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Enigma of Growth Alteration
in Snails Infected with Trematodes
 The
change of snails' growth pattern following the infestation with
the parthenogenetic generation of Trematodes has been known since
Wesenberg - Lund (1934) noted that infected snails attained a larger
size than uninfected ones. The classical laboratory study by Rothschild
and Rothschild (1939) on the growth of Hydrobia ulvae confirmed
that the growth rate increased following infestation. This phenomenon,
called "gigantism", has become quite an enigma. The phase of initial
collecting of phenomenological data had been finished in the early
1980's. It is now clear that infestation by trematode parthenites
may not only accelerate growth but also induce stunting effects
on growth rate (Moose, 1963; McCleland, & Bourns, 1969; Sturrock,
& Sturrock, 1970; Meuleman, 1972; Sluiters et al., 1980; Wilson,
Denison, 1980; Huxham et al., 1993). The existing hypotheses — 'adaptive’
(Minchella,1985) and 'nonadaptive’ (Sousa,1983)
- attempting to explain the alterations in host growth operate with
life history variations of snail species and predict different growth
responses to infestation in semelparous and iteroparous molluscs.
Most of the observations forming the basis of those views involved
comparisons of the growth of healthy snails with ones infected with
single species of parthenitae. Few researchers have adopted the
approach of Sousa (1983) — comparing a growth response of certain
snail host following infestation with different trematode species.
However, this approach looks quite productive. For example, it has
been demonstrated that the growth response of infected Hydrobia
ulvae varies significantly with the trematode species. Most
probably due to its lower pathogenicity, sporocistoid parthenitae
induce increased host growth in contrast to redioid ones which cause
no effect on growth rate (Gorbushin, 1997). This phenomenon produces
the serious objection to both 'adaptive’ and 'nonadaptive’ hypotheses.
In short, the differences between growth responses of infected semelparous
and iteroparous snails may not be the result of different life histories
of ones but that of different pathogenicity of parthenitae parasiting
in these groups of hosts.
Only four studies have investigated
the influence of trematodes on snail growth using a field approach
(Hughes, Answer, 1982; Sousa, 1983; Fernandez, & Esch, 1991a;
Huxham et al., 1995). All but one (Huxham et al., 1995) failed to
show gigantism under field conditions. This fact made Fernandez
and Esch (1991a) describe the phenomenon as a laboratory artefact.
Huxham et al. (1995) found that all large specimens of H.ulvae
in the Ytha estuary, northeast Scotland were infected. However,
this result can not be considered as conclusive evidence for gigantism
as itself (see Sousa, 1983). Whether or not gigantism is naturally
occurring phenomenon Fernandez and Esch (1991a) speculate on the
principal question.
The purpose of this study was to investigate
the species - specific, population and region dependent effects
of trematode parthenites on the growth pattern of six gastropod
species, using a new method that allows an estimation of the growth
rate of snails under natural conditions.
The investigation was supported financially
by the Russian Foundation
for Basic Research (RFBR) grant N 96-04-48-965 and by program
'Universities of Russia', grant N 3918
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