How sea stars stay cool to beat the heat

A new study has shown that sea stars a.k.a starfish absorb cold seawater into their bodies to endure rapid changes in temperature during high and low tides.

The study published in The American Naturalist showed that sea stars soak up cold water into their bodies during high tide as buffer against potentially damaging temperatures brought about by direct sunlight at low tide.

"Sea stars were assumed to be at the mercy of the sun during low tide," said the study's lead author, Sylvain Pincebourde of François Rabelais University in Tours, France.

 "This work shows that some sea stars have an unexpected back-up strategy," Pincebourde added.

During high tide, these sea stars are fully submerged in cool seawater and when tides receded, they are often left on rocky shorelines, baking in the sun. The scientists suspected that there must be some way of beating the heat.

For the study, researchers placed sea stars in aquariums and varied the water level to simulate tidal patterns. Heat lamps were used to control temperature, with some stars experiencing hotter temperatures than others.

They found that stars exposed to higher temperatures at low tide had higher body mass after the high tide that followed. Since the stars were not allowed to eat, the increased mass must be from soaking up water.

"This reservoir of cool water keeps the sea star from overheating when the tide recedes again the next day, a process called 'thermal inertia,'" Pincebourde said.

What appears to be happening, the researchers say, is that a hot low tide serves as a cue telling the star to soak up more water during the next high tide. And the amount of water the stars can hold is remarkable.

"It would be as if humans were able to look at a weather forecast, decide it was going to be hot tomorrow, and then in preparation suck up 15 or more pounds of water into our bodies," said co-author Brian Helmuth of the University of South Carolina in Columbia.

However, researchers are concerned that rapid climate change may put this novel cooling strategy in danger.

"This strategy only works when the sea water is colder than the air," said co-author Eric Sanford of the University if California, Davis.

"Ocean warming might therefore break down this buffering mechanism, making this sea star susceptible to global warming. There are likely limits to how much this mechanism can buffer this animal against global change."