White Spot Disease in Japanese Spider Crabs
White spot disease is a serious viral infection affecting many crustaceans, including the Japanese spider crab. It is caused by a virus called WSSV (White Spot Syndrome Virus). This virus attacks the animal's shell and internal organs. It i...
What is it?
White spot disease is a serious viral infection affecting many crustaceans, including the Japanese spider crab. It is caused by a virus called WSSV (White Spot Syndrome Virus). This virus attacks the animal's shell and internal organs. It is greatly feared in aquariums and breeding tanks because it can quickly kill large numbers of individuals.
The Japanese spider crab, the world's largest crab by leg span, is a fragile animal sensitive to changes in its environment. Infection with this virus can be fatal within days if left untreated.
Key point
A disease with potentially very high mortality and rapid spread in group settings.
Commonly observed signs
These signs are neither exhaustive nor specific. Only a veterinarian can establish a diagnosis.
Related signs — each one links to its page:
The most distinctive sign is the appearance of small white or whitish spots under the shell, sometimes difficult to see in this species because of the natural color of its cuticle.
The animal becomes lethargic, remaining motionless or swimming erratically. It loses its appetite and refuses food normally accepted.
The shell may appear more fragile, with discolored areas or wounds that do not heal normally. In advanced cases, death often occurs suddenly, sometimes with no obvious signs beforehand.
When to consult without delay
This list does not replace veterinary advice. It gathers situations that justify a prompt consultation.
Causes and risk factors
The disease is caused by WSSV, a DNA virus that infects cells in many tissues of the crustacean: the cuticle, gills, digestive tract, and lymphoid organs.
This virus is highly resistant in water and can survive several days outside a host, which facilitates its spread in tanks and aquariums.
Transport stress, poor water quality, sudden fluctuations in temperature or salinity, high animal density in the same tank, and the introduction of new individuals without a quarantine period increase the risk of disease onset and spread.
Transmission
The virus spreads through direct contact between animals, contaminated water, infected raw food (particularly other carrier crustaceans), and contaminated aquarium equipment that has not been disinfected. Birds and other animals visiting ponds can also mechanically spread the virus.
Diagnosis and care
Diagnosis is the veterinarian's responsibility, based on clinical examination and laboratory tests. Care is decided case by case.
Diagnosis is based on observation of clinical signs by a professional familiar with crustacean diseases, supplemented if possible by specialized laboratory tests (detection of the virus using molecular biology techniques) performed on tissue samples.
Treatment
There is no recognized specific antiviral treatment for this disease in crustaceans. Management is essentially supportive: immediate improvement of water quality, stress reduction, and isolation of affected animals to prevent spread to the rest of the tank.
Treatment depends on the cause and the animal's condition: it must be determined by a veterinarian. No self-medication.
Prevention
Prevention is based first and foremost on strict quarantine of any new animal before introduction into a shared tank. It is essential to maintain water quality that is irreproachable, with stable parameters (temperature, salinity, oxygenation).
Equipment used for one aquarium should be disinfected before being used for another. Avoiding feeding crustaceans with raw prey from uncertain sources also limits the risk of introducing the virus.
Possible complications
Progression can be extremely rapid, leading to the animal's death within a few days. In a shared tank, infection can spread to all individuals present, with very high mortality rates.
When to see a veterinarian
Appearance of white spots beneath the shell
Abnormally still or lethargic behaviour
Sudden loss of appetite
Unexplained death of other crustaceans in the same tank
Disordered swimming or unusual behaviour
Prognosis
The prognosis is guarded to poor once clinical signs have appeared, as there is no specific curative treatment. Survival depends heavily on how quickly environmental management is implemented and on isolating affected animals.
Frequently asked questions
Can this disease be transmitted to humans?
No, the virus responsible for white spot disease only infects crustaceans. It is not dangerous for humans or other domestic animals.
Can spider crabs be treated when they are afflicted?
There is no treatment that directly eliminates the virus. Only supportive care and improved living conditions can help the animal better resist it.
How to prevent disease from spreading throughout an entire aquarium?
Systematic quarantine of all new animals and thorough disinfection of equipment are the best protections against collective contamination.
Are white spots always visible?
No, in the Japanese spider crab, the color and texture of the carapace can make these spots discreet. A professional should carefully examine the animal if there is any doubt.
References
WOAH — Organisation mondiale de la santé animale (ex-OIE)
Content reviewed by the TunisieVet editorial team · Updated 06 September 2026
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