Emerald Crab – Detailed Guide: Care, Diet, and Breeding

Emerald Crab (Mithrax sculptus)

The Mithraculus sculptus, commonly known as the Emerald crab, is probably the most heavily traded crab in the marine aquarium industry. Although they can be very beneficial to have in your tank, there are also a lot of debates about this species (aggression, feeding habits, breeding, etc). Therefore, in this detailed guide, I have gathered all information about this species based on existing studies, experiments, researches and experience of aquarists.

Besides being easy to care for, the Emerald crab is one of the crab species known by controlling nuisance algae, particularly Bubble algae (Valonia spp). Ornamental crabs of the genus Mithraculus frequently become part of a group of animals in a reef aquarium, “algae cleaning crew”.

Interesting fact: The interest in crabs for aquariums has risen in the past decade. These days they are the second in the marine aquarium market, after caridean shrimp.

Quick Notes about Emerald crab

Name Emerald crab
Other Names
Emerald Mythrax Crab, Green Crab, Green Clinging Crab, Jade crab
Scientific Name Mithraculus sculptus
Tank size (minimal) 10 gallons (~40 liters)
Keeping Easy-Medium
Breeding Very Difficult 
Size 6 cm (~2.5 inches)
Optimal Temperature 24 – 28°C  (~75°F – 82°F)
Water type SG = 1.023 – 1.025
Optimal PH 8.0 – 8.4 (7.5 – 9)
Optimal KH 8 – 12
Nitrate Less than 20 ppm
Diet Omnivore
Temperament (Reef safe) Considered safe, may be aggressive
Life span up to 4 years
Color Form Green

Natural Habitat of the Emerald Crab

These crabs are abundant in shallow waters of the tropical western Atlantic Ocean, from as far north as Miami, throughout the Bahamas and Caribbean, and south to northern Brazil. They are common in reefs, living in rock cavities, corals or sponges from intertidal to a maximum 55m depth.

Description of the Emerald Crab 

male Emerald Crab (Mithrax sculptus)The Emerald crabs have a bright green (emerald) 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. In the wild, they prevent them from being carried away by the current.

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. When they extend them out, the length of the claws is about 10 cm (4 inches) long.

They can live from 2 to 4 years.

The Behavior of the Emerald Crab 

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. This will allow your crabs to seek comfort in the dark corners and crevices in the tank just as they do in the wild.

Once they get adapted to a tank, they can slightly change their night/day behavior. They will come out during the day and you will see them a lot more.

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. They will be a crucial part of keeping your tank cleaning crew.

Emerald Crabs can be aggressive to each other. Therefore, if you have a small tank, it will be better to have only one crab or a pair of the opposite sex. 

Emerald Crab and 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. This process is called molting.

Once that outer shell is shed, it is common to think that the crab is dead, but it isn’t. During this period of time, the Emerald crab will hide in the rocks or the crevices until a new shell replaces the old one and becomes hard (mineralized).

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. You can learn more about it in my article “How to Supplement Shrimp and Snails with Calcium”.
Note: In general, molting frequency, growth, and survival of crustaceans are dependent on diet, temperature, and water quality. 

Sexing Emerald Crab

Female and male Crab differenceIt is rather easy to tell the difference between a male crab and a female crab. If you want to know the difference between a boy and a girl Emerald crab, you will need to flip them over and look at the underside of the 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. In addition, males are a little bit bigger and have larger claws compared to females.

Test: Bubble algae and Emerald Crab 

Emerald crabs have become increasingly popular in the aquarium hobby, revered by reef aquarists for their ability to control or eliminate the Bubble algae (Valonia spp. and Ventricaria ventricosa).

Note: Reef tanks are commonly invaded by the 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. These algae often live on hard substrates and corals, blocking light to zooxanthellae and may cause coral bleaching and death.

However, are Emerald crabs so good at eating the Bubble algae?

Emerald Crab (Mithrax sculptus) eats bubble algaeThe 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. So where is the truth?

Luckily, I found an interesting study about this matter and it answered my questions as well. 

