Crinoids are marine invertebrates that belong to the class Crinoidea within the phylum Echinodermata, which also includes starfish, brittle stars, sea cucumbers, and sea urchins.
They possess a cup-like body structure called the crown or theca that confers a unique, flower-like appearance. These animals are found in both shallow and deep waters, feeding on suspended food matter, like plankton and detritus.
They exist in two body forms: the sea lilies (named for their resemblance to the terrestrial counterparts) that remain attached to the sea floor by a stalk, and the feather stars, which are free-moving in nature and lose their juvenile stalk as they become adults. As echinoderms, crinoids have a pentamerous (five-fold) symmetry, but most modern forms have arms in multiples of five (ten or more).
Once dominant in the Paleozoic Era, there are currently only about 700 living species of crinoids worldwide. These species are grouped into 4 extant orders: Comatulida (feather stars), Cyrtocrinida, Hyocrinida, and Isocrinida. The last three orders all constitute sea lilies.
Though the average stem length of most living stalked forms does not exceed 0.75 m, some deep-sea species reach up to 1 m (3 ft). The largest fossil crinoid, Taxocrinus saratogensis, had a stem length of about 130 ft (40 m).
They have a central, pentamerous, cup-like crown called the theca and an additional stem or stalk (only in sea lilies) for attaching to a substrate. Both the mouth and the anus are located on the dorsal (upper) surface of the theca, unlike in other echinoderm, like starfish, where the mouth lies on the underside.
Generally, in primitive crinoids, a set of five branched and feathery arms emerges from the theca. However, in most modern forms, the five primary arms branch further to form ten or more arms. In feather stars, these arms may number up to two hundred in total. These arms are segmented and are lined by finer appendages called pinnules.
This part of the body is homologous to the rounded disc of other echinoderms (like starfish) and houses the digestive organs. Its base is composed of a set of bony plates or ossicles arranged in the shape of a cup. In contrast, its upper part comprises a poorly calcified membranous disc called the tegmen.
The tegmen is partitioned into five distinct grooves called ambulacral areas and five interambulacral grooves nestled between them. The ambulacral grooves lead to the mouth, which is located either near the center or on the margin of the tegmen. Feather stars have numerous grasping, claw-like structures on the underside of their theca, which help the animal crawl on the substrate.
This elongated structure is present only in stalked crinoids, the sea lilies. While feather stars initially possess a stalk during their juvenile stage, it is lost as they mature into adults.
The stalk, or column, consists of bony, disc-like plates called ossicles (or columnals) stacked on top of one another. These ossicles are porous and connected by ligamentary tissue, providing flexibility and strength.
At the base of the stem, where it anchors to the ocean floor, the structure transforms into a flattened holdfast or a series of tiny, root-like projections, also called cirri, which particularly help in anchoring the animal to the substrate. These cirri may extend upward toward the base of the theca. In species living on soft sediments, these cirri are slender and rod-like, while in species that attach to hard surfaces, they are sturdier and curved.
Crinoids have a characteristic water vascular system that aids in both locomotion and feeding. Unlike most echinoderm groups, this system opens into the coelom through multiple ciliated pores located within or near the theca, rather than a single madreporite.
The primary ring canal gives rise to numerous calcified stone canals, which open internally into the coelom. Many branches radiate from this ring canal and extend into the stalk and the arms. Finer branches spread into the pinnules, extending as tube feet. These feet are controlled by the hydraulic pressure within the water vascular system.
While crinoids lack a true circulatory system with blood and a heart, they possess an elongated, spindle-shaped structure along the oral-aboral axis of the body called the axial organ. A central component of the hemal system, this organ is composed of a central mesh of fluid-filled lacunae and peripheral glandular tubules. It regulates fluid flow and helps distribute nutrients throughout the body[1].
Their body coelom is divided into several interconnected, fluid-filled compartments. Three coelomic branches extend into each arm (two on the oral side and one on the aboral side) and continue into the pinnules. The movement of gases and nutrients within the body is facilitated by cilia lining these branches.
Gaseous exchange primarily occurs across the thin epithelial surface of the tube feet. Additionally, the large surface area of the arms also allows for gaseous diffusion.
The mouth is followed by a U-shaped gut comprising a short esophagus, which acts as a conduit to the intestine. This group lacks a true stomach, and digestion occurs primarily in the intestine.
