Starfish and their relatives, together known as "echinoderms", have the peculiar distinction of being unlike any other animal. They have neither heads nor tails, and they lack eyes (though some starfish do have light sensitive areas at the end of their arms). They do have a nervous system of sorts, though because there is no head there is no actual brain! There are about 6000 known species worldwide, but none can be found on land or in fresh water.
more stars of the sea:
common sunstar (Crossaster papposus): enormous starfish, usually has 13 arms, sometimes any number between 8 and 14.
edible urchin (Echinus esculentus): a voracious grazer, whole areas of kelp forest can be ‘clear felled’ by them.
brittlestar (Ophiothrix fragilis): may be found together in groups as dense as 1500 per square metre.
There are five main classes of echinoderm:
- featherstars and sea lilies (Crinoidea)
- starfish and cushion stars (Asteroidea)
- brittlestars and basketstars (Ophiuroidea)
- sea urchins and sand dollars (Echinoidea)
- sea cucumbers (Holothuroidea)
The name echinoderm means ‘spiny skin’ (from Greek) and refers to the calcareous plates or ‘ossicles’ that provide the animal with a rigid skeleton or 'test' (as in the sea urchins), or which are simply embedded in the skin (as in the starfish).
One of the most striking characteristic of the group is their 'pentamerous radial symmetry' - that is, the body can usually be divided into five parts arranged around a central axis.
Starfish (and other echinoderms to a lesser degree) have a remarkable ability to regenerate lost or damaged limbs. Even a single arm on its own, as long as it still has part of the central disc attached to it, can regenerate a further four arms to make a completely new starfish! However, it may take as long as a year for the re-formation to be completed.
food and feeding
Predatory starfish have evolved a rather grotesque way of dealing with their victims. The favourite food of the common starfish Asterias rubens is the common mussel Mytilus edulis. Upon detecting a likely victim. the starfish will wrap its arms around the two shells of the mussel and begin the slow process of forcing them apart. Once a slight gape is detected. the starfish exudes its stomach through its mouth and inserts it through the narrow hole into the mussel. The stomach then secretes digestive enzymes to break down the muscle tissue of the musel into a ‘soup’. This soup is then absorbed by the lining of the stomach. For a large mussel. the meal can take eight hours to complete!
Books about echinoderms can be ordered from the MCS shop
