Brittle star
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| Brittle Star Fossil range: Ordovician to Holocene |
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 "Ophiodea" from Ernst Haeckel's Kunstformen der Natur, 1904
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Brittle stars, or ophiurids, are echinoderms, closely related to starfish. They crawl across the sea-floor using their flexible arms for locomotion. The ophiuroids generally have five long slender, whip-like arms which may reach up to 60 centimeters (2 feet) in length on the largest specimens.
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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[edit] Range
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 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.
[edit] Disc and internal organs
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.
Of all echinoderms, the Ophiuroidea may have the strongest tendency toward 5-segment radial (pentaradial) symmetry. The body outline is similar to the Asteroidea, 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.
[edit] 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. Ophiuroids have no eyes, as such. However, they have some ability to sense light through receptors in the epidermis. These are especially found at the ends of their arms, detecting light and retreating into crevices.
[edit] Diet
The mouth is rimmed with five jaws, and serves as an anus (egestion) as well as 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 esentially ceca and extend into the arms, just like sea stars. The stomach wall contains glandular hepatic cells. They are generally scavengers or detritivores, which are selective due to their inability to digest mass mud intake unlike sea stars. 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 move around like starfish, and have tube feet.) Certain tube feet, derived from the ectoderm, can act as chemoreceptors. Nonetheless, brittle stars consume small organisms if available. In large, crowded areas, brittle stars eat suspended matter, and sea-floor currents vindicate this. In basket stars the arms are used to rhythmically aweep food to the mouth. Pectinura will consume beech pollen in the New Zealand fiords (since the trees thereof hang over the water). Eurylina will cling to a coral branch browse on the polyps of the reef.
[edit] Respiration
Gas exchange and excretion occur through cilia-lined sacs called bursae; each opens onto the interambulacral area (between the arm bases) of the oral (ventral) surface of the disc. Typically there are 10 bursae, and each fits between two stomach digestive pouches.
[edit] Reproduction
The sexes are separate in most species, though a few, e.g., Amphipholis squamata, are hermaphroditic. Gonads, found only in the disc, open into the pouches in the integument between radii, called genital bursae. Gametes are then shed by way of the bursal sacs. Many species actually brood developing larvae in the bursae. The ophiuroid coelom is strongly reduced, particularly in comparison to other echinoderms. In a few species the female carries a dwarf male, clinging to it.
[edit] 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.
[edit] Regeneration
Ophiuroids can readily regenerate lost arms or arm segments unless all arms are lost. Ophiuroids use this ability to escape predators, similar to how lizards deliberately shed (autotomize) the distal part of their tails to confuse pursuers.
[edit] Musculoskeletal System
[edit] Skeleton
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 the vertebrae are along the median of the arm. The body and arms also bear calcite plates (ventral and dorsal) and delicate spines (lateral), which protect the vertebral column. The spines, in ophiuroids, compose an 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.
[edit] Muscles
In living ophiuroids the vertebrates are linked by well-structured longitudal 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 a`fter 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.
[edit] Locomotion
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. Their movement has some similarities with animals with bilateral symmetry.
[edit] Water-vascular system
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.
[edit] Brittle stars in the environment
[edit] Ecology
Brittle stars live in areas from the low-tide level downwards. Six families live at least 2 mi, the genera Ophiura, Amphiophiura, and Ophiacantha range below 4 mi. Shallow species live among sponges, stones, or coral, or under the sand or mud, with only its arms protruding. Deep-water species tend to live in or on the sea floor or adhere to coral or an urchin.
[edit] Parasites
The main parasite to enter the digestive tract or genitals are Protozoa. Crustaceans, nematodes, trematodes, and polychaete annelids, also serve as parasites. Algal parsites like Coccomyxa ophiurae cause spinal malformation. Unlike sea stars and sesa urchins, annelids are not a typical parasite.
[edit] Human relations
Brittle stars are not food but are non-venomous. However, when the food chain of commercially important species involves brittle stars, their large population is considered.
[edit] References
- Andrew B. Smith, Howard B. Fell, Daniel B. Blake, Howard B. Fell, "Ophiuroidea", in AccessScience@McGraw-Hill, http://www.accessscience.com, DOI 10.1036/1097-8542.471000
- David L. Pawson, Andrew C. Campbell, David L. Pawson, David L. Pawson, Raymond C. Moore, J. John Sepkoski, Jr., "Echinodermata", in AccessScience@McGraw-Hill, http://www.accessscience.com, DOI 10.1036/1097-8542.210700
Palaeos: Ophiuroidea
