RSS An introduction to coral crabs, good and bad

MASA Admin

8 May 2007
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There is surprisingly little information specifically about coral crabs in aquaria.  Every now and then some hitchhikers arrive on our aquacultured and wild corals and we are able to gain a bit more insight into the life of coral crabs.

Some crabs like the pale face Tetralia cinctipes can be beneficial to our SPS corals while others like the Gorilla crab can irritate coral and even try take a nip at our fish as they get bigger. Good or bad we’ve come up with a breakdown of some of the commonly found hitchhikers and what you can do if you find them on your corals.

One of the good crabs found hiding in a Pocillopora coral

Good coral crabs can be found in Acropora, Pocillopora, Stylophora and Seriatopora (Birdnest) corals. These coral crabs will often prefer one species of coral and will be found living uniquely among that species. The most common species of good coral crabs are the Tetralia and Trapezia species and it is fairly uncommon to find more than one species of crab inhabiting the same coral.

Tetralia and Trapezia coral crabs can be found living solitary or in pairs within the branches of the coral host and will defend the coral from other crabs or predators like starfish. These crabs do not harm the coral, rather they feed off nutrient rich mucus produced by the coral. Many hobbyist may not realize that coral crabs provide a beneficial symbiotic relationship with the corals and that you can even purchase coral crabs to add to your home aquarium.

The pale face coral crab Tetralia cinctipes is a commonly found good crab which provides a beneficial symbiotic relationship with SPS corals

You may have already come across the Pale face coral crab (Tetralia cinctipes), which is easily identified by its white body with a distinctive black line running across the crabs face. These crabs are often found living at the base of the coral tucked in between the branches, feel free to leave these little guys living in your coral as they are no harm to your coral or your aquarium.

If you do wish to keep these good crabs in your aquarium you will want to remove them from the coral before you dip, as coral dips will shock the crabs into coming off the coral host, or outright kill them. You can use any thin tool you have to gently coax the crab out of the coral and into a holding container with some saltwater while you dip.

Coral crabs are difficult to identify as they can look different dependent on their life stage

Coral crab with eggs

We recently came across a few Pocillopora and Acropora corals with some unidentified good crabs, we’ve added the pictures here for you, dipped our corals and then put them back in the system. There is still no guidebook to coral crabs so identifying these little guys can be tricky.

But as we know it can’t all be good, there is one crab you must be on the lookout for as this hairy beast will munch away on coral tissue until you have nothing left. The Gorilla crab (Xanthid sp.) is found primarily on Acropora, and we have never seen this species to occur on the Pocillopora, Stylophora or Seriatopora.

The Gorilla crab is a coral pest and should be removed from your tank at first sight

The Gorilla crab is easily identified with it’s rounded knobby body, blue eyes and hairy gorilla legs. You will want to remove the Gorilla crab as soon as you spot it, as the Gorilla crab grows to a larger size they can cause some havoc in your reef tank and begin to prey on smaller reef tank animals like fish or shrimp. You can remove Gorilla crabs from you coral by spearing the body and then removing the crab.

One crab we did not find this time but have seen in the past is the Gall crab. These small crabs are found exclusively on Seriatopora and will be found burrowing into the body of the coral. If you happen to find a Gall crab their tiny size does not seem to cause a threat to the coral so it’s up to you what to do with it.

Guard crabs defend coral home from crown of thorns starfish.wmv - YouTube
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