A WEAK AND INCONSISTENT BERGMANN'S RULE IN NORTHEASTERN PACIFIC PATELLOGASTROPODS

A substantial body of observation and theory focuses on the relationship between latitude or climate and body size distributions within and among species the species in a clade. Bergmann's Rule, a tendency for body size to increase with latitude, has been documented in a variety of disparate clades. However, there are also a number of clades that show no apparent relationship between body size and latitude. One of the best known of these is the Bivalvia. Roy et al. (2000) studied size distributions of marine Bivalve species in the Northeastern Pacific and found body size distributions to be invariant across 75 degrees of latitude. Here we examine size trends across the same region in a different clade of marine intertidal mollusks, the Patellogastropods (true limpets). Rather then using a single exemplar size measurement for each species as in many previous studies, we apply a novel high-throughput morphometric imaging method to measure the sizes of more than 12,000 individual specimens collected from more than 300 sites ranging from southern Baja California to northern Alaska. Our dataset includes most known species, with many represented by hundreds or thousands of individuals. We find that at the community level the mean size of individuals does tend to increase with latitude, but that the increase is driven primarily by relative rarity of small individuals at high latitude rather than by an increase in maximum size. Within-species trends are variable, with most species exhibiting either a positive relationship or no significant relationship between latitude and mean size. Additional study of potential inter- and intraspecific biotic interactions, as well as more comprehensive analyses of potential environmental influences such as tidal height and seasonality, will help to determine what processes are responsible for the generating the weak Bergman's Rule pattern observed here.