Sea stars


Asteroidea, but not A. vulgaris or A. forbsii

Sea stars (group name Stelleroidea) are sometimes called starfish, though lacking both vertebrae and fins, they are not real fish. There are two sub-types of sea stars: Asteroideas are the true sea stars and sun stars, whereas Ophiuroideas are brittle stars and basket stars.

Ophiuroid means ‘snake-like’, referring to the form and motion of the arms. The arms of brittle stars are easily broken off, but will regenerate in a few days to weeks. Sea stars can also regenerate arms that are broken off, but for most species, the central region of the body must remain intact Class Asteroidea, the true sea stars, contains about 1700 living species of these echinoderms. Unlike the superficially similar brittle stars, true starfish have no sharp demarcation between arms and central body, and they move using tube feet, or podia, rather than wriggling movements of the whole arms. Each arm, or ray has a light sensitive organ called an eyespot, enabling it to detect light and general direction.

Most starfish are predators, feeding on sessile or slow-moving prey such as molluscs and barnacles. A few species, such as the spiny star of the North Atlantic, eat other sea stars! Many, but not all, starfish are able to turn a portion of their stomachs out through the mouth (called eversion), and thus digest food outside of the body. This star-shaped carnivorous animal is usually a dull yellow or orange, but can also be brightly coloured. As a natural defence mechanism, the starfish is able to change its body color to hide or escape from predators.

Starfish vary greatly in size from a few centimetres over one metre. The arms of the starfish are used for movement, catching prey and digestion. Unlike other animals, the starfish is able to grow a new arm if one is lost. Many starfish have five arms, but there are some deep-water species that carry more than fifty. The Northern Sea Star (asterias vulgaris) and the similar but slightly smaller Forbes Sea Star (asterias forbsii) are relatively common in these waters and we would ask you to exclude them from any surveys that you undertake for the Global Dive Log, although of course you are very welcome to include them in your on-line dive log.