The pesticide malathion is a cholinesterase inhibitor in arthropods that was used extensively the 1980’s to control mosquitoes and the West Nile virus. Since first being observed in the USA in New Jersey in 1988, the Asian shore crab Hemigrapsus sanguineus has spread north to Maine and south to North Carolina. In many estuaries, the Asian shore crab has displaced native, panopeid mud crabs. To determine if malathion affects Asian shore crabs and native mud crabs similarly we examined lethal and sublethal effects of the pesticide on these crabs. Crabs were collected from Long Island (NY, USA) in 2014 and 2016. Sub-lethal effects were examined in a series of behavioral assays. Individual crabs were exposed to low, medium and high concentrations of malathion (0, 32-64, and 100-250ppb, respectively). After 48 hours of exposure to malathion, crabs were flipped on their backs and their righting time used to evaluate the pesticide’s impact. Lethal effects were evaluated in an LC50 experiment. Asian shore crabs and panopeid mud crabs (five crabs replicate-1) were exposed to various concentrations of malathion (0 – 10,000 ppb) for 4 days. Nonlinear regression of the log-transformed malathion concentration against toxicity was used to determine an LC50 for each crab.
In righting time assays, although the Asian shore crab did not seem to be affected by medium and high concentrations of malathion, the native mud crab (Eurypanopeus depressus) had significantly longer righting times at medium and high concentrations. The LC50 for Asian shore crab and panopeids were 1935 ppb and 707 ppb, respectively. Our results indicate that the Asian shore crab can tolerate higher concentrations of the pesticide malathion. This tolerance may have facilited the Asian shore crab’s invasion of Eastern USA estuaries. The competitive displacement of native species by invasives should be considered in the context of local environmental pollution, particularly in highly polluted estuaries.