Parasitism by copepods and crustaceans parasitic on conger eels

What is it?

The conger eel (Conger conger) is an anguilliform marine fish which can host various external parasitic crustaceans, mainly copepods (small microscopic to millimeter-sized crustaceans) attached to the skin, the gills or in the oral cavity. These parasites live at the expense of the fish by feeding on mucus, blood or tissues. This parasitism is common in wild fish and can also affect conger eels kept in aquariums or ponds.\nThe presence of a few parasites is often well tolerated by a healthy conger eel, but a significant infestation weakens the animal, especially in captivity where stress and promiscuity favor their multiplication.

A parasitized conger eel may rub itself against rocks, sand or the walls of the pond to try to get rid of the parasites clinging to its skin.\nWe sometimes observe areas of irritated skin, small dots or filaments visible on the surface of the body, on the fins or near the gills.\nThe fins may appear damaged or frayed.\nIn the event of a heavy infestation, the fish swims abnormally, remains less active, eats less and can gradually lose weight.\nSignificant damage to the gills can hamper breathing, which results in more marked opercular movements.

Several species of parasitic copepods (e.g. genera attached to the skin or in the gills) and other small crustaceans can colonize conger eels. They attach to the surface of the body or partially penetrate tissues through specialized appendages, and feed on skin secretions, blood or tissue debris.

Commonly observed signs

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

Transmission

Transmission generally occurs by direct contact in water between carrier fish and healthy fish, or by the presence of free stages of the parasite in the aquatic environment which attach to a new host.

References

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

Content reviewed by the TunisieVet editorial team · Updated 25 September 2026