PARAMETERS OF MARINE INVERTEBRATE POPULATIONS 



37 



A wide range of physical conditions influences the growth of shellfish 

 (Cole, 1956; Coulthard, 1929; Fox & Coe, I943)> though the separate effects 

 of environmental conditions may be difficult to evaluate (Kristensen, 1957). 

 However, since these species, particularly the sedentary forms, are unable to 

 escape their environments, it is possible to investigate the range of factors 

 influencing growth without the complications of migration. Seasonal 

 changes in temperature affect feeding habits or availability of food and are 

 reflected in the seasonal growth pattern. Oysters cease feeding during the 



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o 

 u 



2 



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UJ 20 



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1955 



1956 



1957 





dj'f'm'a'm'j'j'a's'o'n'd 



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j'f'mIa'mIjIj'aIsIoIn'd 

 1958 



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1959 



Fig. 6. —Shell lengths and cooked meat volumes of cockles from Llanrhidian Sands, using cockles 

 settled in 1955-8. Each value of meat volume was an average of usually fifty individuals. 



winter (Savage, 1925), and the rate of feeding of scallops is lowest when the 

 sea temperature is at a minimum (Mason, 1957) • The meat volume of cockles 

 is much reduced during the winter (Fig. 6). Ciliary currents are however 

 maintained at low temperatures in mussels (Dodgson, 1928) and oysters 

 (Cole, 1954), while cockles have been shown to continue shell deposition 

 during the winter, though shell length remains constant (Kristensen, I957)- 

 In filter feeding animals the period of immersion is obviously important, 



