emerald crab

Emerald Crab Care: From Appearance to Molting, and Diet

Caring for emerald crabs can be a rewarding experience, especially if you know the basics. This species of crab is prevalent in marine aquariums for its stunning color and appetite for algae, making it an excellent addition to any tank. However, to ensure your emerald crabs live a long and healthy life, there are a few things you should know about them.

This guide will cover everything from their appearance to diet, to their molting process, as well as how to keep their reef safe and tips for breeding them in captivity.

emerald green
A vibrant emerald crab with distinctive white spots, clings onto a coral reef, showcasing its unique and striking appearance

Species Summary

The emerald crab (Mithraculus sculptus) prefers the shallow waters of the Caribbean and the Gulf of Mexico, where they can often be found around reefs and rocky outcrops. In captivity, they not only provide aesthetic value with their unique coloring, but they also provide vital services such as cleaning algae off the glass, rocks, and other surfaces.


Emerald crabs have a unique shape, featuring a flattened carapace that helps them easily crawl under rocks for shelter, and the top of the shell has a natural rocky texture. The legs of the crab are thin and long, with the eight back legs hairy, while the front claws are large and spoon-shaped.

Interestingly, this species gets its name from its deep green hue, which can sometimes be accompanied by white spots around the carapace and claws.

How to Identify Male and Female Emerald Crabs?

Emerald crabs (Mithraculus sculptus) are a type of marine invertebrate that is commonly kept in saltwater aquariums. While they are not always sexually dimorphic, meaning that there are no obvious physical differences between males and females, there are a few methods you can use to try and determine the sex of an individual crab.

  1. Carapace size: In general, male emerald crabs tend to have slightly larger carapaces (the top shell) than females. However, this can be difficult to determine as carapace size can also be influenced by factors such as age, diet, and overall health.
  1. Abdominal shape: Male emerald crabs often have a more triangular-shaped abdomen, while female abdomens tend to be more rounded.
  1. Pincer size: Male emerald crabs often have larger and thicker pincers than females.
  1. Observing breeding behavior: Observing an emerald crab during the breeding season may give you a clue as to its sex. Males will often display more aggressive behavior and may even fight over a female. Additionally, a female carrying eggs will have a rounded and swollen abdomen, which can be a sign that she is gravid.

Note: It is important to note that these methods are not foolproof and the best way to determine the sex of an emerald crab with certainty is through a dissection. However, this method is not recommended as it can harm or kill the crab.

Are Emerald Crabs Reef Safe?

The ability of the emerald crab to cohabit with coral reefs is one of the most common concerns voiced by marine aquarium keepers when it comes to this crustacean. These crabs can, for the most part, coexist peacefully with reefs without any problems.

However, you really must keep a close check on them.

Crabs of the emerald kind are not picky eaters and will consume virtually everything they can get their claws on. In most cases, they feed on algae as well as discarded food. On the other hand, they are able to transform into reef polyps in certain circumstances.

This condition is almost always caused by the crab not receiving enough food. They could even take tiny fish or susceptible snails if they find them while they’re devouring your coral.

As long as you give them plenty to eat and make sure they are content, they should not bother the coral polyps. Maintain continuous vigilance over them, and if you discover that your reef has been harmed, remove the crab immediately from its environment.


Between two and four years is the normal amount of time an emerald crab will live. They may not have the longest lifespan of any of the invertebrates, but they can survive the majority of the cheaper cleaner shrimp.

As usual, there is no way to provide a definitive estimate of life expectancy. These crabs are at the mercy of their surroundings as well as the care that you give to them.

Maintain vigilance with regard to the tank’s upkeep, and do everything you can to ensure the crabs receive nutritious food. If you don’t do this, the longevity of your crabs might be significantly reduced.

Average Size

When they have reached their full maturity, emerald crabs often reach a size of around two inches in length. When it comes to this species, however, there is a significant amount of variation to be found in terms of size.

You could come across specimens measuring closer to 1.5 inches in length or ones that are significantly longer and measure up to 2.5 inches. The majority of this variation is due to differences in genetic background.

Emerald Crab Care

Caring for an emerald crab does not have to be a very challenging endeavor if one has some prior experience and is equipped with the appropriate knowledge. This species is resilient, does well in the environment of an aquarium, and will always be on the lookout for something to eat. To put it another way, given the correct conditions, these crabs are capable of taking care of themselves fairly adequately.

