also known as: thread worms, pin worms, roundworms
Nematodes are a large group of round, thread-like worms, with both free-living and parasitic species. They are common fish parasites, often seen on the surfaces of internal organs. Nematodes have both direct and indirect life cycles (requiring multiple hosts). While most free-living and fish nematodes are not transmissable to humans or other mammals, the exception is Anisakis, which is a problem when raw infected fish is eaten by a mammal.
In fish, nematodes can cause damage to organs directly, and indirectly through the host immune response, which can lead to buildup of 'adhesions' within the body cavity, and can lead to host sterility (e.g. Philonema).