Primary Productivity and Habitat Depth Shape Developmental Mode in European Marine Gastropods

Prolonged larval development in marine gastropods is less frequent in high latitudes, with non-pelagic larval development much more common in these regions. This pattern has been historically referred to in biogeography as the Thorson's rule. The most invoked theoretical explanation for this pattern is that pelagic larval duration becomes too long as temperatures decrease, thus increasing exposure to predators and transport towards habitats not suitable for recruitment. However, more factors rather than only water temperature could influence pelagic duration and overall larval performance, like initial embryo and juvenile sizes, currents and phytoplankton fluctuations. On the other hand, phylogeny and environmental conditions affecting the adults, like the stability of benthic habitats in time, could also play an important role in the evolution of developmental modes. Besides, Thorson's latitudinal pattern could be an artefact arising from insufficient, incomplete and uneven sampling. In this work, we gathered an up-to-date dataset for 94 species of European gastropods from the literature, including developmental mode and other life history traits together with variables related to bottom habitat, water column and primary productivity. Thorson's rule was not recovered when the proportion of species with non-pelagic development was analysed with fine spatial resolution at regions with at least 20 gastropod species present. Moreover, temperature and phylogeny played a negligible role in determining developmental mode, while greater depths and marked chlorophyll-a seasonality significantly favoured non-pelagic development. Thus, we infer that the increased temporal shifts in bottom habitats at shallow waters in regions with a non-seasonal and more constant phytoplankton availability drive the evolution of pelagic larval forms in gastropods.