The Sea Urchin is a spiny marine (saltwater) echinoderm. It is an animal that lives on the bottom of the ocean. It is related to starfish (sea stars). It is also called a Sea Hedgehog.
The Sea Urchin has a hard shell, called a test. It is spherical (round like a globe), shiny and spiny. Young urchins have bilateral symmetry, and as they become adults, they have fivefold symmetry (similar to starfish with five appendages). Its mouth, with a small jaw, is in the centre of the urchin on its underside.
It does not have eyes. It is sensitive to touch, light, and chemicals, due to the numerous sensitive cells in its spines, tube feet, and around its mouth.
It measures about 10 centimetres (4 inches) across.
It can move, but very slowly. It crawls using its tube feet. It can survive many hours outside water.
The Sea Urchin feeds on algae. Its predators include otters, starfish, eels and fish.
The Sea Urchin is found in all oceans, and in all climates from tropical to polar. It prefers rocky shores and sea beds. It is benthic, living on the sea floor.
There are male and female Sea Urchins. Females lay eggs that float freely in the sea. When fertilized, they becom larvae. The larvae sink to the bottom of the ocean and undergo metamorphosis, turning into a juvenile urchin.
The lifespan of the Sea Urchin is about 20 years.
Photographer: Martina Nicolls
Martina Nicolls: SIMILAR BUT DIFFERENT IN THE ANIMAL KINGDOM