Sublethal Effects of Repeated Pesticide Exposure on Blue Crabs (Callinectes sapidus) in the Gulf of Mexico

Insecticides targeting terrestrial arthropod pests may enter aquatic habitats via overspray and runoff, affecting populations and behaviors of nontarget species. Blue crabs (Callinectes sapidus) play a significant role in estuarine food webs as both predators and prey, and alterations in their abundance or behavior can significantly affect community structure and function. Like insects, blue crabs are arthropods, and are susceptible to insecticides applied for pest control, which may increase their mortality. Although not as thoroughly investigated, sublethal effects of insecticide exposure can alter blue crab foraging and predator avoidance ability, affecting estuarine food webs. Repeated exposure to insecticides may increase negative effects, but few studies have assessed cumulative effects on nontarget organisms after multiple exposures. I examined how two consecutive exposures of 50 ppb of Malathion, an insecticide often used for mosquito abatement, affected blue crab behavior, and examined crab recovery following pesticide removal. Malathion can occur in concentrations as high as 800 ppb in coastal waters and increases blue crab mortality at 1 00 ppb. To focus on sublethal effects of Malathion, I used a concentration of 50 ppb because this concentration interferes with blue crab neuromuscular function but does not significantly increase mortality. I then assessed changes in neuromuscular function, foraging ability, and responses to predation risk cues in blue crabs exposed to Malathion in three static non-renewal treatments: control, repeated exposure, and recovery (following single exposure). Sublethal concentrations of Malathion altered the behavior of both adult and juvenile blue crabs. Malathion exposure led to a 40% decrease in the blue crabs' ability to right itself when placed on their backs, but, crab righting time behavior returned to levels observed in the control treatment after crabs were placed in malathion-free water for 96 hours. Malathion exposure also affected eyestalk reflexes, causing normal responses to decline by 50% iii in adults and 75% in juveniles, with partial recovery taking place in both life stages following pesticide removal. Malathion exposure affected foraging ability, causing blue crabs to seek food more frequently, even in the presence of alarm cues from injured blue crabs. Despite being more willing foragers, adult blue crabs were less able to locate food after pesticide exposure. Malathion, at environmentally-occurring concentrations, interfered with blue crabs' ability to forage and avoid predators. However, two exposures to Malathion at 50 ppb did not increase mortality nor further impair behavior, and crab behavior resumed to levels measured before exposure after 96 hours in seawater without insecticides.