Diadema paucispinum, commonly known as the Long-spined Sea Urchin, possesses a small test with very long, moveable primary spines that can reach up to 25 cm (10 in) in length, approximately four times the diameter of the test. These primary spines are typically bluish-black, with younger individuals sometimes displaying pale bands. Shorter, brittle, secondary spines are also present; these are venomous and easily break off in the skin. Photosensitive spots in the urchin's epidermis allow it to detect shadows, prompting it to angle its secondary spines towards potential threats. Near the apex of its test, a clearly visible anus is raised on a small projection, surrounded by a distinct orange ring, with five narrow, bright blue lines radiating outwards.
This nocturnal sea urchin inhabits calcareous rocky shelves in shallow coastal waters, typically at depths of 1-5 metres (3-16 ft). It uses its spines to wedge itself securely into crevices and hollows in the rock during the day. At night, it emerges to graze on filamentous algae as it crawls over rock surfaces. Despite its substantial defensive spines, Diadema paucispinum is preyed upon by triggerfishes.
- very long, slender, sharply pointed spines
- bluish-black primary spines, often with pale bands in young
- clearly visible anus surrounded by orange ring on small projection
- five narrow, bright blue lines radiating from apex
