- Argulus japonicus is a fish louse (a branchiuran crustacean) that parasitises a wide range of freshwater fish hosts.
- It has a flattened body and can be up to 9mm in length.
- The head extends back into pair of carapace lobes that completely conceal the thorax and its 4 pairs of swimming legs.
- The bilobed abdomen is visible at the rear end of the body.
- Males are similar to females in body shape, although usually a little smaller. The third and fourth swimming legs of males are modified for sperm transfer during mating.
- Juvenile Argulus attach to the host by means of claws on their head limbs, but adults attach by means of a huge pair of modified suckers.
In Europe, this species can be confused with Argulus foliaceus, the common European fish louse. Distinguishing between these two species involves careful comparison of the shape of the third and fourth swimming legs in the male, and the shape of the abdomen in the female.
Fish lice belong to the Branchiura, a group of crustaceans once confused with the siphonostomatoid copepods because both possess tubular sucking mouths. Branchiura differ from copepods in body plan and in having a pair of large compound eyes on the head. Copepods lack compound eyes.