Emerald Crab 101: Care, Molting & Reef Safe Advice

Emerald crabs are a fun and beneficial species to keep in your marine tank. Their neat colors, lively personality, and tank-cleaning habits make them a great creature to consider getting!

But before you do, there are some things you’ll need to be aware of.

These crabs add a lot to your tank, but they also need specific conditions. Potential aggression and particular water parameters are two things that you’ll need to navigate as an owner.

Don’t worry though, this guide on emerald crab care will make the process easy. You’ll learn about their diet, molting process, lifespan, if they’re reef safe, and more!

Species Summary

The emerald crab (scientific name: Mithraculus sculptus) can be a quintessential member of your tank’s cleaning crew. Sometimes called the emerald mithrax crab or green clinging crab, this species is very popular in the marine fish trade.

Aside from their stunning good looks, these crabs are sought-after because of their healthy appetite for algae. They have a reputation for consuming forms of algae that other scavengers avoid.

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A female emerald crab

The emerald crab is native to shallow waters around the Caribbean and the Gulf of Mexico. They can often be found around reefs and rocky outcrops with plenty of food to scavenge.

In captivity, emerald crabs can support the entire closed ecosystem. Plus, they’re lots of fun to watch!


The emerald crab gets its name from its color. All specimens are covered in a deep green hue. You might see some white spots around the carapace and claws, but the crustacean is predominantly green.

They also have a unique shape. The body is longer than it is wide, and the carapace is flat (which helps them crawl under rocks for shelter). However, the top of the shell has a natural rocky texture.

The legs of the crab are thin and long. The eight back legs are hairy, while the front claws are large and smooth. The claws are also spoon-shaped, which helps the crab consume algae.

Author Note: Telling males and females apart is very easy. All you have to do is take a look at the folding apron-like structure on their underside. For males, the apron is narrow and pointed. For females, it’s wide and round.

Are Emerald Crabs Reef Safe?

One of the biggest questions marine aquarists have about the emerald crab is whether or not it can coexist with reef aquariums. For the most part, these crabs do just fine with reefs.

But you must keep an eye on them.

Emerald crabs are opportunistic eaters that will feed on anything they get their hands on. Usually, they stick to algae and leftover food. However, in some situations they can turn to reef polyps.

In most cases, this only happens when the crab is underfed. In addition to eating your coral, they may grab small fish or vulnerable snails.

As long as you keep them well-fed and happy, they should not go after coral polyps. Monitor them closely and make sure to remove the crab from the habitat if you notice damage to your reef.


The typical emerald crab lifespan is between two and four years. They’re not the longest-living inverts around, but they can outlive most cheap cleaner shrimp.

As always, there’s no way to guarantee life expectancy. These crabs are at the whims of the environment and the care you provide.

Be vigilant about tank maintenance and do your part to provide the crabs with a healthy diet. Otherwise, the lifespan of your crabs may shorten dramatically.

Average Size

The average emerald crab size is around two inches when fully matured. However, there’s a lot of variety in terms of size when it comes to this species.

You might see smaller specimens that stay closer to 1.5 inches or larger ones that reach up to 2.5 inches. For the most part, this variance comes down to genetics.

Emerald Crab Care

With a bit of experience (and the right information), emerald crab care isn’t a very difficult task. This species is hardy, adapts well to life in an aquarium, and will constantly search for sustenance. In other words, these crabs are quite self-sufficient in the right living conditions.

That said, you can’t trust them to take care of everything! You must do your part to provide a healthy environment where they can thrive.

Here are some guidelines to help you do just that:

Tank Size

We recommend a tank size of least 20 to 30 gallons of water for a single Emerald Grab.

These small crabs can adjust to small tanks and flourish in big ones, but they do need a minimum amount of space to scavenge. Emerald crabs can get territorial in cramped quarters.

Some aquarists have seen success with a density as high as one crab per 10 gallons. But it’s always best to err on the side of caution to avoid aggressive behavior.

Water Parameters

Emerald crabs are adaptable to most standard reef and marine tank setups. They’re not particularly picky, but they do have some preferences.

The crabs will thrive in warm waters, as they live in shallower waters in the wild. They also do best when the pH level is more on the alkaline side.

Keep the water quality close to the following parameters for the best results.

  • Water temperature: 72°F to 82°F (above 75 degrees is best)
  • pH levels: 8.0 to 8.4
  • Water hardness: 8 to 12 dKH
  • Specific Gravity: 1.020 to 1.025 (around 1.023 is ideal)

Author Note: Take some time to perform water tests (with one of these kits) when you first introduce them to the aquarium. This will allow you to nip any unexpected parameter shifts in the bud (before they harm your crab).

What To Put In Their Tank

In the wild, emerald crabs stick to rocky areas teeming with life. To make your crabs as comfortable as possible in captivity, it’s best to replicate this natural environment as closely as possible in their aquarium.

