Brittle star
Brittle stars or ophiuroids are echinoderm
Echinoderms are a phylum of marine animals. Echinoderms are found at every ocean depth, from the intertidal zone to the abyssal zone....

s in the class Ophiuroidea closely related to starfish. They crawl across the seafloor using their flexible arms for locomotion. The ophiuroids generally have five long slender, whip-like arms which may reach up to 60 centimetres (23.6 in) in length on the largest specimens. They are also known as serpent stars.

Ophiuroidea contains two large clade
A clade is a group consisting of a species and all its descendants. In the terms of biological systematics, a clade is a single "branch" on the "tree of life". The idea that such a "natural group" of organisms should be grouped together and given a taxonomic name is central to biological...

s, Ophiurida
The Ophiurida are an order of echinoderms within the class Ophiuroidea. It includes the vast majority of living brittle stars.-Characteristics:The upper surface of the disk of Ophiurida is covered with scales....

 (brittle stars) and Euryalida (basket stars). Many of the ophiuroids are rarely encountered in the relatively shallow depths normally visited by humans, but they are a diverse group.

There are some 1,500 species of brittle stars living today, and they are largely found in deep waters more than 500 metres (1,650 feet) down.


The ophiuroids diverged in the Early Ordovician, about 500 million years ago.
Ophiuroids can be found today in all of the major marine provinces, from the poles to the tropics. In fact, crinoid
Crinoids are marine animals that make up the class Crinoidea of the echinoderms . Crinoidea comes from the Greek word krinon, "a lily", and eidos, "form". They live both in shallow water and in depths as great as 6,000 meters. Sea lilies refer to the crinoids which, in their adult form, are...

s, holothurians, and ophiuroids live at depths from 16–35 m, all over the world. Basket stars are usually confined to the deeper parts of this range. Ophiuroids are known even from abyssal (>6000 m) depths. However brittle stars are also common, if cryptic, members of reef
In nautical terminology, a reef is a rock, sandbar, or other feature lying beneath the surface of the water ....

 communities, where they hide under rocks and even within other living organisms. A few ophiuroid species can even tolerate brackish water
Brackish water
Brackish water is water that has more salinity than fresh water, but not as much as seawater. It may result from mixing of seawater with fresh water, as in estuaries, or it may occur in brackish fossil aquifers. The word comes from the Middle Dutch root "brak," meaning "salty"...

, an ability otherwise almost unknown among echinoderms.
A brittle star's skeleton is made up of embedded ossicles.


There are roughly 1900 extant species in 230 genera, grouped in the three orders currently living: Oegophiurida
Oegophiurida is an order of Ophiuroidea.The physical characteristics of this order include well separated lateral plates, which exposes the oral surface of radial ossicles. It also has vertebrae with streptospondylous articulation, which thus allow for vertical and horizontal movements. The order...

, Phrynophiurida
Phrynophiurida is an order of brittle stars containing Basket stars.-Characteristics:Phrynophiurida have bursae for as exchange and excretion. Their ventral arm shields are rudimentary, dorsal shields usually absent. Arms sometimes branched, can coil vertically. The vertebrae are joined by...

, and Ophiurida
The Ophiurida are an order of echinoderms within the class Ophiuroidea. It includes the vast majority of living brittle stars.-Characteristics:The upper surface of the disk of Ophiurida is covered with scales....

. There is also a Paleozoic order, the Stenurida.

The relationships among ophiuroids and all other echinoderms provide an enduring problem in invertebrate evolution. Developmental and other studies based on modern organisms imply that asteroids and ophiuroids are not closely related within the echinoderms. Stenurid morphology, in contrast, suggests a close common ancestry for the two; the nature of the ambulacral plates is important, but even their general form is transitional.

