Family Cymothoidae

This isopod family live in freshwater and marine environments. There are currently around 380 species in 40 genera. Young cymothoids attach to fish hosts, bite through the skin, secrete an anticoagulant and suck their victim's blood. Then they leave this host and look for their final host. Cymothoids seems to be fairly host specific and will often only attach to particular parts of the fish. Some live in the gills, others burrow into the flesh of their victim. One species, which has achieved widespread notoriety because of this behavior, enters its victim's mouth and eats its tongue. The isopod then lives in the space left behind.

Other cymothoids are more benign. While they certainly cause their hosts to use more energy, through increased drag, when they swim, they don't suck blood. Instead they feed on skin mucus and the bacteria it contains.

These externally attached isopods have an unusual sex life. They start off as males and in the absence of inhibitory hormones from an attached female, turn into females. There is only ever one female attached to each fish.

Cymothoid isopod outbreaks in fish farms can cause major damage to their host species with resulting loss of productivity for the fish farmer.

Cymothoid parasitic isopod