Aspects of the biology, ecology and seasonal fluctuations in biochemical conlposition of Donax serra in 'the East Cape

Abstract

Donax serra Roding (Pelecypoda) is a large sand mussel which forms vast populations on some East Cape shores. It is most abundant on exposed beaches where the sand is nottoo coarse and phytoplankton production is high. Adultsoccupy , zone just above the mean level of spring tide. Spat settle subtidally and move upshore as they grow. Growth is initially rapid, to about 32 mm at one year and 48 mm at two years. Thereafter growth is very slow and most adult productions goes into reproduction, with a small summerspawning and a large winterspawning each year. Somatic production is low, as large adults totally dominate the population and have low growth and low mortality. Reproductive production is relatively high and P 18 values of 0,2 and 0,5 are suggested for growth and reproduction. D. serra was found to be very tolerant offluctuating salinities for up to four days and should be ableto penetrate river mouths, but is limited by restricted swash action. Total dry-tissue mass, energy and biochemical constituents showed some seasonal fluctuations related mainly to the reproductive cycle. Carbohydrate appears to be a more important energy reserve than lipids.