The Potential Use of the South African River Crab, Potamonautes warreni, as a Bioindicator Species for Heavy Metal Contamination

Abstract

In 1995, preliminary water and sediment analyses of the river bed and burrow sediments from 9 locations along the Mooi River, NW Province, South Africa had shown cadmium concentrations up to 0.009 mg l⁻¹±0.003 and up to 0.33 and 0.89 weight % with scanning electron microscopy (SEM) and x-ray microanalysis. Samples of the adult river crab (Potamonautes warreni) were collected from the Mooi River at Noordbrug (26°40′S/27°05′E), 1 km north of Potchefstroom Town, and exposed to 0.2 or 2.0 mg Cd²⁺ l⁻¹ in situ to determine tolerance, uptake and bioaccumulation of cadmium. Using flame atomic absorption spectroscopy (FAAS) the gills, haemolymph and digestive gland of naturally exposed P. warreni showed wet mass values of 0.74±0.27 μg Cd²⁺ g⁻¹, 0.007±0.007 μg ml⁻¹ and 0.12±0.09 μg g⁻¹ respectively. The tolerance of crabs to aqueous Cd reached its limit (ET₅₀=42 hours) at 2.0 mg l⁻¹ aqueous Cd exposure. At an exposure to 0.2 mg Cd²⁺ l⁻¹ for 21 days, the greatest Cd (n=11; 9.99±5.09 μg g⁻¹ wet mass) and Cu concentrations (n=11; 17.90±4.66 μg g⁻¹ wet mass) were associated with the gills, and to a lesser extent the digestive gland (n=11; 0.38±0.20 μg g⁻¹ wet mass), whereas concentrations of Zn were variable in both organs. In the haemolymph Cd levels were relatively small (n=11; 0.012–0.006 μg ml⁻¹) with exposure and time and Cu, Zn concentrations varied. Changes in the uptake of Cd in P. warreni indicated that transport, storage and possibly regulatory mechanisms are likely to operate in adult crabs. The potential of P. warreni as a bioindicator species of pollution is also discussed.