Published February 2005
Dear Sea Scoop,
How do starfish move?
– JP Nugent
Chicago, IL.
Dear JP,
Sea stars (better known as "starfish") are members of the phylum Echinodermata (ee-KINE-o-derm-ah-tah) which translates to "spiny skin". This phylum also includes sea urchins, sea cucumbers, sand dollars, sea biscuits and a few others. All Echinoderms have the same form of locomotion – a water vascular system. This internal system works by a combination of muscles and water pressure. In a sea star, sea water is taken in through the madreporite (mad-ra-PORE-ite), which is a small disk-shaped intake valve on the aboral (top) surface of the animal's center. The water moves to a circular duct, the ring canal, around the mouth and leads out to canals in each of the starfish's arms. Each of the arm canals, called radial canals, lead into hundreds of smaller muscular, balloon-type reservoirs called tube feet. The bulb at the top of the tube foot is called the ampulla. The muscles surrounding the ampulla contract or relax allowing the sea water inside to move the foot. When the ampulla contracts, it squeezes water into the tube foot. This causes the tube foot to extend. When the ampulla relaxes, the tube foot retracts. This system of muscle and water pressure allows the sea star to move around, tightly attach itself to rocks and also allows it to hold onto its prey.
Today's Tip – Did you know that sea stars have the ability to regenerate (regrow) a lost arm? If a predator tries to catch it, a sea star can release one of its arms so the predator only gets part of it. This is a process called autotomy, the basic idea being that it is better to lose part of your body than all of it. A new arm will then regenerate from the disk in the center of the sea stars body, eventually retuning the sea star to its basic original form. A sea star can also reproduce asexually from an arm that has been torn off. As long as a substantial part of the central disk is still attached, an entirely new sea star can grow from the lone appendage.
For more information on sea stars and other echinoderms:
Tree of Life Web Project – Echinodermata
http://tolweb.org/tree?group=Echinodermata
Enchanted Learning Printout – Sea stars
http://www.enchantedlearning.com
Natural Perspective – Starfish
http://www.perspective.com/nature/animalia/starfish.html
Have a question for about the world beneath the waves? Write it down and send it to Sea Scoop! Please remember to include your name and where you're from.
For more information on marine science in the Virgin Islands, visit the University of the Virgin Islands' Center for Marine & Environmental Studies.
Elizabeth Ban is the University of the Virgin Islands Marine Adviser for St. Thomas and St. John. She works to inform and educate citizens about ocean resources and promote coastal ecosystem health. She is based at UVI's Center for Marine and Environmental studies on the St. Thomas Campus. For more information about UVI's Marine Advisory programs, please call 340-693-1392.
