The iconic image of the seashore is the five armed starfish, every child’s favourite. Slow to escape but with a strong hold on the substrate, these tube footed creatures come from a large group that includes, cucumbers, urchins, sea lilies and brittle stars.
The stars have a fascinating mechanism of locomotion, they use hydrostatic pressure to move tubes on their legs and on these they crawl. This pressure is regulated by an organ called the madreporite, which seems to balance the water contained inside the creature against that on the outside. Asteroids (starfish) have a basic body plan, with a mouth underneath and tube feet, often having suckers. Not all though have five legs; the common sun star has up to fourteen. Most are carnivores, some can extrude their stomachs to aid digestion of prey, others are filter feeders and these wave arms in the water try to catch passing items.
Reproduction is external, fertilization takes place in the water and a planktonic larva (bipinnaria) then free swims for several weeks. Eventually, small arms develop and the organism attaches to the substratum where it metamorphosises into an adult body plan. Not all use the same strategy, some do not free swim, other brood eggs. It is true they can regenerate limbs, I once found two arms attached to a central disc, all that remained of a spiny star, it happily crawled away apparently unaffected by such a major loss of limbs.
Cushion stars are often found mid to low shore, look under rocks for these 50p size plump scavengers. It is reported in the SW they mature at two years old as a male, then change to females at four years, laying eggs in May the young are copies of the parent and do not fee swim. Two species are found but only Asterina gibbosa is common. A smaller A. phylactica has a dark star marking in the centre, these are better parents and protect their young for a few weeks. Adults can live for four years, the larger species for seven years.
The spiny star is the commonest large starfish on the North Coast, it can grow to dinner plate size. Unmistakable with very prominent blunt spines they come in several shades. If you try to lift one off a rock it has a surprisingly strong grip and often loses feet in the process, so please lift them gently or just watch them crawl out of sight following the tide or hiding under large rocks to avoid being left high and dry. The larger stars are normally found only on big tides.
Brittle-stars are on the lower shore, under rocks or in the mud/sand. They live up to their name and easily lose a leg if you handle them, so place them in a container to view. Several species can be found but identification is not easy, you need good pictures of the plates (at the body end of the arms) and arms. The arms are extremely flexible and can twist easily this makes them surprisingly fast (by starfish standards). The tube feet lack suckers, these are more for use in feeding than walking. Some of the larger species (Common brittle-star) can live for ten years. Where found there are often large numbers of mixed species.
The stars of the beach are fascinating to watch but they are fragile so please treat them kindly.