Ontogenetic development of the digestive system and growth in coral trout (Plectropomus leopardus)

Abstract

Histological studies were used to describe the ontogenetic development of the digestive tract and accessory glands in coral trout, Plectropomus leopardus, from the time of hatching to 54days after hatching (DAH). According to the structural changes in the digestive system, four significant stages were established: the endotrophic stage (Stage I), the endo-exotrophic stage (Stage II) and the exotrophic stage (subdivided into Stages III and IV). The first stage spanned from hatching to the onset of exotrophic feeding at 2DAH. During this stage, the digestive system started to differentiate into the buccopharynx, the oesophagus, the incipient stomach, the midgut, the hindgut and the rectum. During Stage II (2–3DAH), the coral trout larvae mouth opening reached a 0.24±0.05mm gape size, much smaller than most teleosts. Therefore, first-feeding coral trout must feed on small prey like protozoa or small size rotifers (Brachionus rotundiformis). Stage III (3–19DAH) started after the depletion of the yolk sac. During this stage, goblet cells, taste buds and pharyngeal teeth appeared. The pharyngeal teeth may be a sign that the larvae can feed on some live prey with a hard outer cover, such as copepods. Eosinophilic supranuclear vacuoles and lipid vacuoles were found in the hindgut and midgut, respectively, indicating the beginning of protein and lipid absorption. The last stage (after 19DAH) started when the gastric glands and pyloric caeca were formed. Gastric glands were only observed in the cardiac and fundic stomach and not in the pyloric region. The appearance of gastric glands and pyloric caeca could be regarded as the sign for coral trout to start weaning. The ultrastructure of the liver in the 54-DAH coral trout larvae proved that the liver was crucial for lipid storage and metabolism. This study suggested a relatively rapid development of the digestive system in coral trout and provided a theoretical basis for the nutritional requirements and digestive capacity of coral trout.