Northeastern University

A hard bargain for sea stars

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Photo courtesy of Brian Helmuth.

Here’s an image for you: A sea star eats by basically vomiting up its stomach, sticking it inside the shell of a tasty morsel, say a mussel, and hanging out all day like this until it’s fully digested the prey. That’s according to Brian Helmuth a professor of marine and environmental sciences who holds a joint appointment in the School of Public Policy and Urban Affairs. I spoke with him the other day about some ongoing research he’s been doing looking at the effects of climate change on marine species interactions.

As you might imagine, this whole out-of-body stomach eating experience is rather energetically expensive for the poor sucker. Once a sea star goes into eat-mode, it’s pretty committed. While the interaction begins under water, the tide slowly gives way leaving the sea star and its prey exposed to the air…and the heat.

Without any water around to cool it off, the sea star must now rely on its own physiology to keep from overheating. They do this by “moving water around in their body,” Helmuth told me. Sea stars’ cores, just like ours, contain the vital organs important for survival. Keeping them at optimum temperature is more important than the arms. “But if they suck water back from the arms,” Helmuth said, “they potentially lose an arm.”

You may remember from visiting the aquarium as a kid that sea stars can shed arms like it’s no big deal, regrowing them when the time is right. Historically, scientists have thought they only did this to escape predators, but from Helmuth’s work it looks as  though they also do it to maintain optimum body temperatures. The only problem, though, is that if a sea star’s core is its “everything,” it’s arms are everything else. They use them to move, to get food, to store resources, and, get this, the arms also contain the sea star’s sexy parts.

So, sacrificing arms isn’t really all that desirable an option. Of course, the alternative–death–isn’t any better.

It turns out that the very fact that sea stars are capable of regulating temperatures at different parts of their body is a big deal. It’s called “regional heterothermy” and while it has been observed in insects and a few other organisms, this is the first time anyone has seen it in a marine species.

“These simple organisms are more complex than we give them credit for,” said Helmuth. “Most of their nervous system is in their stomach, but they have these intricate ways of keeping themselves alive.” 

I asked him how this whole issue was going to be affected by climate change, since that, I gathered, was the original question. He said that it’ll be a slow process of change. We might sea higher rates of arm loss, decreases in growth, sterility. But the big blow may not come for a while. As with many species, he said, everything will appear to be okay until one day there’s a huge drop off in numbers. But, if we pay attention to the early warning signs, perhaps we won’t be so surprised by the devastation…and maybe we could even do something to prevent it.

Helmuth’s whole schtick is the idea that climate change is a long and variable process and the impacts it has on species will likewise be long and variable. “Local scale conditions are going to matter more than global scale to many organisms,” he said, noting that “local” could  be as minute as the rock you happen to be lounging on in addition to the beach you’re native to. With information like this sea star research, scientists can tell a little better which populations are likely to survive and which are not. “If it’s a habitat where the organisms can stay out of the sun, they’re more likely to survive,” he said. “It’s these nit picky details that make a huge impact.”


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