Evolution of burrowing in marine gastropods: observations on rocky-shore species

Trueman & Brown1 have suggested that the burrowing habit observed in representativ es of at least 20 families of marine gastropods2 could most easily have evolved on shores of mixed rock and sand. Gastropods are primitively adapted for locomotion over hard substrata3 but if faced with a habitat in which rock and sand alternate, it would clearly be advantageous for them to develop the ability to crawl over soft substrata as well, if only to get from one patch of rock to another. Evolution of the burrowing habit might follow, together with the ability to extricate themselves from sand if buned The latter would be of particular benefit in intertidal habitats where marked sand movements are in evidence The whelk Bumupena catarrhacta (Gmelm) is essentially a rocky-shore species which nevertheless crawls easily over sand,* employing the same smooth motion observed when it crawls over rock. When disturbed, its pattern of locomotion over sand changes to a discontinuous stepping motion, by which means it can also burrow1. Such stepping is characteristic of burrowing in prosobranchs inhabiting soft substrata." This whelk can also extricate itself if buned, by thrusting vertically upwards. We have pursued this topic further by observing the behaviour on sand of a number of rocky-shore prosobranchs from the coast of the Cape Peninsula, South Ainca, in the field or the laboratory, or both. We find it appropriate to group the rocky-shore species studied into four categories with regard to this behaviour.