``The data showed that, during the 60-year interval between the two animal surveys, annual mean water temperatures increased on average by about 1.3 F [0.7 C],`` says Somero. More significantly, he adds, peak summer temperatures in August rose nearly 4 F (2.2 C).

Although these temperature increases seem relatively small, Somero believes they may have been substantial enough to push some species over the edge of what he calls their thermal tolerance range.

``When thermal stress pushes body temperatures to values that are unnaturally high or low, biochemical structures and the physiological processes they support - such as the heart and nervous system - may be severely, and perhaps lethally, upset,`` Somero observes.

Climatologists predict that, if global warming continues at its current pace, the average temperature of the Earth could increase another a 6 F (3.3 C) in the next 50 years.

What effect will these rising temperatures have on marine organisms - especially on vulnerable intertidal creatures that frequently are exposed to the hot rays of the sun during low tide?

To find out, Somero, postdoctoral scholar Lars Tomanek and former graduate student Jonathan Stillman (now at Occidental College) decided to investigate thermal tolerance limits in two groups of common Pacific invertebrates - porcelain crabs (genus Petrolisthes) and snails (genus Tegula).

The researchers wanted to see if intertidal crabs and snails are more susceptible to heat than their subtidal cousins, which spend their entire lives under water.

Lessons from crabs

Somero and Stillman collected 20 species of porcelain crabs from intertidal and subtidal habitats in four Pacific regions: temperate coastal waters off California and Chile, and subtropical and tropical areas
'"/>

Contact: Mark Shwartz
mshwartz@stanford.edu
650-723-9296
Stanford University
14-Nov-2000