Second only to the Caridean shrimp, the Emerald Crab is now one of the most popular crustaceans for saltwater aquariums. Their captivating color, robust character, and algae-eating distinction all have understandable allure.
But what about the stories of Emerald Crabs eating coral and attacking fish? In this care guide, we’ll get to the bottom of the diverse opinions on keeping these crabs and provide you with the best information on how to keep your Emerald Crabs healthy and happy for the long haul.
Emerald Crab Species Overview
|Emerald Crab Info
|Emerald Crab, Mithraculus Crab, Emerald Mythra, Emerald Crab Crab, Green Crab, Green Clinging Crab, Jade crab
|Caribbean and Gulf of MeEmerald Crabico
|Easy for a marine species (more difficult than freshwater crabs)
|Minimum Tank Size
|MaEmerald Crab Size
|1.5 – 2.5 inches
|74 – 82 degrees F
|8.0 – 8.4
|1.023 – 1.025 (29-35 PPT at 77°F)
|Mostly! Please see the info in the FAQs
|Difficulty to Breed
|Difficult to breed
Origin and Background
The Emerald Crab (Mithraculus sculptus) is native to the Caribbean Sea and the Gulf of Mexico.
Also known as the ‘Green Clinging Crab’, this popular crustacean is found at depths down to around 180 ft and naturally inhibits various environments, including coral reefs.
Emerald Crabs have a diverse diet but are famous for feeding on various types of algae both in the wild and in aquariums.
The Emerald Crab is a handsome fellow! Its entire body is a similar color to the green algae that it loves to feed on, hence its name. This coloration surely provides them with excellent camouflage, helping them to remain undetected by potential predators such as larger fish.
Emerald Crabs also have impressively hairy legs that sometimes catch debris in the water. You can sometimes watch these omnivores pick the debris from their legs and eat them!
Their legs have quite pointed ends which help them to grip onto rocks and substrate, preventing them from being swept away in strong currents. They also have big, strong claws for their size!
Emerald Crabs vary considerably in size. Smaller individuals will only grow to around 1.5 inches long, but larger ones can reach 2.5 inches.
This is an important consideration because larger Emerald Crabs can bully or even prey upon smaller ones!
Emerald Crab Care Guide
One of the most important factors when keeping Emerald Crabs is their tank size.
Although they’re fairly small invertebrates, Emerald Crabs are territorial and feel more relaxed when given adequate space per individual.
The smallest tank size for an individual is 20 to 30 gallons, with an extra 20 gallons needed for each additional crab.
Another extremely important component of the Emerald Crab’s well-being is the right tank environment.
Emerald Crabs are naturally nocturnal invertebrates, meaning they need plenty of rocky caves to take refuge in during the daytime. Without enough hiding places, they could easily become stressed and aggressive towards each other and their tank mates.
When first introduced to your tank, Emerald Crabs might spend most of their time hiding in these crevices but will usually come out more often, even in the daytime, once they begin to feel safe among their surroundings.
Keeping Emerald Crabs in Reef Tanks or Live Rock Aquariums
But while Emerald Crabs do look stunning in a reef aquarium, they can also be kept in live rock marine aquariums without coral, especially where some vegetation such as turtle grass is provided.
Keeping them without coral sidesteps the coral-eating issue, which we’ll discuss in the FAQs!
Emerald Crabs are tropical invertebrates that definitely require an aquarium heater. They will accept a wide range of warm water temperatures, from 74-82°F, so are compatible with almost all tropical saltwater fish in this way.
It’s important to note, though, that most marine species are sensitive to sudden changes in water temperature, so acclimatizing them properly in a new tank is essential to avoid thermal shock.
Emerald Crabs are a fairly hardy marine species that tolerate a range of water conditions, but they still have their preferences.
The optimum pH levels are said to be 8.0 to 8.4, with a water hardness of between 8 to 12 dKH.
Salt concentration or specific gravity should be between 1.020 to 1.025 salinity (28 – 35 PPT).
A good aquarium filter is essential for any marine aquarium to maintain good water quality and adequate oxygenation.
Hang-on-back filters and internal power filters are good options for smaller tanks. For tanks over 50 gallons, consider a canister filter.
With their pointed legs, Emerald Crabs are good at clinging onto rocks, meaning they can handle a strong filter flow with relative ease.
Emerald Crabs are opportunistic omnivores that will eat almost anything that they can get their claws on, including any uneaten food.
Their reputation for eating algae is well warranted, but these crabs are natural scavengers, so need some meaty supplements on their menu, too.
Popular supplements include sinking pellets (see notes below), algae wafers, nori flakes, and fresh and frozen foods such as brine shrimp, krill, and mysis shrimp (mysids).
Stubborn types of algae like bubble algae and hair algae might be less popular with them, and they may seek out other food sources before resorting to eating them!
A Note on Sinking Pellets
One scientific study reported that they’ll eat bubble algae only when supplemented with mysis shrimp, but not crab pellets – so selecting the right type and right amount of supplements to feed them is imperative if you want them to clean up your nuisance algae!
For questions regarding Emerald Crabs eating coral polyps, see the FAQs further down the page!
Emerald Crab Aggression
One of the biggest questions prospective buyers have of Emerald Crabs is: Are Emerald Crabs aggressive?
The answer to this question depends upon the individual crab, their setup, and tank mates.
Where Emerald Crabs have sufficient hiding places, feeding opportunities, and tank mates that don’t antagonize them, they can often be relatively peaceful crabs.
But when kept in small aquariums without enough caves and other bottom-dwelling tank mates competing for territory, Emerald Crabs can become aggressive and attack or kill other crabs, shrimp, snails, and fish.
Ways To Reduce Aggression in Emerald Crabs
- Provide your crabs with a large tank and plenty of hiding places.
