Echinoderms

Seastar / Choriaster granulatus / Family - Oreasteridae

There are at least 800 species of echinoderm on the Great Barrier Reef, including many that are rare. Scientists believe there are more species on the Reef than anywhere else in the world.

About Echinoderms

The three most common characteristics of echinoderms include a symmetrical five-part body plan, tube feet and a skeleton of plates.

Sea stars are the most common echinoderms on the Reef. Others are sea urchins, sea cucumbers and crinoids.

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.

All echinoderms are marine animals. Their amazing water-vascular system, 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.

Echinoderm physiology

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 on echinoderms range form sausage and stare shapes to a ball of spikes.

Habitat

Echinoderms occupy all habitats including coral reefs, mangroves, seagrass and soft-bottom areas.

Reproduction

Many echinoderms are able to reproduce when parts of their bodies break off and grow. This is the asexual form of reproduction.

Sexual reproduction involves mass spawning. Many sea cucumbers, like snakes, sway in the water as eggs and sperm are released from a pore on their head.

The crown-of-thorns is one of the most active spawners releasing over 100 million eggs each year.

Feeding

They 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 use tooth-bearing jaw structure known as 'Aristotle's lantern' to scrape algae from rocks. Starfish don't send food to their stomach, they send their stomach to the food. The starfish sit on top of the food and everts their stomach through the mouth to cover and digest the food externally. Sea cucumbers feed like a vacuum. They eat sand to extract food particles and pass out clean sand from the other end.