Why do Nematodes Select Narrow Microhabitats in the Host?
Nematode site (microhabitat) selection is so predictable and so precise
that the location of a species within the host gut is a robust taxonomic
character. These worms are found in all regions of the gut, from nose to
anus, but their specific distributions are typically very narrow, a phenom-
enon generally referred to as niche restriction (Crompton, 1973; Holmes,
1973; Rohde, 1979). Even within discrete organs in the gut, the worms are
very selective (Wertheim, 1970). For example, pinworm species were long
thought to share the same habitat within the rectum, but, in a study where
eight cohabiting species of pinworms were partitioned both linearly and
radially in the rectum of their turtle host, there was little overlap among
the species (Schad, 1963).
One of the earliest explanations for the precision of habitat selection
was proffered by Holmes (1961, 1973). He felt that niche restriction was a
legacy of past interspecific competition. In this scenario, two parasite
species initially competing for the same habitats would, over time,
segregate into distinct niches (microhabitats) through ecological and
physiological specialization (Hair and Holmes, 1975). Eventually, as
competing species became extinct for one reason or other, the surviving
worms would remain in their narrow niches. There were several
criticisms of this model. Apart from the difficulties of dealing empirically
with the ‘ghost of competition past’, it was not clear why narrow habitats
did not expand as the competitors became extinct. Studies of habitat
selection in both guts and gills suggested that many potential parasite
habitats were unoccupied (Rohde, 1979; Rohde and Hobbs, 1986).
Rohde argued that competition for food and space did not adequately
explain habitat specificity, because of the many available habitats (Rohde,
1981; Rohde and Hobbs, 1986), and suggested that the reason for the
restriction to narrow niches was to increase intraspecific contact and
facilitate mating (Rohde, 1977, 1994). Clearly, at the low densities typical
of most parasite infections, this behaviour would be adaptive, because
it would ensure that the sexes got close enough for sexual attraction
to occur. However, there are also some problems with this hypothesis.
For example, it predicts that dioecious worms will be more restricted in
their habitats than hermaphroditic worms, and this is not necessarily so
(Adamson and Caira, 1994). In addition, some parasitic nematodes mate
before reaching their final habitats. For example, femaleDracunculus
medinensis (human guinea-worm) mate with their males before they
migrate to their specific habitats in the subcutaneous tissue of the leg
(Roberts and Janovy, 2000). In this parasite, the final site is determined by
the need to shed infective propagules through a cutaneous ulcer in the
human foot, and not by the need to encounter mates.
There have been additional suggestions that parasite habitat selection
might be driven by many other factors, including the evolution of parasite224 M.V.K. Sukhdeoet al.