To maintain their identity, i.e. to avoid interspecific hybridization, species need isolating mechanisms, which may be differences in microhabitats, hosts, geographical area, etc. Examples for site segregation can be found in the section on microhabitat restriction (see, for example, Figs 9.1 and 9.2), and an example of geographical isolation is two species of Grubea on the gills of Scomber spp. (legend of Fig. 9.6). Further examples can be found in Rohde (1993). Intraspecific chemoattraction is another factor that may be involved. The discussion above (section on mate finding) has shown that trematodes, for example, exhibit stronger intra- than interspecific attraction, and that selection of appropriate sperm (i.e. sperm belonging to the same species) may occur. Synxenic monogeneans (i.e. monogeneans infecting the same host species) are excellent examples to demonstrate the relative importance of site segreation and morphological differences in copulatory sclerites for the maintenance of specific identity. Closely related species (usually belonging to the same genus) that possess identical copulatory organs use
different sites, whereas closely related species with markedly different copulatory organs may inhabit the same site (Figs 9.2 and 9.6). This suggests that site segregation has not evolved to avoid interspecific competition, but has the function of preventing interspecific hybridization (Rohde, 1991). These conclusions are supported by the recent study of nine species of the monogenean Dactylogyrus, coexisting on the gills of the roach, Rutilus rutilus, by Simkova et al. (2000, and further references therein). The authors found differences in the structure of copulatory organs between species aggregating interspecifically and no evidence for a major role of interspecific competition. A review of published work suggests that polyopisthocotylean Monogenea have fewer types of copulatory sclerites and, consequently, rely more on spatial segregation than monopisthocotylean Monogenea with their much greater variety of copulatory sclerites.
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