The distribution of offshore benthic molluscs provides new insight into South Africa's marine biogeography

Abstract

South African marine biogeography has received a great deal of attention in the past. However, offshore biogeographic breaks are based on abiotic variables or specialist opinion rather than data-driven approaches. Here we investigate biogeographic breaks based on the distribution of offshore marine benthic molluscs collected during the Natal Museum Dredging Programme (1981–1997) along the coast of South Africa, between the South East Atlantic and South West Indian Oceans. Spatially constrained (depth, latitude and longitude) cluster analyses were applied to three resemblance measures; the Jaccard coefficient, popular in biogeographic studies, and gamma+ and theta+ coefficients, where taxonomic relatedness is considered when comparing sites. We used these resemblances to 1) find the most consistent biogeographic breaks and 2) compare the results of different measures to better understand the patterns and possible processes involved in the biogeographical classification. Although there were differences in the overall results from the three resemblance measures, they produced comparable findings at depths shallower than 80 m. Our data-driven approach indicated that the biogeographic break between the warm-temperate and sub-tropical assemblages was 140 km north of the current inshore break. The break between the sub-tropical and tropical mollusc assemblages agreed more closely with the present Ecoregions. Compared to Jaccard, where relatedness is not considered, gamma+ and theta+ results revealed a more substantial depth effect, and samples collected in the same depth range were more closely related and shared a more recent evolutionary history than geographically close samples from different depths. Furthermore, assemblages found deeper than ∼80 m demonstrated greater diversity in terms of relatedness, an essential consideration in marine spatial planning and consequent designation of protected areas. This study reveals the importance of including in-situ data and interspecies relationships in future marine spatial planning to ensure that these evolutionary rich animal communities are considered adequately.