TankBud

Coral guard crab

Trapezia cymodoce

Also known as Red-Spotted Guard Crab, Blue Coral Crab, Lilac Coral Crab, Pocillopora Crab

SaltwaterCrab

Coral guard crab (Trapezia cymodoce)
Diego Delso · CC BY-SA 4.0

Care at a glance

Temperature
75–81°F
24–27°C
pH
8.1–8.4
Adult size
0.8 in
2 cm
Minimum tank
11 gal
40 L
Temperament
Mostly peaceful
Social behavior
Social
Swim level
Bottom
Difficulty
Easy
Breeding
No record
Rarity
Rare
Reef safe
Reef safe
Family
Trapeziidae
Origin
Indo-Pacific, living among the branches of Pocillopora and other stony corals.

Overview

The coral guard crab is a tiny, flat crab for a reef tank with a branching stony coral. It is easy to keep.

Did you know

Coral guard crabs defend their coral so fiercely that they will grab and fend off crown of thorns starfish many times their size.

Caring for the coral guard crab

Give the coral guard crab a branching Pocillopora or Acropora colony to live in, in a tank of at least 11 gal (40 L). Keep the water at 75 to 81°F (24 to 27°C) with a pH of 8.1 to 8.4.

It feeds on its host coral's mucus and fat-rich tips, which give it most of its food. It will also take small meaty scraps.

Usually only one crab or a mated pair holds a single coral head, since guard crabs are territorial toward each other. They are reef safe and safe with corals and other tank mates.

Good to know

The coral guard crab, Trapezia cymodoce, belongs to the family Trapeziidae. It is an obligate coral symbiont, meaning it cannot live long without a host coral. It repays its host by driving off coral predators. It is often lilac, red or orange with fine spotting. It is also sold as the lilac coral crab, red-spotted guard crab or Pocillopora crab.

In the wild

The coral guard crab lives across the Indo-Pacific, spending its whole life tucked among the branches of a single Pocillopora or Acropora colony. It rarely leaves its home coral.

Symbiotic relationships

The coral guard crab and its Pocillopora

Guard crabs live deep in the branches of cauliflower, birdsnest and cat's paw corals and rarely leave. The coral feeds them: it produces fat-rich mucus and small tissue bodies the crab eats. The crab earns it. When a crown-of-thorns starfish or a coral-eating snail moves onto the colony, the crab pinches and pushes until it leaves, and when sediment settles on the branches the crab clears it, which is the difference between the coral living and smothering during a sediment pulse.

Small Pocillopora colonies often arrive with a guard crab already in residence; look for a fingernail-sized crab with a smooth, spotted or plain shell in the branches and leave it there. One crab or a mated pair per colony is the rule, they are harmless to everything else in the tank, and they need no feeding beyond the coral they live in. Without a host coral the crab has nowhere to be and does not last.

Every symbiotic relationship in the catalog

Common questions

What size tank does a coral guard crab need?

A coral guard crab needs at least 11 gal (40 L). It spends most of its time in the bottom.

What temperature and pH does a coral guard crab need?

A coral guard crab is comfortable at 75–81°F (24–27°C) with a pH of 8.1–8.4.

What does a coral guard crab eat?

A coral guard crab gets most of its food from its host coral, feeding on the coral's mucus and fat-rich tips. It will also take small meaty scraps.

How big does a coral guard crab get?

An adult coral guard crab reaches about 0.8 in (2 cm).

Related species