Adaptive phenotypic differentiation across the intertidal gradient in the alga Silvetia compressa

Populations of intertidal species span a steep environmental gradient driven by differences in emersion time. In spite of strong differential selection on traits related to this gradient, the small spatial scale over which differences occur may prevent local adaptation, and instead may favor a single intermediate phenotype, or nongenetic mechanisms of differentiation. Here I examine whether a common macroalga, Silvetia compressa, exhibits phenotypic differentiation across the intertidal gradient and evaluate how local adaptation, developmental plasticity, and maternal effects may interact to shape individual phenotypes. Reciprocal transplants of both adults and embryos showed a "home-height advantage'' in two of the three populations tested. In laboratory trials, the progeny of upper-limit individuals survived exposure to air significantly better than lower-limit progeny from the same population. I compared the emersion tolerance of full-sib families generated from gametes produced in the field to those produced under common garden conditions. The relative advantage of upper-limit lineages was robust to maternal environment during gametogenesis; this pattern is consistent with genetic differentiation. The possible role of local adaptation has historically been ignored in studies of intertidal zonation. In S. compressa, phenotypic differentiation may have important consequences for vertical range, both within and among sites.