Echinoderms are among the strangest of invertebrate animals. They have no real head or tail end. Instead their bodies are built on a radial pattern, often in a form with five sides. They have three main characteristics including a symmetrical five-part body plan, tube feet and a skeleton of plates. All echinoderms are marine animals. Their amazing water-vascular system, consisting of water-filled tubes ending in numerous finger-like projections (tube feet) that stick out through the skin, is their most interesting feature.

Tube feet, on the end of the suckers are used for movement. The water-vascular system works on water pressure, creating a network of tube feet that look like hundreds of tiny, hydraulically operated legs. The name echinoderm, meaning spiny skin, relates to their outer surface that is covered with limestone plates that are often formed into spines. The variations of shapes range from sausage and star shapes to a ball of spikes.

Echinoderms occupy all habitats including coral reefs, mangroves, seagrass and soft-bottom areas. Many echinoderms are able to undergo asexual reproduction when parts of their bodies break off and grow. Sexual reproduction involves mass spawning. Echinoderms exhibit a wide range of feeding techniques including suspension feeders, deposit feeders, carnivores, browsers and parasites. Some species use modified arms to filter food from the water. Others, like sea urchins, use their tooth-bearing jaw structure known as 'Aristotle's lantern' to scrape algae from rocks.

Seastar from our southern waters.
Courtesy Parks Victoria

When disturbed or frightened, some sea cucumbers pour out a mass of sticky white threads to confuse or trap their enemies. Others are capable of releasing toxins which in aquaria have been known to kill all the animals, including the sea cucumbers themselves.

Sea cucumbers were once the basis for an important international fishery. Macassans from the island now called Sulawesi, used to come to northern Australia to collect them. After processing, they would be sold as béche-de-mer.

Brittle stars
Brittle stars have long flexible arms that radiate out from a small central disc and are used for swimming. They do not rely on tube feet for movement and brittle stars are the fastest moving of the echinoderms. Brittle stars are often found living under rocks during the day. When disturbed, they move quickly away using their arms in a rapid snake-like motion. Small species feed on drifting plankton by raising their arms into the water above them. Some large specimens have been known to feed on fish that they have caught while the fish were sleeping.

Feather stars
Feather stars, or crinoids, are among the most beautiful of reef creatures. The arms of a feather star are greatly divided and extend into the water, so they look quite different from sea stars and other echinoderms. The mouth of a feather star is located on top of its body. Feather stars can be found sitting on sea fans or in areas of high current. Feather stars are nocturnal plankton feeders. They do not move around very much but find a position on the Reef where currents bring small animals that they filter through the net of their arms.

Sea urchins
Sea Urchins have tube feet between their spines and the feet are attached to the animal's water vascular system. The tube feet operate on hydraulic principles and allow the urchin to move about, although rather slowly.

Sea urchins are globe-shaped with no arms. They have a compact skeleton with closely fitted plates, and moveable spines with a ball and socket joint. Sea urchins also have powerful scraping jaws known as 'Aristotle's lantern'. The sea urchin is nocturnal, hiding in crevasses during the day and emerging at night to feed. Sea urchins can be found in both warm and cold water.

Predators of Sea Urchins include octopus and triggerfish, which bite off the spines enabling the fish to crack open the body of the urchin.