Size-at-age or structure shift: Which hypothesis explains smaller body size of the fiddler crab Leptuca uruguayensis in northern populations?

Abstract

The mean body size decrease is known as the third most important global consequence of climate change to wild life. Rising temperatures may lead to decreased mean body size of organisms and change their ecological role in the environment. Herein we investigated why the fiddler crab Leptuca uruguayensis is smaller at its northern distributional limit by using the ‘size-at-age’ and ‘structure shift’ as alternative hypotheses. For the first hypothesis, we evaluated whether the smaller mean body size of L. uruguayensis from a northern population is a phenotypic response to the thermal environment. For that, we tested whether the crabs grow less and reach the onset of sexual maturity earlier at high temperatures. We also evaluated their oxygen consumption at different temperatures to test whether higher metabolic rates due to warmer temperatures leads to smaller body sizes. For the second hypothesis, we evaluated whether smaller mean body size in a northern population is a result of differential survivorship between age-classes. We tested whether the temperature itself or a predator model with a range distribution linked to temperature (Minuca rapax) could negatively select larger L. uruguayensis sizes. We showed that crabs grow less, reach sexual maturity earlier and have lower survive in response to high metabolic costs imposed by higher temperatures. The predator chose a large L. uruguayensis size, a finding that could mean selective pressure where prey populations overlap with this predator. Thus, global warming may decrease the mean body size of the fiddler crabs at lower latitudes, affecting their ontogenesis and by selective pressure against larger individuals.