Genomic Analysis of Physiological Responses of Southern Flounder (Paralichthys lethostigma) Pre-Metamorphic Larvae from the Texas Coast to High Temperature and Salinity

Southern flounder (Paralichthys lethostigma), declining in much of its range, is considered a viable candidate for stock enhancement and aquaculture.  Efforts to understand its decline and implement stock enhancement are hampered by limited information about physiological mechanisms for responses to environmental stressors across larval stages.  We evaluated survival of pre- metamorphic larvae (30-da old, 12-mm TL, produced in a hatchery by wild broodstock) from three localities along the Texas coastal gradient, after acute exposure to a range of temperature, salinity, and pH treatment combinations.  Surviving larvae across selected treatments, were used to prepare 18 RNA libraries and analyzed with Illumina, Next Generation RNA sequencing to assemble a “De Novo” transcriptome.  Genes and gene expression were compared for control treatments (18 C° and 32 ppt salinity) versus high temperature (28 C°) and high salinity (40 ppt) treatments.  In total, 29,054 transcripts were expressed (average FPKM > 1.0), and gene functions were identified.  Hierarchical clustering showed that replicates of the same experimental treatments were grouped together (similar expression profiles).  Across locations, differential expression was stronger for salinity than temperature treatments. The identity of differentially expressed genes provided novel insight into responses of southern flounder pre-metamorphic larvae under extremes for temperature and salinity.