Stenurida (extinct)

This is a Paleozoic (Ordovician–Devonian) order, bearing a double row of plates (ambulacra) that abut across the arm axis either directly opposite one another or slightly offset. In contrast, modern ophiuroids have a single series of axial arm plates termed vertebrae. In stenurids, as in modern ophiuroids, lateral plates are present at the sides of ambulacrals, and prominent lateral spines are typical. Stenurids lack the dorsal and ventral arm shields that are found in most ophiuroids. Proximal ambulacral pairs can be partially separated, forming a buccal slit, an expansion of the mouth frame. The arms of some stenurids are slender and flexible, but those of others are broad and comparatively stiff. The central disk varies from little larger than the juncture of the arms to an expansion that extends most of the length of the arms. The content of the order is poorly established, and fewer than 10 genera are known.


Of all echinoderms, the Ophiuroidea may have the strongest tendency toward 5-segment radial (pentaradial) symmetry
Symmetry (biology)
Symmetry in biology is the balanced distribution of duplicate body parts or shapes. The body plans of most multicellular organisms exhibit some form of symmetry, either radial symmetry or bilateral symmetry or "spherical symmetry". A small minority exhibit no symmetry .In nature and biology,...

. The body outline is similar to that of starfish, in that ophiuroids have five arms joined to central body disk. However, in ophiuroids the central body disk is sharply marked off from the arms.

The disk contains all of the viscera. That is, the internal organs of digestion and reproduction never enter the arms, as they do in the Asteroidea. The underside of the disc contains the mouth, which has five toothed jaws formed from skeletal plates. The madreporite
The madreporite is a lightcolored calcerous opening used to filter water into the water vascular system of echinoderms. It acts like a pressure-equalizing valve. It is visible as a small red or yellow button-like structure, looking like a small wart, on the aboral surface of the central disk of a...

 is usually located within one of the jaw plates, and not on the upper side of the animal as it is in starfish.

The ophiuroid coelom
The coelom is a fluid-filled cavity formed within the mesoderm. Coeloms developed in triploblasts but were subsequently lost in several lineages. Loss of coelom is correlated with reduction in body size...

 is strongly reduced, particularly in comparison to other echinoderms.

Water-vascular system

The vessels of the water vascular system
Water vascular system
The water vascular system is a hydraulic system used by echinoderms, such as sea stars and sea urchins, for locomotion, food and waste transportation, and respiration. The system is composed of canals connecting numerous tube feet...

 end in tube feet
Tube feet
Tube feet are the many small tubular projections found most famously on the oral face of a sea star's arms, but are characteristic of the water vascular system of the echinoderm phylum which also includes sea urchins, sand dollars and sea cucumbers and many other sea creatures.Tube feet function in...

. The water vascular system generally has one madreporite. Others, such as certain Euryalina, have one per arm on the aboral surface. Still other forms have no madreporite at all. Suckers and ampulla
An ampulla was, in Ancient Rome, a "small nearly globular flask or bottle, with two handles" . The word is used of these in archaeology, and of later flasks, often handle-less and much flatter, for holy water or holy oil in the Middle Ages....

e are absent from the tube feet.

Nervous system

The nervous system consists of a main nerve ring which runs around the central disk. At the base of each arm, the ring attaches to a radial nerve which runs to the end of the limb. The nerves in each limb run through a canal at the base of the vertebral ossicles.

Ophiuroids have no eyes, or other specialised sense organs. However, they have several types of sensitive nerve ending in their epidermis, and are able to sense chemicals in the water, touch, and even the presence or absence of light. Moreover, tube feet may sense light as well as odors. These are especially found at the ends of their arms, detecting light and retreating into crevices.


The mouth is rimmed with five jaws, and serves as an anus (egestion) as well as a mouth (ingestion
Ingestion is the consumption of a substance by an organism. In animals, it normally is accomplished by taking in the substance through the mouth into the gastrointestinal tract, such as through eating or drinking...

). Behind the jaws is a short esophagus and a large, blind stomach cavity which occupies much of the dorsal half of the disk. Ophiuroids have neither a head nor an anus. Digestion occurs within 10 pouches or infolds of the stomach, which are essentially ceca, but unlike in sea stars, almost never extend into the arms. The stomach wall contains glandular hepatic cells.

