Nested structure is a pattern originally described in island biogeography to characterize how a set of species is distributed among a set of islands. In parasite communities, nestedness has been intensively studied among individual fish from a locality. However, nested patterns among parasite assemblages from different host populations (localities) have scarcely been investigated. We recorded the occurrence of parasites in 9 fish species widely distributed along the southeastern Pacific coast to determine whether the ecto- and endoparasite assemblages of marine fishes show a nested structure associated with host distributional range. Nestedness was tested using Brualdi-Sanderson index of discrepancy (BR); and 5 null models incorporated in a ‘Nestedness’ programme (Ulrich, 2006). The ecto- and endoparasite richness do not show similar patterns of latitudinal gradients among fish hosts, with 33–66% of analysed ectoparasite assemblages, and 25–75% of endoparasite assemblages showing nested structures through the host distributional range. For ectoparasites, species richness gradients and nested structure (when present) might be associated with decreased host densities or could reflect negative environmental conditions in the distributional border of the host species, whereas for endoparasites might be caused by geographical breaks of prey or changes in prey availability (intermediate hosts). The sampled extension of the distributional range of the host species, as well as the lack of specificity of some parasites, could influence the detection of nestedness.

Two kinds of community structure referred to, nestedness and bimodal distribution, have been observed or were searched for in parasite communities. We investigate here the relation between these two kinds of organisation, using marine fishes as a model, in order to show that parasite population dynamics may parsimoniously explain the patterns of ectoparasite species distribution and abundance. Thirty six assemblages of metazoan ectoparasites on the gills and heads of marine fish showed the following patterns: a positive relationship between abundance and the variance of abundance; a positive relationship between abundance and prevalence of infection; a bimodal pattern of the frequency distribution of prevalence of infection; nestedness as indicated by Atmar and Patterson's thermodynamic measure (a mean of 7.9°C); a unimodal distribution of prevalence in parasite assemblages with a temperature lower than the mean, and a bimodal distribution in assemblages with a temperature higher than the mean. We conclude that patterns are the result of characteristics of the parasite species themselves and that interspecific competition is not necessary to explain them. We emphasize that a holistic approach, taking all evidence jointly into account, is necessary to explain patterns of community structure. Ectoparasite assemblages of marine fish are among the animal groups that have been most thoroughly examined using many different methods, and all evidence supports the view that these animals live under non-equilibrium conditions, in largely non-saturated niche space in which interspecific competition occurs but is of little evolutionary importance.