On the bionomics ofCrepidostomum metoecus(Braun, 1900) andCrepidostomum farionis(Müller, 1784) (Trematoda: Allocreadiidae)

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Abstract
The life-cycles of the two species ofCrepidostomumoccurring in a stream in North Wales are investigated. Both species share the same arthropod and definitive hosts, viz.Gammarus pulexandSalmo trutta, respectively, but have different molluscan hosts, viz.:Lymnaea peregraforC. metoecusandPisidium casertanumforC. farionis.C. metoecusandC. farionishave annual cycles, hence periodicity of occurrence in their hosts. InC. metoecusthe dynamics of occurrence in the three hosts are correlated. The inverse relationship inG. pulexandS. truttaas well as the existence of a time-lag of 3–4 months between the establishment of this worm inG. pulexand inS. truttaare discussed.Neither premunition nor age-resistance occurs in the parasitization ofS. truttaandG. pulexbyC. metoecus. The increase in the rate and degree of infestation with the age and length of fish, except in the oldest and longest group, is shown to be due to trophic factors.C. metoecusis more abundant and widely distributed in the stream thanC. farionis. This fact is associated with the occurrence of their molluscan hosts.Experimental observations were made on the stages in the life-history ofC. metoecus, including the mode of emergence of the cercaria, invasion of shrimps and encystment, as well as development of the metacercariae inG. pulex.More cercariae are shed by day and at higher temperatures, and they live for about 5 days at 8 °C. The metacercaria undergoes considerable development before becoming infective.C. metoecusdoes extensive damage toL. peregra, has no observed ill-effect on trout, and apparently affects the life-span of shrimps adversely only in cases of very heavy infestations.The non-parasitization ofCottus gobioby the two flukes is probably due to the prevailing ecological factors.My thanks are due to Professor H. B. N. Hynes and Dr J. C. Chubb for many helpful criticisms at all stages of this study; to Miss Joan Venn for helping with the sampling programme; and the Commonwealth Scholarship Commission in the United Kingdom for financial support.