- Give each Emerald Crab at least 20 gallons of tank space to avoid them fighting with each other.
- Give your crabs enough food so they don’t hunt snails, shrimp, and bottom-dwelling fish for sustenance.
- Identify the sex of your crab using the method given further down under ‘sexual dimorphism’. Since males are more aggressive than females, see if you can swap a problematic male for a female at your local pet store.
- Remove an aggressive Emerald Crab if none of the above works! Their temperament varies considerably among individuals so you might have better luck with another one!
Compatible Tank Mates
Because Emerald Crabs will sometimes pick fights with or prey upon other bottom-dwelling species like other crabs, gobies and cleaner shrimp, the best tank mates for Emerald Crabs are peaceful fish that inhabit the middle and upper layers of the water column.
Other bottom-dwelling species can be considered only in larger tanks, where each species has at least 20-30 gallons of tank space each. There are plenty of reports online of Emerald Crabs eating gobies and peppermint shrimp – so be prudent!
Avoid large, predatory fish, since they might try to eat your Emerald Crab for breakfast!
Emerald Crab and Molting
Like all crabs, shrimps, and lobsters, Emerald Crabs need to shed their old shells and grow new ones as they grow larger. This process is known as molting and happens in cycles according to how fast your crab is growing.
In this fascinating process, Emerald Crabs move out of their old exoskeleton, leaving it behind on the tank floor. Because this hollow shell looks like a carbon copy of your crab, many an aquarium owner has panicked, thinking that their crab is dead!
This is exacerbated by the fact that the crab will hide for many days after molting until a new shell has hardened to replace the old one.
Although it might look creepy, it’s better not to throw away the old shell since it contains an important source of calcium and important minerals for your invertebrates to reabsorb as they nibble at it.
Health and Disease
As with all marine invertebrates, Emerald Crabs are intolerant of copper in the water, so be sure to avoid any copper-based medicines used for treating fish diseases.
One of the most prevalent health complaints among Emerald Crabs is shell disease. Normally caused by viral or bacterial infection, shell disease is an umbrella term for any health condition that causes the deterioration of the crab’s shell.
The early symptoms of shell disease are pockmarks on the shell and legs, but the disease can also eat right through the shell and begin to infect the crab internally.
Mites and parasitic infections are another potential health concern. While Emerald Crabs are not directly affected by marine ich (white spot disease), they can carry the parasites and infect fish in the same aquarium.
To prevent these health issues from affecting their fish stocks, some marine fish keepers prefer to keep new fish and invertebrates in a quarantine tank before introducing them to their main tank.
Also, remember that preventing illnesses is much easier than treating them! Good tank maintenance and close observation of all of the tank’s inhabitants is the best defense against possible infections.
To tell if your Emerald Crab is a male or female, carefully turn it over and look at its underside.
The key difference between male and female Emerald Crabs is the width of the ‘pleon’ – the central abdomen section on the crab’s underside. In males, the pleon is a narrow, slightly triangular shape, in females it is wider and more square.
Males also have slightly larger claws than females and tend to be more aggressive.
Breeding Emerald Crabs is not a simple endeavor, so can’t be recommended for beginners. If you’re serious about taking on the challenge of breeding Emerald Crabs, a great resource is the well-researched guide at aquariumbreeder.com
Emerald Crabs are not the longest-lived type of crab and typically live for 2-4 years in captivity when kept in good conditions. You can help them fulfill their maximum lifespan by following excellent tank maintenance.
As one of the most popular marine invertebrates, Emerald Crabs are fairly easy to find in most marine aquarium stores. Because they’re so difficult to breed in captivity, commercial stocks come from the wild.
You can expect to pay between $5 to $25 per crab in 2023.
Do Emerald Crabs Eat Coral?
Perhaps the biggest concern people have about Emerald Crabs is: do they eat coral?
This is a controversial subject because, while some aquarists have kept Emerald Crabs alongside corals without problem, others have had them attack their soft coral and SPS coral polyps straight away.
After researching many reports on aquarium forums, it’s clear that it depends a lot on circumstances, as well as individual differences between crabs.
How Do I Stop My Emerald Crab From Eating Coral?
Firstly, do your Emerald Crabs have enough to eat? As already discussed, Emerald Crabs are omnivorous and require meaty treats on their menu as well as algae.
If they’re not provided, your Emerald Crab may search for meaty foods elsewhere, including snails, fish, and coral polyps. Note that the more aggressive males may be more prone to this behavior than females.
Try supplementing your Emerald Crab’s diet with mysis shrimp or krill 2-3 times per week and see if that reduces coral predation.
If your crab continues to attack your coral, you may have to remove it to save your coral. But Emerald Crabs are so good at hiding, that removing them can prove surprisingly difficult!
How Can I Catch My Emerald Crab?
One rather invasive method of catching Emerald Crabs is to remove all rocks from the aquarium so your crabs have nowhere left to hide.
A less invasive method is to coax them out of their hiding holes with their favorite food. Some people have reported success doing this nori flakes.
Try placing food items inside a net on the bottom of the tank. Once the crab enters, quickly raise the net to catch the crab!
Help! My Crab is Dead!
Before jumping to conclusions, gently poke what looks like a dead crab with a pair of tongs. Many people have mistaken the old shell of a molted crab for a dead body.
Remember that your crab will probably be hiding for several days after molting!
Emerald Crabs are beautiful, fascinating invertebrates for saltwater tanks that can help to keep the tank free from troublesome algae.
But many fish keepers have also had trouble with them becoming aggressive towards other tank mates and eating coral.
This means you’ll need to keep a close eye on your Emerald Crabs and be willing to adjust their environment and feeding regime to mitigate any problematic behavior.