Comparison between properties of dorsal and ventral skin mucus in Senegalese sole: response to an acute stress

Abstract

Intensive culture of Senegalese sole, Solea senegalensis, represents an option to diversify Mediterranean aquaculture. As a flatfish species, the fish body presents marked asymmetry between the dorsal or ocular side and the ventral or blind side. Taking into account that different surroundings come into contact with each side, in the present study we aimed to compare the properties of skin mucus from both sides, which is the most external barrier and protection for the fish. Skin mucus has recently been proposed as a novel minimally invasive target for the study of physiological responses. Here, a group of ten Senegalese sole were used to measure basal mucus parameters; then four additional groups of ten specimens each were used to study a hypoxia stress challenge (post-stress groups: 15 min-PS, 1 h-PS, 6 h-PS and 24 h-PS), consisting of a 3-minute air exposure. Physicochemical parameters (viscosity, osmolality, pH and main osmotic ions) were analysed in dorsal and ventral mucus. Thereby, mucus components related to stress responses such as cortisol, glucose, lactate, soluble protein and antioxidant power, were measured and compared between the mucus types and in response to hypoxia. Rheograms revealed similar non-Newtonian behaviour of dorsal and ventral mucus viscosities, which suddenly increased in the 15 min-PS group, thus providing fish with immediate insulating and mechanical protection. However, ventral side viscosity of the 24 h-PS group was lower than basal values and its functionality was compromised. Whereas chemical parameters did not differ between sides or in response to hypoxia, components that are soluble in mucus showed asymmetry with soluble protein and lactate being twofold higher in ventral mucus. The classical stress response of increased plasma cortisol accompanying glucose and lactate release was observed. In parallel, mucus glucose and lactate showed rapid but transitory exudation in the 15 min-PS group, while cortisol was detected at minimal levels in skin mucus, and therefore is not recommended as a mucus indicator for this species. Mucus-soluble protein also rose in the 15 min-PS group, which indicates greater exudation of defensive-related proteins. Finally, the antioxidant power of dorsal mucus increased and accumulated, without returning to basal levels. All these findings indicate that flatfish skin mucus properties depend on animal side, and further studies will be necessary to elucidate whether other mucus defensive parameters show this asymmetry and their relevance in these benthonic species.