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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.
Results
The Basic Method

Hydrobia ulvae

Hydrobia ventrosa

Onoba aculeus

Littorina saxatilis

Littorina obtusata

Littorina littorea

Discussion

References

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

Go to...

Trematodes - Life cycles
Competition between Hydrobia spp.
Regeneration in Hydrobia
The Method of Aging and Growth Measurements
Enigma of Gigantism of Snails infected with Trematodes