A hard bargain for sea stars

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.”