The arm-waving star

One of the seabed’s commonest animals

The common brittlestar is one of the most numerous animals on the seabed around Europe. And one of the most fragile. It is a small relative of the starfish. But it does not look like one at first. Its body is a neat central disc about the size of a fingernail. And from it spread five slender arms like the spokes of a wheel. The arms are so thin and brittle that they snap off at the lightest touch.

This is the source of the name. A predator that grabs an arm gets only the arm. While the brittlestar escapes. The lost arm grows back. In some places the animals pile up in dense carpets. Thousands to a square metre. Covering the floor of a shallow sea with a moving grey and purple mesh.

They are a key part of the seabed food web. And are eaten by fish, crabs and larger starfish. They are also a favourite of anyone who turns a stone in a rock pool. Because the arms wriggle and break as the animal tries to get away. The species has no commercial value and no common name beyond brittlestar. But it is one of the most studied of all echinoderms.

Family tree

Ophiotrichidae, the brittle stars

The common brittlestar belongs to the phylum Echinodermata. The group that also holds the starfish. Sea urchins, sea cucumbers and feather stars. Within it the animal sits in the class Ophiuroidea. The brittle stars. One of the largest classes of animals on the seabed. The class holds well over 2000 living species worldwide. The species sits in the genus Ophiothrix, a group of small. Spiny brittle stars in warm and temperate seas.

Otto Friedrich Müller described it in 1776. The genus name Ophiothrix comes from Greek words for snake and hair. A reference to the slender, whip-like arms. The family Ophiotrichidae is defined above all by the fine spines that cover the arms. And those spines are the easiest way to tell the group from the smooth-armed brittle stars. The common brittlestar is one of the most widespread members of its genus. It ranges from the cold north Atlantic to the warmer Mediterranean and Black Sea. It is not close to the true starfish. Though the two share the five-part body plan.

The common brittlestar is a distant relative of the spiny starfish. A much larger animal of the same phylum. The spiny starfish has a broad body and short, thick arms. And moves on tube feet. The brittlestar has a tiny disc and long, fragile arms. And moves by rowing those arms. The two are easy to tell apart. But they often live side by side. And the starfish is one of the brittlestar’s main predators.

Five arms, tiny disc

How to identify one

The common brittlestar is built for lightness and flexibility rather than strength. The central disc measures 1 to 2 cm across. And the five arms each run 8 to 15 cm long in a grown animal. The whole animal weighs almost nothing. The disc is flat and rounded. And is covered on the upper side by small plates and fine spines. The underside of the disc carries the mouth and a set of five jaw-like plates. There are no eyes and no brain. A ring of nerve tissue around the mouth does the work of a brain. The arms are the most striking part.

Each arm is a column of small jointed bones. Covered by four rows of short. Glassy spines that give it a rough, almost furry look. The spines carry sense organs and help the animal feel its way. The arms break with almost no force. At a clean joint so the animal loses as little as possible. A broken arm can regrow fully within a few weeks to a few months. Depending on size and water temperature. The colour is extremely variable. The disc and arms may be purple.

Red, yellow, brown, grey or a mix. And local populations often share a single colour. The underside is paler. The animal has no blood in the usual sense. It uses a water vascular system. A network of fluid canals, to move and breathe. Tube feet run along the lower side of each arm. But they are small and used more for feeling and holding than for walking. The arms do the walking. By curling and uncurling in a rowing motion that can carry the animal fast across the seabed.

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Beds on the seabed

Where it lives

The common brittlestar lives on the seabed in a wide range of depths and habitats. It is found from the low water mark down to about 200 m. And is most abundant on the shallow shelf at depths of 10 to 100 m. It needs a hard or firm surface to hold. It lives on rock, cobble. Shell gravel and the branches of dead coral and sponge. And is a common resident of shipwrecks. It avoids soft mud where it cannot grip. Though it will sit on a pebble on mud. It is a creature of the northeastern Atlantic.

From Norway and the Faroes south through the British Isles. France, Spain and Portugal. And also through the Mediterranean and Black Sea. It is one of the most common animals of the North Sea. In the right place it forms huge aggregations known as brittlestar beds. These beds can cover many hectares. And the density within them can reach 2000 to 3000 animals per square metre. A single bed may hold many millions of individuals. Such beds form where the current brings a steady supply of food and predators are few. The animals stack on top of one another in layers.

