Diversity patterns and zoogeography of the Northeast Atlantic and Mediterranean shallow-water sponge fauna

Recognizing and understanding present-day biodiversity and biogeographical patterns and how these relate to contemporary and past climate is pivotal to predict the effect of future climate on marine biodiversity and promote adequate conservation policies. Sponges constitute an important and dominant component of the marine benthos and are therefore an excellent model group for such investigations. In this study, we assessed the diversity patterns and the zoogeographical affinities of the Northeast Atlantic and Mediterranean shallow-water demosponge assemblages. Data on the distribution of 745 species throughout 28 areas was compiled from the literature and used to build a presence/absence matrix. Diversity patterns were assessed from estimates of species richness (S) and taxonomic distinctness (AvTD). The Mediterranean Sea proved to be more diverse both in terms of species richness and taxonomic distinctness (S = 539, AvTD = 94.74) than the Northeast Atlantic (S = 480, AvTD = 92.42) and the two regions together were found to constitute a diversity hotspot harbouring approximately 11% of the global demosponge diversity. We found an Atlantic N–S and a Mediterranean NW–SE gradient of increasing taxonomic distinctness that is strongly correlated to both contemporary (R ² = 0.5667; P < 0.01) and historical values (R ² = 0.7287; P < 0.01) of sea surface temperature (SST) at the Last Glacial Maximum (LGM). The zoogeographical affinities examined through classification (cluster analysis) and ordination (non-metric multidimensional scaling, nMDS) based on the Bray–Curtis similarity index, revealed the presence of three groups approximately corresponding to the Northern European Seas, Lusitanian and Mediterranean provinces outlined in the ‘Marine Ecoregions of the World’ (MEOW) classification system. Geographical distance and oceanographic circulation were shown to constitute important factors in shaping the zoogeographical affinities among areas. The vast majority of the species occurring in the Northeast Atlantic and the Mediterranean (67 and 57%, respectively) was shown to have extremely restricted geographical ranges, as single-area or narrow-range (2–3 areas) endemics, which raises some concerns regarding their conservation.