Life-history strategy and intertidal distribution in sympatric species of pulmonate limpets of the genus Siphonaria

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Three species of the intertidal pulmonate limpet genus Siphonaria are sympatric on the south coast of South Africa and all lay gelatinous benthic egg masses. Siphonaria capensis and S. concinna hatch as planktonic larvae, whereas S. serrata has direct-developing larvae. Intertidal zonation of these species contradicts a model predicting direct development in species that occupy the high shore of the intertidal, and planktonic development in species that inhabit the low shore. Siphonaria concinna and S. serrata reach maximum densities on the middle to high shores, often co-existing in the same microhabitats. Siphonaria capensis is most abundant higher in the intertidal, especially in shallow pools where it lays its egg masses. Adults of both S. concinna and S. serrata are usually found on bare rock, though they deposit egg masses in other microhabitats. Whereas Siphonaria serrata uses microhabitats where desiccation is reduced, S. concinna is, in addition, able to use harsher habitats. Laboratory desiccation and field translocation experiments indicated that spawning where low-tide desiccation is reduced enhances embryonic survival. In various microhabitats, embryonic survival reflects interspecific differences in egg mass physical structure and egg capsule size that result in trade-offs between respiratory gas diffusion and desiccation. The relationship between mode of larval developmental and intertidal zonation is complex. While there may be no direct relationship between larval development and the intertidal height occupied by adults, the physical structure of egg masses and the microhabitats used for spawning are adaptive and complementary in terms of desiccation. In the absence of a capacity to migrate vertically to spawn, populations can be reproductively successful only by occupying intertidal heights with microhabitats suitable for the survival of egg masses.