Having stated that, you cannot put your faith in them to handle everything for you! You have a responsibility to do what you can toward creating a wholesome setting in which crabs can thrive.

Tank Size

A single Emerald Grab requires a tank capacity of at least 20 to 30 gallons of water, according to our recommendations.

These little crabs are able to adapt to small tanks and thrive in large ones, but they do require a certain minimum amount of room in order to forage for food. When confined in close quarters, emerald crabs can become aggressive and territorial.

Some aquarists have reported having success with a density of one crab for every 10 liters of water in their tanks. However, in order to avoid engaging in violent conduct, it is always preferable to err on the side of caution.

Water Parameters

Emerald crabs can easily adjust to the majority of the common reef and marine tank configurations. They are not very finicky, but they do have a few choices they gravitate toward.

Since crabs often inhabit warmer, shallower waters in naturally, they will flourish in warm water environments. Additionally, they thrive in environments with a pH level that is higher on the alkaline side.

For optimal results, ensure that the water quality is maintained within a tight range of the following values.

Water Temperature: 72°F to 82°F (above 75 degrees is best)

pH: ranges between 8.0 and 8.4

Hardness: ranges from 8 to 12 dKH.

Gravity: 1.020 to 1.025 grams per cubic centimeter (around 1.023 is ideal)

When you initially introduce them to the aquarium, be sure to run water testing using one of these kits. This will ensure that the water is safe for the new additions. Because of this, you will be able to head off any unexpected shifts in the parameter values (before they harm your crab).

What To Put In Their Tank

In the wild, emerald crabs tend to congregate in rocky places that are rich in other forms of life. In order to provide your crabs with the most level of comfort possible while they are held captive, it is important to recreate their natural environment as accurately as possible within their tank.

To begin, spread a thin layer of fine sand. The next step is to arrange the living rocks in some fashion.

When they are first brought into a tank, emerald crabs spend much of their time sleeping. They will spend most of the day hiding in cracks and caverns to avoid being discovered. After some time has passed, you will gradually begin to observe an increased number of them during the daytime. But despite this, the protection provided by those rocky outcrops is quite valuable.

The rocks have the ability to gather algae, which may then become an important source of food. Coral is not required, however, it can be used in order to give the impression of a more natural setting.

Even uncomplicated plant life might be beneficial. Crabs will consume creatures that reside on leaves if given the opportunity. They enjoy Turtle grass and several other ground covers in particular.

Emerald Crabs’ Common Diseases

There are no diseases that are known to specifically target the emerald crab. On the other hand, this species is prone to all of the same health concerns that can be experienced by marine invertebrates in general.

Shell sickness is one of the most prevalent issues that arise when keeping emerald crabs. In most cases, an infection with a virus or bacterium is the root cause of this condition. It has the potential to cause pockmarks on the legs and shell. In extreme circumstances, you could even witness the sickness eating its way through the crab’s shell so that it might infect the crab on the inside.

Infections caused by mites and other parasites are also a possibility. It’s even possible that emerald crabs carry marine Ich. It’s interesting to note that the sickness doesn’t seem to have a direct impact on the crab’s health. Instead, the crab serves as a vector for the infection to spread to the other fish in the aquarium.

In the event that your crab is afflicted with a sickness, it is important to isolate them in a quarantine facility and locate an appropriate therapy. Copper-based drugs should be avoided at all costs since invertebrates are unable to process the metal.

Note: Most illnesses are preventable. Maintain a spotless environment and pay great attention to the conditions in the tank. If you decide to introduce any additional animals to the tank, you must be careful to quarantine them so that any hitchhikers may be eliminated.

Food & Diet

The feeding process for emerald crabs is fairly simple. They never struggle to find something to eat and will happily swallow almost anything without giving it any consideration.

The crab will devote a significant portion of its time to searching for algae and leftover crumbs of food. This species is the exception to the rule when it comes to aquarium cleaners since it consumes bubble algae as well as hair algae, which are two of the most tenacious varieties of aquarium algae. These creatures will also keep an eye out for debris and organisms that can be eaten.

You could even catch a glimpse of them munching on the food that gets stuck in the hairs on their legs.

You may supplement their diet with additional food if the algae in your tank do not give a sufficient amount of food for them. Dried seaweed, shrimp chunks, and pellets purchased commercially all perform quite well.

Behavior and Temperament

The behavior of an emerald crab is heavily influenced by its surrounding environment. These crabs won’t pay any attention to the other inhabitants of the tank as long as the conditions are right. As they search for food, they won’t bother anyone else except themselves.