According to that study, biologists gave Emerald crabs alternative foods to the Bubble alga and tested the preferences. The results were quite interesting:

  • Emerald crabs preferred pellets to the Bubble algae, suggesting that these might be chemical, tactile and/or visually more attractive than the algae.
  • Algal consumption was similar to when they were provided with mysids. Pellets have higher profitability than algae, so algal consumption decreases. Mysids have similar profitability to algae, so algal consumption does not decrease.
  • Small crabs consumed significantly less than medium and large-sized Emerald
  • Medium and large size crabs did not differ significantly in the amount of algae consumed.
  • Smaller crabs were better in controlling algae in areas difficult to access, such as crevices.

As we can see, the Emerald crabs have clear preferences in food choice. Therefore, it is important to evaluate how the presence of alternative foods affects their performance efficiency.

Interesting fact:

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. Basically, they consume and spread the algae at the same time!

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. As a result you might think that they do not do their job.

Feeding Emerald Crab 

One of the best (and unfortunately the worst, see below) things about Emerald Crabs is that they are not fussy eaters. Although Emerald crabs are often sold as an herbivore, they are actually omnivorous and require very little care, when it comes to being fed or feeding patterns.

Basically, they will eat anything extra in the tank. This may include meaty foods leftovers, and (or) the algae that grow naturally in the tank.

However, it will be wrong to think that they do not require a specific diet because they are scavengers. Although Mithraculus have modified, spooned-shaped claws to feed on both fleshy and filamentous algae, they may consume alternative foods offered in a reef aquarium.

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. Along with this, they may begin to prey on any smaller fish, shrimp or crab that are in the same 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. It is like a vicious circle.

We already know that Mysis shrimp does not affect their appetite. There is a high chance that Krill will not as well. Be very careful with any algae products.

Note: When well-fed, in general, Emerald crabs can become very tolerant of tank mates and highly compatible and reef environments. That is why, some aquarists prefer to supplement their diet about 2 – 3 times a week.

Are Emerald Crabs Reef Safe?

Once again, we have an opposing view. 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, etc. They call them Terrors and say that these crabs are not to be trusted, never again, the worst crabs, etc.

Why are they safe in some cases and disruptive in others? Is it really a hit or miss with the Emerald crabs?

The reason is pretty obvious. Like the majority of crabs, the Emerald crabs are opportunistic. Their herbivorous or carnivorous tendencies depend on food availability. Therefore, if they have access to easy and profitable food they can choose it.

Personally, I would keep Emerald crabs under suspicion until proven innocent. In addition, stick to these rules and it will help you to improve the odds:

  • Keep an eye on it for the first week or so.
  • Do not let it get hungry.
  • Choose females. They are smaller and less aggressive.

Keeping and Caring for Emerald Crab 

Emerald crabs have become very popular among aquarium hobbyists also because of their hardiness. These crabs only require an established aquarium with plenty of live rock and reef work for hiding places and to scavenge for algae but are otherwise undemanding and very easy to maintain.

Density wise, do not keep more than 1 crab per 40 liters (10-gallons), else it can lead to aggression amongst themselves. In most cases, there is no need to have more than one in your tank at a time.

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). Crabs, shrimp, and crayfish do not tolerate copper-based medications or fluctuating water parameters.

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”. The principle is the same with Emerald crabs. 

Basic Tank Equipment (links to check the price on Amazon)

Breeding Emerald Crab 

Unfortunately, this one area of the Emerald crabs is not known in detail. After all these years, even the most experienced aquarists and invertebrate breeders have not been able to successfully (systematically) raise them and create a breeding protocol.

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. This is the compilation of different facts.

Females can produce from several dozen (usually a few hundred) to 1000 eggs during each spawn. The eggs are then carried under the female’s abdomen for a few weeks at which point the larva are released into the water.

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. In most experiments, survival to the megalopa stages was extremely high and mortality normally began on the second or third day of megalopa.

At the end of the experiments, the overall survival rate was lower than 10 – 20 % in all treatments.

Emerald Crabs Larval Diet

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. Starved larvae fail to develop past zoea 1 stage.

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. To enrich Artemia nauplii biologists used Algamac 3050™.

