Effects of low temperature on the burrowing rates of four sandy beach molluscs

Burrowing rates have been measured as a function of size and temperature in four molluscs from warm temperate South African sandy beaches: two bivalves of the genus Donax and two whelks of the genus Bullia. Animals were acclimated to a summer temperature of 21 °C and then subjected to drastic temperature drops, aimed at simulating coastal temperatures recorded during local upwelling. Burrowing times increased from 6–16 s at 20 °C to 18-∞ s at 10 °C, the lowest temperature recorded during summer upwelling. The degree of retardation of burrowing that occurred at low temperature was inversely proportional to the height of penetration into the intertidal zone by the different species. Large animals burrowed more slowly than small animals but were less affected by lowered temperature. Comparison of burrowing rates at 10 °C against swash frequencies recorded under upwelling conditions suggests that under extreme conditions of upwelling, burrowing in three species may not be rapid enough to prevent some individuals being swept up the beach by the swash. The most dangerous time in this respect is at the high tide during choppy sea conditions.