The class Ophiuroidea contains brittle stars and basket stars. There are approximately 2,000 species of brittle stars and basket stars. Like most echinoderm classes, ophiuroids are found in most oceans and depths. Brittle stars are often very colourful and active. Closely related to brittle stars are basket stars. These are highly specialised ophiuroids that are highly adapted to suspension feeding. They are often seen at night looking like giant baskets of tentacles.

• To be a member of the Echinodermata.
• To have a disc-shaped body and long jointed arms. Your tube feet never have suckers on them.
• To move by pushing yourself along the substrate.
• To have a digestive system that is incomplete (meaning that it only has one opening).
• Be either a carnivore or deposit feeder.

Brittle stars, Basket stars, ophiuroids.

Brittle stars are star-shaped organisms that look at first glance like asteroids (sea stars). The similarity is only superficial, however. Brittle stars have long, flexible, jointed arms arising from a small disc shaped body. The central disc is not very thick, and so the distance from the mouth (oral) to the upper surface (aboral) of brittle stars is very short. Two other key differences to the Asteroidea are that the madreporite is on oral surface and that the tube feet do not have suckers. Brittle stars (Ophiuroidea) often have spines along their arms, probably functioning to discourage predators. Brittle stars also avoid predators by coming out at night to forage. The reduced light levels at night mean that the risk of predation is lower. Brittle stars have jointed arms that are supported by vertebra-like sections. The muscular movements of these jointed arms are used to push brittle stars along the substrate. This is fundamentally different to asteroids that primarily use their tube feet in movement. Brittle stars do not have suckers on their tube feet but are able to walk up smooth surfaces like the inside of fish tanks. This was somewhat mysterious at first, but turns out to be due to the production of a powerful adhesive by glands at the tip of each tube foot.

Brittle stars and basket stars are found in all oceans and depths. They live generally among the rocks, stones and coral on the bottom of the ocean. They are often very numerous: some places may have several small brittle stars for every per square centimetre! Brittle stars are quite cryptic and have colour patterns that match the type of substrate they are living on. In cases, this can result in the bright colours of sponges being adopted over evolutionary time by brittle stars that live on sponges.

Brittle stars show a great variety of feeding methods. Many large and active species are either predators or scavengers. Other species trap particles from the surface of the sediment at the bottom of the sea. These brittle stars are detritivores. They feed by extending their arms from its hiding place among the gravel substratum. Small pieces of debris are collected and passed from tube foot to tube foot, then to its mouth. The particles are then eaten. Often, divers will see brittle star arms extending from hiding places. When touched these arms will suddenly withdraw. Basket stars are a group of specialised ophiuroids adapted to suspension feeding. They have a feeding strategy that is somewhat similar to crinoids. Surprisingly, they belong to the class Ophiuroidea as opposed to the class Crinoidea. Nocturnal feeders like the crinoids, basket stars unfold long branching arms that ensnare particles and small water-borne animals.

Fish are probably the main predators on ophiuroids. Many brittle stars that suspension feed are nocturnal. Some tropical species are bioluminescent which is thought to be an anti-predation device. The light that these species produce when disturbed may help frighten away potential predators.

Brittle stars generally have separate sexes which are difficult to tell apart. If their eggs develop through a larval stage, those larvae are called ophioplutei. Ophioplutei are similar in structure to echinoplutei (the larvae of echinoids) and have a skeleton made of calcium carbonate that supports a series of ciliated bands. These bands of cilia function in swimming and feeding. Some species brood their young. In this case, there is an interesting relationship between the size of the adult size and the tendency to brood. Species with small adult size tend to brood while larger species tend to broadcast spawn. The answer to why this is so relates to the relative efficiency with which the larger brittle star species can circulate water around the eggs inside the bursal brood chambers. Larger species have lower surface area to volume (of egg) ratios and hence the ability to circulate water and oxygen to brooded eggs is less.

Brittle stars are generally solitary animals, but they do live in association with lots of other organisms. For example, there are a number of species of brittle stars that live in close (probably mutualistic) associations with animals like sponges and soft corals. These live on particles that settle on these organisms and help clean the surface of the sponge

Brittle stars are generally harmless and have little commercial value.