Meet the Tank: Sea Stars

By Meghan West
Gallery Educator
Science Museum of Virginia
 
In our “Beach Science: It’s a Shore Thing” exhibit we have a saltwater tank housing some sea creatures that can be found off the coast of Virginia. For those of you who have been to “Beach Science: It’s a Shore Thing” you may have already seen or even touched one of our sea stars. For those of you who have not, let me introduce you.
 
Sea stars, formally known as starfish, were renamed because they don’t look like a fish, don’t swim like a fish, and are not a fish. Because of this they dropped the name fish and added sea (same thing happened to sea jellies, formerly known as jellyfish). They are in the phylum echinodermata, which means spiny skin and anyone who has touched one or even handled a dead one can feel the bumpy, spiny skin. They are in the same phylum as sea urchins and sand dollars, even though they don’t look a lot alike.

Strange creatures are our sea stars; they have no blood, no brains, and if we chop them up, as long as there is a fifth left, they will grow everything back. As for the no brains thing, anyone who has seen “SpongeBob SquarePants” can attest that Patrick Star, SpongeBob’s best friend, is not the sharpest knife in the drawer. Comedy is not the only reason Patrick is a little slow on the uptake. The creator of SpongeBob, Stephen Hillenburg, taught marine biology at Orange County Ocean Institute in California and puts weird facts like that into the story and characters. Sea stars actually have something going on upstairs, but it’s just a nerve ring instead of a brain.                

Breathing is another thing that our dear sea stars don’t do like most of the creatures we come in contact with. They absorb sea water through a small dot normally located somewhere on the top facing side of the sea star; this is called a madreporite. The water they absorb is used in their circulatory system (yes, you read that right, sea water being used for blood). While they have the water they might as well make the most of it and absorb the oxygen out of it.

For vision the sea star uses a tiny dot on the end of each arm to see. If you find a sea star large enough you may notice the tiny dot (it looks like someone put the point of a highlighter on the very tip of the arm). Their vision is not like ours and is more like dark and light (sun’s out - sun’s not out).

To get around, the sea star uses its arms with hundreds of little, tiny tube feet on each arm. None of the arms are dominant. Our Forbes Sea Stars have 5 arms each and have been clocked at a whopping five inches a minute! That is a sea star run! Full speed, petal to the metal, run! (That’s .005 mph.) When you don’t have to run down your food and most things don’t want to eat you or will only take a bite that you will grow back, speed is not a major concern. Their favorite food is most bivalves (animals with 2 shells) like oysters, mussels, and clams. The creatures that they are most concerned about avoiding are crabs, bottom dwelling fish, sea gulls, sea urchins, lobsters and (be surprised) humans.

To eat, the sea star wraps its arms with tube feet around a bivalve. The bivalve slams shut; it doesn’t want to get eaten. After roughly 10 hours the amount of pressure the sea star exerts on the bivalve forces it open, just a little bit. Then the sea star takes its stomach out through its mouth and begins to eat the squishy inside of the bivalve. When the sea star finally removes itself from the bivalve all that is left is shell (licked clean!). Mussels are easily the favorite food of the sea stars in our tank. I am not sure if it’s because they are easier to open or if it’s the fact that they prefer the taste. Our sea stars go through about a pound of mussels in a week. With some of the larger clams in the tank, our sea stars appear to attack them in a group, which amazes me since they have no brain. So I wonder - can they organize? Is it instinct? Communication beyond our understanding? We may never know.
 
Mysteries of the ocean are being unfolded every day. Remember we know more about the planet Venus than we do about our own oceans. Till the next “Meet the Tank”, take care.