A flat, glossy-green reef crab famous as one of the few reliable eaters of nuisance bubble algae (Valonia). Mostly reef-safe and hardy, it is a popular spot-treatment for algae but can turn opportunistic in underfed tanks.
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Small; body about 1-2 in (2.5-5 cm) across the carapace, with flat hairy legs.
Lifespan
2–4 years
Social needs
solo
Native region
Western Atlantic and Caribbean reefs, including Florida and the West Indies
Origin
New World
Climate
🌴 Tropical
Water type
🌊 Marine
Family
Mithracidae
Genus
Mithraculus
Part of the Marine Crabs
Reef-dwelling crabs kept mostly as algae-grazing and scavenging cleanup crew. Many are useful and largely reef-safe when well fed, but most are opportunistic and can turn on snails, corals, or small fish if hungry — so stock conservatively and watch behavior.
From the minimum an animal needs to be kept humanely, up to the ideal setup. Bigger is almost always better — minimums are floors, not targets.
Photo coming soon
Minimum
Reef aquarium
30 gal (≈ 114 L) reef
Mithraculus sculptus eats bubble algae — keep singly in a 30+ gal reef with rockwork. Mostly reef-safe but may pick at SPS or eat zoanthids when underfed. Supplement with seaweed sheets and frozen mysis.
Photo coming soon
Recommended
Established reef aquarium
55 gal reef, rockwork caves
A 55+ gal reef with rockwork hides and a varied community. One emerald crab per ~30 gal as a bubble-algae control — too many become a problem.
Photo coming soon
Ideal
Mature display reef
75+ gal mature reef
A larger mature reef where the emerald acts as part of a balanced cleanup crew. Generous space and supplementary feeding keep it from sampling corals.
Life & growth stages
How this animal changes through its life — each stage often has its own care, diet and space needs.
Photo coming soon
Larva
Most marine invertebrates hatch into microscopic planktonic larvae (such as the zoea of crustaceans or the bipinnaria/veliger of echinoderms and mollusks) that drift and feed in the water column. The larva looks nothing like the adult and undergoes major reorganization.
Photo coming soon
Juvenile
After settling out of the plankton, the juvenile takes on a recognizable miniature of the adult body plan — a tiny shell, a small star, or a translucent shrimp. Crustaceans grow by molting, shedding the exoskeleton to enlarge.
Adult
Adults reach full size and reproductive maturity with the species' mature shell, shape, or coloration. Many continue to molt or grow throughout life, and some show sex differences in size or claw/appendage shape.
Habitat & enclosure
Best in established reef tanks of at least 75 L (about 20 gal), giving enough rockwork and algae to forage without competing for food. Keep stable reef parameters: temperature 24-27 C (75-80 F), pH 8.1-8.4, specific gravity 1.023-1.026, with copper kept at zero.
It needs plenty of live rock with caves and overhangs where it hides by day and clings while grazing. Lighting is irrelevant to the crab itself; any reef light works, and moderate flow helps it find food. Generally one emerald crab per tank to avoid territorial fights.
Substrate
Live rock with deep crevices is essential for hiding, grazing, and molting cover. A sand or crushed-coral bed is fine; the crab spends most time on rock rather than substrate.
Equipment & setup
Standard reef gear: live-rock biofiltration, a stable heater, protein skimmer, and moderate powerhead flow. No special lighting needed for the crab; lighting is dictated by your corals.
Diet
Primarily herbivorous, grazing bubble algae (Valonia/Ventricaria), hair algae, and other nuisance algae plus detritus. In tanks scrubbed clean of algae it will accept dried seaweed, pellets, and meaty foods, and a hungry crab may then pick at corals, snails, or small fish. Target-feed a meaty morsel a couple of times a week if algae runs low to keep it off livestock.
Behavior & temperament
Largely reef-safe and shy, hiding in rockwork and emerging to graze, often at night. Risk rises with size and hunger: large, underfed individuals may harass snails, nip soft corals, or grab a sleeping fish, so monitor well-fed behavior. Keep singly in most tanks; two crabs often fight over territory unless the system is large.
Health
Hardy if water is stable and copper-free, but sensitive to salinity and pH swings during acclimation. Molting is normal and leaves a hollow green shed that is easily mistaken for a dead crab. The main 'problem' is behavioral — opportunistic predation when starved — which is prevented by ensuring an algae or supplemental food supply.
Tips, DIY & hacks
Buy emerald crabs specifically to combat a bubble-algae (Valonia) outbreak, where they outperform most other grazers. Drip-acclimate over 30-60 minutes, keep them fed when algae is low, and remove any individual that develops a taste for corals or snails. Choose smaller specimens, which tend to be less predatory.
