COMPARISON ON EMBRYONIC AND LARVAL DEVELOPMENT OF Pangasianodon  hypopthalamus (SAUVAGE, 1878) AND Pangasius nasutus (BLEEKER, 1863)

River catfish, Pangasianodon hypophthalamus is a popular fish in the aquaculture industry due to several reasons, high growth rate, omnivorous feeding habits, strong resistance to disease and tolerance to low dissolved oxygen level. Pangasianodon hypopthalamus has been successfully produced through artificial fertilization and cultured in Malaysia and several South East Asian countries. Nevertheless, its close relative, Pangasius nasutus production is almost nonexistent due to major constraint in seed production.  Low fertilization, hatching, fry survival and growth were observed for P. nasutus produced through induce breeding technique. Important information on the basic biological features of oocytes and eggs, and larval development of this species is still lacking and could limit aquaculture production. Thus, the present research was conducted to observe the embryonic stages and determine the morphological differences between P. nasutus  and P. hypopthalamus, which may contribute to the success of it's seed production.  

Ovulations were induced by two injections of commercial hormone, Ovaprim (Syndel Asia) 0.1 ml/body weight for first injection and 0.5 ml/body weight for second injection at a 14 h interval. Artificial fertilizations were carried out 8 h after second injection. The embryonic stages were observed under a microscope. Larval growths were measured weekly and survivals were counted on the fourth week for both species. Results showed percentage of fertilization for P. hypopthalamus and P. nasutus were 80.0 % and 40.0 %, respectively. Meanwhile, incubation period was relatively longer in P. nasutus (25 h) compared to P. hypopthalamus (21 h). This result showed that the embryonic development is faster in P. hypopthalamus than in P. nasutus. Larvae commence first feeding at 38 h after hatching (hAH) for P. hypopthalamus, which is earlier than P. nasutus (42 hAH), while yolk sac absorption was completed at 4 day after hatching (dAH) and 5 dAH, respectively.

Growth and larval survival were higher in P. hypopthalamus (total length, 2.01±0.58 mm; survival, 70.0 %) than P. nasutus (total length, 1.97±0.37mm; survival, 15.0 %). Species-specific and temperature are two main factors influencing embryonic development for both species. Cannibalism was observed in P. nasutus which affects the larval survival. In this case, it is recommended that first feeding should be provided sufficiently at the right time to prevent cannibalism. Even though these species are under the same family, they showed species-specific biological features. Thus, this finding can be used as guidelines to improved and managed eggs incubation and larval rearing of P. nasutus.