Abstract:Cobia (Rachycentron canadum) is an important marine fish with marked potential for aquaculture in China. Skeletal malformations are fundamental problems that severely decrease the production efficiency and profitability of cobia aquaculture by affecting external morphology, growth, and survival. In this study, we documented the skeletal deformities in tank-reared cobia juveniles (25 days after hatching; n=180). Based on observations of cleared and double-stained specimens, skeletal deformities have been described. Overall, 72 of 180 fish were deformed (with a deformity rate of 40.00%) and 22 types of skeletal malformations were identified: Meckel’s cartilage deformity, basihyal deformity, basihyal allotopia, lowered hyoid arch, lordosis, vertebral deformity, bifurcated neural spines, bifurcated haernal spines, haernal spine fusion, cartilage redundancy, epural deficiency, epural fusion, epural deformity, hypural deficiency, hypural and parhypural fusion, hypural deformity, bifurcated pterygiophore, pterygiophore deformity, pterygiophore redundancy, pterygiophore allotopia, anal fin ray redundancy, and dorsal fin spine deficiency. Malformations were most common in the cranium (17.22% of specimens) and caudal fin (15.56%), and the skeletal deformities with the highest incidence were Meckel’s cartilage deformity (12.22%) and epural deficiency (7.78%). All fish with these skeletal deformities showed no visible morphological changes. In addition, a significant difference in total length (TL) was found between normal fish and all deformed individuals (P<0.01), individuals with cranial deformities (P<0.01), and individuals with caudal fin malformations (P<0.01). The TL of individuals with dorsal and anal fin deformities was significantly different to that of individuals with cranial (P<0.05) and caudal fin malformations (P<0.05). These results provide insights into the skeletal developmental process for optimizing cobia rearing conditions.
