The common fish louse – Argulus

Argulus is perhaps one of the most interesting and enigmatic fish ectoparasites that we are likely to encounter (Figure 13.1). It is common on the skin and may be found on flat non-respiratory surfaces inside the gill cavity. Argulus foliaceus is large enough to be a familiar sight for most freshwater fishermen and has been the subject of many studies since Linnaeus named it as Monoculus foliaceus in 1758 (see Baird, 1850). In spite of all this attention, it has proved extremely difficult to obtain answers to the following simple and basic questions about its biology: “how does the parasite feed?”; “what does it feed on?”; “how is sperm transfer achieved?”; “what are its nearest relatives?”.