At a glance the quick version
Looks how to spot one
Hard, flat carapace with hairy legs
A small crab with a flat, glossy, deep green shell (about 1.2 cm across the carapace) and paler, hairy walking legs. The claws have white tips. Its strong legs let it grip rock very firmly.123
Personality what they are like
Semi-aggressive by nature but usually easygoing when well fed.3
Daily activity how busy a day looks
Mostly nocturnal, hiding in crevices by day and scavenging at night.13
Grooming and care the hands-on bits
Easy to keep. Add seaweed, meaty foods and pellets so it stays well fed, and keep water stable and copper-free.3
Home where they are happiest
An established marine reef aquarium with plenty of rockwork and hiding places. LiveAquaria gives 72-78 °F (about 22-26 °C), pH 8.1-8.4, dKH 8-12 and specific gravity 1.020-1.025. Wild crabs live on reefs and in seagrass from the shallows to about 55 m. No minimum tank size was given by the sources.32
Food basics what goes in the bowl
Omnivorous scavenger that eats algae (including bubble algae) and leftover meaty foods.13
Company who they like around
Can live with most reef tankmates when well fed. In the wild it sometimes associates with sea anemones.32
The story where it all began
The emerald crab was described by Lamarck in 1818 from the western Atlantic. Reef keepers adopted it as a natural helper against bubble algae, a use supported by research.21
Fun facts for the dinner table
Recognised by the official bits
- Taxonomy (via SeaLifeBase)
- Described by Lamarck in 1818 (originally as Maia sculptus).2
The care rhythm Furry Diary remembers it for you
A typical home-care rhythm for a Emerald Crab. It is our own starting plan, not a vet's advice, and you can change every line in the app.
- FeedingDepends on species, many feed from the light and the watervaries
- Salinity checkevery week
- Water testevery week
- Top-upTop up evaporation with fresh RO water, not salt waterevery day
- Water changeAbout 10 to 20% of the waterevery 2 weeks
- Light checkevery month
Owner questions the things everyone asks
How often should I feed my Emerald Crab?
Feed small amounts of algae, meaty foods, or pellets daily or every other day, adjusting based on what it eats. Furry Diary's starting plan notes that feeding depends on the species, so watch your crab and consult the food label or a vet for exact amounts.31 Furry Diary plan
What does an Emerald Crab eat, and what should it never eat?
It's an omnivorous scavenger that loves algae (including bubble algae) and leftover meaty foods. Never expose it to copper, as the water must stay copper-free; avoid any foods or supplements containing copper.31
How much space does an Emerald Crab need?
No minimum tank size is given, but it needs an established marine reef aquarium with plenty of rockwork and hiding places. A larger tank helps keep water stable and reduces aggression.32
What temperature, light or water does an Emerald Crab need?
Keep water at 72, 78°F (22, 26°C), pH 8.1, 8.4, dKH 8, 12, and specific gravity 1.020, 1.025. Furry Diary's plan suggests checking lights monthly and topping up evaporation daily with fresh RO water.3 Furry Diary plan
How often should I clean an Emerald Crab's home?
Furry Diary's plan recommends a 10, 20% water change every 14 days, plus weekly salinity and water tests. Top up evaporation daily with fresh RO water to keep conditions stable. Furry Diary plan
Can an Emerald Crab live with others?
Yes, it can live with most reef tankmates when well fed. In the wild it sometimes associates with sea anemones, but it's semi-aggressive, so ensure it has enough food to stay easygoing.32
How long does an Emerald Crab live?
Lifespan varies and isn't given in the sources. Ask a good breeder or an exotic-pet vet for typical longevity in home aquariums.
Can I handle an Emerald Crab?
Handling is not recommended, it's semi-aggressive and grips rock very firmly with its strong legs. It's best to observe it at night when it's active and leave it undisturbed.31
Written from the sources on this page, a few checked guides (VCA, ASPCA, RSPCA) and Furry Diary's own starting care plan. A friendly guide, not a vet's advice.
Like what we are building? Buy Archy a bone.
All of this is free for everyone, with a source on every fact, and it will never show annoying ads. It was a big job to build, so if it helped you, buy Archy a bone. Any amount is lovely, and no is a perfectly fine answer too.
Archy is our Australian Cattle Dog and chief taste tester. Tips go to 4V3D OÜ, the small family studio behind Furry Diary.
Where this comes from
Every breed page shows where each fact comes from. If we got something wrong, sorry, we are pet people, not a science magazine. Tell us at support@furrydiary.app and we will fix it.
- Mithraculus sculptus (Wikipedia) en.wikipedia.orgused for names, range, size, appearance, behaviour, diet, fun facts
- Mithraculus sculptus, SeaLifeBase sealifebase.seused for author and year, range, depth, carapace width, anemone associations
- Emerald Crab, LiveAquaria liveaquaria.comused for care level, temperament, water parameters, diet, max size
- VCA Animal Hospitals: Feeding times and frequency for your dog vcahospitals.comused for owner questions
- ASPCA: Dog grooming tips aspca.orgused for owner questions
- ASPCA: Cat grooming tips aspca.orgused for owner questions
- ASPCA: People foods to avoid feeding your pets aspca.orgused for owner questions
- RSPCA: What to feed your rabbit rspca.org.ukused for owner questions
Checked 2026-09-30. Text is written in our own words from these sources.