Thai micro crab

Water parameters

Tolerated range for this species. Aim for the middle of each band rather than the extremes.

Tank and habitat

Substrate: any.

Behavior

Plant interaction: plant safe.

Typically wild-caught; acclimate slowly.

Feeding

Filter feeder and biofilm grazer. In mature tanks with established biofilm on plants, moss, and hardscape, they feed themselves by filtering particles from the water and grazing surfaces. Supplemental feeding with powdered food (spirulina powder, crushed flake dust, baby shrimp food) helps in newer or very clean tanks. They use their small chelae to pick particles from surfaces and their maxillipeds to filter the water column. They don't eat standard fish food pellets or wafers; the food needs to be fine enough for their tiny feeding apparatus. Feed every other day if supplementing.

Nocturnal feeder; drop food after lights out.

Compatibility

• One of the smallest freshwater crabs available at 1–1.5 cm. Fully aquatic, unlike most freshwater crabs that need land access.
• Extremely delicate and easily eaten. Species-only tanks or shrimp tanks are the safest option. Even small fish like neon tetras may harass or eat them.
• Peaceful and reclusive. They hide among plant roots, moss, and fine-leaved plants during the day. Expect to rarely see them unless you look carefully.
• Best housed with small shrimp (Neocaridina, Caridina) and snails. The crabs ignore shrimp and shrimp ignore the crabs.

Origin and habitat

Limnopilos naiyanetri, the Thai micro crab, is a tiny filter-feeding crab in the family Hymenosomatidae, the false spider crabs, named for their spidery look when the legs are spread. It is endemic to a single river, the Tha Chin in Nakhon Pathom Province, Thailand, where it was found clinging to the trailing roots of floating water hyacinth in slow, plant-choked water, and no confirmed population is known anywhere else. Chuang and Ng described it in 1991, the year it was discovered; the species honours Professor Phaibul Naiyanetr, and the genus name points to its freshwater home and hairy body. Unusually among freshwater crabs it is fully aquatic, living, feeding, moulting, and breeding entirely underwater rather than needing land or brackish water. The carapace reaches only about 1 cm across, with a leg span near 2.5 cm, and the semi-translucent grey-to-brown body is covered in fine setae that trap detritus and help it filter food and stay camouflaged. It feeds by extending feathery legs and claws to strain fine particles and biofilm from the water. Its single-river range is a real conservation concern, though no formal IUCN assessment exists. It reached the hobby around 2008 and remains a specialty animal.

Breeding

Breeding is poorly understood and rarely, if ever, reliably achieved in aquariums. Females carry a small clutch of eggs under the abdomen, and the larvae are assumed to complete development in fresh water rather than needing salt, but this has not been firmly confirmed, and there are no well-documented captive spawns, only occasional reports of progress and the odd juvenile turning up in a mature, planted tank. Sexing is hard at this size, with females showing a broader abdominal flap. For now the trade runs on wild-caught animals.

Common problems

They are so small and reclusive that the main complaint is simply never seeing them; they bury into moss, plant roots, and filter sponge and come out only briefly, so dense moss like java or weeping moss is needed for them to settle and filter-feed. Starvation is a real risk in a clean, new tank with little biofilm, so the tank should be mature before they go in. Like all crustaceans they are sensitive to copper, so copper-based medications are out. They are fragile to handle and should be moved by scooping the moss or plant they are on rather than catching the crab itself. Supply is patchy and they cost more than common invertebrates.

Bioload

Further reading

• How to cycle a fish tank
• Stocking ideas by tank size