If you’re a human, the rocky intertidal zone can be a great place to explore. There are plenty of interesting critters to find. But if you’re one of those creatures, life can be rather rough — in addition to putting up with humans that may pull you out of your home, there’s dangers ranging from predatory birds to punishing waves to dramatic temperature fluctuations that come from being submerged in water and exposed to air over and over. And many of the animals that live in this zone either don’t move or don’t move fast.
This is life for the purple sea star, Pisaster ochraceus, which is a common find on Pacific shores. The sea star (or starfish, if you must) is an ectotherm, meaning that it’s body temperature is determined by its environment, not the organism. But a study published last week in the Journal of Experimental Biology finds that this sea star does appear to have a little control about where that heat goes in its body. The key is hot arms.
A trio of researchers from the University of South Carolina in Columbia and the University of California, Davis started with 70 purple sea stars collected from the California coast. When they viewed the invertebrates with an infrared camera they discovered that the sea stars’ arms were warmer than the core. This was surprising — they had expected the arms to be cooler.
The scientists then subjected groups of sea stars to increasingly warm temperatures by placing them under heat lamps. The temperatures ranged from 26 degrees Celsius (comfortable) to 42 degrees (deadly). At lower temperatures, a sea star’s arms were a couple degrees warmer than its core. At middle temperatures, the arms were also warmer, but after a couple days in this heat, many sea stars would shed one or more arms. And at even higher temperatures, when sea stars’ core temperature got above 35 degrees, they died within 24 hours; their arms, though, were cooler than the core.
The arms are likely acting as heat sinks for the sea stars, the scientists say. That would imply that the invertebrates have the ability to transfer heat from the core to the arms. Though the animals don’t have a circulatory system like we do (that’s what allows humans to transfer heat through our bodies and do things such as conserving heat in our core when it’s cold), sea stars can move fluid within themselves.
“Under this scenario,” the scientists write, “directional movement of fluid within the body might facilitate the transfer of heat from right below the dorsal integument, where solar energy is collected, to the arms where heat can be released to the environment.”
That heat movement alone only works so much, though. If there’s too much heat for the arms to contain, the sea star sheds an arm in an attempt to keep its core below 35 degrees. That’s not a great choice — regrowing an arm is costly — but it’s better than death. But even that won’t work when it’s just too darn hot.
Image courtesy of flickr user Jerry McFarland