Bimodal latitudinal species richness and high endemicity of razor clams (Mollusca)

AimTo examine the global distribution, endemicity, and latitudinal gradients of species richness of razor clams, family Solenidae. LocationGlobal. MethodsA total of 3105 distribution records for 77 Solen and Solena species were used. Species richness was plotted in 5 degrees latitude-longitude cells and related to environmental variables. ResultsThe north-west Pacific and the Indo-West Pacific have the highest species richness (about 85% of all species)-mostly in the Sea of Japan, China Sea, the Gulf of Thailand and the Andaman Sea. Cluster analysis of similarity patterns of species composition (i.e., presence of Solenidae species) for 5 degrees latitudinal-longitudinal grid cells showed 16 significant biogeographical regions that concur with existing marine biogeographical hypotheses. More than half of the species were endemic to specific biogeographical regions. The geographical distribution of species in 5 degrees latitudinal bands showed a significant bimodal pattern. Global patterns of species richness increased from the poles to intermediate latitudes and dipped near the equator. A non-linear relationship between species richness and mean sea-surface temperature (SST) values was compatible with this bimodal pattern. Two inflection points of species richness with correlation of SST at 12 degrees C (low species richness) and 28 degrees C (high species richness) were coincident with the bimodal latitudinal species richness pattern. Species richness was highly positively correlated with mean SST over all latitudes, and within the Northern and Southern Hemispheres. Species richness decreased with SST range over all latitudes and in the Northern Hemisphere. Species richness also decreased with chlorophyll-a concentration and primary productivity, but increased with ocean area in the Northern Hemisphere (only). Main conclusionsThe latitudinal distribution in species richness of Solenidae peaked at 10 degrees N and 25 degrees S rather than at the equator, exhibiting a strongly bimodal pattern that is likely to be temperature driven.