The diadema urchin or blue-black urchin (''Echinothrix diadema'') is a species of tropical sea urchin, member of the

Diadematidae The Diadematidae are a family of sea urchins. Their tests are either rigid or flexible and their spines are long and hollow. * '' Astropyga'' Gray, 1825 **'' Astropyga radiata'' ( Leske, 1778), extant **'' Astropyga pulvinata'' (Lamarck, 1816), ...

family.

Description and characteristics

''Echinothrix diadema'' is a long spined urchin. With its spines, the typical diameter is . The internal organs are enclosed in the

test Test(s), testing, or TEST may refer to: * Test (assessment), an educational assessment intended to measure the respondents' knowledge or other abilities Arts and entertainment * ''Test'' (2013 film), an American film * ''Test'' (2014 film), ...

, covered by a thin dermis and epidermis. It is generally black or blue-black in colour, and always dark (the spines show a blue sheen in the light). The spines are closed at the tip; the anal sac is small and dark. It differs from '' Echinothrix calamaris'' in that the spines are not banded, except in juveniles, and that its anal sac is small and hardly visible, and the spines show a blue sheen (whereas dark morphs of ''E. calamaris'' show a greenish tinge). Another similar species is ''

Diadema setosum ''Diadema setosum'' is a species of long-spined sea urchin belonging to the family Diadematidae. It is a typical sea urchin, with extremely long, hollow spines that are mildly venomous. ''D. setosum'' differs from other '' Diadema'' with five, ...

'', which has longer spines and smaller test, the distinguishing feature being an orange ring around anal sac. Image:Echinothrix calamaris, jeune individu.jpg, Young individual Image:Echinothrix diadema Landaagiraavaru.jpg, Older juvenile Image:Echinothrix diadema Réunion.jpg, Adult.

Feeding habits

The grazing preferences of ''Echinothrix diadema'' in Fiji were found to correlate with its preferred algae/seagrass species. This being ''Codium geppiorum'' in the case of ''Echinothrix diadema''. The grazing activity contributes to echinoid bioerosion.

Distribution and habitat

This species can be found throughout all the Indo-Pacific coral reefs, from the

Red Sea The Red Sea ( ar, البحر الأحمر - بحر القلزم, translit=Modern: al-Baḥr al-ʾAḥmar, Medieval: Baḥr al-Qulzum; or ; Coptic: ⲫⲓⲟⲙ ⲛ̀ϩⲁϩ ''Phiom Enhah'' or ⲫⲓⲟⲙ ⲛ̀ϣⲁⲣⲓ ''Phiom ǹšari''; ...

Hawaii Hawaii ( ; haw, Hawaii or ) is a state in the Western United States, located in the Pacific Ocean about from the U.S. mainland. It is the only U.S. state outside North America, the only state that is an archipelago, and the only state ...

. It is a shallow waters species, living between the surface and 10m deep, but can eventually be found down to 70m.

Behaviour

It is active at night, hiding in crevices, burrows, and under rocks during the day. Females generally choose low-lying locations, presumably so the tiny larvae can have better protection from predators.Parvez, M. S. et al. (2016) Status, prospects, and potentials of echinoid sea urchins in Malaysia. volume 4: 93- 97 It hosts

commensal Commensalism is a long-term biological interaction (symbiosis) in which members of one species gain benefits while those of the other species neither benefit nor are harmed. This is in contrast with mutualism, in which both organisms benefit fro ...

species like the shrimp '' Stegopontonia commensalis''. '' Saron marmoratus'' stays close for protection, like many fish of the families Apogonidae (cardinalfish) and

Centriscidae The Centriscidae are a family of fishes from the order Syngnathiformes which includes the snipefishes, shrimpfishes, and bellowfishes. A small family, consisting of only about a dozen marine species, they are of an unusual appearance, as refle ...

( razorfish and relatives).

References

External links

* Animals described in 1758 Diadematidae Taxa named by Carl Linnaeus {{Echinoidea-stub