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Diadema setosum - overview
Diadema setosum is one of the most common seashore sea urchins in the tropical indo-pacific, especially in coral ecosystems : hence, they are supposed to have an important ecological role, especially in algae grazing. It can be found from the red sea to Hawaii and Pacific archipelagos, from the shore to 20-30 meters deep (sometimes deeper).
Like most Diadema sea urchins, it is a big, dark regular echinoid, with a rather small test and very long spines, which can reach up to 30cm each. These are usually black but some or all of them can be grey or white, and they are banded in young individuals.
This sea urchins looks a lot like its relative Diadema savignyi, and they can be difficult to tell apart. D. setosum has longer spines compared to the test, and a distinct pattern of iridophores, usually with 5 white dots (whereas D. savignyi has more visible iridophores, blue or greyish, usually forming a star - but on some individuals iridophores can be absent in both species).
The most distinctive characteristic of D. setosum is to have an orange ring around its prominent anal papilla, but it may be less visible in some specimens.
Some of the shorter spines are slightly venomous (like in most diadematids), but not really dangerous.