Cymoithoids are ectoparasites, usually of fish, and they include the bizarre "tongue-biter" (Cymothoa exigua), which attaches to a fish's tongue, causing it to atrophy, and replaces the tongue with its own body.

[3] Species of the Cymothoidae are generally found in warmer waters and rarely in the cool and cold climates.

[5] As juveniles, they are not specific in their requirements, and attach themselves temporarily to the skin of any fish.

When they have found the correct species of fish for their adult development, they attach more permanently.

Many host fish have mutualistic arrangements with certain shrimps such as Ancylomenes pedersoni, whereby the fish visits a "cleaning station" and the shrimps remove and feed on the cymothoid parasites.