Coral guard crab (Trapezia rufopunctata) care guide

Group
Other invertebrates
Other names
Rust-spotted guard crab, Red-spotted guard crab, Trapezia crab, Pocillopora crab, Acropora crab, Commensal crab
Family
Trapeziidae
Origin
Indo-Pacific from East Africa to Hawaii and from southern Japan to South Africa; branching corals on reefs and lagoons, 0–30 m
Water
saltwater
Maximum size
5 cm
Temperature
22–28°C
pH
8.1–8.4
Hardness
SG 1.023–1.025, dKH 8–12
Minimum tank
114 litres
Care level
intermediate
Temperament
peaceful
Reef safe
yes

Care

The rust-spotted guard crab is a small, flat crab with a trapezoid shell covered in about 100–200 red or orange spots on a white or pink background, and green eyes. It lives its whole life among the branches of Pocillopora and Stylophora corals (Acropora when those are not available), with pairs sharing a colony; large corals can hold up to five pairs. The wild crab can reach about 5 cm across, though most sold are 1–2 cm. Names: shops often sell guard crabs as Trapezia sp.; several species are traded and T. rufopunctata is the one with dense rusty spots. Role: a true coral guard. It defends its host against coral predators, including the crown-of-thorns starfish, protects it from the mucus nets of vermetid snails, keeps water moving between branches and clears trapped particles and detritus. Hobbyists once saw guard crabs as coral pests, but they may improve the survival and growth of SPS corals. Set-up: an established reef of at least 114 litres (30 US gallons) with a healthy branching Pocillopora, Stylophora, Seriatopora or Acropora colony as host. It is an obligate commensal and cannot survive long without a host coral. It retreats into the branches with claws raised when approached and may pinch fingers working on its coral. Water: 22–28 °C, pH 8.1–8.4, SG 1.023–1.025, dKH 8–12, with calcium, magnesium and trace elements kept up for the corals. It does not tolerate high nitrate. Copper: copper-based medicines and some plant fertilisers are toxic to shrimp, snails and most other invertebrates. Never dose them in a tank holding this animal, and read the label of any treatment, fertiliser or algae remover before use. Sources: LiveAquaria (liveaquaria.com/products/trapezia-pocillopora-acropora-crab); Wikipedia (en.wikipedia.org/wiki/Trapeziarufopunctata). Checked October 2026.

Diet

Feeds on the mucus and some tissue of its host coral, which it releases by massaging the coral with its legs, collecting it on leg hairs and passing it to its mouth, plus trapped particles and detritus. It will also take meaty foods that land on the coral, and the diet can be supplemented with fish food, but it does not need a staple diet in a fed reef tank.

Breeding

Sexes are separate. Pairs court using touch and chemical signals, the female carries the eggs, and they hatch into planktonic larvae that are not reared in home tanks. The pair usually stays together in the same coral afterwards.

Compatibility and tank mates

Reef safe and peaceful. One pair per host colony; extra crabs fight over corals. It defends its coral from other crabs, snails and starfish. Large wrasses, triggers and hawkfish may eat it.

Last updated 8 October 2026. Community-edited care guide on Aquatic-Auctions.

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