When Temperatures Rise, Sea Stars Start Dropping Arms

Ochre sea stars, Pisaster ochraceus. (Photo by D. Gordon E. Robertson via Wikimedia Commons)

When temperatures heat up, we strip off our sweaters. But sea stars remove their arms. The finding, published in the Journal of Experimental Biology, provides some important insights into how Pacific coastal ecosystems might be affected by climate change.

Researchers from the University of California, Davis and the Institute of Research on Insect Biology in Tours, France used heat lamps to mimic the conditions that sea stars face at low tide, when they are exposed to more direct sunlight. Sea stars are ectotherms (cold-blooded); their body temperature depends on warmth from their environment. But that does not mean that their entire body is the same temperature. As the animals warmed up, the heat was not distributed evenly throughout their bodies. And when the sea stars reached a certain temperature—88 to 95 degrees Fahrenheit in the body and a toastier 91 to 102 degrees Fahrenheit in the arms—they responded by shucking their limbs.

Inherently warmer appendages may be linked to how sea stars keep their core body temperatures in check, because the larger surface area of the arms likely releases more heat from their bodies. But the ability to cleave their arms when the mercury rises is not without cost. The arms store some important goods including gonads and little sacs called pyloric caeca that digest food. So losing an arm, even though it will eventually re-grow, means also taking a hit to how well the animals can reproduce and produce energy.

The sea star that the scientists examined, Pisaster ochraceus, is a keystone species along the Pacific coast, meaning that it plays an important role in the entire ecosystems’ future—and perhaps more so as temperatures continue to rise due to climate change. “They influence what the populations of mussels and other animal communities look like,” one of the authors, University of California’s Eric Sanford, told NewScientist. How sea stars respond to warm temperatures affects all the species they are connected within the food web, which could have serious implications in a hotter climate.