Habitat 
Depth 
Group 
Role WHAT DOES IT LOOK LIKE?
The purple sea urchin has a test, a rigid covering made of calcium carbonate plates, generally a few centimetres across. Spines are movable and colour ranges from purple to brown or olive green. Tube feet, small extendable tubes, reach between them and help with attachment and movement.
The mouth faces the seabed. Its five teeth are operated by Aristotle’s lantern, an arrangement of calcium carbonate pieces and muscles. Arbacia lixula, the black sea urchin, is a separate species, not the male of the purple sea urchin.
WHERE DOES IT LIVE?
It occurs on rocks and in Posidonia seagrass beds, mainly in shallow water. French Mediterranean observations also show individuals in hollows, where they anchor themselves and find shelter.
Distribution depends on seabed features. A site with many cavities exposes urchins differently from a bare slab, particularly to waves and predators.
WHAT DOES IT EAT?
It mainly eats algae, scraping or cutting them with its teeth. Other resources can supplement its diet, including small animals. Calling it a grazer describes its dominant activity without implying an exclusively plant-based diet.
Intense grazing can maintain areas with few large upright algae. Urchin density, access to food and predation are among the factors shaping this effect.
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