Broad thermal tolerance and high mitochondrial genetic connectivity in the pinfish (Lagodon rhomboides).

Pinfish (Lagodon rhomboides) are highly abundant in coastal ecosystems of the Gulf of Mexico and western Atlantic Ocean and serve as a crucial link in marine food webs. Despite their ecological relevance, little is known about this species' susceptibility to anthropogenic climate change. Here, we characterized patterns of mitochondrial genetic divergence and examined the upper thermal tolerance of pinfish across a large portion of the species' range. We found little evidence of population genetic differentiation among distant localities with divergent temperature regimes (e.g., Mexico and North Carolina), using two mitochondrial markers (cytochrome b [CytB] and cytochrome c oxidase I [COI]). This suggests high genetic connectivity, which implies low potential for local adaptation of populations to different thermal conditions along a latitudinal gradient. To further examine population-scale differences in thermal tolerance, we assessed the critical thermal maxima (CT_max) of pinfish from four localities: North Carolina, Florida Keys, Alabama, and Texas. We found that CT_max was similar across sites, with all localities showing an average CT_max within a 1°C temperature range (34.5-35.5°C). This suggests that southern populations of pinfish may be more susceptible to the detrimental effects of ocean warming, as individuals in lower latitudes regularly experience temperatures within a few degrees of their CT_max. Finally, we examined the influence of varying salinity on the upper thermal limit of the pinfish and found that pinfish show no variation in CT_max under salinity conditions ranging from hypo- to hypersaline (15-35 ppt). These results show that pinfish can tolerate a wide range of environmental parameters but may rely on phenotypic plasticity, rather than local adaptation, to distinct conditions to cope with different environmental regimes.