The sensitivity of grass shrimp, Palaemonetes pugio, embryos to organophosphate pesticide induced acetylcholinesterase inhibition.

Grass shrimp, Palaemonetes pugio, are common inhabitants of salt marshes along the Atlantic and Gulf coasts of North America. Grass shrimp embryos are brooded externally on the abdomen of adult females for about 2 weeks prior to hatching. In South Carolina, the spring spawning period for grass shrimp coincides with the period of peak pesticide application on crops grown along the South Carolina coast. Thus, grass shrimp of all developmental stages are at risk of exposure to pesticides present in nonpoint source agricultural runoff. Organophosphate (OP) insecticides are commonly applied agricultural chemicals which produce toxicity by inhibiting the nervous system enzyme, acetylcholinesterase (AChE). The purpose of this study was to examine the development of AChE activity in grass shrimp embryos and to assess their sensitivity to OP-induced AChE inhibition. Embryos were exposed for 24 h to either chlorpyrifos or malathion. All exposure concentrations were nominal and ranged from 0 to 2.00 µg l(-1) for chlorpyrifos and from 0 to 120.00 µg l(-1) for malathion. Quantifiable levels of AChE activity first appeared at Stage V of development and increased as embryonic development progressed. AChE inhibition by the OPs was assessed in Stage VI and Stage VII embryos. Both stages of embryos were more sensitive to chlorpyrifos than malathion. The 24-h Effective Concentration (EC(50)) values for chlorpyrifos were 0.49 µg l(-1) (95% C.I.=0.33-0.77 µg l(-1)) and 0.36 µg l(-1) (95% C.I.=0.33-0.38 µg l(-1)) for Stage VI and Stage VII embryos, respectively. In comparison, malathion 24-h EC(50) values were 55.53 µg l(-1) (95% C.I.=22.08-80.73 µg l(-1)) for Stage VI embryos and 29.93 µg l(-1) (95% C.I.=25.22-44.22 µg l(-1)) for Stage VII embryos. For both OPs, there were no significant differences in the EC(50) values calculated for Stage VI and Stage VII embryos; however, AChE inhibition was significantly (P</=0.05) greater in Stage VII embryos at the two highest exposure concentrations for each insecticide. A comparison of the results of these embryo tests with those found for adult and larval toxicity tests indicated that embryos were at least as sensitive to both the OPs as larval and adult grass shrimp. Embryo bioassays provide a number of important advantages over traditional laboratory toxicity tests including reduced laboratory space requirements, large numbers of embryos from a few ovigerous females, and small volumes of waste.