COLLIN, R.: Biogeographic Patterns in Mode of Development in Marine Invertebrates

Latitudinal patterns in mode of development of molluscs in the North Eastern Atlantic were first demonstrated by Thorson in 1946. He demonstrated that planktotrophic development is more common at low latitudes than at higher latitudes. Since then, it has often been assumed that this pattern occurs in other taxonomic groups and in other regions. Anecdotal evidence and studies of isolated groups of species, however, suggest that there may be inter-oceanic differences. I compiled a world-wide dataset of the development of 80 species of calyptraeid gastropods. Analysis of this dataset shows that the frequency of planktotrophic species also decreases with latitude. However the latitudinal increase in the proportion of direct developers occurs differently in the northern and southern hemispheres: in the northern hemisphere the frequency of direct developers with large eggs increase with latitude and in the southern hemisphere the frequency of direct developers with nurse eggs increase with latitude.