Biogeography and depth partitioning in deep-sea gastropods at the Pacific Costa Rica Margin

AimUnderstanding the global distribution of biodiversity and the factors that influence it are among the central goals of biogeography. How abiotic and biotic factors limit species' ranges has been investigated across a variety of environments and taxonomic groups. However, such investigations across oceanic depths remain underrepresented, particularly for chemosynthetic environments such as hydrocarbon seeps. The depth differentiation (DD) hypothesis suggests that steep environmental gradients in the upper depths of the ocean may limit species' ranges over relatively short vertical distances. The present study aims to investigate the evidence for the DD hypothesis at hydrocarbon seep sites along the Pacific Costa Rica Margin (CRM) by investigating the biogeographic distributions of gastropods sampled there and the factors that influence them.LocationCosta Rica.TaxonMollusca, Gastropoda, Provannidae, Genus: Provanna (Dall, 1918).Methods1813 Provanna snails were collected across 1300 metres (m) of depth from Costa Rican hydrocarbon seeps. To test the DD hypothesis, species partitioning across the depth range was investigated. In addition to gradients in oceanographic conditions, such as temperature, oxygen, and salinity, other factors potentially responsible for filtering species were also considered as alternative hypotheses, including the availability of biogenic substrate, the availability of and reliance on chemosynthetic fluids, and specific preferences for certain environmental conditions.Results and Main ConclusionsThree species of Provanna were identified from the CRM and exhibited partitioning across depth. For the two dominant species (P. laevis and P. goniata), the data suggest that depth-associated oceanographic conditions are more likely than any other factor investigated to explain their observed partitioning across depth, supporting the DD hypothesis. This study also presents novel evidence that such partitioning may be unilaterally determined, wherein species with narrower depth ranges may limit the local depth ranges of others. This study represents an extension of biogeographic range limit literature into chemosynthetic habitats and across oceanic depths.