Ophiothrix fragilis
Encyclopedia
Ophiothrix fragilis is a species
of brittle star
in the order
Ophiurida
. It is found around the coasts of northwestern Europe and is known in Britain as the common brittle star.
. It ranges southwards from the Lofoten
Islands and Iceland
to the Mediterranean Sea
and the Azores
, 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 down to 350 metres.
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
estuary in Holland.
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
d so that it blends in with its surroundings which combine to reduce predation
. 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
under its cuticle
. The host-bacteria association can be upset by acute stress
, and changes in levels of bacteria present can be used as an indicator of stress.
This brittle star usually spawns
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. They metamorphose
into young brittle stars which drift in the plankton
for about three weeks before settling. The smallest brittle stars found have just two segments per arm and a disc diameter of two millimetres.
Species
In biology, a species is one of the basic units of biological classification and a taxonomic rank. A species is often defined as a group of organisms capable of interbreeding and producing fertile offspring. While in many cases this definition is adequate, more precise or differing measures are...
of brittle star
Brittle star
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 in length on the largest specimens...
in the order
Order (biology)
In scientific classification used in biology, the order is# a taxonomic rank used in the classification of organisms. Other well-known ranks are life, domain, kingdom, phylum, class, family, genus, and species, with order fitting in between class and family...
Ophiurida
Ophiurida
The Ophiurida are an order of echinoderms within the class Ophiuroidea. It includes the vast majority of living brittle stars.-Characteristics:The upper surface of the disk of Ophiurida is covered with scales....
. It is found around the coasts of northwestern Europe and is known in Britain as the common 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. The dorsal arm plates are naked and have a longitudinal keel. 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 SeaNorth Sea
In the southwest, beyond the Straits of Dover, the North Sea becomes the English Channel connecting to the Atlantic Ocean. In the east, it connects to the Baltic Sea via the Skagerrak and Kattegat, narrow straits that separate Denmark from Norway and Sweden respectively...
. It ranges southwards from the Lofoten
Lofoten
Lofoten is an archipelago and a traditional district in the county of Nordland, Norway. Though lying within the Arctic Circle, the archipelago experiences one of the world's largest elevated temperature anomalies relative to its high latitude.-Etymology:...
Islands and Iceland
Iceland
Iceland , described as the Republic of Iceland, is a Nordic and European island country in the North Atlantic Ocean, on the Mid-Atlantic Ridge. Iceland also refers to the main island of the country, which contains almost all the population and almost all the land area. The country has a population...
to the Mediterranean Sea
Mediterranean Sea
The Mediterranean Sea is a sea connected to the Atlantic Ocean surrounded by the Mediterranean region and almost completely enclosed by land: on the north by Anatolia and Europe, on the south by North Africa, and on the east by the Levant...
and the Azores
Azores
The Archipelago of the Azores is composed of nine volcanic islands situated in the middle of the North Atlantic Ocean, and is located about west from Lisbon and about east from the east coast of North America. The islands, and their economic exclusion zone, form the Autonomous Region of the...
, and along the west coast of Africa south to the Cape of Good Hope
Cape of Good Hope
The Cape of Good Hope is a rocky headland on the Atlantic coast of the Cape Peninsula, South Africa.There is a misconception that the Cape of Good Hope is the southern tip of Africa, because it was once believed to be the dividing point between the Atlantic and Indian Oceans. In fact, the...
. 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 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
Scavenger
Scavenging is both a carnivorous and herbivorous feeding behavior in which individual scavengers search out dead animal and dead plant biomass on which to feed. The eating of carrion from the same species is referred to as cannibalism. Scavengers play an important role in the ecosystem by...
, 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
Oosterschelde
The Oosterschelde is an estuary in Zeeland, Netherlands, between Schouwen-Duiveland and Tholen on the north and Noord-Beveland and Zuid-Beveland on the south.During the Roman Era it was the major mouth of the Scheldt River. Before the St...
estuary in Holland.
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
Camouflage is a method of concealment that allows an otherwise visible animal, military vehicle, or other object to remain unnoticed, by blending with its environment. Examples include a leopard's spotted coat, the battledress of a modern soldier and a leaf-mimic butterfly...
d so that it blends in with its surroundings which combine to reduce predation
Predation
In ecology, predation describes a biological interaction where a predator feeds on its prey . Predators may or may not kill their prey prior to feeding on them, but the act of predation always results in the death of its prey and the eventual absorption of the prey's tissue through consumption...
. 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
Symbiosis
Symbiosis is close and often long-term interaction between different biological species. In 1877 Bennett used the word symbiosis to describe the mutualistic relationship in lichens...
bacteria
Bacteria
Bacteria are a large domain of prokaryotic microorganisms. Typically a few micrometres in length, bacteria have a wide range of shapes, ranging from spheres to rods and spirals...
under its cuticle
Cuticle
A cuticle , or cuticula, is a term used for any of a variety of tough but flexible, non-mineral outer coverings of an organism, or parts of an organism, that provide protection. Various types of "cuticles" are non-homologous; differing in their origin, structure, function, and chemical composition...
. The host-bacteria association can be upset by acute stress
Stress (biology)
Stress is a term in psychology and biology, borrowed from physics and engineering and first used in the biological context in the 1930s, which has in more recent decades become commonly used in popular parlance...
, and changes in levels of bacteria present can be used as an indicator of stress.
This brittle star usually spawns
Spawn (biology)
Spawn refers to the eggs and sperm released or deposited, usually into water, by aquatic animals. As a verb, spawn refers to the process of releasing the eggs and sperm, also called spawning...
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. They metamorphose
Metamorphosis
Metamorphosis is a biological process by which an animal physically develops after birth or hatching, involving a conspicuous and relatively abrupt change in the animal's body structure through cell growth and differentiation...
into young brittle stars which drift in the plankton
Plankton
Plankton are any drifting organisms that inhabit the pelagic zone of oceans, seas, or bodies of fresh water. That is, plankton are defined by their ecological niche rather than phylogenetic or taxonomic classification...
for about three weeks before settling. The smallest brittle stars found have just two segments per arm and a disc diameter of two millimetres.