Ontogenetic development of the digestive tract and ultrastructure of the anterior intestinal epithelia in tiger grouper Epinephelus fuscoguttatus (Forsskål, 1775) larvae

Abstract

The ontogeny of the digestive tract and ultrastructure of the anterior intestine in Epinephelus fuscoguttatus (Forsskål, 1775) larvae were examined using light microscopy and transmission electron microscopy from hatching to 42 d after hatching (DAH). The first developmental stage started at hatching when the digestive tract was a simple tube. The second stage (2-3 DAH), the endo-exotrophic stage, was the time when the mouth of the tiger grouper larvae developed. The third stage (3-24 DAH) started after the depletion of the yolk-sac (3 DAH). The remarkable changes included the appearance of gastric glands at 9 DAH, eosinophilic supranuclear vacuoles appearing in the posterior intestine at 5 DAH and lipid vacuoles found in the interior intestine at 6 DAH which indicated the beginning of protein and lipid absorption. The last stage (after 24 DAH) started when the gastric glands and pyloric caeca were fully developed. The formation of the gastric glands and pyloric caeca indicated a suitable time for weaning. The ultrastructural features of epithelium cells of the anteria intestine showed large lipid droplets at the beginning of the exotrophic stage. From this time onwards, the lipid droplets became smaller, while the endoplasmic reticulum and Golgi complex were well-developed. Upon metamorphosis, the tiger grouper larvae had eosinophilic granule cells (EGCs) in the intestinal epithelia. This substantially increased immunity capability at this time. This study showed that during a critical period of larval survival, the ontogeny coincided with exogenous feeding, while the ultrastructure showed lipid metabolism, hence highlighting the importance of fatty acid in the development of the larvae.