Sea Stars

Les étoiles de mer se rencontrent sur la plupart de nos rivages.

Where seen? Sea stars are encountered on most of our shores. Even the most "beat up" shore will have some kind of sea star. But our Northern shores appears to have the richest variety of sea stars. Some sea stars are small and well hidden. Others are large and colourful.

What are sea stars? Although often called starfish, these creatures are not fish at all! So it is more correct to call them sea stars. Sea stars belong to the Phylum Echinodermata and Subclass Asteroidea. There are about 1,800 known species of sea stars, of which about 300 are found in shallow waters. Sea stars form the second largest group of Echinoderms after the brittle stars (Subclass Ophiuroidea).

Features: Almost everyone knows what a sea star looks like! 'Asteroidea' means 'star-like'. Like other echinoderms, sea stars are symmetrical along five axes, have a spiny skin and tube feet.

An Armful: Sea stars have arms that blend into one another before joining the central disk. Some sea stars seem to be all arms with long narrow arms and a small central disk. Others have arms that are so short they look like pentagons. Most species of sea stars have five arms, although some may have more. However, sometimes, you might come across a sea star with fewer than five arms. Some kinds of sea stars are really flat, others may have more cylindrical arms, and some are so round that they look like cushions. Each arm is usually tipped with one or more sensory tube feet, and an eye spot that detects light and dark but does not form an image.

Sometimes confused with brittle stars. Unlike sea stars, brittle stars have very flexible and long arms attached to a small central disk. Most brittle stars are much smaller than sea stars, although some have very long arms.

Handicapped Stars: Sea stars are famous for their ability to regenerate lost arms. But this takes time and resources. Some species take up to a year to replace a lost limb.

In the meantime, the sea star is probably disadvantaged. If the central disk is damaged, the sea star may die. Only a few species of sea stars are known to regenerate from a piece of an arm. So you won't necessarily get two sea stars when an arm of a sea star is separated. So please don't purposely mutilate sea stars.

Mouth to the ground: The mouth is on the underside facing the ground. Some sea stars have jaws made up of five or more teeth arranged in a star around the mouth. Some sea stars can extend their stomachs out of their mouths! Part of the digestive system of a sea star extends into its arms.

Not all sea stars have an anus. Those that don't, spit out indigestible bits through their mouth. In those that do, the anus is on the upper surface of the central disk.

In the groove: Radiating from the mouth and extending under each arm are grooves (ambulacral grooves). These grooves usually contain 2-4 rows of tube feet. The margins of the groove are guarded by moveable spines that can close over the groove. Out of the water, a sea star will usually retract its tube feet into the grooves so it looks rather lifeless.