The starfish and sea stars are a Class in the Phylum Echinodermata . There are about 1500 species of sea star and starfish, that range in size from 1 - 100 cm in diameter.
They have a flattened body with five (sometimes more) fairly broad arms (see above). It is quite common for starfish to re-grow an arm that has been lost to predation or injury, but only certain species (see below) can grow 4 completely new arms. There is usually an ocellus (primitive eye) at the end of each arm, but their sense organs are not well-developed. The mouth is on the underside and the madreporite and anus on the upper surface. A groove runs from the mouth to the tip of each arm, and rows of tube feet, most with suckers, are situated along either side of the groove. The water-vascular system opens to the outside through the madreporite. The anus is inconspicuous in most, and a few species lack both anus and intestine.
The skeleton is a loosely organised series of small calcareous plates (ossicles) bound together with connective tissue, and is located beneath the epidermis which has pedicellariae (small forceps, scissors or clamp shaped calcareous structures).
The pedicellariae keep the body surface free of debris, and may function in prey capture; they are also used in identification of species.
Most have separate sexes, but, some Asteroidea are hermaphrodite and are capable of releasing as many as 2.5 million eggs in 2 hours. Reproduction is also possible by fission. To do this the starfish sheds one arm, and this shed arm then grows another 4 arms.
Predatory species may have an eversible stomach. This means the stomach can be inserted into any small hole in the prey's armour and digestion can proceed from there. Bivalves are commonly preyed on in this manner. They are mainly nocturnal. Above right is Plecaster decanus found in southern Australian waters.
The Common starfish Asterias rubens, above, is found is found in the North Atlantic and North Sea coasts down to about 400 m, often in mussel beds and amongst barnacle encrustations. It can grow up to 52 cm in diameter, but more usually 10 - 30 cm. Its colour varies from brown to orange and also violet; the deeper dwelling individuals tend to be paler. When active the tips of its arms are often curled up. Movement is slow at around 15 cm a minute.
It can detect its prey by smell, and prey includes bivalves, polychaete worms, barnacles, marine snails and other echinoderms. It can pull open a mussel using the suckers on the undersides of its arms. With a gap of just 1 mm it inserts a fold of its stomach which secretes enzymes and starts the digestive process. When the prey has been sufficiently liquefied it removes its stomach with the prey soup inside.
Lifespan is thought to be about 7 or 8 years. And a female can release around 2.5 million eggs in one go.
The Common comet starfish, above, can be found in the Indo-Pacific oceans. Its colour ranges from green through to brown, blue and red. The larger arm is the one that originated from the parent starfish that reproduced by fission.
The Spiny starfish, above, can be found in the North East Atlantic and the Mediterranean in rock pools and down as far as 200 m deep. Fully grown specimens can be as large as 30 cm across, and there have been a few individuals reaching 75 cm, making it the largest starfish in British waters! It has 3 rows of spines running down each arm. Its colour varies widely and includes grey, green, blue, brown and pink.
Spiny starfish are predacious with bivalves, crustaceans and fish, either dead or alive forming most of their diet. They have light sensitive organs a the tip of their arms, and this explains the frequently observed behaviour of them raising their arm tips.
Labidaster annulatus above, can be found in the Antarctic ocean and is a type of starfish called a Sun star. It is unusual in that it has forked arms, and can grow up to 50 centimetres across. It will eat almost anything.
Goniaster tessellatus (above) can be found in the topical Atlantic.
Tosia australis, as its name suggests, can be found in Australian waters where it is fairly common from intertidal to 40 metres deep in the open or under rocks. Its common name is the Biscuit star, and it is easy to see why. It reaches about 10 centimetres in diameter.
Protoeaster linkii is found in tidal pools and down to 100 m deep in the Indo Pacific. Its many common names arise from its thick, upright, red tubercles running along each arm.
It is carnivorous and can grow up to 30 cm across. When small it eats algae, then coral, sponges tube worms, clams and almost anything else as it grows. It is active during the day, and because of its striking colours this makes it a popular choice in large aquariums.