Ophiuroids are generally scavenger
Scavenging is both a carnivorous and herbivorous feeding behavior in which individual scavengers search out dead animal and dead plant biomass on which to feed. The eating of carrion from the same species is referred to as cannibalism. Scavengers play an important role in the ecosystem by...

s or detritivores. Small organic particles are moved into the mouth by the tube feet. Ophiuroids may also prey on small crustaceans or worms. Basket stars in particular may be capable of suspension feeding, using the mucus coating on their arms to trap plankton and bacteria. They extend one arm out and use the other four as anchors. Brittle stars will eat small suspended organisms if available. In large, crowded areas, brittle stars eat suspended matter from prevailing seafloor currents.

In basket stars the arms are used to rhythmically sweep food to the mouth. Pectinura will consume beech
Beech is a genus of ten species of deciduous trees in the family Fagaceae, native to temperate Europe, Asia and North America.-Habit:...

Pollen is a fine to coarse powder containing the microgametophytes of seed plants, which produce the male gametes . Pollen grains have a hard coat that protects the sperm cells during the process of their movement from the stamens to the pistil of flowering plants or from the male cone to the...

 in the New Zealand
New Zealand
New Zealand is an island country in the south-western Pacific Ocean comprising two main landmasses and numerous smaller islands. The country is situated some east of Australia across the Tasman Sea, and roughly south of the Pacific island nations of New Caledonia, Fiji, and Tonga...

 fjords (since those trees hang over the water). Eurylina
Basket star
Basket stars are a group of brittle stars. They are treated as a suborder Euryalina or order Euryalida. Many of them have characteristic many-branched arms. They generally live in deep sea habitats.The life span in the wild is up to 35 years. They weigh around 11 lbs, or 5 kg...

 clings to coral branches to browse on the polyps.


Gas exchange and excretion occur through cilia-lined sacs called bursae; each opens between the arm bases on the underside of the disc. Typically there are ten bursae, and each fits between two stomach digestive pouches. Water flows through the bursae by means of cilia or muscular contraction. Oxygen is transported through the body via the hemal system, a series of sinuses and vessels distinct from the water vascular system.

The bursae are probably also the main organs of excretion, with phagocytic
Phagocytes are the white blood cells that protect the body by ingesting harmful foreign particles, bacteria, and dead or dying cells. Their name comes from the Greek phagein, "to eat" or "devour", and "-cyte", the suffix in biology denoting "cell", from the Greek kutos, "hollow vessel". They are...

 "coelomocytes" collecting waste products in the body cavity and then migrating to the bursae for expulsion from the body.

Musculo-skeletal system

Like all echinoderms, the Ophiuroidea possess a skeleton of calcium carbonate
Calcium carbonate
Calcium carbonate is a chemical compound with the formula CaCO3. It is a common substance found in rocks in all parts of the world, and is the main component of shells of marine organisms, snails, coal balls, pearls, and eggshells. Calcium carbonate is the active ingredient in agricultural lime,...

 in the form of calcite
Calcite is a carbonate mineral and the most stable polymorph of calcium carbonate . The other polymorphs are the minerals aragonite and vaterite. Aragonite will change to calcite at 380-470°C, and vaterite is even less stable.-Properties:...

. In ophiuroids, the calcite ossicles are fused to form armor plates which are known collectively as the test. The plates are covered by the epidermis
Epidermis (zoology)
The Epidermis is an epithelium that covers the body of an eumetazoan . Eumetazoa have a cavity lined with a similar epithelium, the gastrodermis, which forms a boundary with the epidermis at the mouth.Sponges have no epithelium, and therefore no epidermis or gastrodermis...

, which consists of a smooth syncytium
In biology, a syncytium is a large cell-like structure; filled with cytoplasm and containing many nuclei. Most cells in eukaryotic organisms have a single nucleus; syncytia are specialized forms used by various organisms.The term may also refer to cells that are connected by specialized membrane...

