offspring, occurs in experimentally infected mice (Southgateet al., 1995),
indicating that mate attraction is not limited to males and females of
the same species. However, hybridization in nature is highly unlikely,
considering the different hosts. Also, specific mate choice systems do
not appear to exist in some other combinations of differentSchistosoma
species, whereas they do in others (Southgateet al., 1998). Nevertheless,
overall, most schistosomes infecting the same final host maintain their
genetic identity (Southgateet al., 1998).
Among the acanthocephalans, endoparasites of vertebrates, aggre-
gation for mating is known to occur in some species. In a study
on Acanthocephaloides propinquus infecting the gobiid fish Gobius
bucchichii, Sasalet al. (2000) determined infection intensities, sex ratios
and testicular size and found that, when the percentage of males
increased (suggesting increased competition between males), the size of
the testes increased as well. The authors also concluded that competition
for access to females was more important than competition for space.
Numerous studies of mating of nematodes have been made, but most
of them deal with the free-livingC. elegans. Copulation involves a series
of complicated steps (e.g. Liu and Sternberg, 1995), copulatory plugs are
used to assure paternity (Barker, 1994), and even mutations that affect
neurons involved in certain copulatory behaviour patterns have been
identified (Loer and Kenyon, 1993). InT. spiralis, differences in distrib-
utions of mixed, single male and single female infections suggest that
males migrate in search of females (Sukhdeo and Bansemir, 1996), and
experimental studies have indeed shown that males and females are
attracted to each other (references in Sukhdeo and Bansemir, 1996). The
same authors report that, when females and males ofHeligmosomoides
polygyrusare transplanted together into the terminal ileum of mice,
males migrate faster to the duodenum (their normal habitat) than females,
leaving the females behind, implying that males choose habitat over
female. This is hardly an argument against the importance of mate
selection, since mating occurs once both sexes are established in the
duodenum.
Mating behaviour of copepods has been studied in a number of
species, but most of them are free-living. Sophisticated behaviour patterns
lead to mate encounters, and males show distance chemoreception, as
well as recognition at close range, which may involve fluid mechanism
signals of short duration and contact chemical signals. i.e. species- and
sex-specific glycoproteins (Boxshall, 1998, and references therein).
Precopulatory mate-guarding appears to be common not only in free-
living species, but in parasitic species as well (e.g. in Lernaeocera
branchialis (Heuch and Schram, 1996)). The mating behaviour of
Lepeophtheirus salmonis and some other parasitic copepods was
described by Anstensrud (1990a–d, 1992) and Ritchieet al. (1996). The
latter study suggests that long-range, short-range and contact chemical
stimuli play a role in pairing and mate recognition, either on their own or
jointly. It must be emphasized that sea lice are highly mobile, both on andNiche Restriction and Mate Finding in Vertebrate Hosts 179