FACTORS GOVERNING PATTERNS OF FORAGING ACTIVITY IN LITTORAL MARINE HERBIVOROUS MOLLUSCS

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Relationships of feeding activity to tidal and day/night cycles in a variety of littoral herbivorous molluscs are reviewed. Most species are active while submerged by the tide, both by day and night. There are also many that are active only while emersed, with variation in relation to day/night cycles dependent upon micro-habitat. The third pattern is for activity to occur while awash by the tide, often with continued feeding while the rocks remain damp. Most species in these three groups have endogenous rhythms. The fourth type is one with no relation to tidal activity, and is confined to supra-littoral and sub-littoral species. Major proposed causes of the evolution of these patterns are: desiccation, together with osmotic and thermal stress which may have selected for nocturnal activity; dislodgement by waves; and predation by aquatic carnivores, which is often avoided by feeding at low tide. Of these desiccation and predation are widely regarded as most important. A study of Patella vulgata suggests that while predation and desiccation are major factors controlling activity patterns, they do not fully explain observed activity. A discussion of the importance of food supply is given, and it is proposed that where competition for food is low, night-time feeding should predominate; where competition is high, day-time feeding should predominate. This pattern fits observations on limpets at Lough Hyne, but needs testing in other field situations.