. In most species, the joints between the ossicles and superficial plates allow the arm to bend to the side, but not to bend upwards. However, in the basket star
Basket star
Basket stars are a group of brittle stars. They are treated as a suborder Euryalina or order Euryalida. Many of them have characteristic many-branched arms. They generally live in deep sea habitats.The life span in the wild is up to 35 years. They weigh around 11 lbs, or 5 kg...

s, the arms are flexible in all directions.

Both the Ophiurida and Euryalida (the basket stars) have five long, slender, flexible whip-like arms, up to 60 centimeters in length. They are supported by an internal skeleton of calcium carbonate plates that are referred to as vertebral ossicles. These "vertebrae" articulate by means of ball-in-socket joints, and are controlled by muscles. They are essentially fused plates which correspond to the parallel ambulacral plates in sea stars and 5 Paleozoic families of ophiuroids. In modern forms the vertebrae are along the median of the arm.

The ossicles are surrounded by a relatively thin ring of soft tissue, and then by four series of jointed plates, one each on the upper, lower, and the lateral surfaces of the arm. The two lateral plates often have a number of elongated spines projecting outwards; these help to provide traction against the substrate while the animal is moving. The spines, in ophiuroids, compose a rigid border to the arm edges, whereas in euryalids they are transformed into downward-facing clubs or hooklets. Euryalids are similar to ophiurids, if larger, but their arms are forked and branched. Ophiuroid podia generally function as sensory organs. They are not usually used for feeding, as in Asteroidea. In the Paleozoic era brittle stars had open ambicular grooves but in modern forms these are turned inward.

In living ophiuroids the vertebrates are linked by well-structured longitudinal muscles. Ophiuroida move horizontally, and Euryalina move vertically. The latter have bigger vertebrae and smaller muscles. They are less spasmodic, but can coil their arms around objects, holding even after death. These movement patterns are distinct to the taxa, separating them. Ophiuroida move quickly when disturbed. One arm presses ahead, whereas the other four act as two pairs of opposite levers, thrusting the body in a series of rapid jerks. Although adults do not use their tube feet for locomotion, very young stages use them as stilts and even serve as an adhesive structure.


The sexes are separate in most species, though a few are hermaphroditic or protandric. The gonad
The gonad is the organ that makes gametes. The gonads in males are the testes and the gonads in females are the ovaries. The product, gametes, are haploid germ cells. For example, spermatozoon and egg cells are gametes...

s are located in the disc, and open into pouches in between the arms, called genital bursae. Fertilization is external in most species, with the gamete
A gamete is a cell that fuses with another cell during fertilization in organisms that reproduce sexually...

s being shed into the surrounding water through the bursal sacs. An exception is the Ophiocanopidae, in which the gonads do not open into bursae and are instead paired in a chain along the basal arm joints.

Many species brood developing larvae in the bursae, effectively giving birth to live young. A few, such as Amphipholus squamata are truly viviparous, with the embryo receiving nourishment from the mother through the wall of the bursa. However, there are some species that do not brood their young, and instead have a free-swimming larval stage. Referred to as an ophiopluteus, these larvae have four pairs of rigid arms lined with cilia. They develop directly into an adult, without the attachment stage found in most starfish larvae. The number of species exhibiting ophiopluteus larvae are fewer than those that directly develop.

In a few species the female carries a dwarf male, clinging to it with the mouth.

Life span

Brittle stars generally sexually mature in 2 years, become full grown in 3 to 4 years, and live up to 5 years. Euryalina, such as Gorgonocephalus, may well live much longer.


Ophiuroids can readily regenerate lost arms or arm segments unless all arms are lost. Ophiuroids use this ability to escape predators, in a way similar to lizards which deliberately shed (autotomy
Autotomy or self amputation is the act whereby an animal severs one or more of its own appendages, usually as a self-defense mechanism designed to elude a predator's grasp...

) the distal part of their tails to confuse pursuers. Moreover, the Amphiuridae can regenerate gut and gonad fragments lost along with the arms. No discarded arms have been shown to have the ability to regenerate.


