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Invertebrates · Crab

Pitho Crab

Pitho mirabilis (Herbst, 1794)

Also called Pithos Crab, Urn Crab

  • CareEasy
  • Reef safeYes
  • TemperamentPeacefulPeaceful; may pick at soft corals if underfed
  • Favorite foods
    1. Hair and turf algae
    2. Bubble algae
    3. Nori / algae wafers
  • Max size1.5 in (3.8 cm)
  • Min tank20 gal
  • LifespanAbout 1-3 or more years with proper…
  • IUCN statusNot Evaluated
Pitho Crab (Pitho mirabilis)
Illustration, not a photo of this species.

Quick answers

How big does the Pitho Crab get?
Up to 1.5 in (3.8 cm).
What size tank does it need?
At least 20 gallons.
How hard is it to keep?
Care level: Easy.
Is it reef safe?
Yes. Sold as reef safe by most sellers (Aquatics Unlimited, Aquarium Depot, Reef by Steele) and judged 'as good an option as Emeralds or Hermits' by Quality Marine.
What does it eat?
  • Hair and turf algae
  • Bubble algae
  • Nori / algae wafers
How long does it live?
About 1-3 or more years with proper care, per one retailer (Top Shelf Aquatics); no published data.
What are good tankmates?
  • Peaceful reef fish
  • Snails and hermit crabs in a clean-up crew
  • Other Pitho crabs, if there is enough algae
  • Most corals in a well-fed tank

The 30-second version

The Pitho Crab is a small, quiet crab from the warm seas of the Caribbean. Its shell is about the size of a coin. It is gray or tan with blotches, so it blends in with the rocks. The girl crabs have big claws, like an emerald crab. The boy crabs have tiny claws and hide by looking like a rock. Fish keepers like it because it eats algae. That is the green fuzz and the little green bubbles that grow on rocks in a fish tank. It hides in the day and comes out at night to munch. If there is not much algae left, give it some dried seaweed so it stays happy and leaves the corals alone.

Water & care

Hardy in a stable, established tank with plenty of rock to graze (Top Shelf Aquatics, Quality Marine). It needs algae: in a very clean tank, add dried seaweed or algae wafers. It is sensitive to copper, and sudden changes in salinity, pH or alkalinity stress it, especially around a molt (Top Shelf Aquatics). It is mostly nocturnal (Aquarium Depot), so you may rarely see it grazing by day.

ParameterKeep it at
Temperature 72–78 °F
Salinity (SG) 1.023–1.025
pH 8.1–8.4
Alkalinity 8–12 dKH

Ranges from Top Shelf Aquatics (72-78 F, dKH 8-12, pH 8.1-8.4, SG 1.023-1.025). Keep parameters stable, since sudden salinity, pH or alkalinity changes stress molting crabs. Calcium, magnesium, nitrate and phosphate are not species-specific, so they are left blank. No copper.

Feeding

DietOmnivore

Favorite foods

  • Hair and turf algae
  • Bubble algae (Valonia)
  • Caulerpa and Dictyota
  • Dried seaweed (nori, ogo)
  • Algae wafers

Mostly an algae grazer: it eats hair and turf algae, brown algae, Caulerpa, Dictyota and bubble algae from rock and other hard surfaces (Reef Cleaners, Quality Marine, Top Shelf Aquatics). It also picks up detritus and leftover food, so some sellers call it an omnivore. When algae runs low, offer nori or other dried seaweed and algae wafers; Quality Marine suggests feeding after lights out at first, and Reef Cleaners hand-feeds Caulerpa and Gracilaria.

Tank & maintenance

Minimum tank
20 gallons
Aquascape
Mature live rock with plenty of grazing surfaces, crevices and hiding places.
Lighting
No needs of its own; lighting that grows some algae helps feed it.
Water flow
No stated preference from most sellers; one (Aquarium Depot) suggests moderate flow.

Routine upkeep

Routine reef upkeep. Check that it still has algae to eat, and add seaweed if the tank is very clean. Leave molts in the tank and do not mistake a molt for a dead crab. It is mostly active at night, so look for it after lights out.

