Updated August 6, 2019
Author: Mike - FishLore Admin
The Emerald Crab (Mithrax sculptus) is a saltwater invertebrate that is often used as part of a clean up crew in marine fish tanks. Emerald Crabs are popular because they help keep the tank clean and have been reported to eat bubble algae (Velonia sp.). Bubble algae can grow under even optimal water conditions and this crab is supposed to help keep the bubble algae population to a minimum. They are a pretty emerald color and stay on the small side, only reaching about 2 inches (5 cm) when fully grown.
Stocking wise, it is recommended to keep only 1 crab per 20 or 25 gallons. Some have reported that keeping them in higher concentrations could lead to aggression amongst themselves. They may also attack smaller snails so if you like your snails, you'll need to keep an eye on your crabs. They may do just fine with snails, but you never know.
Another part of their diet may consist of coralline algae. Yes, the pretty purple and red algae that encrusts the tank glass and live rock. If your tank has several emeralds you may have some issues with this habit.
While they are quite hardy, you will still want to take your time and acclimate your crab slowly into your aquarium. Take an hour and do a slow drip acclimation to help get them adjusted to your water parameters. Once inside the tank they may hide out for much of the time, but you may start to see them out and about as time goes on.
Overall, the Emerald Crab can be a good scavenger for your tank. Use caution if you have a reef tank with snails and corals. They may eat or nip at coral polyps. Fish only setups should have no problems with keeping these crabs.
Emerald Crab Care
Scientific Name : Mithrax sculptus
Common Names : Mithrax Crab
Care Level : Easy
Life span : 2 - 4 years
Size : 1.5 - 2 inches or larger (5 cm)
pH : 8.1 - 8.4
Temperature : 70°F - 80°F (21°C - 27°C)
Specific Gravity : 1.020 - 1.025
Carbonate Hardness (dKH) : 8 - 12°
Origin / Habitat : Caribbean, Atlantic
Temperament / Behavior : They can be peaceful but are considered semi-aggressive since they may scrap with other crabs and shrimp.
Breeding : Not sure of a successful rearing of baby emeralds in the home aquarium. They can and do release larvae.
Aquarium Size : 10 gallon minimum
Tank Mates : Is this Crab reef tank safe? It could be considered reef tank safe, although when it reaches it's adult size of 1.5 to 2 inches it may get more aggressive, especially with other inverts such as snails.
Reef Tank Compatible? : Yes, but watch them closely. They may go after your saltwater snails.
Fish Disease : Saltwater Fish Disease - Diagnose, Symptoms and Treatment
Diet / Foods : A scavenger, their claim to fame is their propensity to eat bubble algae. They have also been reported to eat coralline algae (Doh!) as well as scavenge on the live rock and sand bed. They do a decent job as part of a clean up crew.
Tank Region : Usually on, in or around the live rock or sand substrate.
Gender : Hard to determine differences between male and female.
Site References :
Calfo, Anthony & Fenner, Robert (2003). Reef Invertebrates - An Essential Guide to Selection, Care and Compatibility.
Kurtz, Jeffrey (2005), TFH Publications. The Simple Guide to Mini-Reef Aquariums - Modern Reefkeeping Practices, Coral Compatability.
Emerald Crab Comments
I have read numerous accounts of them eating bubble algae but have yet to see mine (I have two of these emerald crabs) go after the bubble algae in my saltwater tank. Maybe it's not their favorite type of food? They are constantly picking at the rock and sand though and are probably doing a good job keeping the tank clean. I have snails too and no problems thus far. Keeping my fingers crossed. If I do have problems with them, they will be moved to a fish only system.
More Saltwater Invertebrate Profiles
Good scavengers that will burrow under the sand and come out when they detect the smell of food entering the water.
Does a great job at eating aiptasia and you can keep multiples and even breed them in a dedicated saltwater shrimp breeding setup.
Red Fire Shrimp
An amazing looking shrimp and one of the most popular. The fire shrimp don't seem to be as active in the cleaning fish department as the skunk cleaner shrimp but they do in fact clean fish.