Having said that, individuals of this species are entirely capable of showing aggressive behavior.

They have the potential to become hostile toward other emerald crabs if they feel their area is being invaded. If the crab isn’t getting enough to eat, it may always grab some snails, tiny fish, or other types of invertebrates to snack on!

Females are often less aggressive than men. However, when housed in small aquariums with very little food to consume, all emerald crabs have the potential to display some form of aggressive behavior.

In spite of their potentially aggressive nature, emerald crabs are a delight to see. These animals tend to be active throughout the night when they are first discovered, as was indicated before. However, this will change, and as they become more accustomed to their new environment, they will eat for a longer and longer period of time during the day.

This species is quite active and does not hesitate to investigate new areas, including the tank’s bottom. The crabs quickly swallow their meal by using both of their claws.

Tank Mates

Crabs of the Emerald kind thrive well when kept in bigger communal aquariums. Despite this, it is essential to plan their tank mates in a manner that is appropriate.

Emeralds have the ability to dominate other crustaceans despite their little size. Crabs and snails are always a concern because of their slow movement.

The same is true for fish of a lower size. Any fish or other marine creature that you add should have sufficient speed to escape from the crab in the event that there are any problems.

When choosing tank mates for emerald crabs, it’s important to consider the size, behavior, and compatibility of the other inhabitants.

Suitable tank mates for saltwater emerald crabs include:

  • Clownfish: These friendly and hardy fish are a great addition to any saltwater reef aquarium and get along well with emerald crabs.
  • Tangs: Active and hardy tangs such as sailfin tangs and blue tangs can make good tank mates for emerald crabs.
  • Shrimp: Cleaner shrimp and peppermint shrimp can help keep the tank clean and free of parasites and make good tank mates for emerald crabs.
  • Snails: Snails such as nerite snails and astrea snails can help keep the tank clean and free of algae and are compatible with emerald crabs.
  • Gobies: Small and peaceful gobies such as watchman gobies and yellow-headed jawfish can also be good tank mates for emerald crabs.

Unsuitable tank mates for saltwater emerald crabs include:

  • Small or weakfish: Emerald crabs have a tendency to consume smaller or weaker tank mates, so it’s best to avoid keeping small fish or invertebrates in the same tank with them.
  • Aggressive fish: Aggressive fish such as triggerfish, lionfish, and eels may harm or harass the emerald crab.
  • Predatory invertebrates: Predatory invertebrates such as arrow crabs and starfish may pose a threat to the emerald crab or other inhabitants of the tank.

It’s always a good idea to carefully research the species you plan to keep in your aquarium to ensure compatibility before introducing new tank mates.

Reminder: A helpful hint for maintaining a tank that is free from aggressive behavior is to give plenty of places for the fish to hide. When your crabs are feeling worried, they should be able to hide in the rocks that you have provided for them.

The Molting Process of the Emerald Crab

The emerald crab goes through the process of molting periodically, much like other types of crabs. This takes place when the crab’s present shell becomes too small for it.

There is no predetermined timetable. The frequency of your emerald crab’s molting process is determined by the water conditions, the amount of food available, and the creature’s pace of growth.

Emerald crabs will shed their old shells and go into hiding for several days throughout the process of becoming their new shells. Some of them could remain concealed for as long as a week! They do this because their new shells are weak and delicate, so they take advantage of this vulnerability.

Because they lack an exoskeleton that can provide protection, fragile crabs prefer to hide in the sand rather than draw attention to themselves.

There is no need for an alarm even if you find an empty shell at the bottom of your tank. Although it could give the impression of a crab that has passed away, it is most likely merely the exoskeleton. You can use some tongs to turn it over if you are unsure of how to proceed. It ought to be obvious to you that the object is hollow.


In conclusion, emerald crabs (Mithraculus sculptus) are a popular addition to marine aquariums due to their unique appearance and usefulness as algae eaters. They prefer shallow waters in their natural habitat but can adapt to life in captivity.

To ensure your emerald crabs live a long and healthy life, it is important to provide them with the right conditions. This includes a sufficient tank size, proper diet, and attention to their molting process. Although they are generally reef safe, they can sometimes harm coral polyps if they do not receive enough food, so it is important to keep a close eye on them.

With proper care and attention, emerald crabs can be a rewarding addition to your aquarium and a fascinating species to observe.