Relatives of sea stars can look very different!
Here's a quick look at some of the spiky echinoderms seen on Singapore shores!
Spiky sea urchins may seem unlikely relatives of sea stars. But like the stars, sea urchins also have five-part symmetry.
The five parts are not so obvious in the White sea urchin (possibly Salmacis sp.). This feature is clearer in the Black sea urchin (Temnopleurus sp.). These sea urchins are seasonally common on our Northern shores, often seen among seagrasses. Both kinds of sea urchins tend to 'carry' stuff: bits of shell, seaweeds, pebbles. These are held over the body by little tube feet. Why do they do this? Possibly as camouflage, or to shade from the light.
Sea urchins have an internal skeleton (called the test). The test is a rigid, hollow sphere. There are little knobs all over the outside of the skeleton. The spines move on these little knobs, articulating somewhat like the ball-and-socket joint of our knees.
Like us, sea urchins have a skin covering all the spines and the test. When a sea urchin dies, the skin decays rapidly and all the spines fall off, leaving only the spherical test. You might sometimes see these strange skeletons washed up on the sea shore. Now you know what they were!
Some sea urchins like the Long-spined sea urchin (Diadema setosum), have long skinny spines that can poke painfully. This sea urchin is more commonly seen in deeper waters by divers. Others like the pretty pink Pencil sea urchin (Prionocidaris bispinosa) have thick fat spines with little spines on the big spines! This sea urchin can be seasonally common on our Northern shores, among seagrasses.
Heart urchins look like oval sea urchins with potato-shaped bodies. They too are covered in spines, though sometimes more sparsely. Living heart urchins are rarely encountered as they usually remain buried in the sand, some as deep as 20cm below the surface.
But their dead skeletons are often seen washed ashore. The five-part symmetry is clear in the skeleton, the flower-like pattern of petals is where the tube feet emerge.
These flat animals are like flattened, two-dimensional sea urchins! The Cake sand dollar (Arachnoides placenta) can be very common on some of our Northern shores. Really tiny ones, each hardly bigger than a 20cent coin, can be found in enormous numbers at Tanah Merah.
Like sea urchins, sand dollars are covered with spines too. But really tiny spines! These spines are moveable and used like miniature spades to dig into the sand or to move around. The dense layer of spines also keeps sediments off so there is a flow of oxygenated water across the body.
The Keyhole sand dollar (Echinodiscus truncatus) is sometimes seen on our Northern shores, including Changi. It has two mysterious slots in its body. Suggested functions of these slots range from helping the animal to burrow, right itself, find food or to prevent the waves from lifting it out of the sand. The last is the most widely accepted explanation. Sort of the same principle of spoilers on the back of sports cars. The spoiler breaks the flow of air over the car and helps keep the car on the road instead of lifting off as it speeds along.
Like the sea urchin, sand dollars also have an internal skeleton (the test), and a skin covering the spines and the test. When the sand dollar dies, the skin decays rapidly and all the spines fall off, leaving only the flat coin-like test. You can clearly see the five-part symmetry in the skeleton. The arrangement of petals in the centre are tiny holes where the tube feet emerge from the skeleton.
More FAQs about poky echinoderms
I can't see any sand dollars! Sand dollars are usually found in ... well... sand! During a hot day at low tide, sand dollars usually lie buried in the sand. Look for tell-tale signs on the surface. And be careful where you walk as you may unknowingly crush these delicate animals as you walk on what seems to be bare sand.
Can I take the dead sand dollar home? Do make sure the sand dollar is really not alive. A living sand dollar is covered with fine spines and appears velvety. If you gently place a living sand dollar on your flat palm, its tiny moving spines will tickle you. Don't bring living sand dollars home as they will die if they are left out of seawater. A dead skeleton is smooth, without any spines, and the details of the skeleton can be seen more clearly. The skeleton is fragile and will shatter at the slightest pressure.
How do upside down sand dollars right themselves? If you look carefully, you will see that sand dollars have an upper side (usually slightly convex, with the petal pattern in the centre) and an underside (usually flat with a hole in the centre - its mouth). An overturned sand dollar will dig one side into ground and stick the other end out. Eventually, the waves and currents flip it over. This is laborious and it usually needs to be in water to achieve this. So please put sand dollars back the right way around. That is, the flatter side with the mouth, facing down.
I didn't see any sea urchins! Sea urchins appear to be seasonally common. Sometimes, the shore is full of them. At other times, none appear to be found. Even when they are 'in season', some sea urchins like the White sea urchin and Black sea urchin carry stuff so they are well camouflaged. They may be under a pile of seaweeds, or uninteresting clump of dead shells, pebbles and bits of rubbish. Look closely and walk carefully. The crunch you hear when you step on a pile of seaweed or shells could be the death of a sea urchin!
Are our sea urchins dangerous? The Long-spined sea urchin can poke painfully, it is said even through dive gloves. It is also said that the spines tend to break and thus remain embedded in the skin. So please don't handle them. The other sea urchins commonly seen are not as dangerous. But it's probably best not to handle them either.
Do sea urchins have any usefulness to humans? The roe (egg mass) of some sea urchins are relished as a Japanese delicacy and sea urchins are commercially harvested for this reason in various parts of the world. Sea urchins are also harvested alive and killed for their skeletons which are made into cheap trinkets.
Unfortunately, overharvesting of sea urchins can upset the natural balance. Sea urchins eat seaweeds. When there are too few sea urchins, seaweed can grow unchecked and smother other marine life such as reefs. Please don't buy souvenirs made from marine life.
Sea urchins have also been extensively studied to better understand egg fertilisation and embryo development for other applications. This is because their eggs are large and easy to study.
More links to spiky echinoderms
Lane, David J.W. and Didier Vandenspiegel. 2003. A Guide to Sea Stars and Other Echinoderms of Singapore. Singapore Science Centre. 187pp.
Relatives of sea stars can look very different!