Emerald Crabs

Published 2018


Written by Dan

Emerald Crabs (Mithraculus scuptus) are a very interesting addition to a saltwater tank.  They do however have their quirks and it is best to be cautious when adding them. Each one has its own personality some are very outgoing and have no problems being front and center in the tank while others will hide and make you think they have disappeared.  Some will also be more aggressive than others.


What this article covers

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Quick Facts 
Care LevelEasy
Minimum Tank Size5g
TemperamentCan be aggressive
Reef SafeWith Caution
Water Conditions72-78° F, dKH 8-12, pH 8.1-8.4, sg 1.023-1.025
Max Size2.5"
Natural HabitatCaribbean
Scientific NameMithraculus sculptus

Common Names

Emerald Crab, Green Clinging Crabs

Where is the Emerald Crab Found?

The emerald crab hails from the Carrabin seas as well as the Gulf of Mexico.  They like plenty of rockwork to hide in during the day and are found scavenging at night among the reefs in these regions. Save Save Save Save Save

Are Emerald Crabs reef safe?

Debatable! Lots of people, myself included have had great experiences with these little guys.  I as of this writing have 4 in the 180 reef.  In a well-established tank that is feed well they typically do well.  In super low nutrient tanks with no algae to speak of and sparse feeding, hunger will turn this little guys from Bruce Banner into the Hulk.  This is where many of the horror stories come from.Save Save Save Save

What do Emerald Crabs eat?

Emeralds are opportunistic omnivores.  This means that their primary diet consists of algae.  They are even known to devour bubble and hair algae, but should not be considered the cure to an outbreak of it. They have been seen eating other inverts, coral polyps, and even small fish.  This normally happens when they are starving or just in the right place at the right time.  Starving animals will feed. This is where the opportunistic portion comes in to play.  If you see them snacking on an overturned Margarita Snail, or a small sickly fish; don’t be surprised.  Margaritas can’t turn themselves back over when they fall off of rocks and dead fish will get eaten by most of your scavenger crew.Save Save Save Save Save Save

Acclimating Emerald Crabs

I personally recommend a 1-2 hour drip acclimation to best adjust them to your tank.  This is the best option when acclimating any new creature to your tank.  Many people, myself included, have had success doing a floating acclimation though to adjust them to the temperature while adding a quarter cup of tank water to the bag you are floating every 15 minutes for the same time frame. Save Save Save Save Save Save Save

What type of behavior can you expect from an Emerald Crab?

I personally find the inverts in my tank often more fascinating to watch than the fish.  The Emerald Crab contributes largely to the reason of that.  They will spend most of their time scavenging for food. They do not tend to be very shy creatures either and have no problem being out on the rocks while you stare into the tank at them.  They will tend to scurry to the closest crevice when you stick your hand in the tank though.  They are also not afraid to snip at your fingers if you get to close to that crevice. They will constantly dig and scrape at the rocks for algae and any food particles that may have settled into the small nooks and crannies of the rocks.  If you have more than one in a tank they will gravitate to their own small territory if enough space is available.Save Save Save

How do Emerald Crabs reproduce?

Emerald crabs will mate with the male and female locking together for the male to fertilize the eggs.  The eggs are then carried under the female’s abdomen for a few weeks at which point the larva are released into the water. The larva will float up to the faster moving portions of the water to reside in the plankton for a period of time while they develop. This phase in their development is why they are not typically bred in captivity.  The standard filtration systems does not allow for them to exist in this stage long without getting picked up by a filter sock, protein skimmer, or other type of filtration medium designed to remove organic particles from the water.Save

If you have had any luck breeding these, please share your experience in the comments section below.

What are some good Tank Mates for the Emerald Crab?

For starters anything small enough so they can not fit one in their mouth.  Most fish large enough to consume one whole are not a good option.  Also natural predatory fish known for hunting and eating coruscations should be left out.  Things such as puffers, triggers, and especially hawkfish should not be included in a tank with your emerald crabs. Anything considered reef safe is a good pick.  Emeralds do have a reputation for eating other inverts but most inverts are opportunistic omnivores and will eat an easy meal if a large supply of their preferred meals are not available. Save Save Save Save

What is the Lifespan of an Emerald Crab?

A typical life expectancy in a fish tank is 6 months to a year inside an aquarium based on a pool of several reef keepers.  There are the outliers that have them for several years too one case indicating they had one for 2-3 years.  If you run a very low nutrient system and do not have ample algae spot feeding them will help.Save Save Save Save Save

How long have yours lived? Let me know in the comments .

What Tank size is recommended for the Emerald Crab?

With the typical specimen being purchased at around ½ – 1” in size and max size around 2” base body diameter there really is no minimum tank size.  It more depends on your other inhabitants.  You honestly could keep one in 2.5 – 5 gallon nano tank with live rock and a few coral frags.  I would not stock any other crabs in a tank that size though as your emerald will likely claim it all as his kingdom. A good rule is 30 gallons per emerald crab when keeping multiples ensuring there is ample rock work in those gallons for them to separate throughout the tank.  I have 5 in my 180g and keep about 90lbs of rockwork in the display portion of the setup where the crabs are. Save Save Save Save Save

Pink Emerald Crabs!!!

A popular color variation of this crab is the Pink Emerald.  Identical in all visual characteristics except color.  These have a pale blue coloring that fades to white near the edges of the shell.  Under actinic lights they will appear blue.  These are much easier to spot in the tank than their green cousins who blend in much more.  I love watching mine move around the tank. Save Save Save Save


Emerald Crabs make a nice addition to a saltwater tank.  You should avoid placing them in predatory FOWLR setups as most fish you would keep in these will eat the crabs.  Also keep an eye on them around your corals just like people different crabs have different personalities and some will nip at small polyps like Zoas while others will leave them alone entirely.  I love watching mine scurry around the tank digging in the cracks and crevices for bits of food.

What is your experience with this crab?  Let me know in the comments below.

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