Migratory differences between ecotypes of the snail Littorina saxatilis on Galician rocky shores

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Local migration patterns may be crucial to gene flow in species of marine gastropods which do not broadcast pelagic larvae. In some species, dispersal over distances of a few metres may influence micro-scale population structures. We investigated the migration pattern in Galician populations of the snail Littorina saxatilis in which populations of contrasting morphologies occupy different tidal levels of the same rocky shore. Two distinct morphs, one at the upper and one at the lower shore, overlap in distribution in a small mid-shore region where hybrids are produced. We documented the distances and directions of migration of both parental morphs and hybrids 1 month after they had been marked and released at different shore levels. When placed at their native shore level, snails migrated less than about 2m and usually in independent directions. This supports the suggestion of a low local gene flow. At an alien shore level, however, the morphs often moved further and more directionally compared with native morphs. These differences may help to keep the two morphs separated at different shore levels. As fitness of an individual is highest in its native habitat, this seems to be an adaptive strategy.

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