Geographic variation and mass mortality in the black abalone : the roles of development and ecology Public Deposited

http://ir.library.oregonstate.edu/concern/graduate_thesis_or_dissertations/70795c167

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  • Studies of geographic variation are central to understanding ecological and evolutionary processes. Geographic variation in the shell of the black abalone (Halioth cracherodii), a marine intertidal gastropod, involves both clinal and local variation along the eastern Pacific coast in the number and size of tremata (=respiratory pores) on the shell. The goals of my studies were to determine the ecological causes of this morphological variation and to evaluate the contribution of developmental processes to morphological patterns. My field studies included 22 surveys conducted over three years (1987-1990), during which I monitored 2715 tagged abalone, at two study sites in California: Afio Nuevo Island and Santa Cruz Island. I depicted the ontogeny of trema number and size using multivariate ontogenetic trajectories: a path describing trema development relative to changes in allometric size. Because variation in the slope of ontogenetic trajectories was associated with latitudinal variation in the habitat of intertidal abalone, I proposed that dissociation between size and trema development by heterochrony was responsible for geographic variation. Spatial and temporal variation in the abundance of drift algal foods had strong effects on the intertidal distribution and feeding preferences of abalone, which are largely sedentary. The availability of drift algae was primarily controlled by the abundance of local algal stocks and the extent of water movement. On Santa Cruz Island, marked fluctuations in the abundance of drift algal foods occurring during the 1986-87 El Niño resulted in mass mortality of abalone in 1986-88. Survivorship during mass mortality had strong interactions with abalone shore-level size gradients and seasonal movement patterns, and was an important factor influencing geographic variation in population structure. Variation in trema number along geographic and wave-exposure gradients was an allometric consequence of variation in shell growth rate, which in turn, was largely controlled by the abundance of drift algal foods. In contrast, shore-level gradients in trema number resulted from a combination of growth-induced allometry, differential survivorship, and complex movement patterns. Trema size varied with the extent of water movement as a response to variation in respiratory efficiency. Overall, morphological patterns resulted from environmentally-induced phenotypic plasticity and not heterochrony.
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