Sea lilies and feather stars

Phylum Echinodermata Class Crinoidea Number of families 25

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Stalked or stalkless organisms with a crown composed of a calyx, five or multiple arms, an anal cone, and a mouth pointing upward

Photo: Feather star arms trapping plankton near Sipadan Island, Borneo, in the South China Sea. (Photo by ©Jeff Rotman/Photo Researchers, Inc. Reproduced by permission.)

Evolution and systematics

Crinoids are a living lineage of echinoderms more than 500 million years old. The first crinoids were stalked forms (the sea lilies), whose probable ancestors are the extinct rhombiferans or the extinct edrioasteroid echinoderms. The first fossil record dates from the Lower Ordovician (510 million years ago[mya]). During the Paleozoic era (550-245 mya), there were at least two major expansions and declines in crinoid diversity. In the early Carboniferous (360 mya) crinoid diversity reached its zenith, exceeding the total diversity of all other echinoderm taxa. During the Permo-Triassic extinction (240 mya), the crinoids suffered a catastrophic decline and only one lineage survived, which gave rise to the earliest subclass, Articulata. Throughout the Mesozoic era, this lineage had begun to diversify and, about the time of the early Jurassic (210 mya), the order Comatulida (stalkless crinoids, the feather stars) appeared. The disappearance of stalked crinoids from shallow waters and their restriction to deeper sites coincides with the Mesozoic radiation of predatory bony fishes. About 6,000 species of crinoids have lived and died out in past geological ages.

There are about 600 feather star species distributed among 150 genera and 17 families in one order, and 95 extant sea lily species distributed among 25 genera (50% of them are monospecific), 8 families, and 4 orders. The living crinoids orders are: Millericrinida, Cyrtocrinida, Bourgueti-crinida, and Isocrinida (all sea lilies); and Comatulida (feather stars).

The class Crinoidea is the ancestor group of all other echinoderm classes. The relationships among extant orders are still obscure, but some attempts have been made to elucidate them. Among the orders, Millericrinida and Isocrinida are the most ancient. The comatulids diverged from a group of Isocrinida, and the bourgueticrinids, due to the retention of larval stem, diverged from the comatulids. Cyrtocrinids possibly diverged from the millericrinids.

Physical characteristics

Crinoids are pentamerous organisms that differ from other echinoderm classes because of the upward position of their mouth. Numerous calcareous plates, more or less firmly joined together, form their endoskeleton. The main body part is the crown, which is made up of the calyx, the tegmen, and the arms. The calyx, a rigid cup formed by the calcareous plates, carries the digestive tract, the mouth, esophagus, gut, rectum, and anus. An upper membrane, called the tegmen, bears the openings of mouth and anus and is perforated by numerous small pores that connect the interior of the crinoid with the external environment. The mouth is usually located near or at the center of the tegmen, although it is displaced peripherally in the family Comasteridae. The anus, displaced from the center, is elevated at the tip of a cone or tube. In all but adult comatulide there is a cylindrical or polygonal stalk (stem, column) below the crown, which elevates the crown above the substratum. In comatulids, a cluster of appendages, called cirri, takes the place of the column. The cirri may or may not be present along columns of stalked crinoids. All crinoids have five arms, developing from the calyx, that usually branch one or more times, giving rise to up to 200 arms. Small branches called pinnules border each arm. Those nearest to the month are called oral pinnules. Gonads usually occur in the next group of pinnules, called the gonadal pinnules, although they may also occur in the arm axis, but almost never in the central mass of the body. After the gonadal pinnules are the so-called distal pinnules. Ambulacral grooves (food tegmen anus mouth marsupium lappets testes anal tube tegmen anus mouth

A. Oral view of a comasterid; B. Close-up of a male's arm; C. Feather star anatomy; D. Close-up of a pinnule; E. Doliolaria larva. (Illustration by Emily Damstra)

covering plates of grooves

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A. Oral view of a comasterid; B. Close-up of a male's arm; C. Feather star anatomy; D. Close-up of a pinnule; E. Doliolaria larva. (Illustration by Emily Damstra)

grooves) occur in the oral surface of the calyx and reach the distal extremity of each arm and pinnule. Ambulacral podia (tube feet) line each groove, but the pinnular podia are organized in groups of three podia of different sizes (each podia with different functions during feeding).

In living crinoids, the arms range in size from 0.39 to 13.8 in (1 to 35 cm), depending on the species. The stem of living sea lilies reaches about 3.3 ft (1 m) long, but was much longer in some fossil species, up to more than 65.6 ft (20 m). Comatulids may be of almost any color, white through black, purple, red, green, brown, or violet. The species may be uniform in color or have a combination of colors. Usually, the deeper the organism, the paler the color.


Crinoids are found from substidal fringe zones to great depths in tropical, temperate, and polar waters, although they are more diversified in coral reefs of the tropical Indo-Pacific and Caribbean (although fewer species are present in the Caribbean). Stalked crinoids are restricted to the deep sea, with just a few species living at depths of 200-490 ft (60-150 m). There are three major areas of sea-lily biodiversity: the tropical West Pacific, where three members of the order Isocrinida predominate, the pentacrinids at 660-1,970 ft (200-600 m) and the bathycrinids and hy-ocrinids at 4,920-9,840 ft (1,500-3,000 m); the tropical western Atlantic, where more diversity occurs at upper water levels; and the Northeastern Atlantic, where more diversity occurs at a deeper water level.

Feather stars (order Comatulida) on a gorgonia sea fan. (Photo by Bill Wood. Bruce Coleman, Inc. Reproduced by permission.)


Crinoids frequently live on hard substratum. Some live in areas of high current flow, usually use the vertical filtration fan posture (described in the next section). Others avoid high streams and use the radial feeding posture. Nevertheless, crinoid community is probably determined by substratum complexity, independent of water flow. A highly complex substratum may trigger a high diversity crinoid community, and a homogenous substratum carry a low diversity crinoid community.


Feather stars usually live in clumps, preferring to attach to crevices, lateral surfaces, or in other places in which they can hide their central mass. This behavior prevents and avoids injuries to vital body parts caused by predators, and also optimizes filtration by enhancing the baffle effect, which improves the chance of food particles touching the feeding structure. They frequently emerge at night, exposing part, or all of, the arm, or even the entire body, although some species emerge during daylight, and others are exposed both during the day and at night.

Stalked crinoids also occur in dense clusters, but do not have a diel pattern of emergence because of the lack of light in deep water. Most can also be found attached to a hard sub stratum. The depth distribution of stalked-crinoid diversity seems to be controlled by variations of both crinoid hydro-dynamic vulnerability and abundance of food particles reaching the sea floor.

Crinoids can also regenerate lost body parts. Feather stars can regenerate their arms as long as at least one arm and an intact dorsal nerve center remain. Sea lilies can regenerate an entire crown.

Feather stars are able to crawl over the substratum utilizing their arms. Some comatulids have been observed swimming. They swim by alternating their arms up and down, and descend through the water by extending their arms out like parachutes. Only a few sea lilies are able to crawl over the substratum, and none have been observed swimming.

Feeding ecology and diet

Feather stars assume a vertical filtration fan posture in areas of high current flow. In this posture, the arms are deployed in a planar fan, with pinnules held in the same plane and the food grooves usually directed downstream. The vertical filtration posture serves to present the maximum cross-sectional area of food-collecting surfaces to the incoming water flow, and also acts to baffle through-flowing water, pos

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