Every week on Facebook we see numerous ID posts about crabs…. Acro crabs, ‘Gorilla’ crabs, ‘Hairy’ crabs and others. Generally, crabs make their way into our systems as hitchhikers on liverock and/or corals.

What we know as Acro crabs are generally all quite safe commensal critters which live in and on branching SPS corals. There are several species across a few different genus’ But most acro crabs are from two main genus…Tetralia and Trapezia spp. They are quite easy to identify once you know what to look for. Each species has its preferred SPS coral and often has a pattern or colouring that matches it’s host. The crab literally guards its host against predators and picks it clean of detritus and debris.

Emerald crabs often get a mention as great for cleaning up nuisance algae, in particular…bubble algae (Valonia ventricosa). The bad news is that you will never see one here in Oz. The Emerald crab (Mithraculus sculptus) is an Atlantic Ocean species (Carribean and Gulf of Mexico) and Australian law prohibits the importation of inverts.

The infamous “Gorilla” crab….. This is a common name and is given loosely to a number of species because they appear to be hairy! The ‘hairs’ are actually sensory organs called Setae and all crabs (all crustaceans actually) have them. The setae are used for various purposes including…feeding filters, sensory organs, defensive tools etc. Decorator crabs use modified ones shaped like hooks to hold on to their decorations.

Most crabs in the aquarium (apart from the commensal SPS crabs) are no more than a nuisance or at the most, a bit destructive but among the pain-in-the-butt types you will occasionally find yourself with a member of a deadly crab family (yes, deadly).

Xanthid crabs often look just like your common, every day reef crab however, they contain lethal amounts of toxin very similar to Tetradotoxin. This is otherwise known as pufferfish toxin. It’s produced by a bacteria known as Vibrio which is very dangerous in its own right.

There are other families of crustaceans that we commonly see in the hobby such as squat lobsters, hermit crabs and so on but there are literally thousands of individual species which makes them well worth covering separately.