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Brittlestar

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Brittlestars

Brittlestars - WWC Archives

The Brittlestar (Ophiura ophiura), its body comprises of a central, flattened disc and five brittle, narrow arms. This is how it gets its name. It has five teeth surrounding its mouth, through which it is able to push out its stomach to digest prey.

DescriptionEdit

O. ophiura has a circular central disc up to 35 mm (1.5 in) wide and five radially arranged, narrow arms each up to 140 mm (6 in) long. The general colour is mottled reddish-brown with a paler underside. Both the top and the underside of the disc are covered with calcareous plates. The arms are joined to the top rather than the edge of the disc and further small, articulating plates allow the arms to bend from side to side. There are small spines on the arms which lie flat against the surface. There are four larger plates across the root of each arm with the outer pair having a comb-like edge, with twenty to thirty fine papillae in each. There are a pair of pores between the underside plates at the root of the arms.[1]There are five large mouth-shield plates on the underside of the disc which surround the central mouth. The teeth are in a vertical row above each of the five jaws and there are about five mouth papillae on each side of the jaw.

Distribution and habitatEdit

O. ophiura is found on the sea floor in the north east Atlantic Ocean and the North Sea from Norway and Sweden south to Madeira and the Mediterranean Sea. It is found below low tide mark in the neritic zone down to a depth of about two hundred metres, on sandy bottoms. It shows a preference for sediments with a fine grain size and approximately 35% mud content. It is a common species with twenty individuals occurring per square metre in some years in the North Sea with a maximum of fifty.

BiologyEdit

O. ophiura is an active brittle star, moving with a jerky swimming action of its legs and sometimes burrowing. It is a filter feeder, feeding on a wide range of food, but also a bottom-feeding carnivore and detrivore. It can regenerate its arms if they are damaged or torn off.

Sexual reproduction takes place during the summer. The larvae are the typical ophiopluteus larvae of brittle stars and later settle on the sea bed and develop into juveniles.

EcologyEdit

The copepod, Parartotrogus richardi, is an ectoparasite of O. ophiura.

In the Clyde Sea fishery for scampi (Nephrops norvegicus) in Scotland, the unwanted invertebrates that get caught up in the trawl include O. ophiura as well as the starfish Asterias rubens. A study was undertaken to discover the survival rate of these animals when discarded and returned to the water. It was found that uninjured A. rubens had a mortality rate of 4% whereas virtually all the O. ophiura died within 14 days, even when they were returned to the sea immediately after being caught.

Another study examined the rate at which the discarded invertebrates sank to the bottom and their ultimate fates. O. ophiura sank relatively slowly and were preyed upon by seabirds, and the arms were eaten by fish. On the sea bed it was found that a succession of benthic scavengers thrived on their remains with crangonid shrimps and crabs such as Carcinus maenas and Liocarcinus depurator being prominent. In six hours little remained except the limbs of crustaceans and the discs of ophiuroids. The crab Pagurus bernhardus was the most likely scavenger to consume O. ophiura in baited traps.

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