early stage. Nevertheless, specimens of this species with the cirrus
inserted into their own metraterm were seen (Fig. 4 in Rohde, 1973).
Older references (in Rohde, 1993) indicate that self-fertilization
appears to be common in the hermaphroditic cestodes, but cross-
fertilization may occur. Schärer and Wedekind (1999) found that the
facultatively self-fertilizing cestode Schizocephalus solidus produced
fewer eggs when paired, but that egg production lasted longer.
Thus, in summary, both self- and cross-fertilization occurs in the
parasitic Platyhelminthes, but some species at least do not self-fertilize,
and it is not known whether or not species for which self-fertilization has
been demonstrated need cross-fertilization over longer periods and many
generations to maintain the viability of the species.
In this context, the detailed studies of the free-living hermaphroditic
nematode Caenorhabditis elegans are relevant. Lamunyon and Ward
(1995) found that the male’s sperm outcompete the hermaphrodite’s own
sperm in mating between male and hermaphroditic worms, suggesting
that this trait maximizes outcrossing and increases genetic diversity and
heterozygosity of offspring whose parents may be highly homozygous due
to self-fertilization. However, mating occurs at a cost. Mating with males
reduces the lifespan of hermaphroditic worms for reasons not related to
egg production or receipt of sperm, whereas males do not seem to be
affected (Gems and Riddle, 1996).
It has been suggested that the nematodeStrongyloides rattirepro-
duces by pseudogamy, but Vineyet al. (1993) have shown by minisatellite
DNA fingerprinting that there is genetic exchange apparently due to
sexual reproduction.Mate Finding
Lo (1999) examined 365 humbugs,Dascyllus aruanus, a small (few cm
long) coral-reef fish, infected with a single species of gill monogenean,
Haliotremasp., and recorded the number of ‘couples’, i.e. monogeneans
showing microhabitat overlap, on fish with low intensities of infection
(fewer than five monogeneans per gill). A very high percentage of
monogeneans occurred in couples, suggesting (possibly chemical)
attraction by worms, because intraspecies microhabitat overlap should
be very scarce if coupling occurred on a random basis. Attraction seems
to be effective over relatively great distances, for example between gill
arches. Haseeb and Fried (1988) also suggested that adult monogeneans,
Diclidophora merlangi, migrate towards each other. Lo (1999) concluded
that ‘mating is a main focus of monogenean life’, supporting Rohde’s
mating hypothesis. Kearnet al. (1993) made extensive studies of the
monogeneanE. soleae, a parasite of the sole,Solea solea, in England.
They concluded that the parasite is unable to self-fertilize in the wild.
Mathematical modelling showed that random locomotion with searching
movements of the body would lead to mate finding on small fish withinNiche Restriction and Mate Finding in Vertebrate Hosts 177