One of many Common Starfish, Asterias rubens Linnaeus, cast ashore near Burry Holms on Rhossili Bay in April. This one was still alive but most of them perished. Starfish are Echinoderms and have five broad triangular arms which are between 3 to 5 times as long as the diameter of the central disc. This distinguishes them from the Brittle Stars which have arms up to 10 times as long as the central disc. The so-called arms are also called ‘fingers’ – hence the common name of Five-Fingers. The scientific term for the arms is rays; and the axes (or way they are arranged) are termed radii.
The arms are swollen, and have a warty-looking surface due to the presence of papulae. Common Starfish can grow up to 500 mm in diameter (I’ve never seen one that huge) and they can be any of a number of colours from orange, red, purple to yellowish or mixtures of these with a mottled appearance. They are common on all types of substrate and are often seen stranded on sandy shores.
The picture above gives a detail of the upper surface of the starfish. This is the side away from the mouth and is called the aboral surface. There are hard spines in the skin and these can be various shapes and designs on different parts of the body and serve different functions. Most of the white structures in the picture are spines but the larger roundish thing (at about 6 o’clock on the central disc) is the madreporite which monitors and controls the intake of sea-water for the all-important hydraulic system.
This picture shows the underside or adoral surface of the starfish. It looks very different from the top surface. There is a groove along each of the arms with rows of mobile tube feet operated by hydrostatic pressure; each tube foot ends in a circular sucker disc by which it can grip onto objects and ‘walk’ along. Spines line the outer edges of the grooves and all grooves lead to the mouth.
Starfish are related to Sea Urchins – both are Echinoderms with pentamerous symmetry (symmetry based on fives). Much smaller than the specimen above, and a different colour, the juvenile common starfish pictured below was seen near Spaniard Rocks on the same day as the starfish above. You can see how small it is by the marks on the pebble where acorn barnacles were once attached.
Revision of a post first published 17 May 2009
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