The relationship between the widespread and common oligochaete Chaetogaster limnaei limnaei (CLL) and its snail hosts is usually considered commensal or mutualistic. However, its effects on snails have rarely been measured. We used laboratory experiments to examine the fitness-relevant behavior, growth, and reproductive output parameters of host snails relative to different degrees of CLL infestation. The pulmonate freshwater snail Physa acuta was used as the host species. At high infestation rates, snails used a smaller proportion of their time for foraging and a larger proportion for resting. Consequently, the infested snails had lower growth rates. Moreover, their overall reproductive output was reduced and their reproductive strategy was shifted toward producing a larger number of egg masses that contained fewer and smaller eggs. Furthermore, fewer of these eggs survived. Based on our results, the relationship between CLL and P. acuta can be described as epizoic antibiosis at high infestation rates. Our study demonstrates the context dependency of the relationship between CLL and their hosts because other investigators found that snails can profit from CLL infestation in the presence of parasitic trematodes or remain unaffected by CLL. CLL could be a much more important factor influencing snail communities than previously assumed given this context-specific potential to influence the fitness of its hosts strongly and its host-species selectivity.
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