''Ophiothrix fragilis'' is a

species In biology, a species is the basic unit of classification and a taxonomic rank of an organism, as well as a unit of biodiversity. A species is often defined as the largest group of organisms in which any two individuals of the appropriate s ...

brittle star Brittle stars, serpent stars, or ophiuroids (; ; referring to the serpent-like arms of the brittle star) are echinoderms in the class Ophiuroidea, closely related to starfish. They crawl across the sea floor using their flexible arms for locomot ...

in the order

Ophiurida The Ophiurida are an order of echinoderms within the class Ophiuroidea. It includes the vast majority of living brittle stars. Characteristics Ophiurida have bursae for respiration and excretion, and dorsal and ventral arm shields are present a ...

. It is found around the coasts of western Europe and is known in Britain as the common brittle star. It is also found along the coast of South Africa where it is known as the hairy brittle star.

Description

The common brittle star is extremely variable in colouration, ranging from violet, purple or red to yellowish or pale grey, often spotted with red. The arms are usually white or grey with pink bands. The central disc is about one centimetre in diameter with the five arms being about five times as long. The disc is clothed in five rays of spines radiating from a spiny centre. Between these are five pairs of triangular plates, each pair forming a heart shaped pattern. The slender tapering arms are quite distinct from the disc and are covered with overlapping scales.John Barrett and C M Young, ''Collins Pocket Guide to the Sea Shore'' (1958) p.180 The dorsal arm plates are naked and have a longitudinal keel.Marine Species Identification Portal
/ref> Each arm segment bears seven glassy, toothed spines. The arms are extremely fragile and easily shed, coming away either whole or in pieces.

Distribution and habitat

This species is common around the coasts of Britain and Ireland and in the

North Sea The North Sea lies between Great Britain, Norway, Denmark, Germany, the Netherlands and Belgium. An epeiric sea, epeiric sea on the European continental shelf, it connects to the Atlantic Ocean through the English Channel in the south and the ...

. It ranges southwards from the

Lofoten Lofoten () is an archipelago and a traditional district in the county of Nordland, Norway. Lofoten has distinctive scenery with dramatic mountains and peaks, open sea and sheltered bays, beaches and untouched lands. There are two towns, Svolv� ...

Islands and

Iceland Iceland ( is, Ísland; ) is a Nordic island country in the North Atlantic Ocean and in the Arctic Ocean. Iceland is the most sparsely populated country in Europe. Iceland's capital and largest city is Reykjavík, which (along with its s ...

to the

Mediterranean Sea The Mediterranean Sea is a sea connected to the Atlantic Ocean, surrounded by the Mediterranean Basin and almost completely enclosed by land: on the north by Western and Southern Europe and Anatolia, on the south by North Africa, and on the ...

and the

Azores ) , motto =( en, "Rather die free than subjected in peace") , anthem= ( en, "Anthem of the Azores") , image_map=Locator_map_of_Azores_in_EU.svg , map_alt=Location of the Azores within the European Union , map_caption=Location of the Azores wi ...

, and along the west coast of Africa south to the Cape of Good Hope. It is most common on tide-swept rock and on coarse sediments, preferring hard substrates including sand and shingle. It is often found in empty shells or under stones, from the

littoral zone The littoral zone or nearshore is the part of a sea, lake, or river that is close to the shore. In coastal ecology, the littoral zone includes the intertidal zone extending from the high water mark (which is rarely inundated), to coastal a ...

down to 350 metres.

Biology

The common brittle star sometimes congregates offshore in vast numbers and as many as two thousand individuals have been recorded in a single square metre. Higher up the shore it normally lives a solitary existence under seaweed or pebbles or in crevices. It moves by jerking a pair of limbs forward and pulling itself along. The common brittle star is a scavenger, feeding on dead organisms. It is also a suspension feeder, raising an arm and extending the tube feet in order to catch particles floating by. It then passes the food to the mouth with its arms. Its abundance varies according to environmental conditions including temperature and the availability of food. After mild winters, it has been found in very large numbers in the

Oosterschelde The Eastern Scheldt ( nl, Oosterschelde) is a former estuary in the province of Zeeland, Netherlands, between Schouwen-Duiveland and Tholen on the north and Noord-Beveland and Zuid-Beveland on the south. It also features the largest natio ...

estuary in the Netherlands. The common brittle star is itself eaten by other species and can be found among the stomach contents of most common predators. However its glassy spicules may make it unpalatable, it lives in concealed places and is well

camouflage Camouflage is the use of any combination of materials, coloration, or illumination for concealment, either by making animals or objects hard to see, or by disguising them as something else. Examples include the leopard's spotted coat, the b ...

d so that it blends in with its surroundings which combine to reduce

predation Predation is a biological interaction where one organism, the predator, kills and eats another organism, its prey. It is one of a family of common feeding behaviours that includes parasitism and micropredation (which usually do not kill th ...

. A study of skeletal bands suggests that it may live for as many as ten years. Like some other brittle stars, ''Ophiothrix fragilis'' has symbiotic

bacteria Bacteria (; singular: bacterium) are ubiquitous, mostly free-living organisms often consisting of one Cell (biology), biological cell. They constitute a large domain (biology), domain of prokaryotic microorganisms. Typically a few micrometr ...

under its cuticle. The host-bacteria association can be upset by acute

stress Stress may refer to: Science and medicine * Stress (biology), an organism's response to a stressor such as an environmental condition * Stress (linguistics), relative emphasis or prominence given to a syllable in a word, or to a word in a phrase ...

, and changes in levels of bacteria present can be used as an indicator of stress.Newton, L.C. & McKenzie, J.D., (1995). Echinoderms and oil pollution: a potential stress assay using bacterial symbionts. Marine Pollution Bulletin, 31, 453-456. This brittle star usually

spawns Spawn is the eggs and sperm released or deposited into water by aquatic animals. As a verb, ''to spawn'' refers to the process of releasing the eggs and sperm, and the act of both sexes is called spawning. Most aquatic animals, except for aquat ...

between May and January. Each individual is thought to spawn just once during each breeding season. A week after spawning, the planktonic larvae appear in the

water column A water column is a conceptual column of water from the surface of a sea, river or lake to the bottom sediment.Munson, B.H., Axler, R., Hagley C., Host G., Merrick G., Richards C. (2004).Glossary. ''Water on the Web''. University of Minnesota-D ...

. They

metamorphose Metamorphosis is a biological process by which an animal physically develops including birth or hatching, involving a conspicuous and relatively abrupt change in the animal's body structure through cell growth and differentiation. Some insec ...

into young brittle stars which drift in the

plankton Plankton are the diverse collection of organisms found in water (or air) that are unable to propel themselves against a current (or wind). The individual organisms constituting plankton are called plankters. In the ocean, they provide a crucia ...

for about three weeks before settling. The smallest brittle stars found have just two segments per arm and a disc diameter of two millimetres.

References

External links

ARKive: video showing ''O. fragilis'' feeding
* {{Taxonbar, from=Q937173 Ophiotrichidae Animals described in 1789