Starlets waltz and glide around our rocky shores
Starfish are well-known symbols of the seaside. They belong to a group of life forms called Echinoderms. The group is unusual in that all 6000 or so of its members found worldwide are strictly marine.
Many groups of life forms have members that are found in the sea, in freshwater and on land. Echinoderms are singularly marine with no representatives at all in freshwater or on land. A few species do venture into estuaries, but none has adapted fully to living in brackish water.
Most echinoderms are large, are slow-moving and are colourful so they are attractive. The main members of the group are the well-known starfish and sea urchins; less well-known sub-groups are the sea cucumbers and sea lilies.
In shape, the typical starfish has a central disc part with five arms radiating from it. The disc may be very large or very small and the arms may be very long and skinny or very short and fat.
The ones with a tiny central disc and very long, very skinny, snake-like arms are known as brittle stars because their arms break easily. Some species are common on rocky seashores and may be found on the undersides of stones overturned when the tide has ebbed. The best hunting ground is close to the water’s edge at extreme low water.
The Common Sun Star is at the other end of the spectrum of shapes in starfish. It has a huge central disc with twelve small, stubby arms radiating from it. It is red in colour and looks like a stylised drawing of the Sun. It is an offshore species, so it is seldom seen by most people.
Cushion stars are the starfish that occupy the central part of the spectrum between the extremes of shape exhibited by the brittle stars and the Common Sun Star. The Starlet, pictured above, is common on rocky seashores.
When the tide floods, the Starlet waltzes around its rocky world gliding along on tube feet. Tube feet are long, flexible, contractile, finger-like tubes emerging in interlinking rows from the undersides of the five arms. The animal has a hydraulic system that pumps fluid into the tube feet enabling the Starlet to walk at speeds of up to 2.5cm per minute; very slow-moving by human standards.
When it encounters a food item it everts its stomach on top of the food and withdraws the morsel into its body.