---
title: Diadema Urchin (Echinothrix diadema)
scientific_name: Echinothrix diadema
common_name: Diadema Urchin
taxon: Sea urchin
published: 2024-01-09T20:21:07.408Z
updated: 2026-08-15T19:25:34.828Z
tags: Diadema Urchin, Echinothrix diadema, Sea urchin, Red Sea, Marine Life, Diving
url: https://redseacreatures.com/taxon/invertebrate/echinoderms/sea-urchin/diadema-urchin
---

# Diadema Urchin (*Echinothrix diadema*)

## Quick facts

- **Group:** Sea urchin (*Echinoidea*)
- **Size:** up to 20 cm
- **Depth:** 1-70 m
- **Diet:** Herbivore
- **Activity:** Nocturnal
- **Rarity:** Common
- **Habitat:** Reef and rubble, under ledges by day
- **Reproduction:** Broadcast spawner, Gonochoric
- **Hazard:** Dangerous, Venomous

## How to recognise it

- uniformly dark spines with no banding, often with a green or purple sheen
- spines all much the same length, unlike its banded relative
- blue-green iridescent spots on the dark test
- spines to 20 cm

A black urchin with an iridescent sheen and no banding on the spines - which is what separates it from the two long-spined urchins it shares the reef with.

The trio is worth learning as a group. The true *Diadema* has very long, needle-thin spines and blue spots on the test; the banded *Echinothrix* has two clearly different spine types, long and banded plus short and fine; this one has a single set of uniform dark spines with a metallic gloss. All three are nocturnal grazers, all three are venomous, and all three shelter other animals.

Grazing is what they are for. Urchins are the reef's other lawnmower alongside the herbivorous fish, and where fish are overfished, urchins take over the job entirely - which works until something happens to the urchins.

Spines break off in a wound and dissolve slowly. Hot water helps the pain; a fragment near a joint or deep in a hand is a matter for a doctor.

## Also known as

- **English:** blue-black urchin, Schwarzer Diademseeigel
- **French:** Oursin diadème à gros piquants, Oursin bleu-noir
- **Italian:** Riccio diadema nerazzurro
- **German:** Diadem-Seeigel
- **Japanese:** ガンガゼモドキ
- **Chinese:** 冠刺棘海膽
- **Russian:** Эхинотрикс-диадема

## Scientific synonyms

- *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)

## Range

Eastern Africa & Madagascar, Federal Republic of Somalia, Indian Ocean, Kenya, Madagascar, Maldives, Philippines, Red Sea, Republic of Mauritius, Seychelles, South Africa, South Andaman Islands

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*Source: [Red Sea Creatures](https://redseacreatures.com/taxon/invertebrate/echinoderms/sea-urchin/diadema-urchin)*
*Updated: August 15, 2026*