Copepod and fish lice infestation in the Japanese spider crab

What is it?

The Japanese spider crab (Macrocheira kaempferi) is the largest known jointed crustacean and is highly prized in major public aquariums. Like many marine crustaceans, it can harbour small parasitic crustaceans on its shell, gills or joints called copepods and fish lice. These organisms live attached to the animal's surface and feed on mucus, skin debris or occasionally blood. Light infestation often goes unnoticed, but heavy parasitism can weaken the animal, especially in captivity where stress and crowding promote their spread.

Commonly observed signs

These signs are neither exhaustive nor specific. Only a veterinarian can establish a diagnosis.

Causes and risk factors

Copepods and fish lice are small external parasitic crustaceans belonging to different groups but sharing a similar lifestyle: they attach to the external surface of a crustacean or fish host to feed. In the Japanese spider crab, they colonise mainly the shell, legs, gill chambers and sometimes soft areas after moulting. Their presence in small numbers is often tolerated without serious consequence, but proliferation can occur when environmental conditions favour them.

Overcrowded tanks or aquariums, poor water quality (inadequate filtration, unsuitable temperature, low water exchange rate) and introduction of new animals without quarantine increase infestation risk. A weakened crab, stressed by transport or recently moulted (shell still soft) is more vulnerable. Proximity to other parasitic crustaceans or fish also favours transmission.

Transmission

Parasites are transmitted through direct contact between animals in the same tank, or via water when free-living copepod or fish lice larvae move in search of a new host. Introduction of a new animal or contaminated material without prior quarantine is a common route of introduction into an aquarium.

Diagnosis and care

Diagnosis is the veterinarian's responsibility, based on clinical examination and laboratory tests. Care is decided case by case.

Treatment

Management relies on baths or antiparasitic treatments suitable for marine invertebrates, to be used with extreme caution as many standard antiparasitic treatments are toxic to crustaceans. Improvement of housing conditions (water quality, stress reduction) always accompanies treatment. Any intervention must be decided and supervised by a professional familiar with the particular sensitivity of crustaceans to chemical products.

Prevention

Possible complications

A severe untreated infestation can cause secondary skin lesions, breathing difficulty if the gills are heavily colonized, general weakening and increased susceptibility to other diseases, particularly opportunistic bacterial or fungal infections on irritated areas.

When to see a veterinarian

Prognosis

Prognosis is generally favorable when infestation is detected early and housing conditions are corrected. Severe and neglected infestation can however permanently weaken the animal and complicate its management in captivity, particularly in public aquariums where stress is already a significant factor.

Frequently asked questions

References

WOAH — Organisation mondiale de la santé animale (ex-OIE)

Content reviewed by the TunisieVet editorial team · Updated 06 September 2026