Emerald Crab: A Delightful Guide for Aquarists

Emerald crabs, scientifically known as Mithraculus sculptus, are a popular addition to marine aquariums not only for their captivating appearance, but also for their invaluable role in supporting the ecosystem. Native to the Caribbean and the Gulf of Mexico, these fascinating creatures are often found in shallow waters around reefs and rocky outcrops.

In your aquarium, these colorful invertebrates are more than just a pretty sight; they are also an essential part of the clean-up crew. Known for their ability to devour uneaten food and tackle nuisance algae, including the dreaded bubble algae, emerald crabs are small heroes working behind the scenes.

As you learn about the care, diet, and compatibility of these splendid marine creatures, you’ll discover that an emerald crab can be your aquarium’s best friend. Get ready to give your saltwater setup a beneficial and captivating boost by welcoming an emerald crab into your aquatic family.

Species Overview

The Emerald crab (Mithraculus sculptus) is a popular choice for marine aquariums, valued for their vibrant coloration, lively personality, and ability to help clean tanks.

Property Description
Scientific name Mithraculus sculptus
Common names Emerald crab, green clinging crab
Distribution Caribbean Sea
Size 2.0 inches (5 cm)
Lifespan 2-3 years
Diet Omnivorous: algae, detritus, meaty foods
Temperament Peaceful
Minimum tank size 30 gallons
Temperature 72-78°F (22-26°C)
pH 8.1-8.4
Water hardness 8-12 dKH
Care level Easy
Filtration/Water Flow Moderate
Water type Saltwater
Breeding Egg layers
Breeding difficulty Difficult
Compatibility Reef safe, but can be aggressive if not fed enough

As nocturnal creatures, Emerald crabs enjoy spending their days hiding in caves, emerging in the evening to clean up algae and detritus. Ideally, you should provide plenty of hiding spots within the tank for them to feel secure and maintain a healthy diet to discourage aggression towards other tankmates. Providing the correct water parameters and a suitable environment will give your Emerald crabs the best opportunity to thrive in their new home.

Emerald Crab Basics

Origins and Natural Habitat

The Emerald Crab, scientifically known as Mithraculus sculptus, originates from the warm waters of the Caribbean. They are commonly found in reefs, hiding among the rocks and corals in search of algae to eat. Their natural environment is typically rich in hiding spots, where they can blend in with their surroundings and avoid predators.

Size and Shape

Emerald Crabs are small in size, with their body measuring about 2 to 2.5 inches in length. They have a compact, flat body shape, which is perfect for navigating through tight spaces in the reef. Their legs are relatively short in comparison to their body, but they still possess impressive climbing abilities.

Color and Markings

As their name suggests, Emerald Crabs are primarily green in color. However, their shade of green can vary, with some individuals displaying a darker hue while others have a lighter, more translucent appearance. Their green color is ideal for blending into the reef environment, providing them with excellent camouflage. They may also have brown or black markings on their body.


Emerald Crabs have an average lifespan of around 2-4 years in a well-maintained aquarium. Proper care, including providing them with adequate food and a suitable environment, is crucial for their health and longevity. Regular monitoring of your tank’s water parameters, such as temperature, pH, and nitrate levels, will be helpful in maintaining their ideal living conditions and promoting a healthy ecosystem for them to thrive in.

Diet and Feeding

Emerald Crabs, scientifically known as Mithraculus sculptus, are popular members of a reef tank’s cleaning crew. Their primary diet consists of algae, so they can be an excellent natural solution for maintaining a clean aquarium environment for you.

Variety is Key

To ensure your Emerald Crab stays healthy, provide a varied diet including different types of algae, such as hair algae and bubble algae. Supplement their diet with meaty foods like small pieces of shrimp or fish for optimum health.

Appropriate Feeding

You may wonder how often you should feed your Emerald Crabs. In most cases, their natural algae consumption is sufficient. However, if your tank has a low algae population, offering additional food once or twice a week can be beneficial.

Remember to observe your Emerald Crabs’ behavior and adjust their diet accordingly. Ensuring proper nutrition will keep your crabs thriving and allow them to continue being a valuable part of your tank’s ecosystem.

Behavior and Temperament

Emerald Crabs (scientific name: Mithraculus sculptus) are nocturnal creatures, meaning they become active during the night. In your marine tank, providing proper rocks and stones allows them to seek comfort in dark corners and crevices, just like in their natural habitat.

These crabs have a lively personality and exhibit fascinating behaviors, such as climbing on rocks and scavenging for food. They are excellent tank cleaners, as they feed on algae and help maintain water quality.

When it comes to temperament, Emerald Crabs are generally peaceful creatures. However, they may occasionally become territorial or aggressive, especially when food is scarce. To prevent conflicts, ensure that your tank has multiple hiding spaces and that you provide a well-balanced diet to your crab.

Finally, remember that these crabs can live for 2 to 4 years, and their intriguing behaviors make them a delightful addition to any marine tank. By understanding their behavior and temperament, you can create a harmonious environment where your Emerald Crab and other tank mates can thrive together.

Care and Tank Requirements

Tank Size

When setting up a tank for your Emerald Crab, start with a minimum of 10 gallons. This will provide ample space for your crab to thrive.

Water Parameters

Keep the water temperature between 75 and 80 degrees Fahrenheit. Ensure that the pH level is slightly higher than neutral, around 8.4.

