The maintenance and stability of the shell color polymorphism in the rocky intertidal gastropod Thais (Nucella) emarginata Public Deposited

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  • Thais (Nucella) emarginata is a common predatory gastropod of the rocky intertidal system of western North America. It is usually found at low tide on top of its sedentary prey, barnacles and mussels. Thais is also highly polymorphic for shell color, and breeding of isolated females has shown that the polymorphism has a genetic basis. This study of the Thais polymorphism is divided into two parts. In the first part, I describe the variation, its genetic basis, consider patterns of variation and change for ten Oregon populations, and assess micro- and macrohabitat relationships of the different morph types. The second part is a study of snail movement and its relationship to morph frequencies at one of the sites. Of the eight shell color categories examined at the Oregon sites, a solid colored Black morph and a striped Grey-Black morph are clearly the most common types. There were no consistent patterns of shell color frequency change associated with any environmental variable recorded. These included latitude, wave action, subtidal type (sand, gravel, or solid rock), relative abundance of any prey type, direction of facing (N, S, or W), Thais density, or Thais distribution. One pattern did emerge from this investigation. Six of the ten sites had significant shell color frequency change between 1978 and 1979, and three others had marginally significant change, indicating the polymorphism is a very dynamic one.. There was a significant association between shell color and microhabitat type. However, Black and Grey-Black snails did not show such an association, and the causes of this relationship in the minor categories remains obscure. There were also a significant association between shell color and macrohabitat (reef versus cobble type habitats). A comparison of the frequencies of Black and Grey-Black snails at seven of the sites (those having both habitat types) exhibited a slight, but very consistent relationship between shell color and macrohabitat. I postulate that this relationship is due to a differential response to wave action in the two macrohabitat types. The study of movement was carried out at Cascade Head on the north central Oregon coast. Preliminary experiments with a marking method de signed to minimally disturb the animals suggested that movement was responsible for the high losses of tagged animals reported in the literature. Snails on a particular boulder at Cascade Head were painted with nail polish and their shell color frequencies were recorded. When a painted snail moved off that rock, it was given a numbered tag. Comparisons of the frequencies of painted snails, tagged snails, and overall frequencies of Black and Grey-Black snails in the population show a close concordance indicating that movement plays a role in the dynamics of shell color frequency change in the population. There was no association between probability of movement and shell color frequencies of those that moved and were tagged. Black and Grey- Black tagged snails were not different in size, nor was there any difference in mean distances moved. There was, however, a significant difference in the directions the two morphs moved, which when taken within the context of the geography of Cascade Head, predicts a net loss of Black snails from the population. There was, in fact, a significant negative regression of the frequencies of Black snails over time, consistent with the idea that movement is very important in the dynamics of the polymorphism. In the discussion of these results, voluntary versus involuntary movement is contrasted, and I note that Thais has the ability to find suitable habitat after a move. Differential directions of movement by the two morphs is postulated to be a response, as with the macrohabitat correlations, to wave action. I suggest that shell color, or genes closely linked to shell color loci, are causing the response in both cases. This movement study suggests that delineation of a population of Thais is a formidable task, and the question of what a biological population of Thais emarginata is remains obscure.
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