Biogeography of polychaete worms (Annelida) of the world

The global biogeography of polychaete worms has never been assessed previously. In the present study, we studied the world distribution patterns of polychaetes based on datasets obtained from the Global Biodiversity Information Facility, the Ocean Biogeographic Information System and our recently published checklist of Indonesian polychaete species. Polychaete biogeographic regions were visualized using ‘Infomap Bioregions’, and the latitudinal species richness gradient of the animals was examined using 3 metrics, i.e. alpha, gamma and estimated species richness (the last metric was adjusted for sampling bias). We identified 11 major polychaete biogeographic regions. The North Atlantic, Australia and Indonesia were the top 3 species-rich biogeographic regions in the world. The total number of polychaete species was higher in the southern hemisphere (~2100 species, 67 families) than in the northern hemisphere (~1800 species, 75 families) despite significantly more data in the latter (>500 000 records compared to >26 000 records). Contrary to the classical idea of a unimodal distribution pattern, the latitudinal gradient of polychaetes was generally bimodal with a pronounced dip north of the Equator (15° N). We suggest that the slightly higher peak of species richness in the southern (30° S) than in the northern (60° N) hemisphere reflects higher southern endemicities. These patterns are unlikely to be due to sampling bias but rather represent a natural phenomenon, and we found them most significantly correlated with sea temperature.