Water parameters and Rearing Setup Conditions

In all treatments:

  • The salinity of 35.
  • The temperature of 28 – 29 C.
  • The pH of 8.0 – 8.2.
  • A photoperiod of 14 h light/10 h dark was maintained during the experiment.
  • Ammonia, nitrite, and nitrate were maintained below detectable levels.
  • Uneaten food must be replaced with fresh Artemia nauplii

The larvae breeding setup should maintain high prey densities, good water quality, and suspension of the larvae in the water column (thus avoiding clumping). A cylindrical shape tank decreases the larvae collision in the tank walls and increases the survival rate.

It will not be a good idea to have rocks in a rearing setup. When post-larvae crab holds on the rock, they hide in the cavities, making the handle without killing any larvae an impossible mission.

Another difficult observed in post-larvae crabs is due to their aggressiveness, even with shelters, they kill on each other until only one remains alive.
After hatching, the larvae were attracted to the light placed in one of the extremities of the tank. The larvae were collected by syphon, counted and acclimatized before entering the larval system.

Emerald Crab and Suitable Tank mates

Some guides and care sheets say that Emerald crabs are safe and peaceful with other tank mates. Well, they are not! We have to take into consideration the fact that there are also countless reports of them attacking fish, snails and other crabs. The danger is a real possibility.

Some basic rules and tips for Emerald crabs keepers:

  1. Think twice before introducing Emerald crabs in your tank. Evaluate the potential risk to corals, fish, snails, shrimp, etc.
  2. Feed the Emerald crabs well. Remember, the hungry crab will start hunting.
  3. Provide lots of hiding places. When they are stressed, they tend to either run away or protect themselves.
  4. It also depends from individual to individual but small females will bring less problems.
  5. The bigger the tank, the better.

Fish

Small crabs can be kept with small, non-aggressive, top or mid-dwelling fish. Avoid keeping them with bottom fish, the Emerald Crabs can feed on them or injure them with their claws.

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. Because of this, make sure that you are only choosing friendly fish types that are less likely to attack your crabs.

Hermit Crabs, Shrimp, and Snails

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. They may do just fine with snails and hermits, but you never know.

Shrimp are too fast for the Emerald crabs but occasionally they can be on the menu as well.

Other Emerald Crabs

It is recommended to only have one crab per 10 gallons (40 liters) because it has been found that with more than one crab can lead to fighting amongst the crabs.

Conclusion

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. However, in some cases, after (or sometimes instead) eliminating the pest, they can also attack and negatively affect other animals or plants in the aquarium.

They are simple creatures to take care of, though they can be pretty hard to breed. Currently, the marine ornamental industry relies heavily on wild-collected specimens, mainly from coral reefs.

References:

  1. Efficiency fusing emerald crabs Mithraculus sculptus to control bubble alga Ventricaria ventricosa (syn. Valonia ventricosa) in aquaria habitats. Journal of the Marine Biological Association of the UK. February 2008.
  2. Larval development and first crab of Mithraculus sculptus (Decapoda: Brachyura: Majoidea: Mithracidae) described from laboratory-reared material. Journal of the Marine Biological Association of the UK86(5). October 2006.
  3. Growth, development, and survival of larval Mithraculus sculptus (Lamark) and Mithraculus forceps (A. Milne Edwards) (Decapoda: Brachyura: Majidae): Economically important marine ornamental crabs. Aquaculture245(1-4):183-191 · March 2005.
  4. Larval rearing of Mithraculus sculptus (Lamarck, 1818) in captivity. UNIVERSIDADE DO ALGARVE by Tiago Miguel Dinísio Mourinho.
  5. Advances in Breeding and Rearing Marine Ornamentals. Journal of the World Aquaculture Society42(2):135-166. April 2011.
  6. Evolutionary transformations of the reproductive system in Eubrachyura(Crustacea: Decapoda). Kienbaum, Katja. Dissertation. 2019.
  7. A rearing system for the culture of ornamental decapod crustacean larvae. Aquaculture. Volume 218, 27 March 2003, Pages 329-339.

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