Emerald Crab (Mithraculus sculptus): Aquarium Care Guide
Emerald Crabs are a fun and beneficial species to have in your marine tank. Their neat colors, lively personality, and tank-cleaning habits make them a great creature to watch!
But before you do, there are a few things to keep in mind.
These crabs add a lot to your tank, but they also need specific conditions. Potential aggression and particular water parameters are two things 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!
The emerald crab (scientific name: Mithraculus sculptus) can be an essential member of your tank cleaning team. Sometimes called the emerald mithrax crab or green clinging crab, this species is very popular in the marine fish trade.
In addition to their impressive appearance, these crabs are sought after for their healthy appetite for algae. They have a reputation for consuming forms of algae that other scavengers avoid.
The emerald crab is native to the shallow waters of the Caribbean and the Gulf of Mexico. They can often be found around reefs and rocky outcrops with plenty of food to forage.
In captivity, emerald crabs can support the entire closed ecosystem. Plus, they’re so much fun to watch!
The emerald crab gets its name from its color. All specimens are covered in an intense green hue. You may see some white spots around the shell and claws, but the crustacean is predominantly green.
They also have a unique shape. The body is longer than it is wide and the shell is flat (which helps them crawl under rocks for shelter). However, the top of the shell has a natural rocky texture.
Crab legs are thin and long. The eight hind legs are furry, while the front claws are large and soft. The claws are also spoon-shaped, which helps the crab consume algae.
Author’s note: Distinguishing males and females is very easy. All you have to do is take a look at the foldable apron-like structure at its bottom. For males, the apron is narrow and pointed. For females, it is wide and round.
Are Emerald Crab Reefs Safe?
One of the biggest questions marine aquarists have about the emerald crab is whether or not it can co-exist with reef aquaria. For the most part, these crabs do well around reefs.
But you must watch them.
Emerald Crabs are opportunistic eaters that feed on anything they get their hands on. They usually stick to algae and food debris. However, in some situations they can develop into reef polyps.
In most cases, this only happens when the crab is malnourished. In addition to eating the coral, they can catch small fish or vulnerable snails.
As long as you keep them well fed and happy, they shouldn’t be chasing coral polyps. Keep a close eye on them and be sure to remove the crab from the habitat if you notice damage to the reef.
The typical lifespan of the emerald crab is two to four years. They are not the longest living inverts, but can outlast most cheaper shrimp.
As always, there is no way to guarantee life expectancy. These crabs are at the whims of the environment and the care you provide.
Keep an eye on tank maintenance and do your part to provide the crabs with a healthy diet. Otherwise, the shelf life of the crabs can be drastically shortened.
The average size of the emerald crab is around two inches when fully mature. However, there is a lot of variety in terms of size when it comes to this species.
You may see smaller specimens that stay closer to 1.5 inches or larger that reach up to 2.5 inches. For the most part, this variation comes down to genetics.
Emerald Crab Care
With a little experience (and the right information), emerald crab care is not 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:
We recommend a tank size of at least 20 to 30 gallons of water for a single Emerald Grab.
These little crabs can adapt to small tanks and thrive in large ones, but they need a minimal amount of space to forage. Emerald crabs can become territorial in tight spaces.
Some aquarists have had success with a density of up to one crab per 10 gallons. But it is always better to err on the side of caution to avoid aggressive behavior.
Emerald Crabs fit into most standard reef and marine tank setups. They are not particularly picky, but they do have some preferences.
Crabs will thrive in warm water, as they live in shallower water in the wild. They also work best when the pH level is more alkaline.
Keep your water quality close to the following parameters for best results.
- Water temperature: 72°F to 82°F (over 75 degrees is best)
- pH levels: 8.0 to 8.4
- Water hardness: 8 to 12 dKH
- Specific gravity: 1.020 to 1.025 (about 1.023 is ideal)
Author’s Note: Take some time to test the water (with one of these kits) when you first introduce them to the aquarium. This will allow you to nip any unexpected parameter changes in the bud (before they harm your crab).
What to put in your tank
In the wild, emerald crabs stick to rocky areas teeming with life. To make your crabs as comfortable as possible in captivity, it is best to replicate this natural environment as closely as possible in your aquarium.
