A paper in last week's Science1 was about the temperature tolerance of the polychete worm Paralvinella sulfincola that lives near deep-sea hydrothermal vents. The study demonstrated that when presented with a range of temperatures from 20º to 61ºC in their aquaria, P. sulfincola moved to regions between 40º to 50ºC. The temperature they prefer to be at is only about 10ºC below 55ºC at which temperature they begin to suffer physiological stress. They die at 60ºC within minutes.
The authors also mention an ostracod and an ant that can survive exposure to temperatures close to 50ºC.
Some aestivating land snails can also survive body temperatures near 50ºC. This was demonstrated in a classic study by Schmidt-Nielsen et al.2 with the snail Sphincterochila zonata (=S. boissieri) that lives in the Negev desert in Israel. These snails aestivate on the surface fully exposed to the sun. Lab experiments demonstrated that 100% of 20 dormant snails survived at 50ºC for up to 8 hours, 30% survived at 55ºC for 2 hours, but none survived at 60ºC for 30 minutes. The diagram below from the paper shows the temperature distribution in and around a snail's shell exposed to the sun on the desert surface.
The body temperature of one dormant snail in the desert was measured to be 50.3ºC. The snails avoid reaching the lethal temperature of about 55ºC by withdrawing deep within their shells. This creates an insulating air space between the snail's body and the very hot surface below the shell. The snails are able to withdraw deep into their shells, because they have shells larger than their bodies as I discussed before here, here and here.
The Science paper mentions that the upper temperature limit for active animals appears to be about 55ºC.
1. Peter R. Girguis and Raymond W. Lee. 2006. Thermal Preference and Tolerance of Alvinellids. Science 312:231.
2. Schmidt-Nielsen et al. 1971. Desert snails: problems of heat, water and food.
J. Exp. Biol. 55:385-398.