Definitions

interambulacral

Brittle star

Brittle stars, or ophiurids, are echinoderms, closely related to sea stars. 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 centimeters (2 feet) 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.

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.

Taxonomy

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 species, the Stenurida.

The relationships among ophiuroids, asteroids, 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.

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.

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. 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 ambulacral groove between the plates is converted into an internal epineural canal, containing the nerve of each limb.

Alimentary System and 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 essentially 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 extend one arm out and use the other four as anchors. Nonetheless, brittle stars consume small organisms if available. In large, crowded areas, brittle stars eat suspended matter, and seafloor 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.

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.

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. An exception is the Ophiocanopidae, which do not open into bursae and are paired in a chain along the basal arm joints. 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. The number of species exhibiting ophiopluteus larvae are fewer than those that diretly develop.

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.

Regeneration

Ophiuroids can readily regenerate lost arms or arm segments unless all arms are lost. The disk is used to do this. Ophiuroids use this ability to escape predators, similar to how lizards deliberately shed (autotomize) the distal part of their tails to confuse pursuers. Moreover, the Amphiuridae can regenerate gut and gonad fragments lost along with the arms. The six-armed Ophiactidae generally exhibit transverse fission, which yields three large arms and three small arms. Viviparous species keep the young in the bursae. No discarded arms have shown ability to regeneration.

Musculoskeletal System

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 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.

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 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.

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. 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.

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.

Brittle stars in the environment

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. Two of the best-known shore 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 an urchin. The most widespread species is the long-armed brittle star (Amphipholis squamata), a grayish or bluish species that is strongly luminescent.

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.

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.

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
  • "brittle star."Encyclopædia Britannica. 2008. Encyclopædia Britannica 2006 Ultimate Reference Suite DVD 17 June 2008 .

Palaeos: Ophiuroidea

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