Rising surface temperatures lead to more frequent and longer burrow retreats in males of the fiddler crab, Minuca pugnax.

The fiddler crab Minuca pugnax occupies thermally unstable mudflat habitats along the eastern United States coastline, where it uses behavioral thermoregulation, including burrow retreats, to manage body temperature (Tb). We explored the relationship between frequency of burrow use and environmental conditions, including burrow and surface temperatures, relative tidal height, and time of day, by twenty male M. pugnax in breeding areas around Flax Pond, New York, USA. We found a highly significant positive correlation between burrow use and surface temperature, with a clear shift to longer times underground above 32 °C degrees. We also experimentally heated live crabs in the laboratory and allowed them to retreat into cooled artificial burrows while continuously measuring Tb. Laboratory data on cooling times were compared to field observations of burrow retreat durations. The median burrow duration in the field of 2.74 min was enough time for our laboratory crabs to capture over 70% of the cooling potential of artificial burrows 10 or 15 °C below Tb. Because crab bodies in burrows experience exponential declines in Tb due to Newton's law of cooling, there are diminishing returns to remaining in a burrow, and many crabs probably leave before coming to equilibrium. For M. pugnax, burrow retreats reduce time spent feeding and courting, activities that only occur on the surface. Current concerns about the impacts of climate change on animals include whether compensatory mechanisms, like more frequent and longer burrow retreats, will come at the cost of other behaviors necessary for survival and reproduction.

Field-based body temperatures reveal behavioral thermoregulation strategies of the Atlantic marsh fiddler crab Minuca pugnax.

Behavioral thermoregulation is an important defense against the negative impacts of climate change for ectotherms. In this study we examined the use of burrows by a common intertidal crab, Minuca pugnax, to control body temperature. To understand how body temperatures respond to changes in the surface temperature and explore how efficiently crabs exploit the cooling potential of burrows to thermoregulate, we measured body, surface, and burrow temperatures during low tide on Sapelo Island, GA in March, May, August, and September of 2019. We found that an increase in 1°C in the surface temperature led to a 0.70-0.71°C increase in body temperature for females and an increase in 0.75-0.77°C in body temperature for males. Body temperatures of small females were 0.3°C warmer than large females for the same surface temperature. Female crabs used burrows more efficiently for thermoregulation compared to the males. Specifically, an increase of 1°C in the cooling capacity (the difference between the burrow temperature and the surface temperature) led to an increase of 0.42-0.50°C for females and 0.34-0.35°C for males in the thermoregulation capacity (the difference between body temperature and surface temperature). The body temperature that crabs began to use burrows to thermoregulate was estimated to be around 24°C, which is far below the critical body temperatures that could lead to death. Many crabs experience body temperatures of 24°C early in the reproductive season, several months before the hottest days of the year. Because the use of burrows involves fitness trade-offs, these results suggest that warming temperatures could begin to impact crabs far earlier in the year than expected.

Exposure to a nicotinoid pesticide reduces defensive behaviors in a non-target organism, the rusty crayfish Orconectes rusticus.

Imidacloprid is the most widely used of the nicotinoid insecticides, the fastest growing class of pesticides on the global market. Although less toxic to mammals and birds compared to organophosphates, nicotinoids have the potential to impact non-target invertebrates, especially through sublehal effects on behavior, physiology, reproduction, and development. We investigated the impact of sublethal doses of imidacloprid on the defensive responses of rusty crayfish Orconectes rusticus exposed to 0, 1, 10, and 100 µg•L-1 of imidacloprid for 10 days (n = 7 crayfish per treatment). Defensive behaviors were examined with the rod test, in which a glass rod was jabbed into the crayfish's container at a 90 degree angle from the bottom and about 0.5 cm directly in front of the crayfish. Crayfish responded to the rod aggressively with claw raising and pinching, neutrally (no response), or by backing or tail-flipping away. The frequency of neutral responses more than doubled after four days in the high (100 µg•L-1) group and after eight days in the low (1 µg•L-1) exposure group. Furthermore, most crayfish in the 100 µg•L-1 treatment were not able to right themselves within 30 s when placed on their backs. Several studies have reported concentrations of imidacloprid contamination in freshwater ecosystems that exceed this study's lowest exposure scenario, 1 µg•L-1. We therefore conclude that imidacloprid contamination reduces the defensive behaviors of crayfish, impairing their ability to survive in habitats where they play important ecological roles.