Globular Sea Urchin

Found singly or in colonies, burrowed into coral, rocks, or limestone reefs, often self-covered by algae or broken shells. With their spines they bore a hole in the substrate to hide in during day-time.
Widespread Eastern Atlantic, Mediterannean

The spines on the side of these sea urchins are worn down by continual abrasion trying to stay in the hole while the longer dorsal spines act as a defence against predators.

Sea Urchins use tubed feet to get around, often in large impenetrable masses for protection.
Sometimes they hitch a lift on the back of crabs.
They have well developed jaws for grinding their prey.
Their anus is on top in most sea urchins, except the heart urchins where it is at the rear.
Predators of sea urchins are triggerfish and large wrasses, who nibble away at their spines before turning them over to eat the fleshy undersides.
Sea Urchins are highly venomous and can piece through a wet-suit.
Some are sensitive to light and have the ability to shoot venom loaded spines at a short distance.
To be avoided.