Aspects of the general biology and bioaccumulation of metals in the freshwater crab, Potamonautes warreni

Rivers all over the world have supported the growth of human civilization since the first towns appeared some 7000 years ago. As a result of this growth and the diversification of activities, most of the world’s rivers have been negatively affected. Freshwater is probably the most important resource of mankind. Not only is it vital for terrestrial life but we consume water in such a range of activities that it can aptly be regarded as a ‘pillar of our civilization’. The development of water conservation recycling and management plans is therefore seen as vital. Cohen (1995) has recently highlighted the importance of freshwater to the future of mankind and in this context it is becoming increasingly important to view water as an economic resource in its own right. Being a third world country, South Africa is a developing country with a constant increase in population size. With the increase in the population, it is accompanied by increasing and expanding mining and industrial sectors. Various metals resulting from these sector’s activities are common pollutants in the rivers of South Africa. Although some metals are essential elements that are necessary for normal growth and metabolism in organisms, all metals can become toxic at elevated levels and therefore the monitoring of the freshwater systems is necessary in order to protect the environment and ultimately to protect humankind. Aquatic invertebrates appear to be excellent biological monitors of heavy metal pollution. They are more tolerant of metals than fish, they accumulate metals in relative proportion to the metal concentration in the water, and they concentrate the metal by some predictable, reproducible factor. However the life cycle of aquatic insects are short which limits the value in bioaccumulation studies. In contrast, crabs are relatively long lived and form an integral part of the food chain, as they are vital components in the diet of several animal species including humans. In the present study an integrated approach was employed in order to investigate the metal pollution in the Klip River, Gauteng. Selected physico-chemical characteristics of the water and sediment were analysed, as were the levels of metals bioaccumulated by the freshwater crab, Potamonautes warreni. Different biological aspects were investigated to identify the natural course of the life cycle, including moulting and reproductive cycles, relative abundance and age structure, as well as several behavioural patterns of this crab species. Water and sediment samples were collected from the Klip River and Sedaven Dam (reference locality) every month from August 1997 up to August 1998. Water quality data revealed that although the quality of the water at all sampling localities were relatively high, Lenasia (locality 1) and Dirk Fourie Stadium (locality 4) appeared to be more impacted than the rest of the sampling localities including the reference locality. Results of the metal analysis in the water samples indicated that levels of zinc, iron, manganese and chromium were below the suggested South African guideline values (laid down for the protection of aquatic life) and that elevated levels of cadmium, lead and aluminium were recorded at localities 1,3 and 4. Results of the metal analysis in the sediment samples indicated that there were elevated levels of zinc, lead, iron, manganese, chromium and aluminium at localities 1 and 2. Potamonautes warreni individuals were sampled every month to coincide with the water and sediment samples and the metal concentrations were determined. The results of this investigation showed that elevated levels of zinc, cadmium, lead, iron, manganese, chromium and aluminium was recorded at locality 1 and this indicated that this locality is subjected to above standard discharge. At locality 3 elevated levels of zinc, cadmium and chromium was recorded and at localities 4 and 5 elevated levels of iron and aluminium was recorded. The results of the present study indicates that metal levels in P. warreni are indicative of the environmental levels of metals to which these crabs are exposed. Higher metal levels as compared to the habitat were found in the organisms at all localities. Locality 1 had a higher environmental level than the rest of the localities. Therefore, these organisms are useful bioaccumulative indicators and should be incorporated into biomonitoring programs in conjunction with fish. It must be stressed that further studies be done on the mechanisms of regulation and elimination of certain metals from the body of P. warreni and on sublethal effects of metals on the physiological processes such as reproduction and growth, especially in juvenile crabs.Prof. G.J. Stey