Like other echinoderms (which also include starfish, brittle stars, sea cucumbers and sea lilies), sea urchins are marine invertebrates.
They occur in oceans, mainly in shallow coastal waters. Lacking the ability to osmoregulate (control the balance of salt and water), they rarely venture into brackish waters (where there is a freshwater content).
Most sea urchins (particularly the regular sea urchins) live on harder ocean substrates, clinging onto rocks in turbulent zones using their tube feet. The irregular sea urchins (including sand dollars and heart urchins) prefer more sandy substrates. Sand dollars live just beneath the surface, whereas heart urchins live deeper (10 cm or more) within burrows.
Read more:
What Are Sea Urchins?
What Are the Main Types of Sea Urchin?
How and What Do Sea Urchins Eat?
How Do Sea Urchins Move?
How Do Sea Urchins Reproduce and Grow?
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