Another group of Echinoderms are the Crinoids which include Feather stars and Sea lilies. Like other Echinoderms they are found in all regions of the sea but most species are found at depths greater than 200m. There are over 600 species of Crinoids of which around 80 are Sea lilies. Crinoids are the oldest of the living echinoderms with a fossil record stretching back 450 million years.

They are usually found living in groups of several thousand. Feather stars swim slowly, by swim by flapping their feathery arms, through the water or crawl along the ocean floor to find food. They often use rocks, corals or sponges to raise themselves above the bottom when they feed and hide in caves and ledges during the day. Feather stars are found in shallow and deep ocean waters but are most common in tropical reef environments. Sea lilies live attached to the ocean floor in depths greater than 100 metres.

Crinoids have an external skeleton made of calcium carbonate plates covered by a thin skin. The plates are held together with ligaments or muscles.Shallow water forms are usually very colourful.

The skeleton is usually divided into four basic parts:

• Holdfast, a disc-like sucker, which anchors the crinoid to the ocean bottom;
• Stem, filled with muscles, which raises the calyx above the substrate;
• Calyx, a cup-shaped central structure , which contains the internal organs; and
• Arms - from five to as many as 200 feeding arms (in multiples of five).

The largest Sea Lily has a large calyx which with its arms gives it a diametrer of 1.5 metres. The largest Feather star has an armspan of 35 cm. The smallest Crinoids are around 3 cm in diameter.

Most species are nocturnal filter feeders consuming plankton and decaying organic matter. To feed they spread their feeding arms to sieve the passing sea water for microscopic organisms and detritus. Mucus, on the tube feet traps their food which is passed down the arms into the mouth by beating cilia. They have a U-shaped digestive system with the anus next to the mouth.

Not much is known about what eats them although fish and other Echinoderms (especially Sea urchins) are known predators. Sea lilies have been observed crawling away from Sea urchins.

Crinoids are either male or female with fertilization taking place in the water. The eggs hatch to form free-swimming larva which do not feed and settle on the bottom after a few days after which they metamorphise into an adult in 8 to 12 months. Some hatch as miniature adults, while some females even hold the eggs in their arms until they hatch.

References

http://en.wikipedia.org/wiki/Crinoid
www.reef.edu.au/asp_pages/secb.asp?FormNo=44
http://blog.reefcharter.com/2010/03/feather-stars.html
http://saltaquarium.about.com/od/starfishcare/a/aafeatherstarinfo.htm
www.wetwebmedia.com/crinoids.htm

Feather star

Sea lily
Image © Sabine Pennison Flickr