The thermal tolerance limit of each species was determined by raising the water temperature in an experimental chamber by 1.8 F (1 C) every 15 minutes, then examining the number of survivors at each temperature interval.

``The rate of 1 C per 15 minutes reflects the temperature increase that porcelain crabs experience during extremely hot low tide periods,`` Somero explains.

The results, recently published in Physiological and Biochemical Zoology, showed that porcelain crab species living at the surface appear more vulnerable to global warming than those that are always submerged in subtidal habitats.

For example, intertidal species from tropical waters off Mexico and Panama succumbed when the thermometer reached 107 F (41 C) - only about 1.8 F (1 C) higher than the maximum temperature they currently experience in the wild.

Similar results were found among intertidal crabs from cooler waters off California and Chile. These animals could tolerate temperatures between 90 to 95 F (32 to 35 C) - only slightly higher than their maximum habitat temperature of 88 F (31 C).

One intertidal crab species included in the study, Petrolisthes cinctipes, was a common inhabitant of Monterey Bay 60 years ago, according to the ocean survey conducted at Hopkins in the 1930s.

``But P. cinctipes showed a significant decline in the 1993 re-survey, a finding that is consistent with our physiological data,`` observes Somero.

Overall, he says, species from intertidal locations around the Pacific already are living at the edge of their thermal limits and might not be able to survive even slight temperature increases.

In contrast, subtidal species from all habitats turned out to have thermal tolerance limits that, while lower than intertidal species, were much higher than the maximum water temperatures they encounter in nature.

Lessons from snails

In another study published in Physiological and Biochemical Zoology, Somero an
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Contact: Mark Shwartz
mshwartz@stanford.edu
650-723-9296
Stanford University
14-Nov-2000