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| Chordata :: sea animals :. Sea star
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 The sea star is traditionally referred to as a "starfish", however they are not actually fish at all. Sea stars are related to brittle stars, sea urchins, feather stars and sea cucumbers and belong to the phylum echinoderm. The word echinoderm is derived from the Greek language meaning "hedgehog skin". This is an accurate description for these animals, as they have a spiny skin covering their skeleton. The skeleton of the sea stars is made up of many plates that move like flexible joints.
Echinoderms are exclusively marine organisms, which are simple in external design, attractive in colour and, in some cases, have sharp spines. They have pentaradial symmetry, which means that the body can usually be divided into five (penta) parts arranged around a central axis. Holothuruid echinoderm, that is sea cucumbers, are bilaterally symmetrical like humans.
Sea stars are incredibly resilient. When they lose an arm from their body it can regenerate into another one within a year. Provided that one fifth of the central disk and at least one arm remains, the original sea star can completely recover.
Echinoderms possess tubed feet, this hydro-vascular system is used in collecting and transporting food and also in locomotion.
Distribution and Habitat
Sea stars can be found in most oceans around the world, on open coastal reefs, in sheltered waters and in open seas. Echinoderms inhabit many different types of terrain - from the bottom of deep-sea trenches, to soft seabed mud, rugged coastlines and coastal reefs, however they are generally bottom dwellers.
Reproduction
Most sea stars are dioecious (they have separate sexes) and reproduce by releasing sperm and eggs into the water (external fertilisation), where they develop from larvae into baby sea stars. The currents throughout the ocean disperse the larvae. When the sea stars are still in the larvae form, they are bilaterally symmetrical with their right and left side being exactly the same. The juveniles will eventually develop 5 arms, typical of most echinoderms and unique to the animal kingdom.
Other forms of reproduction include the sea star brooding their young within their body, which eventually emerge as juvenile sea stars. Some sea stars also lay eggs.
Although the sexes of sea stars are generally separate it is not uncommon for some species to be hermaphroditic.
Food and nutrition
A sea star feeds externally by everting its stomach out side of its body, and digesting its food externally. They are commonly carnivores and scavengers feeding on other invertebrates. They may feed on algae and can also be suspension feeders collecting plankton, detritus, and mud which comes into contact with the body's surface. It is then moved by cilia (fine hairs) on the oral surface of the sea star to the mouth.
Interesting Facts
Sea stars can insert the everted stomach through the minute opening of a clam shell or mussel, a gap of only 0.1mm. The gap increases as digestion takes place when the clam or mussel shell's adductor muscles are attacked, until digestion is completed.
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