Perfectly adapted to the high and low temperature extremes of lives spent half in and half out of the Pacific Ocean, these little crustaceans might not survive if the highs get any higher.
"They're kind of living on the edge already," said ecological physiologist Jonathan Stillman.
The crabs, and many other California animals, are already feeling the heat -- and responding.
Some species, from the coastal crabs to chipmunks in the peaks of the Sierra, have already begun shifting their ranges northward or to higher elevations to compensate for warmer temperatures. But both man-made and natural barriers present sometimes insurmountable challenges and for some species, that strategy is destined to fail.
Climate change can be a powerful catalyst for evolution, as evidenced by the many species that have evolved over thousands and millions of years to fill very specific niches in California's myriad microclimates..
But some of these species may see their habitats disappear faster than they can evolve. If change at the current pace continues, it could lead to mass extinctions of animals and plants.
The evidence that animals are already being affected by climate change is piling up as scientists study species all over the state.
Stillman has a knack for spotting the rocks most likely to have scores of porcelain crabs hiding beneath them. Efficiency is key to gathering enough of the leggy brownish-grey critters before the evening tide comes in and immerses the bouldery intertidal crab haven..
On a trip to Fort Ross in September, Stillman brought an SUV full of graduate students with buckets to help him catch the 300 or so speedy, slippery crabs he needed to sustain his research for several months.
The crabs were headed for Stillman's laboratory at San Francisco State University's Romberg Tiburon Center, where they would have their ability to withstand heat put to the test. These little crustaceans, usually no more than two inches across, are ideal for Stillman's research because there are many species that thrive in different temperature ranges all along the Pacific Coast.
Porcelain crabs live in intertidal zones, and when the tide goes out on a hot, sunny day their body temperature can go up as much as 45 degrees in six hours. The crabs have evolved to tolerate extreme temperature swings, but this comes at the expense of being able to withstand temperatures outside of their normal range.
"If it gets a little hotter, they might not be able to respond physiologically," Stillman said.
In his lab, Stillman measures the crabs' heart beats with electrodes passed through holes made in their shells while they go through temperature trials.
To find their baseline capacity to handle heat, Stillman puts crabs into a tank set at a temperature near the low end of what they experience in their natural habitat. He then slowly turns up the heat to mimic an extremely hot day. The crabs' hearts beat faster and faster as the water gets hotter until at some point they start to fail and slow down.
"That's what we call the critical temperature," Stillman said. "It's sort of the point of no return."
The goal is to see how much these animals can adjust their point of no return to adapt to hotter conditions. After allowing another set of the same species of porcelain crabs to acclimate in a tank with an average temperature at the high end of temperatures in their natural habitat, Stillman puts them to the heat tolerance test again.
He has done this with more than 20 porcelain crab species, and it turns out some can adjust more than others.
Crabs that already live in the hottest habitats can only raise their upper threshold by about 1.6 degrees more when acclimated to the hotter temperatures than at the cooler end.
But crabs that normally live in cooler habitats can push their limit up by 4.5 degrees.
The porcelain crabs living on the coast between Monterey and Fort Ross are in the middle of the spectrum with a critical temperature around 89.5 to 91.5 degrees, and Stillman has measured 89.5 degrees in their habitat on occasion
"The intertidal species around here are definitely potentially experiencing some temperatures that are close to what their thermal limits are," Stillman said. "If it gets just a little hotter on the hottest days, or if those days start occurring more regularly or in longer stretches like we had this summer -- and both of those things are predicted as a consequence of global warming -- then these animals are really going to start feeling it," he said.
It took millions of years for the various porcelain crab species to evolve to live in their specific habitats, but evolution won't be able to keep up with the current rate of climate change.
"Global warming is happening way faster than just about anything is going to be able to respond on an evolutionary timescale," Stillman said. "A million years is pretty fast for evolution."
Stillman views his work with the crabs as a case study of how temperature affects animals that live in the intertidal zone in general, and many other species will likely share their fate.
The only remaining options for some species are to move or go extinct..
One bad day
Many animal species in the ocean and on land already are responding to climate change by moving to cooler habitats.
In Monterey Bay, for example, scientists have documented a change in the types of species that parallels a change in temperature, up an annual average of 1.35 degrees on the surface since the 1920s, and up 4 degrees during the peak summer months.
"That may not seem like a heck of a lot to people because our body temperatures can shift by that much," said physiologist George Somero of Stanford University's Hopkins Marine Station near Pacific Grove. "But these animals are living on the edge of their thermal tolerance, and it's a real challenge to some of them."
From 1931 to 1933, a graduate student at Hopkins Marine Station named Willis Hewatt identified and counted species in 1 yard square quadrants along a transect through the tidal zone in Monterey Bay. Sixty years later, a group of scientists led by James Barry of the Monterey Bay Aquarium Research Institute returned to Hewatt's transect. The study, published in the journal Science in 1995, showed the overall number of species and density of animals had not changed, but the makeup of the populations was different.
