Guardians of the Acropora

Posted on Friday, January 8th, 2010 at 10:45 pm by Felicia McCaulley

There are two genera of Guard Crabs commonly found in the aquarium trade – Trapezia and Tetralia – that are symbiotic on small polyp stony corals such as Acropora, Pocillopora, Stylophora, and Seriatopora corals.

Trapezia crabs can measure up to 1-3/4″ from elbow to elbow. They are also “equal handed,” having same-sized chelipeds (claws). Trapezia crabs are symbionts on Pocilloporid corals. Trapezia cymodoce, Trapezia septata, and the beautiful, red-spotted Trapezia rufopunctata host on Pocillopora corals, while the brown-clawed white Trapezia guttatus hosts on Seriatopora Birdsnest corals. These crabs are perfectly camouflaged to their host coral. The legs of T. guttatus are almost identical to the branches of a Birdsnest coral, and the spots of the T. rufopunctata mimic the host coral’s polyps and color. In captivity if a Pocilloporid coral is not available, a Trapezia crab may host on an Acropora coral.

The tiny Acropora Crabs of the genus Tetralia rarely measure more than 3/4″ and have one claw that is larger than the other. It’s not uncommon to see these crabs living in corals as pairs. Tetralia Crabs are commonly found as hitchhikers on wild Acropora colonies, and only host on Acropora corals. These crabs come in a wide array of colors. They are usually purple, white, or orange and have a brown, black, or blue “mask” across the eyes, giving them the common name “bandit crabs.”

In captivity it’s best to keep large Trapezia crabs on larger SPS colonies. Their activities have (uncommonly) been reported to cause damage to smaller colonies or frags. Guard Crabs are beneficial to their host corals, as they protect the coral from some pests, predators, and settling sediment. In the wild scuba divers see them pinching the underside of the crown of thorns star – a large predatory sea star that consumes coral – until it moves on to an unprotected coral. I’ve personally witnessed an Acropora crab evict a smaller pest blue eyed crab from its host coral. Blue eyed crabs can cause serious damage to the coral colonies they inhabit. Guard Crabs also remove debris that settles on the host coral, preventing tissue necrosis. In turn the crab gets a home and a free meal.

In the aquarium Pocillopora and Acropora Guard Crabs appreciate the occasional target feeding of small mysis, but most of their food is provided by their host coral. They are easy to keep in captivity, but must be given an SPS coral to host on.

Trapezia guttata

Tetralia cinctipes

Trapezia septata

Photos and article by Felicia McCaulley

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Felicia has been keeping aquariums since the early 90s and has a keen interest in taxonomy, aquaculture, and seahorses. she is the former Liveaquaria Diver's Den photographer and now works for Philadelphia area's largest aquarium specialty store The Hidden Reef.