Is the continental life of the European eel Anguilla anguilla affected by the parasitic invader Anguillicoloides crassus ?

Abstract

Quantifying the fitness cost that parasites impose on wild hosts is a challenging task, because the epidemiological history of field-sampled hosts is often unknown. In this study, we used an internal marker of the parasite pressure on individual hosts to evaluate the costs of parasitism with respect to host body condition, size increase and reproductive potential of field-collected animals for which we also determined individual age. In our investigated system, the European eel Anguilla anguilla and the parasitic invader Anguillicoloides crassus, high virulence and severe impacts are expected because the host lacks an adaptive immune response. We demonstrated a nonlinear relationship between the severity of damage to the affected organ (i.e. the swimbladder, our internal marker) and parasite abundance and biomass, thus showing that the use of classical epidemiological parameters was not relevant here. Surprisingly, we found that the most severely affected eels (with damaged swimbladder) had greater body length and mass (þ11% and þ41%, respectively), than unaffected eels of same age. We discuss mechanisms that could explain this finding and other counterintuitive results in this host -parasite system, and highlight the likely importance of host panmixia in generating great inter-individual variability in growth potential and infection risk. Under that scenario, the most active foragers would not only have the greatest size increase, but also the highest probability of becoming repeatedly infected-via trophic parasite transmission-during their continental life.

Influence of introduced vs. native parasites on the body condition of migrant silver eels

“…But this criterion is not the best reflecting the eel pathogenicity due to the short life cycle of A. crassus , and severely damaged swimbladders are shown to harbor very few or even no living nematodes [42, 63]. Nevertheless, using the Swimbladder Degenerative Index, Lefebvre et al [44] recently demonstrated that the most affected eels had greater body length and mass (+11% and +41% respectively) than unaffected eels of the same age. Despite these surprising counterintuitive results, high virulence and severe impacts of A. crassus are expected because A. anguilla lacks an adaptive immune response, and various pathogenic potentially lethal effects (e.g., anemia, energy drain, swimming performance decrease) have been demonstrated, threatening the success of spawning migration in the Sargasso Sea [35, 43, 44, 53, 58].…”

Freshwater eels: A symbol of the effects of global change

“…However, the most serious damage is on the swimbladder itself. The infection may alter the gas composition of the swimbladder, block the pneumatic duct, impairing the organ's function (Kirk, Lewis, et al., ; Lefebvre et al., ) leading to necrosis in the most extreme cases (Molnár, Székely, & Perényi, ; Würtz & Taraschewski, ). The alteration of the swimbladder has a direct impact on swimming capacity (Sprengel & Lüchtenberg, ; Székely, Palstra, Molnár, & van den Thillart, ).…”

Relationship between European eelAnguilla anguillainfection with non‐native parasites and swimming behaviour on encountering accelerating flow

“…More active fish may consume more paratenic hosts and increase the probability of A . crassus infection (Lefebvre et al ., ).…”