Start with a layer of fine sand. Then, create an arrangement of live rocks.

Emerald crabs are largely nocturnal when first introduced to the tank. They will spend most of the day hiding in crevices and caves. Over time, you will start to see more of them during the daytime. But even still, those rocky outcrops are important for shelter.

The rocks can also accumulate algae, turning it into a valuable food source. Coral isn’t necessary, but you can add it to create a more natural appearance.

Simple plants are good, too. The crabs will eat organisms that live on leaves. They are particularly fond of Turtle grass and other ground coverings.

Common Possible Diseases

There are no diseases that attack the emerald crab specifically. However, this species is susceptible to all of the same health problems that marine invertebrates can experience.

One of the most common concerns for emerald crabs is shell disease. This ailment is usually caused by a virus or bacterial infection. It can lead to pockmarks on the shell and legs. In severe cases, you might even see the disease eat its way through the shell and affect your crab internally.

Mites and other parasitic infections can occur, too. Emerald crabs can even carry marine Ich. Interestingly enough, the disease doesn’t affect the crab’s health directly. Instead, the crab acts as a carrier to infect fish in the tank.

If your crab suffers from a disease, it’s important to quarantine them and find the appropriate treatment. Avoid any copper-based medicines, as invertebrates cannot handle the metal.

Most diseases are avoidable. Monitor tank conditions closely and keep the habitat clean. If you add any new creatures to the tank, make sure you quarantine them to get rid of any hitchhikers!

Food & Diet

Emerald crabs are very easy to feed. They always have a healthy appetite and will consume most foods without giving it a second thought!

The crab will spend a lot of its time scavenging for algae and food scraps. This species is known to eat bubble algae and hair algae (two of the more persistent types of aquarium algae), which most other aquarium cleaners avoid. These critters will also look out for detritus and edible organisms.

You may even see them nibbling on food that collects on their leg hairs!

If your tank doesn’t have an adequate supply of algae, you can provide supplemental food to their diet. Dried seaweed, commercial pellets, and chopped shrimp work well.

Author Note: It’s very important to make sure that your crabs are well-fed. Otherwise, they may turn to tank mates to eat.

Behavior & Temperament

An emerald crab’s behavior is largely dependent on the environment. In good conditions, these crabs will not pay any attention to others in the tank. They’ll keep to themselves as they look for food.

That said, this species is perfectly capable of being aggressive.

They can get aggressive with other emerald crabs that encroach on their territory. If the crab isn’t eating enough, they can also snatch small fish, snails, or other inverts to eat!

Females tend to be less aggressive. However, all emerald crabs can exhibit some angry behavior when kept in small tanks with very little food to eat.

Potential aggression aside, emerald crabs are a joy to watch. As we mentioned earlier, these creatures are largely nocturnal in the beginning. This will change though, and they’ll spend more and more time eating during the day as they get comfortable.

This species is very active and not afraid to explore the floor of the tank. They’re also fast eaters. The crabs use both claws to consume food in minutes!

Tank Mates

Emerald crabs do well in larger community tanks. However, it’s still important to plan their tank mates accordingly.

Despite their small size, emeralds can overpower other crustaceans. Slow-moving crabs and snails are always risky.

The same goes for smaller fish. Any fish or marine creature you add should be fast enough to get away from the crab if any issues come up.

Larger aggressive fish should be avoided, too.

If you do add fish, stick to species that occupy other parts of the water column. Try to keep the bottom empty of fish so that the crab can use it to scavenge.

Author Note: One good tip for keeping an aggression-free tank is to provide plenty of hiding spots. Your crabs should have rocks to hide in whenever they get stressed. Other fish need the same.

Emerald Crab Molting Process

Like most crabs, emerald crabs will molt from time to time. This happens when the crab outgrows their current shell.

There’s no set schedule. How often your emerald crab molts depends on the water conditions, availability of food, and the creature’s growth rate.

When they’re molting, emerald crabs will leave behind an empty shell and hide for several days. Some will stay hidden for up to a week! They do this because they’re vulnerable with their new shells being soft.

They don’t have that hard exoskeleton to stay protected, so the vulnerable crab will keep a low profile.

If you see an empty shell at the bottom of your tank, don’t worry! It might look like a dead crab, but it’s likely just the exoskeleton. If you’re not sure, you can use some tongs to flip it over. You should be able to see that it’s hollow.

Author Note: Don’t take the shell out of the aquarium. It’s a valuable source of nutrients and minerals. Your crab can consume it later. Other creatures in the tank can benefit from it, too!            


Emerald crab care really boils down to if you can provide them with a suitable tank environment, and if you know how to manage their potential aggression.

If you feel comfortable with that, then getting one of these critters will be a piece of cake! If not, you can still work your way up by keeping them in a tank by themselves before adding other marine life into the mix.

We hope you enjoyed this guide and feel more prepared to own an emerald crab in your home tank!

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