Solea senegalensis skeletal ossification and gene expression patterns during metamorphosis: New clues on the onset of skeletal deformities during larval to juvenile transition

Farmed Senegalese sole (Solea senegalensis) still show a high incidence of vertebral anomalies that limit its intensive production and hamper its economic profitability. A great effort towards understanding how fish species develops and grows in captivity has been undertaken in the last decade, with particular emphasis to how different biotic and abiotic factors affect its skeletal development. Although some work has been performed on its skeletal development and expression patterns of key genes in developmental signaling pathways, a detailed description of the above-mentioned processes is still lacking. Here, the progression of skeletal development of cranial, appendicular and axial skeleton has been described through the implementation of an acid free double staining protocol; while the gene expression pattern of vitamin A (VA) and thyroid hormones (THs) signaling pathways were assessed through quantitative PCR (qPCR) during larval fish development under a standard larval rearing protocol and in Senegalese sole larvae fed with increased dietary VA levels (8-fold increase) during the Artemia feeding phase (from 6 to 27 dph). The ontogenetic study allowed us to identify the onset and follow-up the development of most skeletal structures in this fish species, and to gather some insights on the potential onset of deformities originated at the larval to juvenile transition affecting Senegalese sole rearing. The pro-metamorphosis stage is when the caudal fin vertebrae are formed, where the most common skeletal deformities are found in Senegalese sole. A highly coordinated expression of VA- and TH-related genes has been revealed during larval to juvenile transition, while a comparative gene expression analysis in larvae fed control and high dietary VA content identified the specific timing of VA and THs signaling disruption through which VA and THs directly or indirectly might increase the incidence of skeletal deformities in this species. The present research work represents an important step forward towards the proper identification of the onset of some skeletal deformities and urge the investigation of nutritional and rearing conditions during the switch of larval behavior - from pelagic to benthonic - in order to overcome them in this important species for southwestern European aquaculture.