Embryonic and larval developmental stages of African giant catfish Heterobranchus bidorsalis (Geoffroy Saint Hilaire, 1809) (Teleostei, Clariidae).

Abstract

The dearth of African giant catfish Heterobranchus bidorsalis seeds poses great threat to its aquaculture and biodiversity, hence detailed knowledge and understanding of its embryology is indispensable for its artificial propagation and conservation programmes. Photomicrographs of extruded oocyte through all developmental cell stages of live embryo to larval stage are documented with the aid of a light microscope. The optical transparency of the developing embryo enabled us to describe its deep structures, distinctive features and characterize the stages pictorially. Extruded oocyte had a mean diameter of 1 ± 0.1 mm, ~20% increase when hydrated, and bounded by double thin perivitelline membranes. The first mitotic cleavage occurred at 69 min post-fertilization (pf) resulting in 2, 4 (2 × 2 array of cells), 8 (2 × 4), 16 (4 × 4), 32 (4 × 8), 64 (2 × 4 × 8) blastomeres, then developed to morula, blastula and gastrula stages. Blastula was featured by formation of enveloping layer and yolk syncytial layer. Onset of epiboly at 3 h 57 min depicted the commencement of gastrula while closure of blastopore at 11 h 8 min marked its completion. Neurulation period was distinct from segmentation where organogenesis was fully active. Embryo sudden muscular contraction was noticed at ~17 h pf, increased prior to hatching with caudal locomotion firstly at 42 s interval. Heartbeat of embryo commenced at ~1 h before its unique eclosion at average of 72 beats/min while first larva emerged at 21 h at a controlled temperature of 28.5 ± 0.5°C. Mean total length (TL) of larvae and their pouch thickness were 5 ± 1 mm and 0.05 ± 0.02 mm respectively. 1 –day old larvae revealed 8 distinctive neuromeres and by day 3, epicanthus folds of the eyes were fully uncovered; and thereafter commenced exogenous feeding. At day 4, larvae recorded mean TL of 9 ± 1 mm and 15 caudal fin rays. The fin bifurcation to dorsal and adipose fins was observed at third and half weeks post-hatchability with the dorsal fin length to adipose fin was 1.7:1. This study, for the first time, presents significant morpho-sequential developmental stages of H. bidorsalis and registers its unique form of eclosion.