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Emerald Crab
Mithraculus sculptus
📍 Caribbean Sea, Gulf of Mexico, and Western Atlantic Ocean
The Emerald Crab is a small, bright green crab with a flattened carapace and distinctive ridged claws, making it an attractive and recognizable addition to reef tanks. It is well known for its appetite for bubble algae (Valonia spp.) and other nuisance algae, serving as a valuable cleanup crew member. Generally peaceful, it can become opportunistic toward slow-moving or injured tankmates when underfed.
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Tanks keeping this 🐟
Kept by 1 hobbyistCommunity tanks featuring Emerald Crab.
Care Guide
Diet
Emerald Crabs are omnivores that primarily graze on algae, with a particular fondness for bubble algae and filamentous algae. They also consume detritus, uneaten food, and meaty supplements such as mysis shrimp or small pieces of seafood, especially in tanks with low algae levels. Supplemental feeding is recommended to prevent them from becoming aggressive toward corals or other invertebrates when food is scarce.
Behavior
Emerald Crabs are generally reef-safe and spend most of their time scavenging along rockwork and the substrate, making them excellent cleanup crew members. They are primarily nocturnal but are often seen foraging during the day in established tanks. While usually peaceful, larger individuals or underfed specimens may harass small fish, snails, or even pick at soft corals and clam mantles.
Breeding
Breeding Emerald Crabs in captivity is extremely difficult and rarely achieved by hobbyists. Females carry fertilized eggs under their abdomen, and larvae are planktonic, requiring specialized rearing conditions with specific foods and water parameters that are very challenging to replicate. Successful captive breeding has been documented only in controlled research settings.
Tank Mates
Active swimmers that generally avoid the crab's territory; compatible in reef setups
Peaceful reef fish that occupies mid-water zones away from the crab
Fellow cleanup crew member; occupies substrate and is generally ignored by emerald crabs
Larger snails are usually safe, but a hungry emerald crab may harass smaller or overturned individuals
Can coexist but may be targeted by larger or aggressive emerald crabs, especially when molting
Peaceful, small fish that stays near the water column and is rarely bothered by emerald crabs
Common Diseases
Molting Complications
Crab appears lethargic, hides for extended periods, or is found motionless near its old exoskeleton
Ensure stable water parameters and adequate iodine levels (via supplements or water changes); provide hiding spots and remove tankmates that may harass the vulnerable crab post-molt
Bacterial Infection
Discoloration, lesions, or necrotic patches on the carapace or limbs; lethargy and loss of appetite
Improve water quality immediately; quarantine if possible; in severe cases, consult a veterinarian for antibiotic treatment, though prognosis is often poor in invertebrates
Low Salinity Stress (Hyposalinity)
Lethargy, loss of coordination, failure to molt properly, or death
Maintain specific gravity at 1.023-1.026; perform gradual salinity correction if levels have drifted; avoid sudden salinity swings
Starvation-Induced Aggression
Crab begins attacking corals, clams, or other invertebrates; visible weight loss or sunken appearance
Supplement diet with meaty foods such as mysis shrimp or nori; ensure adequate algae growth in the tank or provide algae-based foods regularly
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Quick Facts
- diet
- Omnivore — algae, bubble algae, detritus, meaty foods
- lifespan
- 2-4 years
- max size
- 5 cm (2 in)
- tank size
- 30 gallons minimum
Water it likes
- ph
- 8.1-8.4
- ammonia
- 0 ppm
- nitrate
- <20 ppm
- hardness
- 8-12 dKH
- temperature
- 72–79°F (22–26°C)
Legality
No state or federal restrictions on record for this species.
Not legal advice, and possibly incomplete or out of date. Rules vary by state and locality and change over time — always confirm the current regulations with your state wildlife or agriculture agency before buying, keeping, or shipping this species.