It is an irregular animal and not radially symmetric as are most sea urchins; there is a notch at the front and the mouth is forward pointing, while the anus is at the rear.

These findings indicate that Spatangus purpureus has omnivorous feeding habits, with phytodetritus from the algal beds being an important part of the diet.

[3] Another associate that lives among the spines is the polychaete worm Malmgreniella castanea, some 1 or 2 cm (0.4 or 0.8 in) long, which has large purple scales.

The red comb star Astropecten aranciacus is the main predator, and sea breams can crush the test and consume the contents.

In the Mediterranean Sea, the helmet shell Galeodea echinophora drills a hole through the test and inserts its proboscis to digest the soft tissues inside.