Figure 4: Top left: a nuptial male of a lake ecotype of the Three-spined stickleback (Gasterosteus aculeatus species complex, photo Ole Seehausen). Bottom left: a breeding male stickleback and a series of Schistocephalus cestodes that were found in its body cavity (photo courtesy of Kay Lucek). These tapeworms change the behaviour of stickleback and effectively castrate them. Right: mean number of ectoparasitic Gyrodactylus monogenean parasites (log transformed) on Schleswig Holstein lake (grey bars) and river (white bars) sticklebacks, with or without an MHC haplotype G. Occurrence of the haplotype coincides with higher resistance against the parasite in the river ecotype while there is a tendency for the opposite pattern in the lake ecotype. Figure produced with permission from data in Eizaguirre et al. [54].