The differential effects of Ligula intestinalis (L.) plerocercoids on host growth in three natural populations of roach, Rutilus rutilus (L.)

Abstract – This study reports the differential effect of the pseudophyllidean cestode, Ligula intestinalis , on the growth rates of three fish populations of the roach, Rutilus rutilus , under field conditions. Here, we show that only one host-population is affected by parasite-induced gigantism in the first 2 years of host life. Paradoxically, this increase in fish growth is strongly correlated with the parasitic load in plerocercoid stages, in that only the roach population subjected to the heaviest parasitic pressure shows an enhanced growth of cestode-parasitized individuals. This host gigantism observed in only one locality is associated with parasite-induced fish mortality occurring, on average, one year earlier in this fish population when compared to other populations. We discuss the different reasons for this differential parasite effect on growth enhancement across host-population in natural settings. Whether the growth effects represent an adaptive response by the host or a manipulation by L. intestinalis of the host, remains to be elucidated.