Current ecotoxicological studies of the effects of harmful contaminants typically focus on trophic transfer and the associated magnitude of biomagnification (or biodilution), with little focus on organismal-level effects. However, sublethal toxicological exposures have important implications for both the health of exposed individuals and their roles within the ecological community, suggesting that individual-level endpoints are also worth attention. This study investigated the potential for methylmercury(MeHg)-mediated impairment in anti-predator behaviors, specifically the tail-flip startle response and claw pinch strength of the invasive rusty crayfish, Orconectes rusticus. Rusty crayfish are both a benthic indicator species and an important ecological player in impacted watersheds.
Crayfish were collected from the White River, VT and maintained in the laboratory. Animals were randomly assigned to three exposure treatment groups and a control group (N=20 animals/group), stratified on captivity time, source location, sex, and size. Baseline pre-exposure measurements for startle response and claw pinch strength were collected for all individuals. Control animals were fed a formulated crayfish diet, while treatments replaced 20%, 40% and 60% of the diet with MeHg-containing tuna for 16 weeks. Behavioral recordings were made every two weeks throughout the study.
Baseline tail flip escape performance in O. rusticus, measured as the maximum velocity achieved in multiple trials, was consistent with velocities reported in the literature for Procambarus clarkii. In the control group, measurements were variable across individuals but consistent through time for the same individuals, with means of 92.1 ± 25.1 cm/s, 94.3 ± 26.2 cm/s, and 95.5 ± 22.1 cm/s at the baseline, two-week and four-week time points, respectively. Pinch strength scaled with body mass (R2=0.43) and was variable across individuals with a mean of 3.1 ± 2.3 N, including regenerating claws.
Preliminary results after four weeks of exposure suggest little impact of dietary MeHg on either claw pinch strength or tail flip performance (P > 0.2 for the effect of time x treatment interactions in repeated-measures ANOVA); the experiment is ongoing and could reveal cumulative impacts as the experiment proceeds. Impairment in anti-predator behavior may lead to increased predation on impaired individuals, subsequently propagating the movement of MeHg to higher trophic levels. Alternatively, continued consistent performance despite MeHg exposure may indicate a high tolerance of crayfish to low-concentration exposures, although this result is not expected through the duration of the experiment.