Activity rhythms of the pulmonate limpet Siphonaria capensis Q. & G. as an adaptation to osmotic stress, predation and wave action

Abstract

An analysis of the activity rhythms of the pulmonate limpet Siphonaria capensis Q. & G. has shown that individuals in intertidal pools are active during low tide both by day and night, while limpets that are exposed to the air when the tide falls are active only during the nocturnal or late evening low tide. Experimental studies suggest that desiccation prevents the animals which occur on dry rocks from being active during the day, and that one function of homing to a scar is to minimize desiccation. Limpets that occur in pools also home to scars, and further experiments show that this habit minimizes osmotic stress when the pools become filled with rain water or become hypersaline due to evaporation. S. capensis is seldom preyed upon and exudes a mucus that repels predatory whelks and fish. Oystercatchers, Haematopus moquini Bonaparte, and the clingfish, Chorisochismus dentex (Pallas), both avoid Siphonaria capensis in spite of preying avidly on other limpets. Thus homing and activity rhythms are unlikely to have been evolved by S. capensis to reduce predation. On the other hand, S. capensis attaches very weakly to rocks, and is particularly vulnerable to wave action when off its scar and moving around foraging. The restriction of its activity to low tide may reduce the danger of it being dislodged by waves.