If the Earth’s climate continues to warm up as predicted, some crab species along the Pacific coast may face extinction. A study shows that small crabs who live just off the beach in warmer waters have little tolerance for rising temperatures.
Jonathon H. Stillman, a marine biologist at Stanford University, tested the heat tolerance of four species of crabs found in the shallow waters of the Pacific Ocean and reported his findings this week in the journal Science.
Two of the crab species were from the Oregon coast and lived in waters that range from 47 degrees to 59 degrees Fahrenheit (8.33 degrees to 15 degrees Celsius) in temperature. The other two came from the Gulf of California in Mexico and thrive in waters of 54 degrees to 86 degrees Fahrenheit (12.22 degrees to 30 degrees Celsius).
Animals used in the experiment are all in a genus called Porcelain crabs. They are small, 1 to 2 inches across, and generally live near the edge of the beach in what is called an intertidal habitat. The crabs are filter feeders, sucking suspended particles from the water, and are active only when the tide is high. When the tide goes out, they hide under rocks in shallow water.
Live specimens of the animals were placed in aquariums, which were held at a constant temperature just at the upper thermal range of their natural habitat. The crabs were then outfitted with sensors that measured their heartbeats.
Stillman said the temperatures were raised in the individual aquariums by about one-fifth of a degree a minute until the animals’ heartbeats stopped. This established the upper thermal tolerance limit for each of the species.
He said the Mexican crabs who lived in the hottest climate were the least able to adapt to an increase in temperatures above that which they normally encounter in their natural home. Temperatures in the habitat of one of the crabs often reaches 105.8 degrees Fahrenheit (41 Celsius), but the animals died at 106.2 (41.22 Celsius), less than a degree above normal.
One of the Oregon species was acclimated to a temperature of 65 (18.33 Celsius), but survived until the aquarium temperature reached 87 (30.56 Celsius).
"The results were a surprise," said Stillman. "You would expect the animals that live in the hottest habitat are the animals that would be better able to handle an increase in temperature. But it turns out they are the most susceptible."
The crabs who came from the Oregon coast adapted best to a rise in temperature and had the greatest capacity to increase their heat tolerance, he said.
Stillman said the experiments suggests that if the world climate warms over the next century by the 4 degrees to 6 degrees Fahrenheit suggested by some researchers, then the small crabs along the coast of the Gulf of California "will be in real trouble."
"Animals that live high up in the intertidal zone are very near their physiological thermal tolerance level," he said. "They may be severely impacted by the climate change, and that could have a serious effect on the food webs of which they are a part."
Stillman said other studies have already noted that at least one crab species along North America’s West Coast has disappeared from a traditional habitat. Studies also have shown that some other coastal species have moved farther north to escape the temperature rise in southern waters.