Emerald Crab


Dark Green with hairy legs and built like a tank the Emerald Crab is a very affordable scavenger which can be added to your reef aquarium.  As a general rule of thumb crabs in general cannot be considered 100% reef safe, however, the Emerald Crab is just about as close as you can come to owning a crab in your reef aquarium and resting easy that all your other livestock are safe.

Common Names:  Emerald Crab or Green Crab

Skill Level: Beginner

Disposition: Mostly Peaceful

Water Conditions: 72-78° F, dKH 8-12, pH 8.1-8.4, sg 1.023-1.025


The Emerald Crab is probably the best and cheapest addition you can add to your reef tank for handling bubble algae.  With claws specifically designed for cleaning off live rock of nuisance algae an Emerald Crab can keep your tank clean so that you don’t have to.  This being said, an Emerald crab will eat more than just bubble algae. The Emerald Crabs are also scavengers and will also eat bits of food which have gotten away from fish or corals.  I have also caught  the Emerald Crab picking at other forms of nuisance algae as well.

Below is a quick video of an Emerald Crab scavenging the bottom of a tank and finding a piece of raw shrimp the fish missed.


It is possible to keep multiple Emerald Crabs together without them fighting, as long as the food supply is steady.  I made the mistake once of having too many crabs in a tank (I’m a big fan of Emerald Crabs) and once the food supply began to decline I saw them fighting.  I witnessed a larger crab literally rip the claw off a smaller crab.  I assume they were fighting over a newly added piece of live rock which I had added to the aquarium.  I removed the smaller crab and placed him in a frag tank where he happily lives scavenging off the bottom. Although now he only has one claw, which I still feel bad about.

The ripped off crab arm

Once you add your Emerald Crab to your aquarium it’s likely you will very rarely ever get to see them, which is a shame since they are very interesting looking. They usually like to hang on the back or underside of live rock hidden away in small caves or shelters where they feel safe.  Every once in a while you will see your Emerald Crab’s green arm sticking out of some dark hole picking at the live rock keeping your reef tank clean.  I have noticed on a rare occasion that after several months some of the larger Emerald crabs will venture away from the rock work or come out of hiding for extended periods of time to feed.


Just because your Emerald Crab is hanging around corals does not mean he's eating them!

Besides witnessing two Emerald Crabs fighting with each other, which I assume was over food on a piece of live rock, I have never seen them attack any other inhabitants of my tank.  At the beginning of the article I mentioned that no crab can be considered 100% reef safe.  I do believe a hungry Emerald Crab could attack a sleeping fish or snail and kill it if it needed to survive, however, if your tank is well fed you shouldn’t experience any problems and you can enjoy having this hard working and very neat looking scavenger occupying your tank.






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