Monday, September 6, 2010

Brittle Stars

The swaying fronds of red seaweed that fringe rockpools near the low tide level on the seashore are home to a wealth of miniature marine life, less spectacular than the inhabitants of coral reefs but every bit as intriguing. I found scores of these tiny brittle stars, the largest no larger than a centimetre across (including arms), on a visit to the Northumberland coast at the weekend. Brittle stars, or ophiuroids, are relatives of starfish and sea urchins, in the phylum Echinodermata (which means spiny skin - a feature many members of the phylum share). The view above is of the underside of one of the brttle stars, showing the mouth fringed with five teeth formed from calcareous plates.

Seen from above, five arms radiate from the pentangular body. Each arm is formed from articulated segments linked by muscles and these are very flexible, so the animal often curls the tip of an arm around a seaweed frond to stop itself from being washed away by currents. If it's alarmed the muscles between the arm segments contract and then the arms become very brittle.....

... and it doesn't take much force to snap them, as has happened here with the upper arm. This is not a problem, as....

... arms can easily be regenerated, as is happening here with the middle, lower arm. This capacity for shedding and regenerating arms is analogous to the way that lizards shed their tails (autotomy) if they are picked up by that appendage.

At higher magnification you can see the anatomy of the arms more clearly. Each calcareous segment bears spines and a pair of tube feet, that are all interconnected by a hydaulic system of radial canals that run along the arms and a ring canal that runs around the central body. Local relaxation or contraction of muscles, compressing liquid within, elongates or retracts the tube feet.
Unlike the tube feet of starfish which have suckers on their tips and are used for 'walking', those of brittle stars are primarily for sensory purposes and to assist in feeding, by secreting adhesive mucus. In this higher magnification image you can see that the tube foot is hollow.

The ring of tube feet around the mouth on the underside, where the arms converge, help to sweep food particles beyond the five calcareous teeth)....

.... into the muscular oesophagous, and then into ....

... the stomach. The tiny central body also contains gonads, that produce eggs and sperm that give rise to the planktonic ophiopluteus larvae.

When they're fully grown some brittle star species can reach 60 centimetres in diameter (not in Britain, though), but they all begin life as planktonic larvae, often settling into the shelter of seaweeds on the nursery slopes of rock pools or coastal shallow seas, which are of such importance for the health of the oceans.

You can see a YouTube vieo sequence of an adult brittlestar here.

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