Thermal stress: The role of body size and the giant major claw in survival and heat transfer of a fiddler crab (Leptuca pugilator)

Abstract

Mobile intertidal crabs are often exposed to the sun while displaying outside of moist and cooler breeding burrows, which imposes increased heat stress and water. Individuals of a field population in mid-summer of the fiddler crab Leptuca pugilator had body temperatures that declined with increasing body size. All crabs were cooler than the sediment surface. Laboratory experiments simulated a male leaving its burrow and standing on hot sand heated by the sun. Smaller crabs succumbed to thermal stress sooner than larger crabs. Incapacitation keyed to field sediment temperatures occurred usually in about 30 min. Water loss ranged 1–8% at incapacity Small crabs heated up as fast as large crabs. Males with a major claw gained body heat more than clawless males and succumbed sooner. The major cheliped loses water proportionally more slowly than the body, which is likely explained by the more impervious surface of the claw, suggesting the claw might be a heat sink. I hypothesized that heat may be accumulated in the relatively impermeable major cheliped and transferred to the body, increasing thermal stress in the body where crucial organs must function. Several experiments were done to attempt to demonstrate body part differences in heating or heat transfer between cheliped and body. Thermal images demonstrated a temperature excess in the manus, and a lower temperature of the dactyl, consistent with the impermeability of the manus and its larger surface area intercepting heat, and perhaps radiation from the dactyl. Immersion of major claws in warm and cold water demonstrated heat transfer between cheliped and body. If claws are painted black, they heat up faster than unpainted claws, but there was no evidence of the transfer of increased heat to the body when claws were darkened. Thus the potential for heat transfer between claw and body is probably quite limited, which is further suggested by the very narrow connection at the basal ischium segment of the cheliped.The results suggest that smaller male fiddler crabs may have reduced success in the upper intertidal partially because of their higher vulnerability to thermal stress. Smaller males may choose to avoid the hotter upper intertidal until they become larger. Experimental results show no evidence for the role of the major claw in alleviating thermal stress to the fiddler crab carapace.