the total number of receptors was estimated to be about 20,000–40,000
(Rohde, 1989). All stages in the life cycle of this species are parasitic (with
the exception of the egg) (Rohde, 1973); the assumption is therefore
justified that the receptors are important in microhabitat finding, feeding
and/or mating, but, again, functional studies of the receptors have not
been made.
The free-living nematodeC. eleganshas been studied in much greater
detail than parasitic ones. Mating involves a series of steps; by laser
microbeam ablation of male-specific copulatory organs and their
associated neurons, it was possible to identify sensory structures and
neurons responsible for each step (Liu and Sternberg, 1995). Using stereo
images of the amphids (the main chemoreceptory organs of nematodes)
ofStrongyloides stercoralis, three-dimensional reconstructions could be
made, as previously done forC. elegans(Ashton and Schad, 1996; also
Ashtonet al., 1999). A role in host finding and the control of development
has been suggested, but a role in mating is also possible. Ashtonet al.
(1999) have reviewed chemo- and thermosensory neurons in several
parasitic nematodes. These neurons are thought to play a role in host
finding and development. The interpretation is based on comparisons
with studies ofC. elegans. Neurons involved in mating have apparently
not been identified in parasitic nematodes.
In the parasitic copepod L. salmonis, ablation of the tip of the
antennules, which carry many receptors (Grestyet al., 1993), reduces
mating success. However, the mechanism of action and the sensory roles
of the setae on the antennules are not known. Also, even ablated males
still formed pairs, although pair formation and mating were delayed (Hull
et al., 1998).The Role of Niche Restriction in Facilitating Mating
Rohde (1979, also 1994, and further references therein) gave the following
evidence for the mating hypothesis of niche restriction: host ranges and
microhabitats of parasites on the gills (and of other parasites) are often
extremely restricted, although competing species are not present; species
with good locomotory ability often have larger microhabitats than less
mobile species; asexual or larval stages often have wider microhabitats
than adult, sexually reproducing stages; species that can establish large
populations often have wider microhabitats than less populous species;
and species often aggregate, i.e. reduce their microhabitat width, at
the time of mating. Examples for the effect of locomotory ability are
differences in microhabitat width of mobile and sessile monogeneans
and copepods parasitizing fish; examples for differences in microhabitat
width of asexual/larval and sexual stages are larvalTrichinella, protozoan
cysts on the gills and metacercariae vs. adult Trichinella, adult
Monogenea and Trematoda; and examples for the effect of population size
are many gill Monogenea. Examples of aggregation for mating are someNiche Restriction and Mate Finding in Vertebrate Hosts 183