Ionizing radiation is known to induce genetic damage in diverse groups of 
organisms. Under accidental situations, large quantities of radioactive elements 
get released into the environment and radiation emitted from these radionuclides 
may adversely affect both the man and the non-human biota. The present study is 
aimed (a) to know the genotoxic effect of gamma radiation on aquatic fauna 
employing two species of selected bivalves, (b) to evaluate the possible use of 
'Comet assay' for detecting genetic damage in haemocytes of bivalves as a 
biomarker for environmental biomonitoring and also (c) to compare the relative 
sensitivity of two species of bivalves viz. Paphia malabarica and Meretrix casta 
to gamma radiation. The comet assays was optimized and validated using different 
concentrations (18, 32 and 56 mg/L) of ethyl methanesulfonate (EMS), a 
direct-acting reference genotoxic agent, to which the bivalves were exposed for 
various times (24, 48 and 72 h). Bivalves were irradiated (single acute 
exposure) with 5 different doses (viz. 2, 4, 6, 8 and 10 Gy) of gamma radiation 
and their genotoxic effects on the haemocytes were studied using the comet 
assay. Haemolymph was collected from the adductor muscle at 24, 48 and 72 h of 
both EMS-exposed and irradiated bivalves and comet assay was carried out using 
standard protocol. A significant increase in DNA damage was observed as 
indicated by an increase in % tail DNA damage at different concentrations of EMS 
and all the doses of gamma radiation as compared to controls in both bivalve 
species. This showed a dose-dependent increase of genetic damage induced in 
bivalves by EMS as well as gamma radiation. Further, the highest DNA damage was 
observed at 24h. The damage gradually decreased with time, i.e. was smaller at 
48 and 72 h than at 24h post irradiation in both species of bivalves. This may 
indicate repair of the damaged DNA and/or loss of heavily damaged cells as the 
post irradiation time advanced. The present study reveals that gamma radiation 
induces single strand breaks in DNA as measured by alkaline comet assay in 
bivalves and comet assay serves as a sensitive and rapid method to detect 
genotoxicity of gamma radiation. This study further indicates that both M. casta 
and P. malabarica exhibit almost identical sensitivity to gamma radiation as 
measured by DNA damage.