Gnathia

Gnathia is a genus of isopod crustaceans belonging to the family Gnathiidae. These isopods are commonly known as "fish lice" or "parasitic isopods," although the term "fish lice" can also refer to copepods.

Gnathia isopods exhibit a complex life cycle involving distinct larval and adult stages. The larval stage, known as a praniza larva, is parasitic and feeds on the blood of marine fish. Praniza larvae attach themselves to the gills, skin, or fins of host fish and engorge themselves with blood, causing irritation and potential secondary infections.

The adult stages of Gnathia isopods are free-living and do not feed. Adult males are significantly different in morphology from the larval praniza and adult females, possessing enlarged mandibles and complex mouthparts that are not used for feeding. Adult females reside in bottom sediments, where they brood their eggs. The adult stage focuses solely on reproduction and dispersal.

The genus Gnathia includes a wide variety of species, found in marine environments worldwide. Their parasitic nature makes them a concern in aquaculture, as heavy infestations can negatively impact fish health and production. Identifying Gnathia species often requires careful examination of morphological characteristics, particularly the mandibles of adult males.