the reproductive lifetime of the worm. However, it is unlikely that mating
will occur soon after maturation. Attraction by pheromones could ensure
early mating and the thin layer of water, stagnant between the lower
surface of the fish and the sea bottom, would be an ideal environment for
the action of such pheromones. The authors suggest that this may explain
the preferred habitat of the worm: the lower surface of the fish. However,
pheromones have not yet been demonstrated.
Nollen (1997b) has given a review of mating and chemical attraction
in trematodes, Basch (1991) reviewed the reproductive biology in the
dioecious schistosomes and Haas (2000) included a brief section on
mating in his review of the behavioural ecology of echinostomes.In vitro
experiments by Fried and collaborators showed chemoattraction between
immature and mature adults of echinostomes. The free sterol fraction of
lipophilic extracts of the worms was found to be responsible (Friedet al.,
1980; Fried and Diaz, 1987; Fried and Haseeb, 1990). Interspecific
attraction between adult E. caproni and Echinostoma trivolvis also
occurred, but was less significant than that between individuals ofE.
trivolvis. Interestingly,E. trivolvis, which occurs along the whole small
intestine, showed greater intraspecific chemoattraction thanE. paraensei,
which is very site-specific – to the duodenum. Trouve and Coustau (1998)
demonstrated differences in the excretory–secretory products of three
strains ofE. caproni, which may be the basis of selective mating. In this
species, populations are highly subdivided along their microhabitat, the
small intestine, promoting local mate competition (Trouveet al., 1999b).
An alternative mechanism of reproductive isolation, not dependent on
mate attraction, is suggested by the findings of Trouve and Coustau
(1999). These authors examined mating between two geographical isolates
ofE. caproniand another species of the same genus. They recorded
similar attraction for intraisolate, interisolate and interspecific combina-
tions and suggested that reproductive isolation may be due to sperm selec-
tion. Some studies have demonstrated chemoattraction by neutral lipids
in mating of schistosomes (review in Nollen, 1997b).In vitro, excretion of
such lipids is increased in the presence of other worms, and males and
females that were separated and allowed to reunite paired more fre-
quently with the original partner than with new ones. Males of schisto-
somes may have more than one female in their gynaecophoric groove,
or they may share females or also hold males if there is an undersupply
of females. Also, female reproductive organs do not fully develop in
unpaired worms, and females of some species (review in Basch, 1991)
need stimulation by males to continue growth and production of eggs.
Female reproductive organs even regress in the absence of a male, but pro-
duction of eggs begins again after re-pairing. Females obtain glucose from
males, and male extracts stimulate development of female vitellaria. Some
schistosomes, at least, may also reproduce by parthenogenesis (references
in Nollen, 1997b). In the two speciesSchistosoma haematobium(primar-
ily parasitic in humans) andSchistosoma mattheei(parasitic in livestock
and wild ungulates), hybridization, leading to the production of viable178 K. Rohde