Some brittle stars, such as the six-armed members of the family Ophiactidae
Ophiactidae are a family of brittle stars.It includes the following genera:*Amphiactis*Hemipholis*Histampica*Ophiactis*Ophiodaphne*Ophiolypus*Ophiopholis*Ophiopus*Ophiosphaera...

, exhibit fissiparity (division though fission), with the disc splitting in half. Regrowth of both the lost part of the disc and the arms occur which yields an animal with three large arms and three small arms during the period of growth.

The West Indian brittle star, Ophiocomella ophiactoides, frequently undergoes asexual reproduction
Asexual reproduction
Asexual reproduction is a mode of reproduction by which offspring arise from a single parent, and inherit the genes of that parent only, it is reproduction which does not involve meiosis, ploidy reduction, or fertilization. A more stringent definition is agamogenesis which is reproduction without...

 by fission of the disc with subsequent regeneration of the arms. In both summer and winter, large numbers of individuals with three long arms and three short arms can be found. Other individuals have half a disc and only three arms. A study of the age range of the population indicates that there is little recruitment and that fission is the primary means of reproduction in this species.

In this species, fission appears to start with the softening of one side of the disc and the initiation of a furrow. This deepens and widens until it extends across the disc and the animal splits in two. New arms begin to grow before the fission is complete, thus minimizing the time between possible successive divisions. The plane of fission varies so that some newly formed individuals have existing arms of different lengths. The time period between successive divisions is 89 days so that theoretically, each brittle star can produce 15 new individuals during the course of a year.


Brittle stars use their arms for locomotion. They do not, like sea stars, depend on tube feet, which are mere sensory tentacles without suction. Brittle stars move fairly rapidly by wriggling their arms which are highly flexible and enable the animals to make either snake-like or rowing movements. However, they tend to attach themselves to the seafloor or to sponges or cnidarians, such as coral. Their movement has some similarities with animals with bilateral symmetry.


Brittle stars live in areas from the low-tide level downwards. Six families live at least 2 meters deep; the genera Ophiura, Amphiophiura, and Ophiacantha range below 4 meters. Shallow species live among sponges, stones, or coral, or under the sand or mud, with only their arms protruding. Two of the best-known shallow species are the green brittle star (Ophioderma brevispina), found from Massachusetts to Brazil, and the common European brittle star (Ophiothrix fragilis
Ophiothrix fragilis
Ophiothrix fragilis is a species of brittle star in the order Ophiurida. It is found around the coasts of northwestern Europe and is known in Britain as the common brittle star.-Description:...

). Deep-water species tend to live in or on the sea floor or adhere to coral or urchins. The most widespread species is the long-armed brittle star (Amphipholis squamata), a grayish or bluish species that is strongly luminescent.


The main parasite to enter the digestive tract or genitals are Protozoa. Crustaceans, nematodes, trematodes, and polychaete annelids, also serve as parasites. Algal parasites like Coccomyxa ophiurae cause spinal malformation. Unlike sea stars and sea urchins, annelids are not a typical parasite.


Brittle stars are a moderately popular invertebrate in fishkeeping
Fishkeeping is a popular hobby concerned with keeping fish in a home aquarium or garden pond. There is also a fishkeeping industry, as a branch of agriculture.-Types of fishkeeping systems:...

. They can easily thrive in marine tanks, in fact the micro brittle star is a common "hitchhiker" that will propagate and become common in almost any saltwater tank, if one happens to come along on some live rock
Live rock
Live rock is rock from the ocean that has been introduced into a saltwater aquarium. Along with live sand, it confers to the closed marine system multiple benefits desired by the saltwater aquarium hobbyist...


Larger brittle stars are popular because, unlike asteriodae, they are not generally seen as a threat to coral, and are also faster-moving and more active than their more archetypical cousins.
The source of this article is wikipedia, the free encyclopedia.  The text of this article is licensed under the GFDL.