Emerald Crab Care: Appearance, Diet, Tankmates, and Lifespan

Green Emerald Crab
Green Emerald Crab
Dr. Mollie Newton
Published by Dr. Mollie Newton PHD| Senior Editor
Last updated: July 13, 2024
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Are you looking for an efficient way to improve your reef tank and control troublesome algae? Consider adding the emerald crab (Mithraculus sculptus) to your environment. Gain valuable information on caring for this fascinating marine creature with this detailed tutorial. This thorough care guide offers all the essential information you need to keep these remarkable beings happy and healthy.

Article Summary

  • Emerald crabs have a visually striking appearance with a deep green color resembling emeralds.
  • Emerald crabs are omnivores and scavengers, constantly searching for a variety of snacks in the corals and reefs.
  • Other invertebrates should be avoided, as emerald crabs can manage aquarium cleaning on their own and may become upset with intruders.

Species Overview

Common namesEmerald crab
Scientific nameMithraculus sculptus
SpeciesM. sculptus
Size2 inches
Lifespan2-4 yrs
FamilyMajidae
GenusMithraculus
DistributionGulf of Mexico, Caribbean
ColorGreen
DietOmnivore
TemperamentSemi-aggressive
Minimum tank size20 gallons
Place in the tankBottom
Care levelEasy
BreedingDifficult

History and Background

The emerald crab (Mithraculus sculptus) is a reef safe species that is a popular choice for the clean up crew of marine aquariums. First described in 1818 by Lamarck, this species thrives in tropical waters and can commonly be found in reefs along the Caribbean.

What is an emerald crab?

Emerald crabs are a popular reef safe choice for saltwater aquariums due to their affinity for eating bubble algae and green hair algae. This affection for green nuisance algae is appropriate as their name comes from their color.

Where did the emerald crab come from?

This species is native to the tropical waters of the Gulf of Mexico as well as the shallow waters surrounding the islands of the Caribbean sea. They are generally found around landscape features such as rock or reefs.

Appearance

Not only are emerald crabs an excellent part of the tank clean up crew they also have a visually striking appearance. When you get a chance to see these creatures out and active not hiding in caves you’ll be stunned at their beautiful coloration.

What do emerald crabs look like?

The emerald crab has a deep green color resembling that of emeralds. There might be some white spots along the body and claws of the crab but in most cases they are completely green. Unlike most crabs the emerald crab’s body is longer than it is wide and the carapace is flat (which helps them crawl into rocks and reefs). Like most crabs they have 10 legs but the front claws are spoon shaped to better scoop algae into the crabs mouth.

How big does an emerald crab get?

This crab species grows to an average size of 2 inches when fully matured but exact size depends on tank environment and living conditions.

How fast do emerald crabs grow?

In general, crabs will reach their adult size within the first 6 months of life. However, they might grow slightly larger with each molt.

How do you sex an emerald crab?

To differentiate between male and female crabs you have to look at the underside of their shell. This portion of the body is called their apron, male crabs have a narrow, pointed apron whereas females have a wide and round apron.

Temperament and Tankmates

A reef tank with corals and saltwater fish
Beautiful Reef Tank

As these crustaceans are semi-aggressive and very territorial it can be difficult matching other invertebrates and fish to their environment.

What fish can live with emerald crabs?

Fish that mainly occupy the water column are best when it comes to housing them with emerald crabs. As the crabs are territorial they will claim most of the bottom as well as part of any coral reef as their own and will become aggressive and territorial if they feel like their area is being encroached on.

Additionally, you should avoid any fish species that are well known to be aggressive as they will likely make your emerald crab into a snack.

Compatible tank mates

  • Clownfish-a popular reef safe species that will spend most of its time among the upper levels of the tank and in the coral polyps.
  • Angelfish-another popular reef species that is sure to add hours of fun as they swim through the corals of your tank.
  • Wrasse-a generally peaceful fish species that spends most of its time at the upper levels of the tank
  • Blennies-a peaceful species that gets along well in community tanks.
  • Gobies-a smaller fish that is reef safe and would do well if you’re planning to keep a small tank environment.

