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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 hybrid-

ization (Rohde, 1991). These conclusions are supported by the recent

study of nine species of the monogeneanDactylogyrus, coexisting on the

gills of the roach,Rutilus rutilus, by Simkováet 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.

Evolutionary Maintenance of Niche Restriction

Combes and Théron (2000) discussed the mating hypothesis and two

mechanisms that may maintain niche restriction after host speciation or

host switching. Because of the importance of their study, it is discussed

here in greater detail. The mechanisms for maintaining specificity include

alloxenic speciation (i.e. speciation on different hosts) by habitat prefer-

ences (in an ‘encounter arms race’) and alloxenic speciation by assortive

survival (in a ‘compatibility arms race’). The authors point out that

specialization provides constraints and benefits. Among the first, the

most important are the limitation of resources and the increased risk of

extinction; among the second are the increased chances of outcrossing,

especially when infection intensities are low, increasing genetic diversity.

Limitation of interspecific competition and generally ‘better adaptation’

to a specialized habitat may also be important, but effects on genetic

diversity may constitute the main benefit. In an encounter arms race,

natural selection improves the probability of the parasite meeting a host

and it improves the probability of the host avoiding the infective stages of

a parasite. In a compatibility arms race, natural selection favours the

probability of survival of a parasite in or on a host and it favours the

probability of the host killing the parasite. At any time in evolution,

an ‘encounter filter’ and a ‘compatibility filter’, representing ‘crossed

Niche Restriction and Mate Finding in Vertebrate Hosts 187

Fig. 9.6. (opposite) Copulatory organs of monopisthocotylean monogeneans

infecting the gills ofLethrinus miniatuson the Great Barrier Reef, Australia (A–G),

and of polyopisthocotylean monogeneans infecting the gills ofScomberspp. (H, I).

A–C,Haliotremaspp. in overlapping microhabitats. D, F, G,Calydiscoidesspp. in

overlapping microhabitats. E,Protolamellodiscussp. on the pseudobranch. H, three

species ofKuhniaand two species ofGrubeaspatially segregated in different

microhabitats or in different geographical areas. I,Pseudokuhnia minoroverlapping

with four species ofKuhniaandGrubea. (A–G from Rohdeet al., 1994.)