In investigating the parasites of adult thick-lipped grey mullets Chelon labrosus (Risso, 1826) at Plymouth these fishes were found to be infected exclusively with an ancyrocephaline gill monogenean identified as Ligophorus angustus Euzet & Suriano, 1977. When, at a later date, some juvenile C. labrosus were examined ancyrocephalines were again collected; however, quite unexpectedly, these parasites were not juvenile L. angustus but belonged exclusively to a new species of ancyrocephaline subsequently reported as Ergenstrema labrosi Anderson, 1981.

It seemed then that there might be a situation of considerable biological interest in which young fishes were characterized by one host-specific monogenean and older mullets by another. Although a change in the species composition of the total parasite fauna of mullets with age has been reported previously by Reshetnikova (1955) for golden grey mullet Liza aurata of the Black Sea, a change of monogenean species has never previously been recorded.

From an examination of the literature (e.g. Hickling, 1970) on the life history of the British thick-lipped grey mullet it was inferred that the situation described above might be associated with the migratory habits of the fish and in particular the different environments inhabited by young and old mullets; while there may be many factors (e.g. oxygen, temperature, pH, etc.) which differ between the open sea and tidal pools, inhabited by old and young fishes respectively, the most obvious one is that of salinity. Hence, in addition to establishing the identity and frequency of the ancyrocephaline infections with host age, it was decided to make investigations on the salinity tolerances of L. angustus and E. labrosi.