also known as: roundworm, thread worm
Philonema are parasitic roundworms. Juvenile fish become infected in freshwater and carry the infection with them when they migrate to the ocean. Parasite development is then triggered by the rise in hormones that accompanies the fish reaching maturity and preparing to re-enter freshwater. The parasite releases live young when the fish spawns, thus enabling infection of the next generation of juvenile fish.
In heavy infections, Philonema can cause severe visceral adhesions (where the host produces fibrous connective tissue in response to parasite irritation). These adhesions can sterilize the fish.