Tolerance of early life stages of Tilapia (Cichlidae: Tilapiini) to metal stress

Abstract

Ecophysiological indices that characterise animals fitness directly or indirectly measure the components of protein turnover and its associated metabolic costs, Therefore, more likely protein turnover and associated metabolic costs may play a major role in underlying stress tolerance mechanisms. In this thesis a flow-through system was designed and developed to overcome some of the existing basic design flaws in such systems and used to determine responses of different species of' tilapiine fishes (Cichlidae: Tilapiini) under lethal and non-lethal stress using cadmium and copper.

A significant variation in tolerance capability between mouth brooding and substrate spawning tilapia yolk sac-fry to lethal cadmium and copper stress was observed. There was concordance between the relative tolerance capabilities of these two groups to the two metals suggesting a general response. Similarly, tolerance capabilities were in concordance with early life-history growth traits and ass associated metabolic costs measured under non-stressed (control) conditions suggesting individuals with higher growth rates and low maintenance metabolic costs are better capable of tolerating metal stress than the individuals with low growth rates and higher maintenance metabolic costs. Lower cadmium body burden levels were observed in the sac-fry of the more tolerant substrate-spawner T zillii than those of in the more sensitive mouth- brooder () niloticus. Variations in growth performances between mouthbrooders and substrate spawners were attributed to the difference in their developmental rates. Therefore, genetically based phenotypic variations for early life history traits translate into variations in stress tolerance.

Similarly O niloticus yolk sac-fry originating from small eggs were more tolerant to cadmium stress and had lower body burdens than larger conspecifics originating from large eggs. The early life history growth traits and associated metabolic costs measured under non-stress conditions were in concordance with the tolerance capabilities of the two size groups supporting the correlation between higher tolerance and low maintenance metabolic cost. The size of the yolk sac-fry was influenced by maternal age and size, and hence, by egg size. Therefore, translation of pre-determined phenotypic variations for early life history traits into variations in tolerance capabilities to metal lethal stress was supported. Starvation-induced reductions in metabolic rate of tilapia sac-fry carried a fitness advantage by reducing cadmium uptake under lethal stress. Therefore, post adapted physiological acclimation to one type of stress may carry a fitness advantage over metal stress. In all cases tolerance capability to metal stress was correlated to the metabolic status of yolk sac-fry.

Using the most sensitive mouth brooder O niloticus and most tolerant substrate spawner T zillii which demonstrated the largest difference in their lethal tolerance to cadmium and copper, the effects of non-lethal cadmium stress were investigated. Significant differences in stress tolerance between the two species was observed. The effects of cadmium on growth and associated metabolic costs were similar for both species suggesting a general response under non-lethal cadmium stress. I here was evidence that both species showed an increase in protein turnover, and hence, an increase in maintenance metabolic costs. It was found that cadmium did not affect the energy supply, but reduced protein growth which appears to be due to investment of more supplied energy on increased protein turnover, and hence, increased maintenance metabolic cost than deposition as growth energy. Therefore, the predicted fitness advantage for lethal cadmium stress was observed for non-lethal cadmium stress.

In conclusion, in tilapia yolk sac-fry there was a general response to lethal as well as non-lethal stress. The tolerance capability have been brought about either by genetically pre-determined or physiologically post-adapted variations in early life history traits of tilapia yolk sac-fry. The observed concordance between the range over which differences in responses occur in terms of more sensitive non-lethal stress indices and lethal tolerance capability of O. niloticus and T. zillii yolk sac-fry suggests there may be a possible link between responses to lethal and non-lethal cadmium stress.