Charles Messing's Crinoid Pages: Home

The Sea Lilies and Feather Stars

Introduction

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This website outlines current understanding of living crinoids: their structure, classification, and ecology. It includes 1) an introduction to crinoid structure, features, terms and symbols; 2) aspects of crinoid ecology; 3) the basics of crinoid systematics; 4) working with specimens, and 5) an artificial key to the families of living crinoids—artificial because detailed phylogenies have not yet been worked out (watch this space).

Feather star Trichometra cubensis clinging to the stalk of Porphyrocrinus daniellalevyae. NW Bahama Is., depth 616 m. (Bioluminescence 2009/NSF)

References

Brusca, R. & Brusca, G.J. 1990. Invertebrates. Sinauer, Sunderland MA. 922 pp.

The Crinoidea includes the most exquisite members of the Echinodermata, far more stunning as a rule than any plodding urchin or commonplace starfish (just to let you know at the outset where my allegiance lies). With a family tree rooted in over 400 million years of history, they are the senior group of living echinoderms. Their typically echinoderm features include:

5-sided adult symmetry derived from a bilateral larva.
Water vascular system - a network of coelomic canals and reservoirs that may serve in respiration, circulation, feeding and locomotion, and that terminates in external podia, or tube feet.
Calcareous endoskeleton consisting of individual plates (ossicles) with a meshwork fine structure (stereom), each formed from a single high-magnesium calcite crystal.
Mutable collagenous (or catch connective) tissue that can alter between rigid and flaccid states under neuronal control.
Deuterostome, enterocoelous embryonic development with radial cleavage (Brusca and Brusca 1990).

Features that distinguish crinoids from other echinoderms are:

Two to four circlets of ossicles (never 4 in living species) fused together as a cup- or box-like calyx that contains or supports the viscera.
Five flexible, usually branched and featherlike rays—extensions of the body wall supported by skeletal plates and bearing food-collecting grooves and extensions of the water-vascular, nervous, haemal and reproductive systems.
The oral surface contains both mouth and anus and orients away from the substrate.

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Most of the several thousand known fossil crinoids and over a hundred modern deep-water species bear a stalk on the side of the body opposite the mouth (the aboral side) and are known as sea lilies. Imagine a feathery sea star on a stick.

Parahyocrinus claguei, N Monterey Canyon, California, depth 2,500-2,893 m.

Feather stars account for the majority of living crinoids. They are often called unstalked crinoids, although this is not accurate, because they do develop a stalk early on. However, they shed it when still small juveniles and retain only the topmost stalk segment, which usually bears numerous hooks (cirri) for anchoring. Feather stars have also been called comatulids, because Order Comatulida formerly only included feather stars. However, molecular data shows that the group also includes some families of sea lilies that retain their stalk as adults, so feather star and comatulid are no longer equivalent terms.

Anneissia bennetti, Batu Moncho I., near Komodo, Indonesia.
Crinoid on the Reef of Batu Moncho Island, Alexander Vasenin, CC BY-SA 3.0

Because most crinoids are at least 80% calcium carbonate skeleton, they have left an enormous fossil record. Over 5,000 fossil species have been described, with the earliest over 400 million years old. Their remains dominate limestone deposits called regional encrinites that may exceed 100 feet thick and span hundreds of square miles. However, fossil crinoids are beyond the scope of this website. Click here for examples of fossil crinoids.

Crinoid Fossils of Jurassic, Kevin Walsh, CC BY 2.0