Cold water temperatures define the poleward range limits of south American fiddler crabs

Abstract

Temperature increase due to climate change has caused shifts in the range distribution of several organisms globally. In coastal intertidal environments most organisms have an amphibious life cycle and their poleward range limits may be delimited by their thermal tolerance during the pelagic larval stages. Fiddler crabs are key species in intertidal environments and their early larval stages occur in coastal waters. We evaluated the mean and monthly minimum sea surface temperature (SST) gradient over the South American coast and compared it to the minimum and maximum critical thermal limits (CTmin and CTmax) of the first larval stage of eight fiddler crab species to assess whether temperature delimits their distributional ranges. We found a clinal decrease in mean SST of 0.28 °C per latitudinal degree along the distribution of fiddler crabs in South America. Cold tolerance differed among the larvae of fiddler crab species, which corresponds to the latitudinal temperature gradient observed in their poleward range limits. Thus, our results suggest that cold water temperature can define the poleward range limits of South American fiddler crabs. The CTmax cannot explain the northern poleward range limits of the fiddler crabs. Fiddler crabs larvae showed a similar tolerance to high temperatures (mean 40.5 °C) regardless of species (except Leptuca uruguayensis) and these are higher than environmental temperatures observed in South America. We also observed an increase in mean SST of 0.9 °C in the last 37 years. This increase in water temperature may explain the recent poleward range expansion of one South American fiddler crab species, Leptuca cumulanta. Therefore, we showed that differential thermal responses at the early larval stage have consequences on the geographic range limits of the fiddler crab species. Our findings allow us to hypothesize that fiddler crabs’ poleward range limits may expand to higher latitudes in the future due to global warming.