Bryozoa from the continental shelf off Tierra del Fuego (Argentina): Species richness, colonial growth-forms, and their relationship with water depth

Abstract

As bryozoans have a rich fossil record, the analysis of their bathymetric ranges and the occurrence of different colonial growth-forms in living assemblages may be valuable for palaeoecological interpretation. The bryozoan fauna from 58 benthic stations (18–92 m) collected on the Atlantic continental shelf off Tierra del Fuego (Southwest Atlantic) was analyzed to assess its biodiversity and the relationship of species richness and colonial growth-forms with depth. In the 58 stations analyzed here, which covered an area of ∼5600 km2, 90 species were found. Of these, 43 species were already known for this area, whereas the remaining 47 are new records. Therefore, the present study increases by 87% the known biodiversity for the study area, which now reaches 101 species. There is a clear increase in bryozoan species richness with depth, but the ratio of erect-rigid/encrusting species remained unchanged, as all the growth-forms increased with depth. Stations were classified by cluster analysis in three groups differing mainly in species richness. Taxonomic turnover (beta diversity) increased with increasing differences in depth between stations. This was due to the presence of a new set of deeper species, but not to the demise of the shallower fauna in the deeper stations. Twenty-two potentially habitat-forming species were found along the bathymetric range surveyed in this study. They were absent at the shallowest station, its number was relatively low at intermediate depths (31–64 m) and then experienced a sharp increase at 70 m. Palaeoecological interpretations should take into account that fragile, calcareous growth-forms could be expected to occur at shallower depths in more protected areas.