Invariant size-frequency distributions along a latitudinal gradient in marine bivalves

In the most extensive analysis of body size in marine invertebrates to date , we show that the size-frequency distributions of northeastern Pacific biv alves at the provincial level are surprisingly invariant in modal and media n size as well as size range, despite a 4-fold change in species richness f rom the tropics to the Arctic. The modal sizes and shapes of these size-fre quency distributions are consistent with the predictions of an energetic mo del previously applied to terrestrial mammals and birds. However, analyses of the Miocene-Recent history of body sizes within 82 molluscan genera show little support for the expectation that the modal size is an evolutionary attractor over geological time.