By Jody Bourton
Earth News reporter
Cool starfish (purple indicates cool areas, red indicates warm)
One starfish has a remarkable strategy to avoid overheating in the sun, scientists have discovered.
The starfish pumps itself up with cold seawater to lower its body temperature when exposed to the sun at low tide.
It is equivalent to a person drinking seven litres of water before heading into the midday sun, scientists say.
However, global climate change may drastically interfere with this vital mechanism by increasing sea temperatures, the researchers warn.
The ochre starfish or sea star (Pisaster ochraceus) is found in the intertidal zone along the Pacific North American coast.
It mainly feeds on mussels while underwater. During low tide it is exposed to the air and cannot move until it is submerged again at high tide.
When exposed to the warm sun at low tide, the ochre starfish can suffer heat stress.
Now scientists based in California, US reveal how it manages this excess heat in the journal American Naturalist.
Pump up the volume
"We have discovered a quite novel thermoregulation strategy in the animal kingdom," says Dr Sylvain Pincebourde, formerly of the University of South Carolina, Columbia but now at the François Rabelais University in Tours, France.
"We found that the weight of the sea stars increased during the days after exposure to high temperature at low tide."
"The sea stars were not allowed to feed. So this increase can be explained only by an increase in seawater uptake. When sucking up water, the body mass of a sea star increases," he says.
The increase in body mass allows the starfish to be buffered against environmental temperature changes in the subsequent low tide.
"Because its body mass is now higher, body temperature increases more slowly," Dr Pincebourde explains.
"The thermoregulatory strategy we have revealed in our study indicates that the sea star is well adapted to such a variable environment."
Water cooled ochre starfish
Dr Pincebourde and colleagues at the University of California, Davis and Bodega Marine Laboratory, Bodega Bay, California, made their discovery by studying the starfish in an aquarium, and replicating tides and environmental conditions.
They used heat lamps to adjust the aerial temperature and an infrared camera to measure body temperature.
Finally, they weighed the star fish to estimate the amount of body fluids.
The scientists were especially surprised by the amount of water the starfish were able to store in a few days.
"It's as if we decided to suck up more than 15 pounds of cold water in the morning to prepare ourselves to the high temperature we will get at noon," Dr Pincebourde says.
They were also amazed to see the degree of control the starfish had.
"This quantity of stored water allows a decrease of almost 4°C in excess body temperature, which is enough to avoid reaching dangerous body temperatures," he explains.
Dr Pincebourde is concerned that this novel strategy may have limitations in a rapidly changing world.
"This discovery is quite important for us to formulate expectations on what could be the impact of climate change on the sea star," he says.
As oceans warm together with air temperature the thermoregulatory mechanism used by the starfish will cease to work, he warns.
"The colder the sea water, the more it is able to lower its body temperature. The efficiency of this thermoregulation strategy therefore might be annihilated by ocean warming."
"Sea stars will reach more frequently high aerial body temperatures, leading to higher mortality rates, for example."