In fish, the production of specific immunoglobulins against gill
monogeneans or other helminths was previously documented (e.g.
Buchmann and Lindenstrøm, 2002; Secombes and Chappell, 1996).
Even though our statistical analyses did not reveal a significant effect
of fish group (parental species and hybrids) on IgM level, the hybrids
tended to reach an IgM level similar to common carp. In addition, we
showed a higher level of IgM in females than in males for all fish groups
(i.e. common carp, gibel carp and hybrids) and in both sampling
periods, which most likely reflects the different costs of reproductive
investment between the sexes.