Global biodiversity and biogeography of rhodolith-forming species

Unattached nodules of calcareous red algae (Rhodophyta), known as rhodoliths, are widely reported and studied in places that extend from the tropics to polar latitudes. Factors controlling the distribution of the rhodolith-forming species remain poorly understood. A review of the global distribution of present-day rhodolith beds was undertaken, collating information on 106 rhodolith-forming species from 10 families, representing 21 genera distributed through 11 realms: 1) Arctic, 2) Temperate Northern Atlantic, 3) Temperate Northern Pacific, 4) Tropical Atlantic, 5) Western Indo-Pacific, 6) Central Indo-Pacific, 7) Eastern Indo-Pacific, 8) Tropical Eastern Pacific, 9) Temperate South America, 10) Temperate Australasia, and 11) Southern Ocean. The Central Indo-Pacific and Temperate Australasia proved to be the most diverse realms. Of 62 provinces across these realms, the Tropical Southwestern Atlantic, the Mediterranean Sea and the Tropical East Pacific feature the highest diversity of rhodolith-forming species. A significant proportion of the 106 species (14.2%; 15 species) are endemic to a single biogeographic province. Species richness is weakly related to sampling effort (r2=0.573) and unrelated to littoral area (r2=0.012). Even when high latitude provinces are excluded from the analysis, no correlation between species richness and littoral was found (r2 = 0.0005). A wider, evolutionary-time framework revealed that the existence of marine barriers and the geological age of their final emplacement are key elements to explaining compositional differences between the rhodoliths of former contiguous areas (e.g., Pacific versus Atlantic shores of Panama and Costa Rica, in the Central America; eastern Mediterranean Sea versus Red Sea and Gulf of Aden). Finally, we propose that the lower diversity of the rhodolith-forming species in the tropical Pacific Ocean when compared to the Atlantic Ocean (23 versus 33 spp.), may be linked to the higher abundance of corals and coral reefs in the Pacific, which act as competitors with coralline algae for space.