Looking like Long-Limbed versions of their close relatives, the starfishes, brittle star occurs in huge numbers around the sea coast. Sometimes thousands of individuals can be found crammed into a single square yard of the sea bed.
Brittle star belongs to a group of marine animals called the echinoderms, along with other, more familiar, creatures such as star-fishes, and sea-urchins. Brittle star resembles starfishes closely-both have five arms radiating from a central body but on the brittle star the arms are much more clearly demarcated from the body and they are longer and thinner, even snake-like.
Indeed, the zoological term for the brittle star, the ophiuroids, comes from the Greek for a snake, ‘ophis’. Their common name refers to the fact that the arms are brittle and break off easily.
Like the other echinoderms, the brittle star is exclusively marine. They may be found under stones and among seaweed at low tide but they are far more common below the intertidal zone. Here, they often occur in enormous numbers, either on the sea-bed or partly buried in sediment The common brittle star (Ophiothrix fragilis), for example, has been recorded at densities of 2000 per square meter and many underwater photographs show brittle star to be the dominant organisms over large areas of the sea-bed.
Depending on the species, the central body of a brittle star may be circular, star-shaped, or pentagonal. The upper surface is covered with tissue which often extends between the five arms round to the underside where the mouth lies. The enveloping tissue does not quite reach the arms themselves.
Instead, it stops short, leaving slits between the sides of the arms and the central disc. These slits are called genital or bursal slits and they lead inside the body to thin-walled sacs called bursae, which act as gills and are also associated with the reproductive organs.
The arms are jointed and can bend in the horizontal plane, but not vertically. Each arm contains a series of bone-like ossicles joined together in a row, in a similar manner to their own backbone. In extreme cases, fractures can occur between ossicles, allowing the arm to break in two and providing the brittle star with a useful means of escaping when caught.
Each ossicle is surrounded by four shields one above, one below and one on each side – giving the arms a jointed appearance, though this is not always obvious in species where the arms are covered by tissue. Each of the side shields usually has between two and fifteen spines emerging from it the arrangement and number of these spines is often an essential clue to identifying different species!
Arms for Moving and Feeding
Brittle star uses the sinuous bending of their arms to move along. Usually, the central disc is held above the substratum and, while one arm trails or leads. The other two pairs thrust the brittle star forward in a rowing action. Less active brittle stars tend to extend one arm, wrap it around a stationary object, and haul the rest of the body along.
Other species, however, can climb and some even manage to swim jerkily. An equally important function of the arms is feeding. This is accomplished by means of suckers called tube-feet that protrude between the shields on the arms. These tube feet are similar to those on the underside of starfish’s arm, but in the latter case, they are used primarily for movement, whereas on the brittle star they are used to collect food material and transfer it to the mouth.
Only occasionally are they used to provide some purchase on the substratum, for example when climbing. Although some brittle star is scavenger most feed on small particles of detritus. Some Teed by using their tube-feet to sort out and pick up particles lying on the sea-bed, while others filter-feed by holding up one or more arms and trapping drifting food particles on their tube-feet or between the spines on their arms.
The tube-feet, with the assistance of small hairs, called cilia, pass the food along the arms to the mouth. The first one or two pairs of tube-feet closest to the mouth act as taste organs as well as helping to select and pass along food, and they are sometimes found well inside the mouth.
The mouth is shaped like a five-pointed star, the five inner projections being known as jaws because they are lined with ossicles acting as teeth. The mouth leads into a large sac-like stomach, but there is no intestine or anus, and indigestible material has to be voided via the mouth.
In most species of brittle star males and females form separate individuals. Reproduction usually occurs by either eggs or sperm being released through the bursal slits into the open sea, where the eggs are fertilized. As with other echinoderms, the adults are bottom-dwelling yet they develop from a free-swimming larva. The larva of a brittle star is microscopically small and consists of a body with long extensions.
The extensions are covered in cilia which beat to create feeding currents and to drive the larva along, so dispersing the species. Eventually, the larva settles on the sea-bed and metamorphoses into the adult form. The young adult may not become sexually mature for another two years.
Some species of the brittle stars are hermaphroditic, having both sexes present in the same individual. These species may not have a larval stage; instead, they brood their young in their bursae. When they’re young emerge, they look like replicas of their parent. As well as sexual reproduction, some brittle star is capable of reproducing themselves asexually by dividing into two.
British Brittle star
Worldwide, there are about 2000 species of brittle star, of which approximately 100 can be found in British waters. The most common species of Ophiothrix fragilis which, as its specific name indicates is very fragile and often seen with arms regenerating. Like many brittle stars its color is variable, ranging from white to brown, red, or even violet.
The disc is about 2cm (in) in diameter and each arm is about five times as long as that. It can often be found under stones or among seaweed low down on rocky shores and is common on the sea-bed. Another species often found associated with Ophiothrix fragilis on the lower shore is the small brooding brittle star (Amphipholis squamata).
This species, one of the few hermaphroditic brittle star, is blue-grey in color and only a quarter the size of the common brittle star and another species, Acrocnida brachiata, burrows into the substratum at the bottom of sandy shores. This brittle star is distinguished by its extremely long arms, which may be up to 15 times as long as its disc diameter.
As the disc is 13mm (4in) across, each arm is therefore about 20cm (8in) long. It burrows into the sand to a depth of about 10cm (4in) leaving just the end of one arm above the surface to catch food particles drifting by. One of the larger
British brittle star is Ophiura texturata; it has a central disc about 3cm (1in) long and arms four times longer It, too, burrows in the sand from the lower shore downwards and is sometimes found washed up on the upper shore.
Brittle star is not toxic due to its robust skeleton and not used as food. Therefore, some species have blunt spines, but they are not venomous or dangerous. They are harmless to humans. Although, when they feel from danger from their predators, they just prefer to escape or discarding an arm. Brittle star is a popular invertebrate in fish keeping. They can easily thrive in a fish tank. Brittle life is concise; they are sexually mature at 2 to 3 years; however; they live up to five years only.
Acrocnida brachiata burrowing into the sand as this species has an extremely long arms-from tip to tip it can measure up to 40cm (16in) across.
All brittle stars have spines on their arms but on the common brittle star, they are particularly noticeable. The spines help to grip the substratum during movement.
The underside of Ophiura texturata, one of our few burrowing brittle stars, clearly showing its star-shaped mouth. The upper surface of this species is orange-brown. The underside of a typical brittle star is shown on the right. A dense colony of a common brittle star on the bottom of Strangford Lough, a saltwater lough on the east coast of Northern Ireland.
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