There are four subspecies:[1] The green sea urchin has a globular test (shell) densely covered in spines and can reach a diameter of around 11 centimetres (4.3 in).

[3] The green sea urchin is often found with pieces of algae, bits of seagrass and fragments of mollusc shell on its aboral (upper) surface, holding them in place with its tube feet.

It is thought that the urchin is photo-sensitive and that these pieces of debris may provide some protection from strong sunlight and ultraviolet light in the clear shallow waters it favours.

However, studies in wave tanks with or without light showed they masked with shell material and aggregated in groups when exposed to surge activity.

[5] The green sea urchin has a structure called an Aristotle's lantern surrounding its mouth on its oral (under) surface.

Its tube feet and spines also play a role in feeding, catching and holding bits of debris that float past.