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 have a unique, flower-like appearance that constitutes a cup-like crown called theca, the base of which is composed of numerous body ossicles, followed by a stalk. They are found in both shallow and deep waters, feeding on suspended food matter, like plankton and detritus.
Crinoids exist in two distinct 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, also known as comatulids that are free-moving in nature and lose their juvenile stalk during adulthood. As echinoderms, crinoids have a pentamerous (five-fold) symmetry, but most modern forms have arms in multiples of five (ten or more).
A group once dominant in the Palaeozoic Era, currently, there are 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 a stem or stalk (absent in adult feather stars) 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 groups where the mouth lies on the underside.
Generally, in primitive crinoids, a set of five branched and feathery arms emerge from the theca. However, in most modern forms, the five primary arms branch further to form ten arms. In feather stars, these arms branch profusely, making up to two hundred branches in total. These arms are jointed and are lined by finer appendages called pinnules.
This part of the body is homologous to the disc of other echinoderms 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 areas between them. The ambulacral grooves lead to the mouth, which is located either near the center or on the margin of the tegmen.
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 called cirri. These cirri may extend upward toward the base of the theca. In species living on soft sediments, the 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 a large number of pores instead of a single madreporite.
The primary ring canal is connected to the coelom through multiple calcified stone canals. A number of 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 expansion and contraction of the ring canal.
While crinoids lack a distinct circulatory system, they possess a large blood vessel known as the axial organ situated at the base of the theca.
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.
Usually, oxygen is absorbed through the tube feet, the most thin-walled parts of the body. 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 directly leads to the intestine. This group lacks a true stomach.
The intestine forms a single loop and often includes multiple long, branched diverticula 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.
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 had 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 and are found in both shallow waters as well as in depths as low as 30,000 ft (9,000 m). They are also abundant in coral reefs.
Sea lilies, in particular, prefer living in deep oceans, especially in the aphotic zone, where light barely reaches, and predation pressure is naturally low.
They typically feed on plankton and 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 mesh-like structure. As the food 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 flick it into the ambulacral groove. From there, cilia guide the food efficiently toward the mouth.
Although many sea lilies (being stalked) remain attached to the sediment, feather stars (as well as some sea lilies) move by crawling on their leg-like cirri. They also swim by moving their arms sequentially in a coordinated, repetitive fashion, 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.
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 ruptured 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, whereas in those found in colder waters, the eggs are stored in specialized sacs on the 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.
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 prolonged period of metamorphosis and finally transforms into a radially symmetric, stalked juvenile. In feather stars, the adults lose this stalk and become free-swimming again.
They are typically preyed upon by benthic invertebrates, especially sea urchins like Calocidaris micans. Fossil records reveal that pelagic crinoids of the genus Saccocoma were consumed by fish (especially placoderms) and cephalopods. Additionally, some prehistoric snails, like Platyceras, are believed to have been significant predators of crinoids.
Like starfish and brittle stars, these animals are capable of regenerating their lost body parts. For instance, they can easily regrow arms damaged by environmental stress or torn off by predators. In fact, they can regenerate their entire visceral mass in a span of a few weeks.
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