---
title: Red Pencil Urchin (Heterocentrotus mamillatus)
scientific_name: Heterocentrotus mamillatus
common_name: Red Pencil Urchin
taxon: Sea urchin
published: 2024-01-03T05:17:25.313Z
updated: 2026-08-12T13:12:48.666Z
tags: Red Pencil Urchin, Heterocentrotus mamillatus, Sea urchin, Red Sea, Marine Life, Diving
url: https://redseacreatures.com/taxon/invertebrate/echinoderms/sea-urchin/red-pencil-urchin
---

# Red Pencil Urchin (*Heterocentrotus mamillatus*)

## Quick facts

- **Group:** Sea urchin (*Echinoidea*)
- **Size:** up to 8 cm across, spines to 10 cm
- **Depth:** 0-25 m
- **Diet:** Herbivore
- **Activity:** Nocturnal
- **Rarity:** Common
- **Habitat:** Shallow reef and surge zone, wedged in crevices by day
- **Reproduction:** Broadcast spawner, Gonochoric

## How to recognise it

- thick blunt spines like red crayons, far heavier than any other urchin's here
- spines triangular in section and often algae-covered at the tips
- wedged into a crack by day, in the top few meters of reef
- test to 8 cm, spines to 10 cm

Thick red spines like blunt crayons, up to 10 cm (4 in) long on a test of 8 cm (3 in), wedged into a crack in the surge zone. Nothing else here looks remotely like it, and the spines are so heavy that a live animal is startlingly solid in the hand.

Those spines are a wedging tool, not a weapon. The urchin jams itself into a hole with them and locks in place, which is how a slow animal survives in the top two meters of a reef where the surge would roll anything loose. It comes out at night to graze algae and returns before dawn.

They are also used by people, and have been for a long time: heavy, uniform and easy to shape, the spines were used as writing implements, files and jewelry, and the name slate pencil urchin is literal.

And they are a record. The spines lay down growth bands and their chemistry tracks the water the animal grew in, so fossil and archaeological spines are read as a temperature archive of shallow reefs going back thousands of years.

## Also known as

- **English:** slate pencil urchin
- **French:** Oursin crayon
- **German:** Griffel-Seeigel
- **Japanese:** パイプウニ
- **Russian:** Грифельный ёж

## Scientific synonyms

- *Acrocladia blainvillei* (Des Moulins, 1837)
- *Acrocladia blainvillii* (Des Moulins, 1837)
- *Acrocladia hastifera* L. Agassiz in L. Agassiz & Desor, 1846
- *Acrocladia mamillata* (Linnaeus, 1758)
- *Acrocladia mammillata* (Linnaeus, 1758)
- *Acrocladia planispina* von Martens, 1886
- *Acrocladia planissima*
- *Acrocladia serialis* Perrier, 1869
- *Cidaris mammillata* (Linnaeus, 1758)
- *Echinometra (Acrocladia) planispina* von Martens, 1886
- *Echinometra blainvillii* Des Moulins, 1837
- *Echinometra carinata* (Blainville, 1825)
- *Echinometra hastifera* (L. Agassiz in L. Agassiz & Desor, 1846)
- *Echinometra mammillata* (Linnaeus, 1758)
- *Echinus (Heterocentrotus) carinatus* (Blainville, 1825)
- *Echinus (Heterocentrotus) postelsii* Brandt, 1835
- *Echinus carinatus* Blainville, 1825
- *Echinus castaneus* Perry, 1810
- *Echinus mamillatus* Linnaeus, 1758
- *Echinus mammillatus* Linnaeus, 1758
- *Heterocentrotus carinatus* (Blainville, 1825)
- *Heterocentrotus mammillatus* (Linnaeus, 1758)
- *Heterocentrotus postelsii* (Brandt, 1835)
- *Heterocentrus mamillatus*

## Range

Jordan, Philippines, Red Sea

---

*Source: [Red Sea Creatures](https://redseacreatures.com/taxon/invertebrate/echinoderms/sea-urchin/red-pencil-urchin)*
*Updated: August 12, 2026*