---
title: Brown Guard Crab (Trapezia digitalis)
scientific_name: Trapezia digitalis
common_name: Brown Guard Crab
taxon: Crabs
published: 2024-06-23T10:34:02.547Z
updated: 2026-08-08T22:01:23.148Z
tags: Brown Guard Crab, Trapezia digitalis, Crabs, Red Sea, Marine Life, Diving
url: https://redseacreatures.com/taxon/invertebrate/crustaceans/crabs/brown-guard-crab
---

# Brown Guard Crab (*Trapezia digitalis*)

## Quick facts

- **Group:** Crabs (*Brachyura*)
- **Size:** 2 cm
- **Depth:** 0-40 m
- **Diet:** Corallivore
- **Habitat:** Branching corals, particularly Pocillopora and Acropora
- **Reproduction:** Broadcast spawner

## How to recognise it

- light tan to deep chocolate brown coloration
- relatively large chelae with tubercles and serrations
- subtle patterned markings on carapace

The Brown Guard Crab, Trapezia digitalis, grows a carapace typically 1 to 2 centimeters (0.4 to 0.8 in) in width. Its carapace is slightly convex with subtle patterned markings. The coloration ranges from light tan to deep chocolate brown, providing camouflage among coral branches. Its chelae (pincers) are relatively large for its body size, featuring a pattern of tubercles and serrations used for both defense and feeding.

This crab primarily inhabits branching corals, particularly those from the genera Pocillopora and Acropora, where it lives from the intertidal zone to depths of 40 meters (130 ft). It forms a mutualistic symbiotic relationship with its coral host, acting as a 'guard' by actively fending off potential threats such as crown-of-thorns starfish and coral-eating snails. Trapezia digitalis feeds on the mucus excreted by its coral host, supplemented by small planktonic organisms and detritus trapped within the coral branches, thereby helping to keep the coral clean. The species is not currently listed as endangered.

The reproductive cycle of Trapezia digitalis is linked to lunar phases. Females typically release their fertilized eggs during new or full moons, a synchronization that ensures larval dispersal by tides and reduces the likelihood of predation.

## Also known as

- **English:** brown guard crab
- **Italian:** Granchio guardiano bruno
- **Spanish:** Cangrejo Paladín Chocolate
- **Portuguese:** Caranguejo-cardinal-tímido
- **Japanese:** クロサンゴガニ

## Scientific synonyms

- *Trapezia digitalis var. typica* Borradaile, 1902
- *Trapezia fusca* Hombron & Jacquinot, 1846
- *Trapezia leucodactyla* Rüppell, 1830
- *Trapezia nigrofusca* Stimpson, 1860
- *Trapezia nigro-fusca* Stimpson, 1860

## Range

East China Sea, Indian Ocean, Mozambique Channel, New Caledonian Exclusive Economic Zone, North Pacific Ocean, Red Sea, Seychelles, South Pacific Ocean

---

*Source: [Red Sea Creatures](https://redseacreatures.com/taxon/invertebrate/crustaceans/crabs/brown-guard-crab)*
*Updated: August 8, 2026*