The Secret Lives of Sea Stars

Where is their mouth? How do they move? Why does the animal care team call them sea stars and not starfish? Learn a few basics and discover how different sea stars can be with aquarist Bethany H. #cleaquarium #natureiscurious

Today we’re going to talk about sea stars. The reason we call them sea stars and not starfish most of the time is because they’re not actually fish. They’re in a group called echinoderms, which means spiny skin. Other echinoderms include sea urchins and sea cucumbers, so it’s a big group of invertebrate animals.

We have several different species here at the Aquarium. We will start by looking at the Bahama sea stars that live in the Aquarium’s Invertebrate Pool.  They have a bottom surface where you can see their mouths are right there in the middle. They also have rows of tube feet coming out in every direction. Those tube feet end in little suction cups, so that’s how this particular sea star is sticking to the acrylic.  Sea stars also have a back surface with little spikes all over it.

The way sea stars move and breathe are really pretty interesting. Both of those work a lot on what is called a water vascular system. You might think of vascular blood vessels. While these guys do have a couple of blood vessels, most of their circulatory system is done with just sea water. That sea water can move their tube feet in and out.

Because we disturbed him, this sea star might decide that this is no longer where he wants to be and start moving very slowly across the acrylic. He’s going to do that by sticking out each tube foot and pulling himself along. Sea stars tend to look like they’re just gliding because their arms aren’t actually moving, just their tube feet. They’re really slow. Their average speed is about six inches a minute. The fastest sea star there is can move about nine feet a minute, which is still quite a bit slower than your walking speed.

Sea stars also use that water vascular system to breathe by sticking little papulae or little hair-like projections out of their backs. All those little white specks in between the big bumps are papulae that are all pulled in right now. Again, sea stars can use hydrostatic pressure inside to move those in and out. When those papulae are sticking out they create a lot of surface area for gas exchange with the water. Then that will circulate through the sea water inside their body to move it all throughout all of their tissues.

This central little dot or light area you can see here is called the madreporite. That’s a big word for something sea stars can open or close to determine how much water is inside of them.

The leather and the pink star in our Coastal Arch exhibit look a lot furrier with all of the papulae out. These are some cold water species. Sea stars do live in different climates all over the world. There are many different species. Most of them have five arms, although there are a few exceptions. 

All sea stars can move their arms independently a little bit, but how much they do so is very much dependent on the species. These are all mottled sea stars and as sea stars go, they are probably some of the most flexible.

Author: Bethany H.