Observations on the occurrence of the plerocercoids of Triaenophorus nodulosus (Pallas, 1781) (Cestoda: Pseudophyllidea) in the perch Perca fluviatilis L. of Llyn Tegid (Bala Lake), Merionethshire

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Abstract
The perch was the principal host for the plerocercoid of Triaenophorus nodulosus in Llyn Tegid.Three stages of plerocercoid development were recognized: non-encapsuled developing plerocercoids, encapsuled but living mature plerocercoids, and dead encapsuled degenerate plerocercoids.The stomach contents of the perch were analysed and planktonic crustaceans were found to be a significant part of the food from May to December. No change in diet in relation to length was found.The period of occurrence of developing plerocercoids in the perch livers was March to June. This in relation to other evidence was shown to indicate the period of infection of the perch by the procercoids.The three stages of plerocercoid development recognized were found to occur simultaneously in a few perch, and thus may represent infections acquired over three successive seasons.There was no seasonal periodicity of occurrence of the plerocercoids in the perch.All the perch examined were more or less equally infected by plerocercoids. There was no tendency for older, longer perch to be more heavily infected.One to five capsules, normally containing one plerocercoid each, occurred in the livers of the infected perch.For the perch examined there was a dynamic equilibrium between establishment of procercoids from the copepod host, and loss of plerocercoids by degeneration within the perch.The mature plerocercoids form the reservoir of infection for the definitive host of T. nodulosus, the pike.

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