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

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 (+11% and +41%,
respectively) than unaffected eels of same age. We discuss mechanisms that
could explain this finding and other counter-intuitive 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.