Figure 1: Top left: three of the four ecotypes and species of Arctic charr (Salvelinus alpinus species complex) found in Thingvallavatn, Iceland (top left). S. thingvallensis, a small benthic ecotype (uppermost) lives mainly in lava crevices, the pelagic ecotype, one morph of S. murta (middle) feeds mainly on plankton in open water, and the large piscivore ecotype, another morph of S. murta (lowermost) preys upon smaller fishes. Bottom left: Diplostomum metacercariae in an eye lens of fish (all photos Anssi Karvonen). These widespread and abundant parasites cause cataracts and have significant fitness consequences for the fish. Species of the same genus are also found in the vitreous humour of the fish eye, like in the Icelandic ecotypes of charr. Right: average total sum of cestodes (white bars) and trematodes (grey bars) in the four ecotypes of arctic charr in Thingvallavatn (SB: small benthic S. thingvallensis, LB: large benthic S. sp., PL: planktivorous S. murta, PI: piscivorous S. murta). The result illustrates the extent of variation in parasite infections between the sympatric and parapatric ecotypes and interactions between different parasite taxa. Figure produced with permission of John Wiley & Sons Inc. from data in Frandsen et al. [61].