Tank Setup and Decorations

Emerald Crabs are nocturnal, so providing rocks and stones for hiding spaces is essential. Offer crevices and dark corners within the tank to mimic their natural environment.

Filtration and Aeration

Make sure to use proper filtration and aeration systems in your tank to provide a healthy environment for your crab. This will help maintain water quality and improve oxygen levels for their overall well-being.

Suitable Tank Mates

When choosing suitable tank mates for your Emerald Crab, it’s important to consider their diet and temperament. Since Emerald Crabs are scavengers that primarily eat algae, opt for tank mates that won’t compete for food with them. Good choices include algae-eating species such as nerite snails and hermit crabs.

Emerald Crabs are generally considered reef safe, so it’s a good idea to choose tank mates that share this trait. However, keep in mind that their size makes them vulnerable. Be sure to avoid tank mates that can easily eat them or have the ability to crack their hard shells.

Remember, Emerald Crabs are nocturnal creatures. To provide them with a comfortable environment, ensure you have proper rocks and stones in your tank with dark corners and crevices where they can seek comfort.

By selecting tank mates carefully, you’ll create a harmonious and healthy environment for your Emerald Crab and its companions, ensuring a thriving marine habitat for all.


Breeding Emerald Crabs is not well-documented, and even experienced aquarists often find it challenging to establish a successful breeding protocol. Despite numerous attempts, there is still a scarcity of detailed information in this area. As a result, you might find it quite difficult or seemingly impossible to breed Emerald Crabs in your aquarium. However, some studies and general observations can be useful for understanding their breeding behavior.

Female Emerald Crabs are known to produce a range of several dozen to 1,000 eggs during each spawning, which she carries under her abdomen for a few weeks before releasing the larvae into the water. The larval development typically includes two zoeal stages, each lasting around 2 days, during which the larvae molt after darkness and actively swim in the water column.

The megalopa stage, which follows the zoeal stages, lasts from 3 to 9 days and is known to have a high mortality rate, resulting in low overall survival rates of less than 10-20% in most experiments. This stage’s challenging nature contributes to the difficulty aquarists face when attempting to breed Emerald Crabs.

So while breeding Emerald Crabs may be a difficult task, having a grasp on their breeding behavior might aid you in better understanding and caring for these fascinating creatures in your aquarium.

Emerald Crab Molting Process

Emerald crabs molt periodically when they outgrow their current shell. The frequency of molting depends on factors such as water conditions, food availability, and the crab’s growth rate. During the molting process, your emerald crab will shed its old exoskeleton and hide for several days, sometimes even up to a week.

This hiding behavior isn’t something to worry about; your crab is simply protecting itself while its new shell is soft and vulnerable. It’s common to find an empty shell at the bottom of your tank during this time. Although it may resemble a dead crab, it’s just the exoskeleton that has been shed.

You don’t need to remove the empty shell from your aquarium. In fact, it’s beneficial to leave it there, as it contains valuable nutrients and minerals that your crab may consume later. Additionally, other tank inhabitants can benefit from the nutrients in the discarded shell.

By leaving the discarded shell in the tank, you’re allowing for natural recycling and providing additional food sources for the ecosystem, ensuring you maintain a healthy environment for your emerald crab and other tank mates.

Common Diseases and Treatments

Emerald crabs are generally hardy creatures, but one common ailment that might affect them is shell disease. This condition is typically caused by a viral or bacterial infection, which can lead to indentation marks on your crab’s shell and legs. If left untreated, it might breach through the shell entirely, affecting your crab’s internal body.

To prevent shell disease in your emerald crab, maintain optimal water quality and cleanliness in your tank. Regularly monitor and address your aquarium’s parameters, such as water temperature, pH level, and ammonia levels. It’s essential to provide a varied and nutritious diet that includes things like fish food pellets and fresh vegetables.

If you suspect that your emerald crab is suffering from shell disease, consider isolating it in a separate tank to avoid possible contamination or aggressive tank mates. It might be helpful to consult with a marine aquarium specialist for further advice on treatments.

Remember, prevention is the best way to keep your emerald crab healthy and thriving. By regularly checking your tank’s environment and providing a balanced diet, you can minimize the risk of shell disease and ensure a happy and healthy crab.

Frequently Asked Questions

Are emerald crabs suitable for tanks?

Yes, emerald crabs are suitable for marine tanks. They are easy to care for and can help manage algae growth in your tank. Ensure that the tank’s pH is maintained around 8.4 and the water temperature is between 70°F – 80°F (21°C – 27°C).

Can they coexist with a reef?

Emerald crabs are generally considered reef-safe and can coexist with a majority of reef species. However, be cautious with hawkfish species, as they may prey on emerald crabs.

Do they get along with shrimp?

Emerald crabs are usually compatible with most shrimp species. As long as you provide enough space and hiding spots for both the crabs and shrimp, they should be able to coexist without any issues.

Are they effective at eating hair algae?

Yes, emerald crabs are known for their ability to eat hair algae and other forms of algae. If you are looking for a natural way to control algae growth in your tank, emerald crabs can be a great addition.

How large do they grow?

Emerald crabs typically grow up to 2.5 inches (6 cm) when fully grown. Their compact size allows them to fit into tight spaces and navigate through your tank with ease.

What do they eat in addition to algae?

In addition to algae, emerald crabs will feed on any scraps or leftovers they find around the tank. It’s essential to maintain the water quality by keeping nitrates below 10 ppm and monitoring phosphate levels to ensure there’s enough algae growth for the crabs to feed on.