The collector urchin (Tripneustes gratilla) is a species of sea urchin. Collector urchins are found at depths of 2 to 30 metres (7 to 100 ft) in the waters of the Indo-Pacific, Hawaii, the Red Sea, and The Bahamas. They can reach 10 to 15 centimetres (4 to 6 in) in size.
Collector urchins are dark in color, usually bluish-purple with white spines. The pedicles are also white, with a dark or black base. Individuals found at Green Island had orange-tipped spines. The spines of some specimens are wholly orange, while those of others are only orange-tipped or completely white. This color disappears when the individual dies or is taken out of the ocean, and is difficult to preserve. Collector urchins reach 10 to 15 centimetres (4 to 6 in) in size. Debris tends to “collect” on these urchins, hence their name.
Unlike some other sea urchins, collector urchins graze continually, day and night. They graze near the substrate, and their diet includes algae, periphyton, and seagrass. Most collector urchins feed on seagrass fronds; this has an ecological impact varying with the season and abundance of the urchins. They feed voraciously between November and January; one study found they consumed up to or in excess of half of seagrass production. On an annual basis, however, the same study concluded that about 24% of seagrass production is consumed by the collector urchin. The seagrass species grazed are mainly Thalassodendron ciliatum and Syringodim isoetifolium, but other algae may also be consumed.
Collector urchins are prey for puffer fish, octopus and humans as food dish.