Mar 4, 2009

Acropora Crabs Commensal Guard Crabs

Trapezia cymodoce in Acropora secale

Keeping Acropora corals and other SPS alive in captivity can be a challenge, But you can enlist the help of tiny Commensal Acropora crabs and Pocillopora crabs of the Trapezia and Tetralia genus. These crabs are extremely beneficial to Acropora and other SPS corals. They protect the corals from some predators and keep the coral clean. If your aquarium houses SPS (small polyp stony) corals like Pocillopora or Acropora corals, these crabs should acclimate well and be easy to keep. These tiny crabs are very territorial, so only one should be kept per coral unless they are a mated pair. These tiny crabs can be found as hitchhikers on SPS corals or can be purchased online.


Trapezia septata lives in Pocillopora
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.

Trapezia guttata lives on Seriatopora
The Birdsnset Coral Crab, Trapezia guttata, lives in Birdsnest corals of the genus Seriatopora. They may live on other species of SPS if no Birdsnest corals are present. Notice the legs of the crab are nearly identical in markings, texture, and shape to the branches of a Seriatopora hystrix. Even false polyps are present as dark markings on the brown legs.


Tetralia spp. live on Acropora
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.


"The scientists showed the importance of trapeziid crabs by gently removing crabs from sections of the two species of branching corals on a coastal reef. This resulted in 50 to 80 percent of those corals dying in less than a month. By contrast, all corals with crabs survived...For surviving corals that lacked crabs, growth was slower, tissue bleaching was greater, and sediment load was higher. Laboratory experiments revealed that corals with crabs not only shed substantially more of the sediments deposited on coral surfaces, but also that crabs were most effective at removing grain sizes that were most damaging to coral tissues." - from Tiny Crabs Project

Pocillopora and Acropora Guard crab food is provided entirely their host coral. They are easy to keep in captivity, but must be given an SPS coral to host on.


See more photos and read more about Commensal coral crabs at http://www.chucksaddiction.com/hitchcrabs.html

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