Lethal and sublethal effects of malathion on three life stages of the grass shrimp, Palaemonetes pugio

The effects of malathion exposure on three life stages of the grass shrimp ( Palaemonetes pugio) were evaluated. After 96-h exposures, malathion was most toxic to newly hatched larvae with an LC 50 of 9.06 μg l −1 followed by an LC 50 of 13.24 μg l −1 for 18-day-old larvae and an LC 50 of 38.19 μg l −1 for adult shrimp. In a separate bioassay, to simulate field conditions, newly hatched larvae were exposed to malathion at 6 h day −1 every 5 days at a salinity of 10‰ until metamorphosis to postlarvae. After four pulse dose exposures, mortality was highest in the two highest concentrations, of 15.0 and 30.0 μg l −1. The number of instars to postlarvae was significantly lower in the highest concentration compared to control. The findings indicate that malathion may not directly affect growth in a measurable way but may alter natural metamorphic rhythms at the highest concentrations. Whole body acetylcholinesterase (AChE) activity was measured on Day 0 and Day 15 from the pulse exposure test. AChE activities were not significantly different from controls. Other factors than just AChE inhibition may have contributed to malathion toxicity.