Crinoid Totally Explained

Everything about Crinoidea totally explained

Crinoids, also known as sea lilies or feather-stars, are marine animals that make up the class Crinoidea of the echinoderms (phylum Echinodermata). They live both in shallow water and in depths as great as 6,000 meters. Crinoids are characterized by a mouth on the top surface that's surrounded by feeding arms. They have a U-shaped gut, and their anus is located next to the mouth. Although the basic echinoderm pattern of five-fold symmetry can be recognized, most crinoids have many more than five arms. Crinoids usually have a stem used to attach themselves to a substrate, but many live attached only as juveniles and become free-swimming as adults. There are only a few hundred known modern forms, but crinoids were much more numerous both in species and numbers in the past. Some thick limestone beds dating to the mid- to late-Paleozoic are entirely made up of disarticulated crinoid fragments.

Morphology

Crinoids comprise three basic sections; the stem, the calyx, and the arms. The stem is composed of highly porous ossicles which are filled with muscular tissue. The Calyx contains the crinoid's digestive and reproductive organs, and the mouth is located at the top of the dorsal cup, while the anus is located peripheral to it. The brachials(arms) display pentameral symmetry and comprise smaller ossicles than the stem and are equipped with cirri which facilitate feeding by moving the the organic media down the arm and into the mouth.

Ecology

Nutrition

Crinoids feed by filtering small particles of food from the sea water with their feather like arms.

Reproduction

Crinoids reproduce sexually by the males releasing their sperm and the females releasing their eggs into the current where that'll develop into a bottom-dwelling non-feeding larval stage and then eventually grow a stalk (in the stalked crinoids), and within 10 to 16 months will be able to reproduce. In some cases the female of the species has been known to temporarly brood the larva.

Motility

Most modern crinoids are free-swimming and lack a stem. Examples of free-swimming crinoid fossils include Marsupitsa, Saccocoma and Uintacrinus.
In 2005, a stalked crinoid was recorded pulling itself along the sea floor off the Grand Bahama Island. While it has been known that stalked crinoids move, prior to this recording the fastest motion of a crinoid was 0.6 meters/hour (2 ft/h). The 2005 recording showed a crinoid moving at 140 meters/hour (460 ft/h).

Evolution

The earliest known crinoids come from the Ordovician. They are thought to have evolved from primitive echinoderms known as Eocystoids. Confusingly, another early group of echinoderms were also the Eocrinoids, but that group is currently thought to be an ancestor of blastoids rather than of crinoids.
The crinoids underwent two periods of abrupt adaptive radiation; the first during the Ordovician, the other after they underwent a selective mass extinction at the end of the Permian period. This Triassic radiation resulted in forms possessing flexible arms becoming widespread; motility, predominantly a response to predation pressure, also became far more prevalent. After the end-Permian extinction, crinoids never regained the morphological disparity they enjoyed in the Paleozoic; they occupied a different region of morphospace, employing different ecological strategies to those that had proven so successful in the Paleozoic.

Crinoids in pop culture

Fossilised crinoid columnals extracted from limestone quarried on Lindisfarne, or found washed up along the foreshore, which were threaded into necklaces or rosaries, became known as St Cuthbert's beads.
Crinoids are the state fossil of Missouri

References

Further Information

Get more info on 'Crinoidea'.

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