The secret lives of starfish

It’s the time of year for swimming and rock-pool gazing and admiring the myriad  sea life around the shores of West Cork.

With a copy of Sherkin Island Marine Station’s excellent publication A Beginner’s Guide to Ireland’s Seashore in the pocket, and a bucket and net in the hands, the scene is set for many hours of wonderful entertainment.

Collecting and identifying shells is always a great pastime and we usually return home with a bucket of shells of all shapes and sizes. Crabs are quite popular too and there are often fights over who gets to keep the delicate empty shells. 

However, the most popular by far of the seashore ‘finds’ are starfish which, it seems, all children love. We do not bring them home but holding a starfish on your open palm is one of the highlights of a rock-pool outing.

Starfish and their relatives the brittle stars, sea-urchins and sea-cucumbers are echinoderms – meaning ‘spiny-skinned’, although all the species don’t have obvious spines. Echinoderms are wholly marine. In addition to their spines, they have another common feature. Their bodies radiate out from a central point into equal parts, rather like a bicycle wheel. 

Therefore starfish are symmetrical, mostly with five arms (or rays) joined together in the centre but lacking a head. However, some starfish have more than five legs. And because starfish can regenerate a leg that is lost, it’s possible to find individuals with just one remaining leg too.

It does not have a skeleton of bone, however, tiny canals of water inside the starfish enable it to move. Although the starfish as a whole is flexible, some parts of its body are fairly rigid. Strengthened by a special skeleton of overlapping parts in the body wall, some starfish have protective spines. In the common starfish, some of them stick through the skin while in the aptly-named spiny starfish these form a conspicuous shield to deter predators. 

As starfish lack a head, they have no special area of the body where sensory organs are concentrated. They have no power of smell, sight or sound. However, there is a primitive light receptor on most varieties – the optic cushion – on the tip of the upper surface of each arm. Although this cannot form an image, it can distinguish between bright and dim light. The remainder of the body is sensitive to touch and to chemicals, which helps the animal to sense its prey. 

The mouth of the starfish is on the underside, at its centre, while the anus is on top directly opposite. Some species take worms and other small prey whole with the aid of the powerful tube-feet.

However some starfish eat bivalve molluscs – such as mussels and oysters – in a quite remarkable way. The starfish straddles the prey and wraps its arms around the shell, gripping the two valves with its tube-feet. Groups of the tube-feet are used in relays to pull repeatedly on either valve until the exhausted victim finally allows its shell to gape a fraction. This enables the starfish to insert folds of its stomach, which it sticks out through its mouth on the underside, to engulf and digest the mussel’s soft body. Finally the empty shell is discarded.

Spiny starfish predate in this way and you will usually find a few on a mussel bed prising open the shells.

Brittlestars are long-limbed relatives of starfish and they are found in great numbers around our coasts. As the name suggests, the arms are brittle and break off easily so very gentle handling is required.

Like most starfish, brittlestars have five arms arranged around a central body. However, on brittlestars, these arms are much longer and thinner and they are very distinct from the body. 

The central body of a brittlestar can be round, star-shaped or five-sided. The arms are jointed and can bend sideways but not up and down. Each arms has a series of bone-like structures (ossicles) joined in a row – rather like the vertebrae in our backbone. If a brittlestar is trapped, fractures occur between the ossicles and the arms breaks in two enabling the animal to escape. 

Sea-urchins are also echinoderms. They move along the shore and sea-bed using their spines and tube-feet. Some can even climb up vertical rocks. 

To protect themselves against sea-bed predators (including starfish), they have an imposing array of defence weapons lining their outer shell. The spines also help the sea-urchin to feed and, in some species, enable the animal to burrow into sand or bore into rock.

Sea-urchins graze on algae. The mouth of the edible sea-urchin (on its underside) is armed with a set of five sharp teeth which are operated by a structure called Aristotle’s lantern. Named after the Greek philosopher who first described the animal in detail, the Aristotle’s lantern can sometimes be found intact on the inside of the empty shell.

The shell of a sea-urchin is called a test. It appears to be an external skeleton, however, it’s also an internal one as is covered in a thin layer of living tissue.

Some of the shells of the sea-urchin are very well known to us – particularly that of the heart urchin. This is also known as the sea potato because of the shape if its distinctive test which resembles a potato washed up on shore. Other urchin shells were used as ornaments and playthings in times past as they are beautiful objects. But urchins were also eaten and were regarded as a luxury food. The only edible parts are the star-shaped ovaries (called sea-eggs) and the most frequently eaten species is the edible sea-urchin. This is now considered ‘near-threatened’ in the IUCN ‘Red List of Threatened Species’.

So, grab your copy of A Beginner’s Guide to Ireland’s Sea Shore and go in search of cushion-stars, bloody Henrys, spiny starfish and many more. Or, for a chance to learn more about these extraordinary echinoderms, visit the UCC/Skibbereen Heritage Centre annual ‘Touch Tanks at Lough Hyne’ event during Heritage Week. This year it’s on Saturday August 20th from 11.30am to 1.30pm.