Tank mates to avoid

  • Other invertebrates– amazingly your emerald crab can single handedly manage cleaning your aquarium and doesn’t need additional members of a clean up crew. In fact, your crab will likely get upset about other invertebrates encroaching on its space and eating its food.
  • Atlantic Chalk Bass-is food aggressive which might cause some issues during feeding time.
  • Blue green chromis-extremely sensitive to water changes this fish will become aggressive if not in a large enough group or if it doesn’t have enough food.
  • Falco Hawkfish-a known predatory fish that will think your emerald crab is food.
  • Snails-your crabs will likely be angry about the encroachment of snails on their space and it will cause a stressful environment for both of them. Despite snails being excellent additions to the clean up crew it is best to keep these two species separate.

TIP

One of the best ways to avoid aggression, even among species that normally get along, is to provide plenty of hiding spots. Whether a hide is in the coral reefs, rocks, or a shell it will help companions separate when they become stressed and prevent tensions from rising.

Tank Requirements

big empty fish tank on white background
Big Empty Fish Tank on White Background

Luckily the emerald crab is not picky when it comes to tank requirements making it an excellent addition to beginner reef tanks.

Water Parameters

Tank Size20 gallons
Water TypeSaltwater
Water temperature75-80
Water pH8.0-8.4
Water hardness1.23 specific gravity

Tank setup:

What kind of Substrate to use?

As your crabs will likely be in a reef tank you should design it with corals in mind. This means you should begin with a layer of fine sand before adding live rock work throughout the aquarium. You will likely notice your crabs spend most of the day hiding in the coral crevices and caves throughout the aquarium. As they become more accustomed they will begin to explore.

TIP

Another benefit of the addition of live rock to your aquarium is that it will accumulate algae and other organic matter as food for your crab keeping your emerald crab well fed and happy.

Do I need a Filter?

It is necessary to have a good filtration system in your reef tank as not only will you crabs need it but so will any coral housed in the tank. Be sure to properly size the filter to your tank, the outflow should be roughly four times the size of the tank. For example, a 20 gallon tank would require a filter with at least an 80 gallon outflow.

Is a Pump necessary?

Pumps are not necessary for emerald crabs however, the increased water flow won’t bother this species. Therefore, you can cater to the other fish in your aquarium.

Should I use a Water heater?

These crabs are found in the Gulf of Mexico and Caribbean Sea which are both tropical environments. Therefore, you should add a water heater to your tank to maintain a temperature range that is as comfortable as possible for this species.

How much Lighting do I need?

Emerald crabs don’t have any specific lighting requirements and in fact are a mostly nocturnal species. You can cater to the lighting requirements of other fish species in your tank. Due to their nocturnal nature you should be sure that your light cycle includes 12-16 hours of darkness to allow enough time for your crabs to explore and feed.

Diet and Health

Fish Food
Fish Food

A large part of keeping your emerald crab healthy is providing a well balanced diet with high quality food.

What do emerald crabs eat?

Emerald crabs are omnivores and scavengers, meaning they are constantly searching for a snack. In the corals and reef of their natural environment you will see them meticulously crawling over every surface in search of algae, coral polyps, or other organic matter. While it may look like they are just scuttling around their tank they are actually feeding on a wide variety of microorganisms.

What to feed an emerald crab?

In captivity you should try to replicate these animals’ wild diet as much as possible. Most often aquarists will allow emerald crabs to eat bubble algae or green hair algae that their aquarium is struggling with. If your tank is lucky enough to not have an infestation of hair algae you can supplement the crabs diet with dried seaweed or algae wafers. In a large community tank it is often not necessary to add outside food sources as the fish in the tank will make enough of a mess for your crab.

How often do you feed emerald crabs?

As emerald crabs are scavengers and are used to spending several days searching for the nutrients they need in the wild, their feeding schedule doesn’t need to be that strict. It is a good rule of thumb to add nori (seaweed) or other algae supplements every three days or so. If you notice your emerald crab eating or pursuing smaller fish it might be a sign that they are not getting enough to eat.

TIP

If you ever see your emerald crab eating another crab or small fish it is likely that they are already dead. Emerald crabs are scavengers and will eat any member of your aquarium that’s not moving such as dead crab, snails, and shrimp.

