Emerald Crab (Mithraculus Sculptus): Ultimate Care Guide

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The emerald crab, also known by its scientific name, Mithraculus Sculptus, could be a great addition to the right saltwater aquarium. Emerald crabs are known to come from Caribbean reefs where they have plenty of places to hide during the day. Since they are nocturnal, you most likely will not see them during the day, so long as the aquarium that they are in has a sufficient amount of rock and places for them to hide. The emerald crab is also relatively small, reaching a maximum length of only 2.5 inches. Despite their name, they may have additional colors other than green; they could have streaks of red, blue, black, and gray throughout their body.

Although they won’t mind their tankmates much, they have been known to be aggressive towards one another, so it’s best to keep one in the aquarium unless it is large enough to contain multiple without there being too much competition for food. If there is a competition for food within the species, they can kill small fish in the tank and potentially cause a bigger issue. Despite being tolerant of their neighbors that belong to other species, they do not have tolerance for changes in their water conditions.

Are Emerald Crabs Reef Safe?

If you have coral in your tank, there is a chance that it will be eaten by an emerald crab. It isn’t a guarantee that the crabs will eat your coral, especially if you keep them well-fed and taken care of, but if you start to notice damage to your reef, it is important to monitor them and remove the crabs from the tank if necessary.

Will Emerald Crabs eat Duncan Coral, Small Polyp Stony (SPS), or zoanthids?

Emerald crabs don’t always go after Duncan coral and Small Polyp Stony, but there is a chance that they will go after them, along with zoanthids and even anemones, purely for opportunistic reasons. Just to be safe, it’s best to keep them away from the potential temptation.

Will Emerald Crabs eat anemones?

Although there are no guarantees that emerald crabs will eat anemones, there are many people who do not trust emerald crabs because they are opportunistic creatures that can be unpredictable in what they choose to eat.

Emerald Crab (Mithraculus Sculptus)
Emerald Crab (Mithraculus Sculptus)

Emerald Crab Tank Mates

Emerald crabs are not picky eaters and as omnivores, they will happily feed on leftover food or any meaty substance that they can find laying around the bottom of the tank. As easy as that may sound, if emerald crabs are not provided with a substantial, supplemental diet, your coral, small fish, shrimp, and other small aquarium members may be in danger.

If the emerald crab is provided with a sufficient diet and enough hiding places, it will likely be able to peacefully coexist with shrimp, along with other invertebrates and fish, but keep in mind that the risk is always there. Anything that is considered to be reef safe would be a good option for a potential tankmate of an emerald crab.

Although they have the potential to be aggressive, their size leaves them vulnerable and any tankmates that can easily eat them or have the ability to crack their hard shells should be avoided. Specifically, emerald crabs should not be in the same tank as hawkfish, pufferfish, or triggers.

Emerald Crab Care

If you’ve decided to go ahead and get an emerald crab, there’s a lot to know regarding their preferred water conditions as they can be finicky if the water parameters do not meet their standards. Usually, a drip acclimation for a couple of hours is suggested before introducing the crabs to the new tank altogether.

Some people like to float new tank additions in a bag placed in the expectant tank for around fifteen minutes. When that time is up, it’s important to take the new member out of the bag, rather than pouring the bag with the water from the store into the already established tank environment.

Drip acclimation and floating can help the new member slowly be introduced to the temperature and water conditions that they are going to live in from this point forward, rather than shocking them with the sudden environmental changes.


The optimal temperature for emerald crabs is between 75 and 80 degrees Fahrenheit.

Tank Setup & Water pH

Emerald crabs also prefer for their pH to be slightly higher than neutral, at around 8.4. Being that they feed on algae and other scraps that they can find around the tank, nitrates should be kept below 10 ppm to satisfy the necessary amount of algae growth. It is also important to keep an eye on the phosphate levels. Emerald crabs are sensitive to changes in their water parameters and high phosphate levels can affect the water and life in the tank. For these crabs specifically, the phosphate level should be below 10. This can be fixed by doing a water change.

Emerald Crab Size and Tank Size

Emerald crabs only grow to about 1.5-2.5 inches, but due to their potentially aggressive nature, their size doesn’t do much to influence the suggestion that each emerald crab has at least 20-30 gallons of water. The aforementioned water should have a salinity of 1.023-1.025 and, similarly to all other tank setups, ammonia, nitrate, and nitrite levels should all be kept below a detectable level in order to set yourself and your aquarium up for success. Being that these crabs can be territorial, it is important that they have their own designated hiding spot in addition to their own 20-30-gallon section of the tank.