"There was a shift from species that were more cold adapted to more warm-adapted species. So species that typically would be found down towards Santa Barbara, Los Angeles were in greater abundance here. Those that you would expect to find around Oregon, but would also occur here, were reduced in abundance," Somero said..
"We're quite sure that these changes that have occurred in the distribution patterns of animals have quite a lot to do with the fact that some animals are now facing temperatures that they're just not evolved to face."
One species that has become significantly more scarce in Monterey Bay is the porcelain crab Stillman collected at Fort Ross. Other porcelain crab species also appear to have shifted north, and a few species seem to have moved deeper from intertidal zones to cooler subtidal zones.
Shifting north has its problems for some species, however. Low tides along the central California coast normally occur early and late in the day when the sun is less intense. But further north, such as in the Puget Sound area, low tide is closer to the middle of the day.
This is particularly bad for animals such as snails and mussels that move slowly and can't get out of the hot sun.
"It just takes one bad day, or maybe one bad hour, as far as overheating and dying are concerned," Somero said..
Without a safe northern escape route, these animals' ranges may shrink rather than shift.
Moving on up
Similar patterns are being documented among animals in California's mountains.
Biologists at the Museum of Vertebrate Zoology at UC Berkeley have been painstakingly counting animals in Yosemite, Lassen and other wilderness areas in California as part of an ambitious project to retrace the steps of the museum's first director, Joseph Grinnell, who set out to count and catalogue the state's wildlife more than 80 years ago.
His team took thousands of pages of field notes, snapped thousands of photos and collected thousands of animal specimens from more than 700 sites across the state; all of the specimens are still housed in the museum.
Grinnell's aim was to document what was living where in California in the early part of the century so that future scientists would be able to recognize the changes he was sure would occur. His foresight is paying off for UC Berkeley biologists John Perrine and Jim Patton who have been revisiting the spots that Grinnell surveyed to see if things have changed.
During four years of surveying animals in Yosemite National Park, Patton has seen some of the changes predicted as a response to warming. In particular, several high-elevation species appear to have retracted their ranges upward.
The alpine chipmunk, found only in California's high Sierra, was spotted by Grinnell at an elevation of 7,700 feet. Patton's team hasn't found the chipmunk lower than 9,700 feet.
"Since they can't go any higher than the tops of these mountains, if they keep retracting upward, eventually they're going to go extinct," Patton said. "Is that something that's of concern to people? I would hope so."
Belding's ground squirrels have also withdrawn their range upward by around 1,500 feet and the golden mantle ground squirrel has lost several hundred feet of elevation at the lower end of its range as well..
Patton found that the pika, a hamsterlike cousin of the rabbit, has withdrawn the lower limit of its range up 1,500 feet, a change seen in pika populations in mountain ranges throughout the west, resulting in local extinctions of some populations.
"These are animals that are apparently very sensitive to temperature increase, and a few degrees of temperature increase in the summer can cause death of individuals," Patton said.
At the same time, species typically found at lower elevations are appearing at much higher elevations than before. The pinion mouse, which didn't exist in Yosemite National Park in Grinnell's day, has expanded the upper limit of its range from outside of the park around 7,800 feet up into the park as high as 10,500 feet.
"I trapped the first one up on Mt. Lyell," Patton said. "When I saw it, I thought, 'What in the world is this animal doing up here? It's not even close to its habitat.' That was a real surprise."
It's possible that for any of these species, Grinnell and Patton happened to survey them at opposite ends of a natural fluctuation in population size. But as more and more species appear to follow the same pattern over a series of years, the likelihood of the shifts being natural diminishes.
Last year, Perrine began retracing Grinnell's trek through Lassen Volcanic National Park and early indications are that the pattern of habitat shifts will be repeated there.
"We are seeing a few little signals that appear consistent," Perrine said. For example, the California ground squirrel has expanded its range to higher elevations in both parks. But heavy winter snows made surveying at higher elevations difficult and Perrine hopes to revisit the area in 2008 to get a better picture.
Patton has already moved on to the White Mountains east of Mono Lake. And the team was recently granted $570,000 for four years from the National Science Foundation to survey more than 50 sites along the spine of California's mountains from Mt. Shasta, the Trinity Alps and the Warner Mountains in the north, through Lassen and Tahoe, to Yosemite and the Sequoia and Kings Canyon National Parks in the south.
"That will really tell us if some of the Yosemite patterns are statewide," Perrine said.
And if the high-altitude range retractions are pervasive, that will be another clue global warming is the likely cause.
"I don't know what else would explain that," Patton said.
Betsy Mason covers science and the national laboratories. Reach her at 925-847-2158 or firstname.lastname@example.org.