Latitudinal differences in host-specificity of marine Monogenea and digenea

Data from 15 surveys of marine trematodes (average of 91 trematode species and more than 80 fish species per survey) and from 12 surveys of marine Monogenea (average of 52 Monogenea species and more than 49 fish species per survey) show that the degree of host specificity of marine digenetic trematodes increases from cold to warm seas; Monogenea do not show such a trend, and the trend is probably reversed in the Pacific Ocean. The difference between the two groups is explained in terms of r ^- and k ^- strategy. Monogenea tend to follow a k ^- strategy (great complexity of adult, few offspring), which results in a high degree of host- and sitespecificity to facilitate mating in low-density populations. Only one or a few related host species can be infected, and as more related host species are present in the warm Pacific, host-specificity there is reduced. Digenea tend to follow an r^- strategy (simple strucure of adult, many offspring), part of which is to infect many ecologically suitable hosts. Host-specificity in cold-temperate seas is reduced because of the less patchy and ecologically less restricted distribution of hosts.