Blue-black sea urchin (Echinothrix diadema)

Classification (WoRMS)

Full classification as recorded by WoRMS: Kingdom: Animalia · Phylum: Echinodermata · Class: Echinoidea · Order: Diadematoida · Family: Diadematidae · Genus: Echinothrix.

Accepted name: Echinothrix diadema · Authority: (Linnaeus, 1758) · Taxonomic status: accepted · AphiaID: 213378.

Synonyms and superseded combinations recorded for this taxon: Astropyga spinosissima (Lamarck, 1816); Astropyga subularis (Lamarck, 1816); Centrostephanus subularis (Lamarck, 1816); Cidaris araneiformis Leske, 1778; Cidaris coronalis Leske, 1778; Cidarites spinosissima Lamarck, 1816; Cidarites subularis Lamarck, 1816; Diadema desjardinsii Michelin, 1845; Diadema spinosissimum (Lamarck, 1816); Diadema subulare (Lamarck, 1816); Echinothrix cincta (A. Agassiz, 1863); Echinothrix petersii Bölsche, 1865; Echinothrix spinosissima (Lamarck, 1816); Echinothrix spinosissimum (Lamarck, 1816); Echinothrix subularis (Lamarck, 1816); Echinothrix turcarum Peters, 1853; Echinus coronalis (Leske, 1778); Echinus diadema Linnaeus, 1758; Garelia cincta A. Agassiz, 1863; Garelia subularis (Lamarck, 1816).

Source: World Register of Marine Species (WoRMS) — https://www.marinespecies.org/aphia.php?p=taxdetails&id=213378 (CC BY 4.0). Accessed 2026-09-23.

Biology and life history (SeaLifeBase)

Measured values published by SeaLifeBase for Echinothrix diadema. Only fields the database actually holds for this species are shown; these are database values, not aquarium guidance.

Maximum length: 20 cm · Depth range: 1–220 m · Habitat zone: reef-associated · Vulnerability index: 10 of 100 · Fishery importance: commercial.

SeaLifeBase notes: Occurs in shallow coral and coral rubble areas at depths of 1 to 40 m (Ref. 81254), coral reefs and sandy areas (Ref. 129602). A generalist herbivore displaying nocturnal feeding behavior (Ref. 81254). Known to graze (Also Ref. 129602) on organic material and adults may also feed on live hard corals. Sometimes associated with commensal shrimp Stegopontonia commensalis (Ref. 800). Commonly found with Diadema spp. Bioeroder. Feeds on small and turf algae over hard substrates (Ref. 129602).

Source: SeaLifeBase species summary (SpecCode 50317) — https://www.sealifebase.se/summary/Echinothrix-diadema.html (CC BY-NC 3.0 — non-commercial use only). Accessed 2026-09-23.

Distribution and occurrence records (GBIF)

3,302 georeferenced occurrence records for Echinothrix diadema are published through GBIF from 53 countries or marine areas. The map above plots 192 distinct locations from a sample of 3,000 records, omitting 20 inland records that carry an institution's coordinates rather than a collection site; it shows where the species has been observed and reported, not a modelled range.

Most records by country or area: Cook Islands (459); United States of America (376); French Polynesia (372); Kenya (276); Wallis and Futuna (189).

The full country table, WoRMS marine regions and the sampling notes are on this species' Distribution page.

Source: GBIF Occurrence Search (taxonKey 5188164) — https://www.gbif.org/occurrence/search?taxon_key=5188164&has_coordinate=true (records CC0 / CC BY); WoRMS distribution records (MarineRegions) — https://www.marinespecies.org/aphia.php?p=taxdetails&id=213378 (CC BY 4.0); Natural Earth 110m basemap — https://www.naturalearthdata.com/ (public domain). Accessed 2026-09-23.

Description and natural history (Wikipedia)

Echinothrix diadema is a long spined urchin. With its spines, the typical diameter is 10 - 20 cm. The internal organs are enclosed in the test, 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, which has longer spines and smaller test, the distinguishing feature being an orange ring around anal sac.

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

This species can be found throughout all the Indo-Pacific coral reefs, from the Red Sea to Hawaii. It is a shallow waters species, living between the surface and 10m deep, but can eventually be found down to 70m.

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.

It hosts commensal species like the shrimp Stegopontonia commensalis. Saron marmoratus stays close for protection, like many fish of the families Apogonidae (cardinalfish) and Centriscidae (razorfish and relatives).

Source: Wikipedia: Echinothrix diadema — https://en.wikipedia.org/wiki/Echinothrix_diadema (CC BY-SA 4.0). Accessed 2026-09-23.

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