Background. The stinging catfish, Heteropneustes fossilis (Bloch, 1794), has recently raised interest among fish farmers, ornamental fish keepers, and pathologists. Its natural populations are threatened due to habitat loss and high fishing pressure. A number of factors may influence the reproductive success of this. The aim of this study was to assess the effect of one of such factors—the water hardness—on the course of the embryogenesis, the structure of the egg shell, the general morphology, and the behaviour of the hatched larvae. Materials and methods. The fertilised eggs were incubated at a constant temperature of 23 ± 0.2°C in water of different hardness: 0ºGH (soft), 9ºGH (moderately soft), 18ºGH (moderately hard). Egg membranes of activated eggs were viewed under a scanning electron microscope. Also egg membranes strength and egg deformations were determined 1.5 h after fertilisation. Images of eggs and newly hatched larvae, recorded with the observation sets described above, were measured and analysed. Results. The study showed the eggs were surrounded by thin, translucent, ~5 µm thick membranes equipped with numerous outgrowths on the external surface and porous on the inner side. The following observations were made within 828 h° (degree-hour) at 23 ± 0.2°C: the highest volume of eggs (1.16 ± 0.092 mm3) was typical for eggs incubated in soft water (0°GH), while the lowest egg volume (0.99 ± 0.113 mm3) was recorded in eggs incubated in moderately soft water (9°GH); yolks spheres were greenish in colour, the embryonic disc and the embryo itself being reddish; the embryo performed diverse movements (quasi-peristaltic, cardiac muscle contractions); after few hours of hatching, the pigments developed resulting in colour appearance; club-like primordial barbels were formed 24 h after hatching; the larvae commenced feeding on day 3 post hatching. Conclusion. Water hardness influences embryonic and larval development and the effects may be diverse. Low water hardness is recommended for egg incubation. However, the situation changed when the egg membrane protection ceased to exist at hatching. Increasing water hardness a few hours prior to the expected hatching time prevents larval deformation.

stinging catfish, Heteropneustes fossilis, embryogenesis, eggs, embryonic motorics, embryonic morphometry, water hardness