METABOLIC ENZYME-ACTIVITIES IN LARVAE OF THE AFRICAN CATFISH, CLARIAS-GARIEPINUS - CHANGES IN RELATION TO AGE AND NUTRITION

The influence of ontogeny and nutrition on metabolic enzyme activities in larvae of the African catfish, Clarias gariepinus, was studied. Af ter start of exogenous feeding, the larvae were reared for 10 days und er three different nutritional conditions: Artemia nauplii, a dry star ter diet, and starvation. The live feed gave the best growth (96 mg wi thin 10 days) whereas the dry diet resulted in low growth (33 mg). Thi s growth difference was reflected in larval RNA and DNA concentrations , but not in the levels of soluble protein. Enzymes representing the f ollowing aspects of metabolism have been analysed: NADPH generation (G 6PDH, ME), glycolysis (PFK, PK), gluconeogenesis (FDPase), amino acid catabolism (GOT, GPT) and oxidative catabolism (CS). All enzymes were present from the start of exogenous feeding onwards, but their maximum specific activities displayed different developmental patterns. In ca tfish larvae fed on Artemia, G6PDH and ME activities steadily increase d with age and weight of the larvae. CS levels remained, after an imme diate enhancement upon onset of exogenous feeding, on a rather stable plateau. The amino acid-degrading enzymes GOT and GPT showed maximum l evels at days 3-5 of feeding or at a body weight of 10-20 mg, but decr eased thereafter. Activities of PFK, PK and FDPase showed low initial levels, and increased significantly with age and size. Based on the on togenetic patterns of metabolic enzymes, in C. gariepinus larvae an ea rly and a late developmental phase can be distinguished. During the ea rly phase, the glycolytic and gluconeo-genetic capacities are low, whe reas they are enforced during the later phase. The oxidative capacity is high both during the early and the late phase. The metabolic change s in catfish development coincide with other major ontogenetic events, e.g., alterations of muscle organization, gill morphology, respiratio n and stomach structure and function. Rearing catfish larvae on a dry diet instead of Artemia partly altered the developmental pattern descr ibed: The ontogenetic elevation of CS, PFK and FDPase was delayed and the early peak in GOT and GPT activities was not realized. Particularl y during the early developmental phase, the enzyme behaviour of the la rvae fed on dry food was similar to that of starved larvae.