Start with a layer of fine sand. Next, create a live rock arrangement.
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 begin to see more of them during the day. But still, those rocky outcroppings are important for shelter.
Rocks can also collect algae, making them a valuable food source. Coral isn’t necessary, but you can add it to create a more natural look.
Simple plants are also good. The crabs will eat organisms that live on the leaves. They especially like turtle grass and other ground coverings.
Possible common diseases
There are no diseases that specifically attack the emerald crab. 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 cause pockmarks on the shell and legs. In severe cases, you may even see the disease work its way through the shell and affect your crab internally.
Mites and other parasitic infections can also occur. Emerald Crabs can even carry Marine Ich. Interestingly, the disease does not directly affect the crab’s health. Instead, the crab acts as a carrier to infect the fish in the tank.
If your crab is sick, it’s important to quarantine it and find the right treatment. Avoid any copper-based medications, as invertebrates cannot handle metal.
Most diseases are preventable. Closely monitor tank conditions and keep the habitat clean. If you add any new creatures to the tank, be sure to quarantine them to get rid of the hitchhikers!
Food and Diet
Emerald crabs are very easy to feed. They always have a healthy appetite and will consume most foods without a second thought!
The crab will spend a lot of time looking for algae and food scraps. This species is known to eat bubble algae and hair algae (two of the most persistent types of aquarium algae), which most other aquarium cleaners avoid. These critters will also forage for detritus and edible organisms.
You can even see them nibbling on the food that accumulates in the hair on their legs!
If your tank does not have an adequate supply of algae, you can provide complementary foods to its diet. Dried seaweed, commercial pellets, and chopped shrimp work well.
Author’s Note: It is very important to make sure your crabs are well fed. Otherwise, they may turn to their tank mates for food.
behavior and temperament
The behavior of an emerald crab is highly dependent on the environment. Under good conditions, these crabs will pay no attention to others in the tank. They will stay on their own while foraging for food.
That said, this species is perfectly capable of being aggressive.
They can become aggressive towards other emerald crabs that invade their territory. If the crab is not eating enough, it can also catch small fish, snails or other inverts to eat!
Females tend to be less aggressive. However, all emerald crabs can display 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 primarily nocturnal at first. However, this will change and they will spend more and more time eating throughout the day as they get comfortable.
This species is very active and is not afraid to explore the bottom of the tank. They are also fast eaters. Crabs use both claws to consume food in minutes!
Emerald crabs do well in larger community tanks. However, it’s still important to plan your tankmates 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 sea creatures you add must be quick enough to move away from the crab if any trouble arises.
Larger aggressive fish should also be avoided.
If you add fish, stick with species that occupy other parts of the water column. Try to keep the bottom clear of fish so the crab can use it to forage for food.
Author’s Note: A good tip for keeping a tank aggression-free is to provide plenty of hiding places. Your crabs should have rocks to hide in when they get stressed. Other fish need the same.
Emerald crab molting process
Like most crabs, emerald crabs will shed from time to time. This happens when the crab outgrows its current shell.
There is no set schedule. How often the emerald crab molts depends on water conditions, food availability, and the creature’s growth rate.
When they are molting, emerald crabs will leave an empty shell and hide for several days. Some will stay hidden for up to a week! They do this because they are vulnerable with their new soft shells.
They don’t have that hard exoskeleton to keep themselves 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 may look like a dead crab, but it’s probably just the exoskeleton. If you’re not sure, you can use tweezers to flip it over. You should be able to see that it is hollow.
Author’s note: Do not remove the shell from the aquarium. It is a valuable source of nutrients and minerals. Your crab can consume it later. Other creatures in the tank can also benefit from it!
Emerald Crab care really comes down to whether you can provide them with a suitable tank environment and whether you know how to manage their potential aggression.
If you’re comfortable with that, getting one of these critters will be a piece of cake! If not, you can still make progress by keeping them alone in a tank before adding other marine life to the mix.
We hope you’ve enjoyed this guide and feel more prepared to have an emerald crab in your home tank!