Horseshoe crab body size cline

Aim Adult body size often exhibits patterns across large-scale
environmental gradients, creating ecogeographic clines. However, the form
of body size clines varies across taxonomic groups, with linear and
non-linear patterns in body size observed in nature. Non-linear body size
clines have received less study, and questions remain about how
environmental gradients interact to produce non-linear clines. We examined
the body size of the American horseshoe crab (Limulus polyphemus), a
widely distributed marine arthropod, and evaluated the hypothesis that
temperature and active season length can interact multiplicatively to
result in a dome-shaped distribution. Location Fourteen states in the
United States of America and three Mexican states, representing the entire
geographic range of the species. Methods We compiled environmental data
and body size measurements from more than 49,000 individual horseshoe
crabs. For each location, we extracted from the literature or calculated
from raw data the mean male prosoma width and the mean female prosoma
width. We applied a General Additive Modeling (GAM) approach to
characterize the body size cline, test a hypothesis regarding temperature
and season length, and explore evidence for the influence of additional
environmental factors. Results Model results indicate temperature and
season length could act multiplicatively to produce dome-shaped clines,
and these findings align with and quantify previous anecdotal reports of a
strong dome-shaped body size cline across latitude for horseshoe crabs.
Main conclusions Active season length appears to become relatively more
influential on horseshoe crab body size in the northern part of their
range, while temperature effects per se appear to dominate in southern
latitudes. For horseshoe crabs, the pattern of size variation is
consistent with the predictions of Optimal Resource Allocation models, but
more study is needed to elucidate mechanistic underpinnings. Considering
climate change projections, results from our study suggest future shifts
in horseshoe crab body sizes.