Differentiation of the Swimbladder Structure in Larvae of the Red Seabream Pagrus major

Abstract

Differentiation of the swimbladder was not observed in larvae 24 hours after hatching (TL 2.88-3.06mm).It was first observed as a germinal swimbladder already protruding upward from the esophageal dorsal wall 48 hours after hatching (TL 2.93 ti 3.30mm).By 60 hours (TL 3.10-3.40mm) the swimbladder had a hollow portion inside and its epithelium was thin (6-8μm) on the dorsal side and thick (10-13μm) on the ventral side.By 66 hours (TL 3.09-3.50mm) the swimbladder caviy became larger and longer, and surrounding mesenchymal tissues became evident.In 3 days, the swimbladder cavity became markedly dilated assuming an oval shape with its major axis coinciding with the body axis.In 5 to 6 days (TL 3.30-3.67mm) gas glands and rete mirabile were formed.In 8 to 9 days (TL 3.60-4.41mm) blood corpuscles were seen for the first time, in the rete mirabile, and larvae (TL 4.01-4.18mm) bearing gas in their swimbladder were observed.The pneumatic duct was present between the posterior end of the swimblad-der and the esophageal dorsal wall.In 10 to 12 days (TL 3.85-4.72mm) larvae with a closed or separated esophageal opening of the pneumatic duct and various stages of regression of the duct were seen.In 19 to 22 days (TL 7.8-10.0mm) the duct had already become rudimentary, attaching to the posterior end of the swimbladder.The gas glands formed complicated folds, occupying the anterior half of the swimbladder.