acanthocephalans and monogeneans. Kamegai (1986) reported that juve-
niles of the monogeneanDiplozoon nipponicumonCarassius carassius
auratusare found on all gills until the fourth day of infection; then they
gather on one gill and copulate for life (further examples in Rohde, 1994).
Some recent studies with information on the mating hypothesis are
discussed in the following. Geetset al. (1997) tested the importance of
interspecific competition and intraspecific factors in ectoparasites of the
white-spotted rabbit-fish,Siganus sutor, on the Kenyan coast and found
that intraspecific factors are much more important than interspecific
factors in choosing microhabitats. They specifically tested the mating
hypothesis in two species of Monogenea and found that niche restriction
on the gills leads to higher intraspecific contact, consistent with the
hypothesis. The finding of Kearnet al. (1993), discussed above, that niche
restriction in a monogenean infecting European sole may play a role in
allowing chemoattraction of mates also supports the hypothesis.
Echinostome trematodes use small parts of the intestine and this is
thought to contribute to mate finding, although chemical communication
also plays a role, as shown by the studies of Nollen (see above). In this
context, the observation is important thatE. trivolvis, which occurs along
the whole small intestine, showed greater intraspecific chemoattraction
thanE. paraensei, which is very site-specific – to the duodenum. Accord-
ing to Sukhdeo and Mettrick (1987) and Sukhdeo and Sukhdeo (1994),
non-directed activation-dependent or contact-dependent mechanisms
may be more important than chemotaxis during migration within hosts;
for mate finding, this would imply an even greater role of microhabitat
restriction.
Nollen (1996b) found thatE. paraenseilives in the duodenum andE.
caproniin the ileum of mice, into which they were transplanted. Never-
theless, 25% of the worms of both species were found within 1 cm of each
other. Lack of interspecies mating, demonstrated by autoradiographic
studies, cannot therefore be due to microhabitat segregation alone.
Adamson and Caira (1994) have argued that the mating hypothesis
implies greater site specificity in gynochoristic than in hermaphroditic
and particularly selfing hermaphroditic species. However, we have seen
above that, even in (selfing) hermaphrodites, cross-fertilization occurs
and that it may be important in all hermaphroditic species over many
generations. It could be argued that in hermaphrodites site restriction is
even more important than in dioecious species, in order to force them
into at least occasional cross-fertilization. Adamson and Caira (1994)
further argued that greater mobility and greater densities of species with
increased microhabitat might be expected even without selection for
greater mating success. These arguments are less convincing if species are
compared that belong to the same parasite group and live in the same
habitat. For example, metacercariae almost always have wide micro-
habitats in their vertebrate hosts, but adult didymozoid trematodes, also
parasitic in the tissues of vertebrates, often have extremely restricted
microhabitats; monogeneans that are permanently sessile generally have184 K. Rohde