Spineless Sea Urchin

The Spineless Sea Urchin (Abatus cordatus) is a marine (saltwater) invertebrate echinoderm. It is also known as the Kangaroo Sea Urchin because the female keeps her eggs in a pocket or pouch. It is related to starfish (sea stars). 

The Spineless Sea Urchin has a hard, spherical (ball-shaped) shell, called a test, with no backbone and no spines sticking out of its body (like the Spiny Sea Urchin). 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 sense cells around its mouth.

It grows to about 10 centimetres (4 inches) across. 

Spineless Sea Urchin, Paris Natural History Museum

The Spineless Sea Urchin is native to the Southern Ocean. It 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. 

It can move, but very slowly. It can survive for many hours outside water.

The Spineless Sea Urchin feeds on algae. Its predators include otters, starfish, eels, and fish. 

There are male and female Spineless Sea Urchins. The female Spineless Sea Urchin keeps about 200 eggs in four protective pockets or pouches. This is different from the Spiny Sea Urchin. The female Spiny Sea Urchin does not have a pouch – her eggs float freely in the sea. 

The young Spineless Sea Urchins stay in the pouch for 8-9 months. Young urchins have bilateral symmetry, and as they become adults, they have fivefold symmetry (similar to starfish with five appendages). 

The lifespan of the Spineless Sea Urchin is about 20 years. 

Spineless Sea Urchin, Paris Natural History Museum

Photographer: Martina Nicolls


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