Wave exposure and ecotypical differentiation in Sargassum cymosum (Phaeophyta—Fucales)

Populations of Sargassum cymosum from Ubatuba (S.P., Brazil), exposed to various degrees of wave intensity, are compared. The differences found between the populations from the extreme exposure indices include plant size, density and biomass as well as the weight ratio between the holdfast and the frond, sex distribution, morphology of receptacles and occurrence of bladders.Transplantation experiments showed that these differences were not due the phenotypic plasticity, but rather to genotypic differentiation. The extreme forms have evolved into ecotypes; the existence of individuals of intermediate morphology between the two typical populations occurring in areas of transition were interpreted as hybrids growing in an ecotone. The extent of transitional populations varies a great deal and depends on the configuration of the coast and its relation to exposure.A new variety, S. cymosum var. nanum, is proposed for the exposed type.