Skeletal deformities in gilthead seabream (Sparus aurata): exploring the association between mechanical loading and opercular deformation

Fish aquaculture is frequently confronted with skeletal abnormalities. In gilthead seabream (Sparus aurata (Linnaeus, 1758)), opercular deformities are one of the most common types of deformities. Many studies point at potential causal factors, mainly genetic or nutritional. However, no clear consensus has surfaced yet, and other factors known to affect bone formation remain unexplored, including mechanical stressors by external forces or muscle contraction. In this study, we investigated whether an altered mechanical use of the gill cover could be associated with opercular deformities, by inducing a change in the respiratory rate and thus gill ventilation. Juvenile seabreams were reared under 80, 100 or 200% dissolved oxygen (DO) to trigger altered respiration behaviour, and the effect on body and opercular shape was analysed. The main hypothesis was that hypoxic conditions would increase opercular ventilation, which would result in a higher prevalence of opercular deformities. The results show that the hypoxic condition (80% DO) did not trigger a significantly higher prevalence of opercular deformations, though the opposite is true for the hyperoxic condition (200% DO). No effect of oxygen treatment was observed on overall body shape, though deformed opercles showed a pronounced, but non-significant difference in shape across treatments. Morphometric results and µCT scans reveal that deformations mainly occur in the dorsocaudal region of the opercular bone. Although no causal link could be demonstrated, we discuss how these results can indirectly suggest that an altered mechanical loading on the operculum could explain its deformation.