FACT SHEET 13
• Sea urchins are solitary animals that sometimes live in loose groups. The general body surface of sea urchins is dominated by spiny extensions of their internal skeleton.
• 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 are used for locomotion. Sea urchins are globe-shaped or midely flattened animals. They have a compact skeleton, called a test, with closely fitted calcareous plates and moveable spines with a ball and socket joint.
• Sea urchins are thought to be crucial in controlling the abundance of algae on reefs. In ecosystems where they have disappeared, algae have generally increased rapidly and in some case, completely replaced coral reefs. • Sea urchins are found in all oceans from the equator to the poles. They are also regularly collected from deep in the oceanic trenches. Most urchins hide in crevasses by day and come out at night to feed.
• Urchins are grazers, using their multi-toothed powerful scraping jaws to scrape algae off the surfaces of rocks. They eat both plants and animals.
• The spines of sea urchins make an impenetrable barrier for potential predators. Several fish, however, are known to prey on sea urchins (e.g. large trigger fish and wrasses), which bite off the spines enabling the fish to crack open the body of the urchin. Sea urchins are not currently endangered, but like most Reef animals, they are susceptible to changes in water temperature and pollution. Some species of sea urchins are poisonous and they should generally be treated with care.
• Sea urchins generally have separate sexes and spawn seasonally every year. Eggs and sperm are usually released to the water column where fertilisation occurs. Development of the young involves several larval stages (i.e. indirect development).
• Sea urchins are counted in a Reef Check Australia survey although only 3 types are noted, the black spined urchin, collectors urchin and pencil urchin.
Interesting Fact: Sea urchins have powerful scraping jaws known as 'Aristotle's lantern'.
• ReefEd: http://www.reefed.edu.au/home/explorer/animals/marine_invertebrates/echinoderms/sea_urchins
• Reef Education: http://www.reef.edu.au/asp_pages/secb.asp?FormNo=42