Contrasting bivalve body-size distributions between Northern and Southern Hemispheres

ABSTRACT Despite similarly frigid climates and low diversity compared to the tropics, the Northern and Southern Hemisphere polar regions—and the high latitudes of the Northern and Southern Hemisphere at large—have different histories, and their biotas differ in a number of ways, but these biotic contrasts remain understudied. In this study, I tested whether the body sizes of Northern and Southern Hemisphere extratropical bivalve faunas differ from each other. The Southern Hemisphere polar bivalve fauna has a significantly smaller median body size than the Northern Hemisphere polar region, and this difference remains whether shared families, shared genera or the whole fauna was compared. There were also significant, pervasive contrasts in body size between the Northern and Southern Hemisphere throughout the extratropics for the whole bivalve fauna and largely for the five major bivalve clades examined (Protobranchia, Pteriomorphia, Archiheterodonta, Anomalodesmata and Imparidentia), and these contrasts largely remained even when only endemics were compared.