Latitudinal distribution of body size in north-eastern Paci®c marine bivalves

Aim To understand the latitudinal patterns in body size distributions of north-eastern Paci®c bivalves, as well as the factors in ̄uencing those patterns. Locations The north-eastern Paci®c continental shelf from 5 °S latitude to 70 °N latitude. Methods We used data on the body size and latitudinal ranges of 915 species of northeastern Paci®c marine bivalves to test for the presence of interspeci®c latitudinal size trends. We tested for trends in both mean body size as well as variance in body size using randomization tests. Results Overall, there is no signi®cant linear relationship between either mean size or variance in size and latitude. Rather, there is a mid-latitude trough and peak in mean body size and variance in body size, respectively, with changes in the sign of the correlations occurring at provincial boundaries. A signi®cant decreasing trend in mean size with latitude exists within the tropical Panamic province. In contrast, variance in body size shows a signi®cant increasing trend with latitude within the tropics, and a signi®cant decreasing trend with latitude in the cold temperate and Arctic provinces. For north-eastern Paci®c bivalves, latitudinal trends in mean body size and variance in body size show very little correlation with latitudinal trends in species richness. Close to an order of magnitude difference in species numbers between tropical and polar latitudes does not appear to signi®cantly affect either mean size or variance in size. Changes in mean body size associated with biogeographic boundaries are signi®cantly larger than changes elsewhere along the range. Main conclusions The spatial distribution of major environmental barriers along the north-eastern Paci®c margin plays a major role in structuring the latitudinal distribution of body size in marine bivalves along this coast.