Temperament & tankmates

TemperamentPeaceful

Described as peaceful or non-aggressive by Reef Cleaners, Top Shelf Aquatics, Quality Marine, Aquarium Depot and Worldwide Corals; Quality Marine says it does not pick fights with fish, other invertebrates or other Pitho crabs. Aquatics Unlimited's data field says 'Semi-Aggressive', though its own text calls it peaceful. The crab is defenseless rather than aggressive: males have no real weapons and rely on camouflage.

Reef safeYes

Sold as reef safe by most sellers (Aquatics Unlimited, Aquarium Depot, Reef by Steele) and judged 'as good an option as Emeralds or Hermits' by Quality Marine. Top Shelf Aquatics rates it 'with caution', and Reef Cleaners notes that some have been reported to pick on soft corals; Quality Marine adds that it may take the odd polyp to decorate its shell. Keep it fed with seaweed when algae runs low, and watch new crabs around soft corals.

Good tankmates

  • Peaceful reef fish
  • Snails and hermit crabs in a clean-up crew
  • Other Pitho crabs, if there is enough algae
  • Most corals in a well-fed tank

Avoid

  • Triggerfish, pufferfish, hawkfish and large wrasses
  • Large or aggressive crabs
  • Prized soft corals if you cannot keep it fed
  • Copper-treated systems

Help it thrive

  • Add it to a mature tank with algae already growing; it is a grazer first.
  • Feed nori or algae wafers when algae runs low, so it does not turn to soft corals.
  • Give it time and look at night: it is mostly nocturnal and may stay out of sight by day.
  • Acclimate gently: float the bag about 30 minutes, then slowly add tank water; crabs may lie still at first after a cold or rough trip (Reef Cleaners).
  • Never use copper in its tank, and keep salinity and alkalinity steady.

Common ailments

Failed or fatal molt
Signs
Crab stuck in its old shell, missing legs after a molt, or found dead in a split shell.
Prevention
Stable salinity, pH and alkalinity; enough food; rock to hide in while the new shell hardens.
Treatment
Leave it alone in a quiet spot; lost legs regrow over later molts.

Starvation
Signs
Inactive crab, picking at corals or other animals, in a tank with little algae.
Prevention
Keep algae available or feed nori and algae wafers.
Treatment
Target-feed seaweed after lights out; move it to a tank with more algae if needed.

Copper or medication poisoning
Signs
Weakness, loss of grip and death after the tank was treated.
Prevention
No copper or invertebrate-toxic medicines in any tank with crabs; treat fish in a hospital tank.
Treatment
Move the crab to clean, matched water and remove the medicine with carbon and water changes.

This is general hobby guidance, not veterinary advice. Quarantine new arrivals and treat in a separate tank whenever you can.

Risks of owning one

  • Some individuals pick at soft corals, especially when underfed.
  • May starve in a very clean tank unless fed seaweed.
  • Trade identification is unverified; similar Pitho species may be shipped under the same name.
  • Copper and many fish medicines cannot be used in a tank containing it.
  • Hides by day, so it is easy to lose track of.

Breeding

DifficultyNot achieved Captive-bredRare or wild-caught

Quality Marine reports that females often mate with males in home aquariums and then carry eggs. No source found describes raising the larvae, and all trade stock is wild-collected.

  1. Keep a male (small claws) and a female (large claws) together and well fed.
  2. Expect the female to lay eggs after mating.
  3. Larval rearing for this species has not been documented, so treat any attempt as an experiment.

