| Brittle Star | |
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| "Ophiodea" from Ernst Haeckel's Kunstformen der Natur, 1904 | |
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| Class: | Ophiuroidea Gray, 1840
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Brittle stars (also called serpent stars, Ophiuroidea) are a class of echinoderms, related to starfish. 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 crawl across the sea-floor using their flexible arms as "legs" for locomotion.
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 more than 2000 species[1] of brittle stars living today, and many of them are found in deep waters more than 500 metres (1,650 feet) down.
Systematics and phylogeny edit
Fossil record has traditionally supported a monophyletic asteroid plus ophiuroid clade (Asterozoa Zittel, 1896). The relationship between asteroids and ophiuroids has been difficult to address using molecular tools but the monophyly of brittle stars is strongly supported[2].
Paleontology edit
The ophiuroids diverged in the Early Ordovician, (ca. 500 million years ago). At the time of their first appearance, the ophiuroid body plan was already established. This suggests a significant but as yet unobserved pre-Ordovician history (vt lk 5, )[3] Somasteroidea? Protasteridae In the Permian–Triassic mass extinction event, the biodiversity of the Ophiuroidea did not undergo an evolutionary bottleneck but apparently increased, and in time, modern groups appeared (however, no Permian genera and species extend to the Lower Triassic[4]).
Systematics edit
The most recent version Smith, Paterson & Lafay (1995)
Anatomy and physiology edit
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 a central disk (also called calyx). 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, unlike in the Asteroidea.
Arms edit
Most of the Ophiurida and Euryalida 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 referred to as vertebral ossicles. These "vertebrae" articulate through ball-in-socket joints, and are controlled by muscles. The body and arms also bear calcite plates and delicate spines. The ambulacral grooves are closed, covered with arm ossicles. Euryalids are similar, 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, but for locomotion. They do not, like sea stars, depend on tube feet, but move often 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).
Skeleton edit
Like all echinoderms, the Ophiuroidea possess a skeleton of calcium carbonate. In ophiuroids, the calcite ossicles are fused to form armor plates which are known collectively as the test.
Digestive system edit
The ophiuroid coelom is strongly reduced, particularly in comparison to other echinoderms. The mouth is situated on the underside center and is rimmed with five jaws. Behind the jaws is a short esophagus and a large, blind stomach cavity which occupies much of the dorsal half of the disk. Digestion occurs within 10 pouches or infolds of the stomach. There is no anus, and the wastes are eliminated through the mouth.
Bursae edit
Gas exchange and excretion usually 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 (the openings are called bursal slits or genital slits), and water circulates in and out of them. Typically there are 5 pairs of bursae, and each fits between two stomach digestive pouches. In the wall of each bursa are also small gonads that discharge their ripe sex cells into the bursa when mating takes place[5].
Nervous system edit
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. In the case of Ophiocoma wendtii and some other species, it has been shown that calcite crystals of skeletal ossicles are also a component of specialized photosensory organs, conceivably with the function of a compound eye able to detect light from a particular direction. The optical performance of the lens array is further optimized by phototropic chromatophores that regulate the dose of illumination reaching the receptors[6].
Water vascular system edit
The vessels of the water vascular system consist of the main ring canal (with polian vesicles) and give off five radial vessels into arms (and also enters to buccal tube feet around the mouth). In arms, the branches end in tube feet. Suckers and ampullae are absent from the tube feet, but they may secrete sticky mucus.
The water vascular system generally has one madreporite located on one of the oral shield ossicles. However, in some forms it is reduced.
Reproduction and development edit
Reproduction edit
The sexes are separate in most species. Gonads in the disc open into the bursae. Gametes are then shed by way of the bursal sacs. Many species actually brood developing larvae in the bursae.
Development edit
Pluteus, ophiopluteus. Two types of larval development[7]
Regeneration edit
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 autotomize, or deliberately shed, part of their tails to confuse pursuers. Some can reproduce asexually by cleaving the disc, each progeny regenerating the missing parts.
Ecology edit
Ophiuroids can be found today in all of the major marine provinces, from the poles to the tropics. In fact, crinoids, holothurians, and ophiuroids rule the floor of the deep oceans at depths below 500 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.
Most ophiuroids are 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.
References edit
- ^ Sabine Stöhr & Tim O’Hara (2007). World Ophiuroidea database. Available online at http://www.marinespecies.org/ophiuroidea. Consulted on 2008-01-15
- ^ Pawson,D.L.,Phylum Echinodermata, Zootaxa 1668: 749–764 (2007)
- ^ J. Dean Shackleton. 2005. Skeletal homologies, phylogeny and classification of the earliest asterozoan echinoderms. Journal of Systematic Palaeontology 3(1):29-114
- ^ Chen,Z.Q.& McNamara,K.J.,End-Permian extinction and subsequent recovery of the Ophiuroidea (Echinodermata), Palaeogeography, Palaeoclimatology, Palaeoecology 236 (2006) 321– 344
- ^ R.C.Brusca, G.J.Brusca. Invertebrates. Sinauer Associates, 2 ed.(2003)
- ^ Aizenberg,J et al.,Calcitic microlenses as part of the photoreceptor system in brittlestars, Nature 412, 819-822 (2001)
- ^ Cisternas, P., Selvakumaraswamy, P. & Byrne, M. (2004) Evolution of development and the Ophiuroidea – revisited. In: Heinzeller, T. & Nebelsick, J.H. (Eds.) Echinoderms: München. Taylor & Francis, London, pp. 521–526
http://www.palaeos.com/Invertebrates/Echinoderms/Ophiuroidea/Ophiuroidea.htm