WHAT ARE SAND DOLLARS?

These urchin-like creatures look very different to their discarded exoskeletons

Sand dollars are small marine invertebrates that live on the seafloor. The 11 sand dollar species are disc-shaped and have an average diameter between seven and ten centimetres. Most of the time these animals are submerged underwater, but they sometimes become dislodged from the seabed and wash up on beaches. Live sand dollars should be returned to the water or they will dry out and die. They can be identified by their red or purple velvetlike coats and star-shaped lines on their surface. Hard, solid, white sand dollars are just the remains of the creature. Their soft tissue is gone and all that remains is their calcium carbonate exoskeleton. The skeleton’s hard, circular appearance resembles large coins – hence the name sand dollar.

When they are alive, sand dollars move by grabbing the seabed with hair-like extensions on their feet. Having just one disc-shaped foot and no arms, legs or tail makes mobility restricted. Predators such as crabs and medium-sized fish have a fairly easy time catching this prey once spotted. To deter them, sand dollars have small stinging cells across their bodies. Sand dollars themselves aren’t picky eaters, as they have to rely significantly on the food that floats their way. Their diet usually consists of phytoplankton, algae and the larvae of crustaceans and fish. To consume these, they use jaws full of toothlike protrusions that grind their food for around 15 minutes before it’s edible.

1 4 LUNULE 5 These slits allow water to pass through so that sand dollars aren’t pushed around too much by the waves. SEA SURVIVAL 4 How this urchin-like creature lives and feeds in the ocean 1 FOOT Tube-like keratin bristles called cilia cover the bottom of a sand dollar. These are used to propel them along the seafloor, as well as to collect food. 2 3 5 FOOD GROOVES Beneath the cilia are grooves that extend out from the centre. Food becomes trapped in these grooves and is directed towards the central mouth. 2 MADREPORITE PLATE This central upper section is covered in tiny pores – openings to the body’s water vessels. The five larger pores surrounding this are where eggs or sperm are released. 3 GAS EXCHANGE Along these petal-shaped lines, the sand dollar’s gills extend to the body’s surface to absorb dissolved oxygen from the water.