Deep dive The science, history and family tree

The science

Pitho mirabilis is a small spider crab (Majoidea: Mithracidae) of the tropical western Atlantic, the same family as the emerald crab. It was described by Herbst in 1794 as Cancer mirabilis, and Othonia rotunda Rathbun, 1892 is a junior synonym (WoRMS). WoRMS records it from the Gulf of Mexico, the Lesser Antilles, Antigua, Saint Martin, Guadeloupe, Bonaire and Curacao, and SeaLifeBase lists it as benthic in the Western Central Atlantic at 0-20 m, on shallow bottoms; Quality Marine describes its wild home as shallow tidal flats, seagrass beds and reefs. In the aquarium trade it is the 'Pitho crab' (often misspelled 'Pithos'). The sexes look different: females resemble emerald crabs, with claws that Reef Cleaners describes as up to double the body size, while males have small claws and rely on camouflage. Reef Cleaners gives maximum carapace widths of about 1.25 in (30 mm) for males and 1.5 in (40 mm) for females; most retail animals are 1-1.5 in across the shell (Quality Marine). It is sold as an algae grazer that eats hair and turf algae, Caulerpa, Dictyota and bubble algae (Valonia), and like other spider crabs it may decorate its shell with bits of algae or the odd coral polyp. Other western Atlantic Pitho species look alike (for example P. lherminieri, P. aculeata and P. anisodon), and trade identifications have not been checked by specialists, so a shipment may not be exactly this species.

Appearance
Small, roundish crab, gray to tan or brown and mottled for camouflage; coloring is variable. Females have a broad shell and large claws and closely resemble emerald crabs (Mithraculus sculptus) but are not shiny green; males have small claws and look quite different. Like other spider crabs it often carries bits of algae on its shell. Look-alikes: emerald crabs are shiny green; other Pitho (urn crab) species of the western Atlantic are hard to tell apart without a specialist.
Native range
Tropical western Atlantic: Gulf of Mexico and the Caribbean, including the Lesser Antilles, Antigua, Saint Martin, Guadeloupe, Bonaire and Curacao (WoRMS); retailers give its origin as the Caribbean.
Habitat
Shallow bottoms from the surface to about 20 m (SeaLifeBase): tidal flats, seagrass beds and reefs (Quality Marine), among rock, rubble and algae-covered surfaces (Top Shelf Aquatics).
Size
1.5 in (3.8 cm). Carapace width. Reef Cleaners gives males 1.25 in (30 mm) and females 1.5 in (40 mm); Top Shelf Aquatics gives females about 1.5 in. Quality Marine says about 1-1.5 in, up to about 2 in across; Aquatics Unlimited says 1-2 in; Aquarium Depot says about 1 in. Female claws make them look bigger.
Lifespan
About 1-3 or more years with proper care, per one retailer (Top Shelf Aquatics); no published data.
Conservation
Not Evaluated. Not assessed on the IUCN Red List (no assessment found on WoRMS or SeaLifeBase). Trade stock is wild-collected in Florida and the Caribbean; Reef Cleaners describes the species as abundant and widespread but sells it in limited monthly quantities

In the aquarium vs in the wild

In your aquarium

Clean-up crew algae grazer for hair, turf and bubble algae and some macroalgae; also scavenges leftovers.

On the reef

A small grazing crab of shallow tidal flats, seagrass beds and reefs, where it feeds on algae and camouflages itself among rock and rubble.

History

Described by Herbst in 1794 as Cancer mirabilis; the genus Pitho was named by Bell in 1836 in an account of South American crustaceans in the Proceedings of the Zoological Society of London (WoRMS). Othonia rotunda, described by Rathbun in 1892, is now a synonym. Reef Cleaners, a Florida clean-up-crew seller, says it was the first company to offer the species to hobbyists, and it is now sold by wholesalers such as Quality Marine and many reef stores as an alternative to the emerald crab.

Evolution & relatives

Pitho crabs are true crabs (Brachyura) in the spider-crab superfamily Majoidea, which includes decorator crabs and has fossils back to the Cenomanian stage of the Late Cretaceous. Within it they belong to Mithracidae, alongside Mithrax, Maguimithrax and the emerald crab's genus Mithraculus. The genus Pitho has ten accepted species in WoRMS, several of them in the western Atlantic.

