Mucosal immune responses in Senegalese sole (Solea senegalensis) juveniles after Tenacibaculum maritimum challenge: A comparative study between ocular and blind sides

Most pathogens start the process of infection at the mucosal surfaces and therefore the mucosal immune response plays an essential role in the course of the infection. Due to the Senegalese sole (Solea senegalensis Kaup) condition of flatfish, the present comparative study aimed to analyse several immune-related enzymes as well as the bactericidal activity in the skin mucus from ocular and blind sides. For this purpose, Senegalese sole juveniles were bath challenged with a sub-lethal dose of Tenacibaculum maritimum for 24 h and sampled at 1, 2 and 3 weeks. The haematological profile and immune-related parameters were also measured in plasma in order to evaluate the systemic immune response after T. maritimum challenge. Results from this study showed that most parameters tested increased in skin mucus of bath challenged fish compared to unchallenged ones. In contrast, the sub-lethal dose tested did not influence the haematological profile including peripheral numbers the different leucocyte types. No variations were observed in plasma lysozyme, peroxidase, protease and haemolytic complement activities between unchallenged and bath challenged fish. This study suggests that the studied innate immune-related molecules are constitutively present in both skin mucus sides but at different levels. Interestingly, the levels of most parameters measured were higher on the ocular side than on the blind side, possibly due to the higher exposure to invasion by waterborne microorganisms on this side. Therefore, the present study brings some insights regarding local immune responses after bacterial challenge in skin mucus from the ocular and blind sides in one of the most valuable flatfish species in southern Europe.