Invertebrates
Feather Star
Crinoidea · also called comatulid, sea lily
A feather star looks like a fern that has learned to crawl. It is not a plant; it is a relative of the starfish, and it belongs to one of the oldest and once most abundant groups of animals in the sea, the crinoids, whose broken stems make up whole beds of limestone.
Overview
The feather stars are the mobile members of the class Crinoidea, the crinoids or sea lilies, found on reefs and rocky bottoms worldwide. They are echinoderms, like starfish and sea urchins, but they hold the body mouth-upward with many feathery arms spread into the current to filter plankton.
It crawls, and it swims
A feather star grips the reef with a ring of small clawed cirri and can crawl by hooking its arm tips onto the rock. If threatened, it can also swim, awkwardly, by sweeping alternate sets of arms.
Arms that drop and regrow
The arms break off readily to distract a predator and grow back, and some feather stars regenerate from a fragment of the central body. A large one can wave 100 to 200 arm-branches at once, and the fan is a home for camouflaged shrimp, crabs, and clingfish.
Status
Not Evaluated and common, though the ancient lineage — 480 million years old, and a major reef-builder for much of that time — is one of the sea’s great survivors.
Feather Star: common questions
Is a feather star a plant or an animal?
It is an animal, and a fairly close relative of the starfish. The confusion comes from its appearance and from the old name sea lily, applied to the stalked, flower-like forms. A feather star has no leaves, roots, or photosynthesis; it has a mouth, a gut, a nervous system, tube feet, and a skeleton of calcite plates, all the features of an echinoderm. It just holds its five-part body with the mouth facing up and the arms spread into the water to catch food, which gives it a botanical look. Its closest living relatives are the deep-sea stalked sea lilies, and beyond them the sea urchins and sea cucumbers.
Can a feather star really swim?
Yes, though it is not graceful. Most of the time a feather star sits anchored to the reef by its cirri with its arms fanned out to feed, or crawls slowly by hooking its arm tips onto the substrate and hauling itself along. But if it is dislodged or seriously threatened, it can lift off and swim by sweeping its arms in coordinated sets, raising one group while lowering the next, which drives it upward and along in a rolling, flapping motion. It cannot swim far or fast, but it is enough to get off the bottom, away from a slow predator like a sea urchin or a crawling snail, and drift to a new spot.
Why do so many small animals live on a feather star?
Because the dense fan of arms is one of the safest addresses on the reef. It provides a three-dimensional thicket to hide in, it is constantly bathed in a food-bearing current, and it moves with the feather star to new feeding sites. Shrimp, squat lobsters, tiny crabs, brittle stars, clingfish, and polychaete worms all live among the arms, and many are camouflaged to match the exact colour and pattern of their host species, so a black-and-yellow feather star has black-and-yellow shrimp and a red one has red shrimp. Some of these tenants steal food from the host, while others just use it for shelter.
How old is the feather star lineage?
Very old. Crinoids appear in the fossil record around 480 million years ago and became so abundant that they built vast underwater meadows, and their broken-up stem segments, small calcite discs often called sea-lily beads or, historically, star stones, make up large parts of some limestone formations. They were hit hard by the great end-Permian extinction about 252 million years ago, and the modern feather stars, which crawl and swim rather than staying rooted, radiated afterward. So a feather star on a reef today is the mobile, modernised descendant of one of the sea's most ancient and successful designs.