Family tree, from kingdom down

  1. Kingdom Animalia Animals Animals are living things made of many cells that must eat food to live. Fish, corals, crabs, snails and sea stars are all animals.
  2. Phylum Arthropoda Arthropods Arthropods have jointed legs and a hard outer shell called an exoskeleton. To grow, they must crawl out of the old shell, which is called molting.
  3. Subphylum Crustacea Crustaceans Crustaceans are arthropods with a hard shell and two pairs of feelers. Most live in water, like crabs, shrimps and lobsters.
  4. Superclass Multicrustacea
  5. Class Malacostraca Malacostracans This big class of crustaceans includes crabs, shrimps, lobsters and krill. Their bodies are built from about the same number of segments.
  6. Subclass Eumalacostraca
  7. Superorder Eucarida
  8. Order Decapoda Decapods Decapod means 'ten feet'. Crabs, shrimps, lobsters and hermit crabs all have ten legs, counting their claws.
  9. Suborder Pleocyemata Egg-carrying decapods
  10. Infraorder Brachyura true crabs
  11. Section Eubrachyura
  12. Subsection Heterotremata
  13. Superfamily Majoidea spider crabs
  14. Family Mithracidae Mithracid spider crabs These spider crabs mostly live on reefs in warm seas. Many hide in rocks by day and come out at night.
  15. Genus Pitho
  16. Species Pitho mirabilis

See it on the Tree of Life Compare with another species

Closest relatives

  • Mithraculus sculptus Emerald Crab Same family (Mithracidae); similar algae-grazing role Read guide · Compare
  • Mithraculus forceps Red-ridged Clinging Crab (Ruby Mithrax) Same family (Mithracidae) Read guide · Compare
  • Pitho lherminieri Broadback Urn Crab Same genus; western Atlantic look-alike
  • Pitho aculeata Massive Urn Crab Same genus
  • Pitho anisodon Oval Urn Crab Same genus

Older scientific names: Cancer mirabilis, Othonia rotunda.

Fun facts

  1. Males and females look so different that they could pass for two species: females have huge claws, males tiny ones.
  2. Reef Cleaners says one Pitho crab removes more bubble algae than one emerald crab.
  3. Like other spider crabs, it may stick bits of algae or a coral polyp on its shell as camouflage.
  4. Reef Cleaners' crabs learned to wave their claws at the keepers to ask for food.

Learn more

Sources

  1. WoRMS taxon record 422019 marinespecies.org
  2. WoRMS genus record Pitho Bell, 1836 marinespecies.org
  3. SeaLifeBase: Pitho mirabilis summary sealifebase.se
  4. Quality Marine: A Pithy Ally in the Fight Against Algae qualitymarine.com
  5. Reef Cleaners: Pitho Crab reefcleaners.org
  6. Reef Cleaners: Acclimation reefcleaners.org
  7. Top Shelf Aquatics: Pitho Crab topshelfaquatics.com
  8. Aquatics Unlimited: Crab - Pitho aquaticsunlimited.com
  9. Aquarium Depot: Pitho Crab care and feeding aquariumdepot.ca
  10. Reef by Steele: Pithos Crabs reefbysteele.com
  11. Worldwide Corals: Pitho Crab worldwidecorals.com
  12. Wikipedia: Majoidea en.wikipedia.org
  13. Wikipedia: Mithracidae en.wikipedia.org

Care facts

The care facts on this guide and its product pages (care level, reef safety, temperament, size, tank, diet, flow, purpose, foods, acclimation and origin) combine 9 independent sources, checked October 25, 2026.

  1. Pitho Crab Top Shelf Aquatics
  2. CRAB - PITHO Pitho mirabilis Aquatics Unlimited
  3. Pitho Crab - Pitho mirabilis Aquarium Depot (Toronto)
  4. A Pithy Ally In The Fight Against Algae! Quality Marine (wholesaler)
  5. Pitho Crab Reef Cleaners (Florida clean-up-crew collector/retailer)
  6. Pithos Crabs Reef by Steele
  7. Pitho Crab Worldwide Corals
  8. Pitho mirabilis summary SeaLifeBase
  9. WoRMS taxon record: Pitho mirabilis (Herbst, 1794) World Register of Marine Species

Scientific names and classification follow the World Register of Marine Species (AphiaID 422019). Last updated October 25, 2026.