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Feather Stars

Feather stars, or comatulids, are echinoderms that belong to the class Crinoidea (phylum Echinodermata) which they share with the sea lilies. Unlike the latter group, however, feather stars are not obliged to remain in one place; instead they can swim or even crawl over short distances before attaching themselves to some support. Swimming movements are achieved by waving the arms up and down in a slow, controlled manner. Feather stars are widely distributed throughout tropical and warm-temperate waters, with the main center of their distribution being focused on the Indo-Pacific region. An estimated 550 species are known.

A feather star's body consists of a basal plate known as the calyx which bears a number of specialized cirri that are used for holding onto rocks or other objects when the animal is feeding or at rest. The mouth is situated on the upper surface of the calyx. Also arising from this small body are the jointed arms which are usually quite short but may measure more than 11.8 in (30 cm) in some species. Cold-water species tend to have much shorter arms than tropical feather stars. Each arm bears a large number of short pinnules (featherlike appendages).

By far the most striking part of a feather star's anatomy is their delicate, ostrich-plumelike arms that are usually highly colored. Some species can have more than 200 arms. Feather stars are suspension feeders and, when feeding, unfurl their arms and extend the many pinnules into the water current. Feather stars usually carefully choose their feeding site; typically they select a site on a high vantage point at an angle to the current. As the water flows between the pinnules, additional tiny tentacles (known as podia) that are covered with mucus trap the many tiny food particles. These are then transferred to special grooves that run the inner length of the pinnules, where they are transferred down and inward to the mouth region. From here the food passes directly through the esophagus which opens into an intestine. Most feeding takes place at night, when the majority of reef fishes are resting, in order to avoid the grazing effects of these predators. Although feather stars react almost immediately to being touched and hurriedly fold away their arms, most species can also shed their arms if attacked. The arms will regenerate in time. As daylight approaches, however, and the risk of predation increases, they move away and hide among the crevices of the reef face.

Feather stars are either male or female and fertilization is usually external. Some species retain their eggs on their arms but this is not the usual pattern of behavior. When the larvae hatch they pass through a series of development stages as free-swimming animals known as vitellaria. Eventually these settle and undergo a transformation which initially restricts them to a sessile (attached, not free-moving) state, as in sea lilies. Eventually, however, they develop the familiar arms of the adult feather star and are able to move around.

See also Sea lily.

Additional topics

Science EncyclopediaScience & Philosophy: Evolution to Ferrocyanide