Early development of the African catfish Clarias gariepinus (Burchell, 1822), focusing on the ontogeny of selected organs

Embryonic development of Clarias gariepinus was studied from oocyte activation to the end of endogenous feeding (164 h post-fertilization, 164 h-PF). The ontogeny of the eyes, the ear, the heart, the digestive tract and the notochord were described histologically: (i) eyes were not pigmented at hatching (40 h-PF) and the anlage of the retina was observed at 96 h-PF. At 164 h-PF, the pigmented epithelium was distinguished as a single layer of cubical cells and the outer nuclear layer was composed of cones; (ii) the inner ear was oval-shaped and two otoliths and a sensory epithelium were differentiated at hatching. Three semicircular canals with rudimentary cristae developed until 96 h-PF, leading to a fully developed auditory vesicle at 120 h-PF; (iii) at 40 h-PF, a beating heart appeared as a thin-walled tube and at 72 h-PF no blood elements were recorded in the two-chambered heart. The four compartments (bulbus arteriosus, ventricle, atrium, sinus venosus) and mature blood elements were found only at 144 h-PF; (iv) the hatchlings possessed an undifferentiated digestive system, and differentiation of a buccal cavity, oesophagus and intestine was only recorded 96 h-PF. The connection between the oesophagus and the intestine and the anus opened at 144 h-PF; (v) the notochord in newly hatched embryos was composed of vacuolated cells and a peripheral layer of notochordal epithelium inside a fibrous sheath. Subsequently, vacuolated cells proliferated and notochordal epithelium changed from cubical to squamous cells. Eye differentiation was concomitant with first feeding, thus suggesting a key role of visual senses in feeding behaviour.