Food & Diet

As previously mentioned, emerald crabs are generally not picky eaters and will eat most things that they are fed. They have been known to eat algae, including bubble algae, hair algae, and Chaeto algae. Emerald crabs should not be relied on in order to control the algae population in the tank, but they will most likely be helpful in the maintenance.

Emerald Crab Lifespan

Despite being relatively low maintenance, emerald crabs do not live for very long. Aside from a select few that have reported having their emerald crab for two or three years, most people only have them for about six months to a year.

Emerald Crab Breeding

During their short lifetime, you will find that breeding them is extremely difficult. While there are basic facts that surround the topic of emerald crab breeding, scientists have not yet been able to figure out a successful breeding protocol. However, we do know that females can produce anywhere from several dozen eggs to a whopping one thousand eggs during each spawn.

Instead of laying her eggs, she has a pouch under her stomach where eggs will remain until the baby crabs are ready to be released directly into the water. Regardless of the high egg production, the overall observed survival rate is very low at only 10-20%.

Determining the gender of a crab can be as easy as flipping them onto their backs and taking a look at their abdomens. Looking closely, you’ll notice that males have a narrower abdomen while females have a wider and more rounded abdomen.

Emerald Crab Disease

Unfortunately, as is the case with all living things, emerald crabs have a weakness, the only difference being that theirs has become a commonly known concern. Emerald crabs can get, you guessed it, emerald crab disease. This is a shell disease that can start with a virus or a bacterial infection and spread through the body. If your crab has this ailment, you may notice pockmarks on its shell and/or its legs.

These crabs can also become infected with ich, which is common in both saltwater and freshwater fish tanks, but the emerald crabs will only act as carriers throughout the tank and will generally remain unaffected.

If you notice that your crab is suffering from an ailment, the best thing that you can do is quarantine them and provide the appropriate treatment. Keep in mind that invertebrates, such as emerald crabs, should never be exposed to copper-based medicines as the metal is on the list of things that they cannot tolerate.

Emerald Crab Molt

Molting is an important part of your crab’s growth and will happen between an estimated ten and twenty times in their lifetime. Molting is when your crab outgrows its current shell and essentially shed it entirely, which also helps get rid of any unwanted organisms that may have attached themselves to the shell along with damaged parts of the shell.

During this process, it’s common for crabs to hide in the crevices of the rocks in the tank as they are vulnerable for a period while they wait for their new shell to harden. After not seeing them for a while, it’s common to think that the crab is dead at first, but if you give it time, you will see them become active again.

Molting isn’t usually like clockwork and a lot of variables can play into the frequency of molting periods. The quality and temperature of the water along with the crab’s diet and other outside influential factors can all play a significant role in how frequently they molt.

It is also recommended that the old shells that have been shed not be removed from the tank or disposed of as they are full of minerals and will eventually become a nutritious part of the crab’s diet.

The molting process can happen more frequently when they are still in their growing stages and can be as often as every few weeks. When they are older and fully grown, the frequency of the molting can shift to being as infrequent as every few months.

Where Can I Find Emerald Crabs for Sale?

Emerald crabs are common in saltwater aquarium stores but may also be found in your local pet store, especially if they have a variety of options for members of a reef cleanup crew. They usually go for around $7-$10, but prices may vary.

Variations of Emerald Crab – Pink Emerald Crabs

The pink emerald crabs are similar to the green emerald crabs in all ways except for their physical appearance. They have a pale coloring that appears to be blue but fades to a white color as it gets closer to the edges of the shell. The drastic difference in color makes the pink emerald crab much easier to spot in the tank.

Would an Emerald Crab be a Good Addition to Your Aquarium?

Taking all of this information into consideration, it is up to you to decide if an emerald crab would be right for your tank. Based on their behavior and opportunistic diet, it can be hard to determine if they will be worth any potential trouble that they may cause, but they would truly be a great addition to any tank.

Due to their scavenging behaviors and eating waste in the tank, they can help to clean up any food that the other tank members may have missed. If excess food sits in the tank for extended periods of time, it will eventually start to decay and turn into ammonia, a level that needs to be kept in check in any and all aquariums.

These crabs can also be helpful when it comes to maintaining and controlling the algae population in a tank which can prevent it from taking over the tank entirely, making the cleaning process easier for the owner.

If your other tank members include medium-sized fish or larger that do not prey on crustaceans, or other reef-safe species, then you most likely won’t have to worry about the emerald crab displaying aggressive behaviors, or the safety and health of your other tank members. 

If your tank meets all of these specifications and standards, then it’s likely that an emerald crab will be a beneficial addition to your saltwater aquarium, rather than a hazard or an inconvenience.