talking points for nature guides
Phylum Echinodermata > Class Echinoidea
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For nature guides: introducing sand dollars and sea urchins
updated Oct 08

Don't step on the sand dollars!
Sand dollars make a good 'first station' to sensitise visitors to the fact that every inch of the shore is alive. Don't point out the sand dollars immediately. Bring the visitors close to a concentration of the sand dollars, then let the visitors try to find them on their own. This illustrates that just because we can't see the animals, doesn't mean there are none.

  • Can you find any sand dollars here? Explain what to look for "Sand dollars are flat and circular animals about this big" (show with your hands the approximate size). Can you see any signs of them in the sand? How about that little lump next to your foot? Go on, touch it and see for yourself.

  • It's alive! It doesn't look like it's alive does it? But let's put this sand dollar in water and see it move! The spines are used by the sand dollar to move around and to burrow into the sand.

  • It's delicate. If we step on a sand dollar it will crack and die. So watch your step!

What are sand dollars?
They are animals!
They are related to sea stars so they have five parts too.

A closer look at sand dollars

  • Five-part animal: see the segments on the sand dollar that is in multiples of five?
  • The animal has an upperside and underside. Don't put it back upside down!
  • The mouth is on its underside, facing the ground.
  • Some, but not all, sand dollars have jaws made of a circle of five plates that meet in the middle.
  • The petal design on the upperside of a sand dollar is called a petaloid. The petaloid is a series of tiny holes in the skeleton. Tube feet emerge through these holes and the sand dollar breathes through these feet!

What are sea urchins?
They are animals!
They are related to sea stars so they have five parts too.

  • Can you find the sea urchin? They sometimes 'carry' things like seaweed and shells so they are hard to spot.

A closer look at sand dollars

  • Five-part animal: see the segments on the sea urchin that is in multiples of five?
  • The animal has an upperside and underside. Don't put it back upside down!
  • The mouth is on its underside, facing the ground.
  • Some sea urchins have a kind of 'worm' around the mouth.
  • Sea urchins have jaws made of a circle of five plates that meet in the middle.
  • Sea urchins have tube feet too! In some, these are used to 'carry' seaweed or other debris.
  • Sea urchins usually have two kinds of spines; one larger and/or longer, and the other smaller. These moveable spines not only protect the sea urchin, but are also used for walking.

  • Here's the skeleton of a dead sea urchin. Let's see what this tells us about sea urchins.
    • Sea urchins have an internal skeleton (called the test) that is a rigid, hollow sphere. The inside of a sea urchin is mostly empty except during mating season when it is full of sperm or eggs.
    • There are little knobs all over the outside of the test. The spines move on these little knobs, articulating somewhat like the ball-and-socket joint of our knees.
    • Where do the spines of a dead sea urchin go? Like us, sea urchins have a skin covering 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.

Role in the habitat

  • Sea urchins by eating seaweeds help maintain the balance of life on the shore. If there are too many seaweeds on a reef, for example, baby corals can't find a place to settle down. More about the role of seaweeds, baby corals and animals that eat seaweeds on the wild shores of singapore blog.

Sand dollars, sea urchins and you

  • Some species of sea urchins are sadly overharvested as a delicacy, e.g., for sushi. Many of these species are threatened by over-collection for the trade.
  • Some large sand dollars are sadly harvested to make cheap trinkets like Christmas ornaments, bathroom decorations and tourist souvenirs. Please don't buy souvenirs made from marine life.

Sand dollar myths

  • Can I take this dead sand dollar home? While a sand dollars may appear dead, it may well be very much alive.
    • A living sand dollar is covered with fine spines and appears velvety.
    • A dead one is smooth, without any spines, and the details of skeleton can be seen more clearly. The skeleton is fragile and will shatter at the slightest pressure. The shattered skeleton will be recycled into calcium for new snail shells and skeleton of other sea creatures.
Handling tips

Finding sand dollars Most are well hidden. Some burrow into the sand leaving only tell-tale signs on the surface.

Finding sea urchins
some sea urchins 'carry' seaweed or bits of debris and are thus well camouflaged. Watch your step!

Don't step on sand dollars and sea urchins They are living animals. They will shatter and die.

Don't touch sea urchins or make visitors touch them.
Spines may poke and break off in the skin. Some sea urchins have toxic spines or other body structures to pinch.

Don't remove them out of water for too long
they need sea water the way we need blood.

Don't break sand dollars!

Don't pull out the spines of sea urchins!

Return them where you found them: They move slowly and some need to be in specific locations. Put sand dollars back the RIGHT WAY up.
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