced rapidly. As in related gastropods 

 hke the dog whelk (Moore, 1936) and the American whelk tingle (Hancock, 

 1959), the slowing of the growth rate following the onset of maturity was 

 accompanied by the continual deposition of layers of shell, resulting in 

 thickening of the shell lip (Fig. 5). The female of the American tingle has a 

 faster growth rate than the male, though this has not been found to be true 

 of the whelk. Among mature lobsters, males grow faster than females. 

 4 



