---
title: Rusty Guard Crab (Trapezia bidentata)
scientific_name: Trapezia bidentata
common_name: Rusty Guard Crab
taxon: Crabs
published: 2023-12-25T12:33:26.970Z
updated: 2026-08-08T22:14:20.510Z
tags: Rusty Guard Crab, Trapezia bidentata, Crabs, Red Sea, Marine Life, Diving
url: https://redseacreatures.com/taxon/invertebrate/crustaceans/crabs/rusty-guard-crab
---

# Rusty Guard Crab (*Trapezia bidentata*)

## Quick facts

- **Group:** Crabs (*Brachyura*)
- **Size:** up to 25 mm
- **Depth:** 0-30 m
- **Diet:** Omnivore
- **Rarity:** Common
- **Habitat:** Reef flats and slopes, living within branches of Pocillopora and Acropora corals
- **Reproduction:** Brooder, Gonochoric

## How to recognise it

- orange body with red-brown markings on legs and claws
- trapezoidal carapace
- two large claws
- reddish eyes with black pupils

The Rusty Guard Crab, Trapezia bidentata, is a small marine crab with a trapezoidal carapace typically measuring up to 1.5 cm (0.6 in) in width. Its coloration is usually orange or yellowish-orange, often featuring reddish-brown spots or blotches on the carapace. The walking legs and chelipeds (claws) display darker red-brown bands or streaks. A key identification feature is the presence of two distinct teeth on the anterolateral margin of the carapace, reflected in its scientific name bidentata, meaning "two-toothed." The crab's eyes are reddish with black pupils, and its two large, smooth claws are primarily used for defense.

This species lives in an obligate mutualistic association with branching stony corals, primarily within the genera Pocillopora and Acropora, inhabiting depths from 1 to 30 meters (3-100 ft). Individual crabs typically reside in male-female pairs within a single coral colony, rarely venturing from its branches. The crab feeds on coral mucus, various algae, and detritus found on the coral branches, contributing to the host's cleanliness. It actively defends its coral host from predators, including Crown-of-Thorns Starfish and parrotfish, using its chelipeds to deter threats. The crab's movements also aid in circulating water around the coral branches, which supports the coral's health. In return, the coral provides shelter and a consistent food source.

## Also known as

- **English:** rusty guard crab
- **French:** Crabe trapèze rouillé
- **Italian:** Granchio guardiano rugginoso
- **Spanish:** Cangrejo Paladín Rojo
- **Portuguese:** Caranguejo-cardinal-enferrujado
- **Japanese:** カバイロサンゴガニ
- **Chinese:** 雙齒梯形蟹

## Scientific synonyms

- *Cancer bidentatus* Forskål, 1775
- *Grapsillus subinteger* MacLeay, 1838
- *Trapezia cymodoce var. edentula* Laurie, 1906
- *Trapezia ferruginea* Latreille, 1828
- *Trapezia ferruginea var. typica* Borradaile, 1900
- *Trapezia miniata* Hombron & Jacquinot, 1846
- *Trapezia subdentata* Gerstaecker, 1856

## Range

Indian Ocean, Mozambique Channel, New Caledonian Exclusive Economic Zone, North Pacific Ocean, South Pacific Ocean

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