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Behavioral and Ecological Aspects of Shore‐Level Size Gradients in Thais Lamellosa and Thais Emarginata
- Bertness, Mark D.
- Ecology 1977 v.58 no.1 pp. 86-97
- Balanus, Gastropoda, beaches, carnivores, geotaxis, littoral zone, Puget Sound
- Four intertidal carnivorous gastropods in the Puget Sound region. Thais lamellosa, Thais emarginata, Thais canaliculata, and Searlesia dira, exhibit intraspecific shore—level size gradients, with shell size increasing with decreasing tidal height, on a majority of the 13 beaches examined. Thais lamellosa and T. emarginata respond by photo—orthokinesis and geotaxis to gradients of light and beach elevation to establish and maintain their nonrandom size—class distributions. A previously published hypothesis that size gradients place the prereproductive members of a population in the least stressful portion of a species' intertidal range is examined. Evidence suggests that thaid size gradients are in response to similar size gradients in their major food source, Balanus spp. This promotes an energetically efficient predator—prey size selection relationship and facilitates intra and interspecific resource partitioning.