Head morphology and piscivory of European eels, Anguilla anguilla, predict their probability of infection by the invasive parasitic nematode Anguillicoloides crassus

Summary The morphology of animal body structures influences their function; intrapopulation plasticity in diet composition can occur where head morphology limits gape size. The European eel, Anguilla anguilla, a critically endangered catadromous fish, shows significant intrapopulation variations in head width, with broader headed individuals being more piscivorous. Infection of eels during their freshwater phase by Anguillicoloides crassus, an invasive nematode parasite, involves paratenic fish hosts. We tested the relationships between their infection status, head functional morphology (as head width/total length ratio; HW:TL) and the proportion of fish in diet (estimated by stable isotope mixing models) across three populations. The extent of piscivory in the diets of individual eels increased significantly as their HW:TL ratios increased. There were no significant differences between infected and uninfected eels in their total lengths and hepatic–somatic indices. However, the HW:TL ratios of infected eels were significantly higher than those of uninfected eels and, correspondingly, their diet comprised a higher proportion of fish. Logistic regression revealed that head morphology and diet were significant predictors of infection status, with models correctly assigning up to 78% of eels to their infection status. Thus, eel head functional morphology significantly influenced their probability of being infected by invasive A. crassus, most likely through increased exposure to fish paratenic hosts. Accordingly, the detrimental consequences of infections are likely to be focussed on those individuals in freshwater populations whose functional morphology enables greater specialisation in piscivory.