The Mithraculus sculptus, commonly known as the Emerald Crab, is a popular and heavily traded crab in the marine aquarium industry. Revered by reef aquarists for their ability to control nuisance algae, particularly Bubble algae (Valonia spp.), Emerald Crabs offer a natural solution to a common aquarium problem. These crabs are abundant in shallow waters of the tropical western Atlantic Ocean, from Miami, throughout the Bahamas and Caribbean, and south to northern Brazil.
Appearance and Natural Behavior
Emerald crabs have a bright green color. Their body is flat and compact with hairy legs. The body of the crab is almost flat, allowing it to fit into tight spaces. At the end of the legs, they have pretty strong points. The biggest of Emerald Crabs will only reach 6 cm (~2.5 inches) when they are fully grown. Along with its beautiful coloration and small stature, Emerald Crabs are also known for their big and strong claws.
Both in the wild and in captivity, the Emerald crabs are nocturnal, so it is important that you have the proper rocks and stones within the tank. Once they get adapted to a tank, they can slightly change their night/day behavior.
The Emerald Crab as a Reef Tank Algae Control
Reef tanks are commonly invaded by Bubble algae. These algae are greenish, grapelike clusters of small (usually less than 1 cm (0.4 inches) in diameter), single-cell, fluid-filled spheres. In your tank, the Emerald crabs will mainly focus on cleaning it from the Bubble algae. Unlike some algae-eating fish, these crabs can crawl into tight spaces and eliminate algae basically everywhere. The Emerald Crab is well respected for its scavenging ability. It will enthusiastically feed on uneaten meaty foods and many types of nuisance algae. Unlike many other animals, Mithraculus sculptus will eat bubble algae and helps clean your aquarium of these algae.
Dietary Needs and Feeding Habits
One of the best things about Emerald Crabs is that they are not fussy eaters. Basically, they will eat anything extra in the tank. The Emerald Mithrax Crab is well respected for its scavenging ability. It will enthusiastically feed on uneaten meaty foods and many types of nuisance algae. Unlike many other animals, Mithrax sculptus will eat bubble algae and helps clean your aquarium of these algae.
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However, it will be wrong to think that they do not require a specific diet because they are scavengers. In fact, if an Emerald Crab is not given a supplemental diet, they may start to mess with any coral that you have in your tank. This is a very delicate moment. On the one hand, aquarists keep the Emerald carbs to control pest algae. On the other hand, they have to give them some other food to reduce (Though, it will not eliminate the risk!) the chance of aggression. However, a supplemental diet can prevent them from fulfilling their main “responsibility”- eating algae.
Many aquarists supplement the Emerald Mithrax Crab's diet with dried seaweed and chopped meaty foods, such as shrimp, as well as a quality pellet fish food. Algae, bits of fish/prawn. The Emerald Crab is well respected for its scavenging ability. It will enthusiastically feed on uneaten meaty foods and many types of nuisance algae.
Preferences in food choice
According to that study, biologists gave Emerald crabs alternative foods to the Bubble alga and tested the preferences. Algal consumption was similar to when they were provided with mysids. Pellets have higher profitability than algae, so algal consumption decreases. As we can see, the Emerald crabs have clear preferences in food choice. We already know that Mysis shrimp does not affect their appetite. There is a high chance that Krill will not as well.
Potential Drawbacks and Considerations
The point is that there are aquarium hobbyists who are not satisfied with them. They say that their Emerald crabs did not touch the algae, or did it very slowly, and without enthusiasm. Do you know that when the Emerald crab tears the algal cell apart, the cell liquid that contains juvenile cytoplasmatic spheres is released into the water? This behavior might contribute to algal dispersal and consequently algal infestation. To avoid that, Emerald crab’s consumption rate must be higher than the growth of the Bubble algae. So, if you have a huge and heavily infested tank, there is a high chance one crab will not be able to clean that alone.
Reef Safe?
Some people can guarantee you that the Emerald crabs are reef safe, and they have never had an issue with them. Other aquarists saw them picking on their SPS corals, plucking interstellar mushrooms off their rocks, destroying colonies of zoas, sponges, etc. Why are they safe in some cases and disruptive in others? The reason is pretty obvious. Like the majority of crabs, the Emerald crabs are opportunistic. Their herbivorous or carnivorous tendencies depend on food availability. Personally, I would keep Emerald crabs under suspicion until proven innocent.
