Neighboring Gulf Killifish ( Fundulus grandis ) Populations Differ in Their Plastic and Genetic Responses to Salinity Fluctuations

ABSTRACT

Coastal development is increasing rapidly, intensifying the frequency and magnitude of watershed runoff and salinity fluctuations in estuarine ecosystems. In developed areas, impervious surfaces can accelerate freshwater runoff during rains, producing rapid and extreme osmotic shifts that challenge coastal organisms. Along the northern Gulf of Mexico (GoM), the Gulf killifish ( Fundulus grandis ) is a species that is consistently present in reference and urbanized tidal creeks. To evaluate responses to osmotic fluctuations, we conducted a laboratory‐based reciprocal transplant experiment using individuals from a reference watershed (Long Bayou, AL) and an urbanized watershed (Weakley Bayou, FL) along the northern GoM. Individuals from each population were exposed to either a simulated tidal event (low fluctuation; 15–10 ppt) or a tide/runoff event (high fluctuation; 15 ppt to 10 ppt to 5 ppt) over an acute period (~7 h) to assess responses via gill gene expression. RNA‐seq analyses identified 112 differentially expressed genes (DEGs), and principal component analysis showed separation by salinity treatment along PC1 (31% variance) and separation by population origin along PC2 (13% variance). The urban population showed a substantially higher number of DEGs compared to the reference population. Processes primarily activated in the urban population under osmotic stress were associated with ion transport, negative regulation of cell cycle, cellular integrity, and energy reallocation. Transcriptome‐derived single nucleotide polymorphisms (SNPs) revealed small but significant genetic differentiation ( F _ST  = 0.021, p  < 0.05), with higher nucleotide diversity and lower inbreeding in the urban population ( π  = 0.106; FIS = 0.004). This study indicates that both adaptation and plasticity can shape responses to abiotic variability in coastal environments at small spatial scales.