Hatching mode and latitude in marine gastropods: revisiting Thorson's paradigm in the southern hemisphere

For much of the last century the developmental mode of marine invertebrates, particularly of prosobranch gastropods, has been thought to reflect a latitudinal pattern known as Thorson's rule; the proportion of species with pelagic larvae decreases with increase in latitude. Although the predictions of this rule have been criticized recently, its validity along latitudinal transects in the world ocean still remains poorly tested. In the present work, we compare the frequency of occurrence of contrasting prosobranch hatching modes (pelagic versus benthic development) along two latitudinal gradients of the subequatorial coastline of South America: the southeast Pacific and the southwest Atlantic. The results are clearly contrasting. While the pattern observed along the Pacific (Chilean) coast fits the predictions of Thorson's rule very well, benthic development predominates all along the Atlantic coast, even at subtropical latitudes. This difference in observed patterns is attributable to the different compositions of the gastropod assemblages on each side of the South American continent, which are determined, in turn, by differences in the ecological conditions on the two sides of the continent. The scarcity of pelagic development among the Atlantic prosobranch gastropods reflects the near-continuous soft-bottom habitat there, and the consequent prevalence of predaceous, soft-bottom taxa that had acquired in their evolution more evolved, non-pelagic patterns of development. In contrast, the Pacific coast is much more heterogeneous, with a diverse mixture of benthic habitats, including rocky substrates that, in part, support grazing taxa. Our results indicate that two factors are important for Thorson's rule to be valid. First, the habitat needs to include rocky substrates, as soft-bottom habitats appear to favour non-pelagic development. Second, a diverse assemblage of taxa need to be compared to avoid the problem of phyletic constraints, which could limit the evolution of different developmental modes.