Effects of Arsenic and Cadmium on the Post Larvae of Giant Freshwater Prawn (Macrobrachium rosenbergii) a Preliminary Study

Abstract

Arsenic (As) and Cadmium (Cd) are two of the heavy metals that gained public and nationalattention of the Sri Lankan community. Globally, As and Cd are reported as two of the mosttoxic heavy metals that cause physiological and biochemical alterations and mortality inexposed organisms. Cadmium levels in the range of 0.001 to 0.138 mg l-1 have been reportedin both ground and surface water sources of Sri Lanka. Few studies in the country havedocumented levels of arsenic in natural water bodies. Furthermore, studies assessing thepotential toxicity of these two metals to species that are of importance to Sri Lanka, arescarce. The aim of this preliminary study was to investigate the effects of environmentallyrelevant levels of As and Cd on the commercially exploited fresh water prawnMacrobrachium rosenbergii, using its post larvae, which are initially released into reservoirsin Sri Lanka. Post larvae were collected from a culture facility in Pambala, Chilaw and weremaintained in glass beakers. They were acclimatized before use for 96 hr acute toxicity trials,where they were exposed to a series of concentrations of either As (from 0.001 to 0.08 mg l-1)and Cd (from 0.001 to 0.05 mg l-1). Constant experimental conditions were maintained inboth the control and treatment, each conducted in triplicate.

The results show that the heavy metals at the tested concentrations induced significantlyhigher levels of mortality (p<0.05). For instance, the lowest test level of As (0.001 mg l-1)induced a twofold higher level of mortality (50%) than the control (20 %), whilst that withthe lowest level of Cd was also similarly high (46.7 %). A strong and positive dose dependenttrend (R2 = 0.87; p<0.05) was, however, evident only for Cd. Comparing the toxicity of theheavy metals in terms of inducing mortality, Cd appears to be potentially more toxic than As.A level of 0.05 mg l-1 of Cd induced 99 % mortality in the post larvae, whilst a higher level ofAs (0.08 mg l-1) induced only around 56 % mortality. It is interesting that both heavy metalsdid not cause marked alterations in growth with no significant differences being evidentbetween the body sizes of the control and treated larvae after exposure. It should however benoted that there was a greater deviation in body sizes in treated larvae than in those of thecontrol, indicating that there may be differential sensitivity among the individual larvae. Theresults of this study are important since heavy metal pollution may threaten the incomegenerating capacity of Sri Lanka’s inland reservoir fishery.