| Brittle Star Temporal range: 488.2–0 Ma Ordovician to Holocene |
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|---|---|
| "Ophiodea" from Ernst Haeckel's Kunstformen der Natur, 1904 | |
| Scientific classification | |
| Kingdom: | Animalia |
| Phylum: | Echinodermata |
| Subphylum: | Asterozoa |
| Class: | Ophiuroidea Gray, 1840 |
| Orders | |
Brittle stars or ophiuroids are echinoderms 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 (24 in) in length on the largest specimens. They are also known as serpent stars.
Ophiuroidea contains two large clades, Ophiurida (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.
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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, crinoids, 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 communities, where they hide under rocks and even within other living organisms. A few ophiuroid species can even tolerate brackish water, 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, Phrynophiurida, and Ophiurida. 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.
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. 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.[1]
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 is usually located within one of the jaw plates, and not on the upper side of the animal as it is in starfish.[1]
The ophiuroid coelom is strongly reduced, particularly in comparison to other echinoderms.
The vessels of the water vascular system end in tube feet. 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 ampullae are absent from the tube feet.
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.[1]
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.[1] 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). 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.[1] The stomach wall contains glandular hepatic cells.
Ophiuroids are generally scavengers 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 pollen in the New Zealand fjords (since those trees hang over the water). Eurylina 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.[1]
The bursae are probably also the main organs of excretion, with phagocytic "coelomocytes" collecting waste products in the body cavity and then migrating to the bursae for expulsion from the body.[1]
Like all echinoderms, the Ophiuroidea possess a skeleton of calcium carbonate in the form of calcite. 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, which consists of a smooth syncytium. 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 stars, the arms are flexible in all directions.[1]
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 gonads are located in the disc, and open into pouches in between the arms, called genital bursae. Fertilization is external in most species, with the gametes being shed into the surrounding water through the bursal sacs.[1] 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.[1] 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.[1]
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) 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, exhibit fissiparity (division though fission), with the disc splitting in half. Regrowth of both the lost part of the disc and the arms occur [2] 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 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.[3]
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.[3]
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). 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 not used as food, even though they are non-toxic.
Brittle stars are a moderately popular invertebrate in fishkeeping. 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.
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.