How to keep Emerald crabs


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The Emerald, or Mithrax crab, is often sold as a natural control for bubble algae. Jeremy Gay explains how to keep it.

Common name: Emerald crab, Mithrax crab

Scientific name: Mithrax sculptus

Origin: Caribbean

Requirements: Mithrax crabs require algae on which to graze, a mature reef tank and lots of places to hide.

Notes: The Emerald crab is one of those popular inverts that is bought solely to do a job for the reef keeper, and that is to control Valonia and Ventricaria bubble algae.

Also known as Sailor’s eyeballs, this large bulbous algae spreads quickly through a reef, growing on pumps, blocking their inlets and even overgrowing corals. In short – it’s a bad thing and should be removed or controlled on sight.

Emerald crabs eat such algae and can be witnessed destroying and eating them with their sharp claws. Add several to a mature reef and they should offer natural control.

An issue with them however, and one that also occurs with Peppermint shrimp, is that these natural predators aren’t always as totally effective as you would like, especially if something more tasty is offered like fish foods. They are useful and popular, but don’t expect miracles or 100% clear-up.

Emerald crabs are totally peaceful and can be sexed, with males being larger, with bigger claws, and females having the egg flap underneath, which will often be full of eggs in a mature reef tank.

Raising the young will be near to impossible, unfortunately, because the larvae are so tiny and pelagic.

Adults often hide out in the daytime, clinging upside down to the underside of some live rock, so provide these crabs with plenty of dark, quiet refuges, and don’t combine them with any large, non-invert-safe predators as then they won’t come out and graze at all.  

Availability: Regularly available from marine wholesaler, Tropical Marine Centre.

Price: Around £10 each.

This article was first published in the November 2009 issue of Practical Fishkeeping magazine. It may not be reproduced without written permission.