By: Devon Kennedy
What do the night sky, Hollywood, and the ocean all have in common? That’s right, they all have stars. What stars are in the ocean you ask? Well, other than the celebrities out for a swim, the ocean is home to an animal you all have heard of, starfish! If you’re like me, then reading about the latest celebrity gossip and starring up at the night sky is just not that interesting. Sea stars, on the other hand, are fascinating. Learning about the way they move, eat, and reproduce will change your view of them forever.
How do they move?
Sea star locomotion almost looks as if it is floating from place to place. This is not actually the case. Sea stars have what are called “tube feet” located on their underside. These tube feet will extend outwards and suction to a surface and then retract, pulling the sea star forward. This process occurs with help from the sea stars water vascular system. The madreporite, an opening on the aboral surface of the sea star, will filter water into the water vascular system. The water will then be collected in the ampulla, a bulb like structure, of each tube foot. When the ampulla is squeezed it forces the water into the tube foot, which then extends and contacts the substrate. When the ampulla is no longer being squeezed, the water will flow back in, pulling on the tube foot and therefore moving the sea star forward.
This video demonstrates how tube feet work.
After taking in all this information you are probably thinking of ways that a sea star can change to increase its overall mobility. I thought, well they use tube feet to move right? So why not just have more tube feet! Turns out, Montgomery and Palmer thought the exact same thing. They did experiments testing whether having an increased number of arms or increased size would have a positive effect on the speed at which sea stars can move. Their results shocked me, so I strongly suggest reading it yourself.
How do they feed?
Starfish could honestly be the focus of a horror movie solely based on the way they feed. A starfish will find its prey, usually any kind of mollusc, and pry it open. This can take a long time to accomplish because, hey, a mollusc does not exactly want to be eaten. The amazing this is that the starfish will not get tired or expend lots of energy to accomplish this task. This is because they have mutable connective tissue. Basically, they can manipulate their muscles into many different postures and not expend any energy holding that position.
Imagine yourself trying to open a particularly resistant jar. You twist and you twist but it just will not open. You may be loosening it bit by bit but you get very tired doing this. A sea star can twist a little bit, manipulate its muscles, and hold that position without getting tired. Then it will twist a bit more and manipulate its muscles again. It may take a long time but eventually the jar will open.
After the starfish has opened the molluscs shell even a tiny bit, it will egest one of its stomachs through the opened crack. The stomach will release digestive enzymes and slowly digest the mollusc…alive. The “mollusc soup” will then be ingested by the sea star and it will leave. All that is left is an empty shell. Now you cannot tell me that would not make a great horror film!
One thing that I still ponder about is how do the sea stars find their food? They all seem to gather when there is food present, but how do they all know it is there? There is a clip from the show “Planet Earth” that shows many organisms, including starfish, gathering on a large food source.
How do they reproduce?
Sea stars, like most marine organisms, undergo mass spawning. The males and females will release their sperm and eggs, in massive quantities, into the surrounding water. If the eggs were to come in contact with sperm then they will become fertilized and eventually become larva. The larva will eventually mature into a starfish and the cycle will begin again.
A really neat thing about star fish is that they have the ability to regenerate lost limbs. Sometimes, if enough of the starfish were removed without damaging its internal organs too much, the two pieces will form two new individuals.
I do not know if I could say the same for everyone, but after learning as much as I have about sea stars, I will never be able to look at them the same again. They may look harmless, but try saying that to the mussels just waiting and praying they are not the next shell to be emptied.
I have mainly been talking about one kind of Echinoderm, Asteroidea. There are in fact many kinds, like the brittle star (Osphiruoidea) for example. This is a link to a blog about really cool research being done on a species of Antarctic brittle star.
“Echinodermata.” Echinodermata. 2012. <http://tolweb.org/Echinodermata>.
“Life – Timelapse of Swarming Monster Worms and Sea Stars – BBC One.”Youtube. 2009 <http://www.youtube.com/watch?v=HG17TsgV_qI>.
Montgomery, E. M. and Palmer, A. R. (2012). Effects of Body Size and Shape on Locomotion in the Bat Star (Patiria miniata). Biol. Bull. 222: 222-232.
“Starfish Locomotion.” YouTube. 2010. <http://www.youtube.com/watch?v=HPhAGyDceLo>.
“Starfish (Sea Star).” National Geographic. <http://animals.nationalgeographic.com/animals/invertebrates/starfish/>.
“Watch Helplessly from a Mussels Shell as It Is Slowly & Inexorably Consumed.” Deep Sea News. 2012. <http://deepseanews.com/2012/09/watch-helplessly-from-a-mussels-shell-as-it-is-slowly-inexorably-consumed/>.