Included within this group are the starfish, brittlestars and sea-urchins. The name ‘echinoderm’ means spiny skin and this is one of the most obvious characteristics of this group of animals. Echinoderms also possess a large number of tiny feet called tube feet. These feet are powered by changes in water pressure inside the animal. The feet are used for movement and sometimes for feeding. All echinoderms exhibit five way symmetry. However, members of the group have very different ways of feeding. Starfish are mainly carnivores. Sea-urchins usually graze on algae while brittlestars feed on particles suspended in the water.

Cushion-star Asterina gibbosa (Pennant)

This tiny starfish starts off as a female and then changes into a male. It is a mature female when it is about two years old and about 1 cm in diameter. It changes into a male when it is four years old and about 2 cm in diameter. In late spring, orange eggs are laid under rocks by the female. The eggs hatch after two to three weeks into miniature starfish.
The cushion-star is a scavenger and feeds on dead animals and seaweed. When feeding, the stomach is everted through the mouth and the food is digested outside the body before being sucked in.

The cushion-star is a southern species found only on the west and south-west coasts of Britain.

• Habitat

  • In rock pools and under stones on the low shore and down to 20 m.

• Key Identification Features

  • Variable colour, olive-green, brown, orange.

  • Five short arms.

  • Up to 5 cm across.

Common Starfish Asterias rubens (Linnaeus)

Common starfish are often found in large aggregations feeding on mussel beds. The starfish pulls apart mussel shells with its tube feet. The shell halves only need to be opened a tiny crack, as little as one tenth of a millimetre across. The starfish can then push its stomach through the crack and digest the mussel within its shell. The resulting liquid is sucked into the mouth of the starfish. Commercial mussel and oyster fisheries, therefore, regard this starfish as a pest. In the past, fishermen would chop up any starfish they dredged up and throw the pieces back in the sea. Starfish, however, have amazing powers of regeneration. As long as a single arm has part of the central region attached it will re-grow the other four arms. Fishermen, therefore, unwittingly caused the multiplication of this pest.
Unlike the cushion star (Asterias gibbosa), the common starfish does not change sex. Eggs and sperm are released into the water in spring and summer. Fertilized eggs hatch into larvae and float in the sea for up to three months before they settle on the seabed.

As its name suggests, this starfish is common and widespread. It is found both inshore and in deeper water.

• Habitat

  • Found on rocks and stony ground and in mussel and oyster beds. From lower shore down to 200 m.

• Key Identification Features

  • Orange or a pale yellow/brown with a cream underside.

  • Five arms (occasionally 4 or 6).

  • Plump, rounded arms, tapering down towards their tips.

  • Upper surface rough with an irregularly spaced row of spines along the mid-line of the arms.

  • Usually up to 15 cm across but can grow bigger.

Common brittle-star Ophiothrix fragilis (Abildgaard)

The brittle-stars are much more mobile than the starfish and can propel themselves along by rapid wriggling of the whole of their arms. The spines on the common brittle-star help the animal to grip the substrate during movement. The arms are also used for feeding. The common brittle-star feeds on food suspended in the water. The arms are held up to trap food and the tube-feet on the arms pass the food down to the mouth. As its name suggests, the arms of Ophiothrix fragilis are very fragile and often break off. However, the brittle-star, like the common starfish (Asterias rubens) can grow new arms.

There are both male and female brittle-stars. In summer, eggs and sperm are released into the water. The larvae that hatch out from the fertilized eggs are then dispersed in the currents before settling on the seabed.

On the seabed in deeper water brittle-stars can form huge aggregations that stretch for several miles.

• Habitat

  • Under stones and in crevices on the low shore and down to 100 m.

• Key Identification Features

  • Variable colour; red, white, brown and sometimes banded.

  • Fragile arms five times the diameter of the central region.

  • Five rows of spines and five pairs of triangular plates are found on the central region.

  • Central region up to 2 cm in diameter.

Green sea urchin Psammechinus miliaris (Gmelin)

The green sea urchin has a varied diet. Its food includes barnacles, bivalve molluscs, seaweed and sea squirts. To help it scrape animal and plant material off the rocks it has teeth arranged in a complex apparatus called an Aristotle’s lantern. The animal moves using its spines and tube feet.
The green sea urchin is mature after one or two years when it is only between 0.5 and 1 cm in diameter. Eggs are released into the water and the larvae that hatch out remain in the open sea for a month before settling on the shore. The green-sea urchin may live for six or seven years.

This spiny animal is often camouflaged by pieces of seaweed trapped amongst its spines.

• Habitat

  • Found under stones, in crevices and amongst kelp holdfasts on the low shore.

• Key Identification Features

  • Rounded with well-developed spines.

  • Green with purple spine tips.

  • Up to 5 cm in diameter.