Ontogenetic changes of body density of larvae and juveniles in seven-band grouper Epinephelus septemfasciatus and kelp grouper Epinephelus bruneus

Abstract

Sinking death syndrome has been considered as one of the causes of early larval mortality during the seed production of seven-band grouper and kelp grouper. To elucidate the rearing period when larvae show strong minus buoyancy, ontogenetic change of body density was examined for larvae and juveniles of two species. Larvae tended to show nearly neutral buoyancy from hatching to growing to 3.5-3.9 mm in total length (TL) when the buds of the second dorsal fin and pelvic-fin spines appeared. Then, body densities greatly increased with increasing the length of fin spines and reached a plateau at 1.060-1.070 g/cm3 after the size of around 11 mm TL. Swim bladder inflation was not observed in the seven-band grouper; however, it was observed in the kelp grouper after 8.2 mm TL and the percentage of kelp grouper with inflated swim bladders largely increased after 18.6 mm TL. Body densities of kelp grouper with inflated swim bladders decreased to 1.024-1.035 g/cm3. It could be speculated that fin spines regulate the larval buoyancy by generating drag forces. It is therefore considered that larvae show strong minus buoyancy in rearing seawater at the initiation of great increase in body density when fin spines begin to elongate.