The intestine forms a single loop and often includes multiple long, branched diverticula, which increase the internal surface area for better absorption of nutrients. It is followed by a short, muscular rectum, which ends in an anus projecting from a tiny, conical structure at the edge of the tegmen.
The nervous system of these animals is divided into three distinct parts, which are well-connected through nerve fibers.
The name Crinoidea is derived from the Ancient Greek words krínon (meaning lily) and the suffix –oid (meaning like).
About 700 species of crinoids are currently divided into 4 extant orders: Comatulida, Cyrtocrinida, Hyocrinida, and Isocrinida. Members of the first order are commonly called feather stars or comatulids, while those of the remaining three orders are collectively called sea lilies.
There exist two competing hypotheses regarding the origin of crinoids. While the traditional viewpoint suggests that this group originated from blastozoans, the alternate hypothesis holds that crinoids probably split early from the edrioasteroids. However, scientists have been uncertain about the true ancestors since all three groups (crinoids, edrioasteroids, and blastozoans) share multiple similarities, like radial symmetry, calcareous plates, and stalks for attachment.
Early Cambrian Period (~540 million years ago)
Ordovician Period (~480 million years ago)
End of the Permian Period (~252 million years ago)
Early Triassic Period (~230 million years ago)
Crinoids are exclusively marine animals, found in all oceans worldwide.
Sea lilies typically inhabit deep-sea environments, especially in the aphotic zone (typically between 660 and 2600 ft), where light barely reaches, and predation pressure is naturally low. According to a 2009 study, some crinoid species, like Bathycrinus volubilis, are spotted at depths as low as 30,000 ft (9000 m) in rocky substrates in the Izu-Ogasawara Trench, Japan.
Feather stars are found in shallow tropical and subtropical waters, particularly in coral reefs.
Feather stars typically consume phytoplankton, zooplankton, and small organic particulates suspended in water, while sea lilies consume bacteria and organic detritus.
Crinoids are suspension feeders that trap their food using their tube feet and pinnules. They extend their arms out perpendicular to the water current, forming a fan-shaped array that acts like a mesh to entrap food particles. As the particles float in, they extend their longest, mucous-laden tube feet along with the erect pinnules.
When food becomes trapped in the sticky mesh, the tube feet direct it into the ambulacral groove. From there, cilia guide the food efficiently toward the mouth.
Although most sea lilies (being stalked) remain attached to the sediment, feather stars move by crawling on their cirri. They also swim by moving their arms sequentially in a coordinated, sequential pattern, producing short bursts of activity that last up to half a minute. Initially, they start swimming upwards vertically but soon switch to a horizontal orientation, moving at a speed of about 7 cm (2.8 in) per second. A few sea lilies, like Metacrinus rotundus, have been observed crawling on the ocean floor using their arms[2].
These echinoderms are dioecious (exist in separate male and female sexes) and reproduce sexually. Both males and females release their gametes (sperm and eggs) into the surrounding water through their reproductive pinnules, where external fertilization takes place.
In some species, like those in the genus Antedon, the fertilized eggs remain attached to the arms of the female by secretions from epidermal glands[3], whereas in those found in colder waters, the eggs are stored in specialized brooding pouches within the pinnules of their arms.
The fertilized eggs hatch into a bilaterally symmetrical, free-swimming vitellaria larva. This larva is barrel-shaped and bears rings of cilia around its body, along with a tuft of sensory hairs at the upper end. In all modern crinoids, the larva is non-feeding or lecithotrophic and relies on the yolk reserves for nutrition.
In a few days, the larva settles on the bottom and attaches itself to the substrate using secretions from an adhesive gland on its underside. It then undergoes a period of metamorphosis (3 to 4 weeks on average, though it varies across species) and finally transforms into a radially symmetric, stalked juvenile. In feather stars, the adults lose this stalk and become free-swimming.
Crinoids are typically preyed upon by other echinoderms, like sea urchins (such as Calocidaris micans), as well as some sea stars. Some fossil crinoids, like those of the genus Saccocoma, were consumed by fish (especially placoderms) and cephalopods, such as nautiloids and ammonoids. Additionally, some prehistoric snails, like Platyceras, are also believed to have preyed upon crinoids[4].
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