And the top layer feeds while the lower layers wait. The bed is a living carpet that shifts and folds as the animals move. Because the species is so numerous. It is a major food source for fish such as cod. Plaice and dab. And for crabs and larger starfish. Its range and numbers make it one of the most important animals of the temperate seabed. Even though most people never see it.

A catcher of plankton

What it eats

The common brittlestar feeds in two main ways. And switches between them with the conditions. It is above all a suspension feeder. It lifts its arms into the current and uses the sticky tips and tube feet to catch particles drifting past. The food is mostly the tiny plants and animals of the plankton. Together with the falling rain of dead material from the surface. The arms are held in a cupped posture that forms a small net. And the animal passes the caught particles down the arms to the mouth. It also browses.

It uses its arms to crawl over the surface and pick up detritus. Small worms and the films of algae on the rock. When food is scarce it can absorb material through the walls of the arms. A useful trick for an animal buried under others in a dense bed. The diet changes with size. Small brittlestars take the smallest particles. While large ones handle bigger prey. The animal is not a hunter in the active sense. It waits for food to come to it.

And makes the most of whatever the current delivers. In a brittlestar bed the combined filtering of millions of arms clears a great deal of material from the water. This links the species to the health of the whole seabed. The food supply is seasonal in shallow water. And the animals grow fastest in the warm months when plankton is abundant. In winter they slow down and live on stored reserves. They tolerate a wide range of salinity and temperature. This is one reason they live both in the open Atlantic and in the partly enclosed Baltic and Black Sea.

Arms that break and regrow

The breeding cycle

The common brittlestar has separate sexes. And reproduces by releasing eggs and sperm into the water. There is no care of the young after the spawn. In most populations spawning takes place in spring and early summer. When the water has warmed and food is plentiful. A female can release a very large number of eggs. Often many thousands at a time. Because the chance that any one egg will survive is tiny. The eggs are fertilised in the open water and hatch within a day or two into free larvae.

The larva is a tiny, transparent ophiopluteus, a form with long. Calcareous arms used to drift and feed on the smallest plankton. The larval stage lasts about 2 to 3 weeks. During which the larva is carried by the current and may travel some distance from its parents. At the end it settles to the seabed and changes into the adult form with its five arms. Growth is rapid at first. A young brittlestar reaches a disc width of about 1 cm in its first year. And is mature at about 1 to 2 years. The adult life span is short for an echinoderm.

Usually 2 to 3 years. Though some live longer in cold water. The broken arms play a part in the numbers too. Predation on the arms is constant. And the ability to drop and regrow an arm means an animal can lose a part many times without dying. The species can also divide the disc in rare cases. But normal sexual spawning is the rule. Because it spawns so many eggs and grows so fast. A bed can recover from a bad year more quickly than many slower echinoderms.

Not Evaluated, abundant

Conservation

The common brittlestar has not been assessed for the global Red List. And is listed as Not Evaluated. That status reflects the lack of a full global review rather than any sign of safety. In practice the species is one of the most abundant animals on the temperate seabed. And there is no evidence of a range-wide decline. It is not fished, has no commercial value. And is not protected by law in any specific way. The threats it faces are the ones that face the whole seabed. Bottom trawling disturbs the beds and turns a dense carpet of animals into a cloud of broken arms.

Once a bed is ploughed by a trawl. The animals must regrow from the survivors. And the structure of the habitat is lost. Dredging for shellfish, the laying of cables and pipes. And the dumping of spoil all damage the firm ground the species needs. Pollution and the lowering of oxygen in enclosed seas can kill the animals in large numbers. Because they depend on a steady flow of clean, oxygenated water. Climate change may shift the range. As warmer water suits some of its competitors and predators better than it suits the brittlestar.

None of these pressures is thought to threaten the species as a whole. Given its huge numbers and wide range. The animal is in fact a useful indicator. Because it is so common and so sensitive to disturbance. A healthy brittlestar bed is a sign of a healthy seabed. And its local decline is an early warning that the bottom community is in trouble. It is also collected in small numbers for public aquariums. Where it is valued because it is easy to keep and endlessly active.

Quick hits

Five things to remember

• Its arms break off at the lightest touch, and the animal escapes while the predator is left holding only an arm that later regrows.
• In the right place it forms beds of 2000 to 3000 animals per square metre, and a single bed can hold millions.
• It has no eyes and no brain, only a ring of nerve tissue around the mouth that coordinates its five arms.
• It feeds by lifting its arms into the current to trap plankton, like a small living net on the seabed.
• Each arm-free animal moves by rowing its arms like oars, which can carry it quickly across the bottom.