A preliminary histological and ultrastructural study of opercular anomalies in gilthead sea bream larvae (Sparus aurata)

The high prevalence of opercular deformities present in fish from Mediterranean marine hatcheries is an important problem, which is causing considerable economic losses. The aetiology of this syndrome is not yet well understood. In this study a histological and ultrastructural description of gilthead sea bream (Sparus aurata) larvae affected with anomalies of the opercular complex was carried out. Samples of larvae were obtained from a marine hatchery located in north-eastern Italy. For each sample, 100 larvae (age range day 0 to 90) were fixed for histological and ultrastructural observations. The first opercular alterations were detected 17 days after hatching, when the opercular support in the branchio-cranial area is still cartilaginous and the only structure of the opercle is a connective lamina composed of a dense connective tissue. Deformation could affect one or both opercles. No degenerative alterations or inflammatory processes were found in the opercular tissues. The electron microscopy (TEM) observations on 30 day old larvae allowed the identification of mineral deposition abnormalities within the dermal ossification in curled opercles. The mineralization process appears to be very heterogeneous and irregular, in some cases being less abundant than normally detected. These preliminary observations suggest that an early alteration in the connective tissue formation and an abnormal mineralization of the fibrous bone can predispose opercular folding.