How much to feed an emerald crab?

Depending on how many crabs you have in your aquarium you should plan to feed a 1 square inch piece of seaweed or two to three algae wafers for each. It is important to note that uneaten food can quickly become a problem in reef ranks if your crabs don’t eat it fast enough and potentially lead to fouling of your tank.

Common diseases of emerald crab?

The most common disease of emerald crabs is shell illness which is caused by a bacterial infection. Symptoms include marks or shallow holes on the shell and legs, in more severe cases you can see through the shell from the holes caused by the disease. Treatment includes immediate quarantining and broad-spectrum antibiotics.

How long do emerald crabs live?

On average emerald crabs live to be between 2 and 4 years old. Lifespan is the same in both the wild and captivity and greatly depends on environmental factors. A healthy diet, clean tank environment, and low amounts of stress will increase your crabs life.

Breeding

It is difficult to breed crab in general and the emerald crab is no exception. As the larvae stage of their life requires them to be in deep ocean water which is impossible to replicate in a reef aquarium.

Can you breed an emerald crab?

Not much is known about emerald crabs breeding in captivity and most stock is caught in the wild in their Caribbean or Gulf homes. To date there has been no success breeding them in home aquariums, but if you’re looking for a challenge we’ve included what little we know about breeding below.

How to breed an emerald crab?

Emerald Crabs breeding

The first step to breeding crab successfully is to identify males from females. The easiest way to do this is to look on the underside of the crab shell at an area called the apron. Male crab will have a long, narrow, and pointed apron whereas female crab will have a rounded one.

To increase chances of reproduction you should put several male and female crabs in your tank, but be sure there is enough space so that males can have their own territory. Additionally, you will want to have a plethora of places for the female to hide if she gets stressed by the male crab advances.

If your crabs successfully mate you’ll notice a large amount of eggs on the female crabs underside. She will not lay these but instead carry them until the larvae are ready to be released.

Emerald crab FAQs

How to acclimate emerald crab?

A common question when you first receive an emerald crab is how to acclimate it to its new environment. The best method is to place the bag of water it came in at the top of your tank and let the water temperature equalize. One of the easiest things to do is leave it for a night and your crab will be acclimated by morning.

What to do after your emerald crab molts?

What to do after molting is a common question, oftentimes people mistake their crab’s molt for a dead crab! If you notice your crab molting the best thing to do is leave it alone. It will likely be in a grumpy mood (you would too if you were getting rid of your skeleton) and spend most of its time in a hide.

Crabs generally stay away from others when molting because this is the shedding of their exoskeleton. The new exoskeleton that replaces the old is soft and leaves the crab vulnerable to others. If you notice an exoskeleton at the bottom of your tank you can choose to remove it or leave it. Most often your crab will eat the old shell, but some people like to keep it to show off.

TIP

If you haven’t kept watch on your crab and are unsure whether it is molting you can always use a pair of tweezers or tongs to flip over the molt shell. If it’s hollow feel free to remove it.

Molting is a natural part of life for invertebrates and can be fun to watch if you have the chance.

Is an emerald crab reef safe?

Yes, emerald crabs make a great addition to reef tanks and won’t bother your corals or reef pieces.

Do emerald crab pinches hurt?

Being pinched by a crab will likely hurt a little but should not happen unless the crab is provoked. The most common occurrence of pinching is when picking a crab up to examine its shell or claws. If you must pick the crab up be sure to do so from the sides behind where the claws are to prevent injury.

Do emerald crabs spread bubble algae?

No, this crab won’t spread bubble algae but it will eat it. Not only will the emerald crab eat bubble algae, it will eat other types of aggressive algae such as green hair algae.

Is the emerald crab for you?

If you’re looking for a tank member that can handle the dirty job of algae cleanup and look good while doing it, the emerald crab is a top contender for your tank.

Conclusion

Emerald crabs are excellent additions to any reef tank as they are not high maintenance, do not require a large amount of space, and will mostly find their own food.

(1) Featured Image – Mark Loch, CC BY-SA 2.0, via Wikimedia Commons

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