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Care needs to be taken to ensure an ample supply of food is available for the Emerald Crab. Otherwise, this opportunistic feeder may turn to corals, invertebrates, or small fish for a food source. However, when well fed, most Emerald Crabs are very tolerant of their tankmates and highly compatible in reef environments. The Emerald Mithrax Crab is well respected for its scavenging ability. It will enthusiastically feed on uneaten meaty foods and many types of nuisance algae.
Tank Mates and Compatibility
When well-fed, in general, Emerald crabs can become very tolerant of tank mates and highly compatible and reef environments. Once familiar with its surroundings, the Emerald Mithrax Crab will venture out of hiding during the day to forage on algae.
Some aquarists say, that Emerald carbs can even eat Vermetid snails. Think twice before introducing Emerald crabs in your tank. Feed the Emerald crabs well. Provide lots of hiding places. Small crabs can be kept with small, non-aggressive, top or mid-dwelling fish. Along with this, having too large of fish can put the crab at risk. Large fish will often find them to be prey that can be fed on. Hermit crabs and snails will be always at risk in the tank. They move very slowly making them ideal prey for the Emerald crab. The size of the crab will play a huge role here.
Husbandry and Tank Environment
The Emerald Crab is one of the most popular and yet debated crabs in this hobby. Aquarists often use them as a cleaning tool in saltwater tanks to control nuisance algae, particularly the bubble algae. Native to the reefs of the Caribbean, the Emerald Crab is nocturnal and hides in caves and amongst rubble during the day. In the home aquarium, this member of the Majidae family requires a well-established aquarium with plenty of rockwork and hiding places.
Density wise, do not keep more than 1 crab per 40 liters (10-gallons), else it can lead to aggression amongst themselves. Emerald crabs really seem to thrive when the water temperature is kept between 24 - 28 C (75 - 82 F), pH 8.0 - 8.4, KH 8 - 12, and SG = 1.023-1.025. Be careful with chemicals like copper (read more). Do not forget to acclimatize them before putting them into the tank. If you do not take the care to do that, you run a higher risk of your crabs dying in the tank because of shock. Read my article “How I Drip Acclimate Shrimp and Why”.
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Sexual Dimorphism
It is rather easy to tell the difference between a male crab and a female crab. In males, the pleon (telson/abdomen, the part folded underneath the crab) is narrower (it looks like a lighthouse) and wider/rounder in females. Choose females.
Breeding Emerald Crabs
Unfortunately, this one area of the Emerald crabs is not known in detail. Basically, there is like zero information about breeding. Everybody says that it is just impossible. But why? I found several studies and researches about breeding the Emerald crabs. Females can produce from several dozen (usually a few hundred) to 1000 eggs during each spawn. The complete larval development of Emerald Crabs consisted of two zoeal stages and one megalopa stage (transformation stage). Both zoeal stages last about 2 days. Larvae molt just after darkness. Emerald Crabs larvae display a very active swimming behavior in the water column. Biologists did not notice any delayed molting in zoeal stages. The duration of the Megalopa stage is more variable. It can last from 3 to 9 days.
Larval diet is one of the most important factors for successful larval rearing of crustacean species. When it comes to feeding, the Emerald crabs larvae need newly hatched Artemia nauplii or enriched Artemia nauplii. It is extremely important that larvae can start feeding right after hatching. Interesting fact: Considering the importance of feeding in larval stages, 3 different food concentrations were tested. The best result was reached for 7 - 15 nauplii mL-1 trial. The larvae breeding setup should maintain high prey densities, good water quality, and suspension of the larvae in the water column (thus avoiding clumping). It will not be a good idea to have rocks in a rearing setup. After hatching, the larvae were attracted to the light placed in one of the extremities of the tank.
Molting
Like all crustaceans, they need to molt to grow. With time, as they outgrow the existing shell (exoskeleton), they begin to shed their shells off. Once that outer shell is shed, it is common to think that the crab is dead, but it isn’t. Tip: Do not throw away old shells. They contain lots of minerals and crabs will gladly eat them after some time. Like all snails, shrimp, and crayfish, crabs, also require a lot of calcium for their exoskeleton.