Interspecific interactions between larval digeneans in the eyes of perch,Perca fluviatilis

Abstract

The changes in prevalence and abundance of the three species of metacercariae in the humour of the eyes of perchPerca fluviatilisin Slapton Ley, Devon, have been monitored over a period of 29 years. Earlier studies had revealed thatDiplostomum gasterosteiwas originally the sole occupant of this niche, butTylodelphys clavatacolonised in 1973 andT. podicipinain 1976. A decline in the number of perch with heavy infections ofD. gasterosteiwas significantly negatively correlated with population abundance ofT. clavataand a decline in recruitment rate ofD. gasterosteicoincided with the population increase inT. podicipinaover the period 1976–1979. It was suggested that the decline in population size ofD. gasterosteiwas due to inter-specific competition, but this hypothesis could not be tested experimentally. Subsequent investigations, reported here, confirmed the decline when the data set was extended to 1985. A severe decline in the perch population over the winter of 1984/1985 resulted in the disappearance ofD. gasterosteiandT. podicipinaand this was followed by a slow recovery from 1990 onwards. This natural experiment provided an opportunity to test the hypothesis. OnlyT. clavatasurvived throughout the perch crash and the population continued at pre-crash levels up to 1999. Its congenerT. podicipinadid not re-appear until 1994 and was probably a re-introduction: it did not attain pre-crash levels until 1999. It is likely thatD. gasterosteisurvived the crash as it re-appeared in 1991, but was confined to young of the year fish and barely approached pre-crash levels even in 1999. Its continual low levels cannot be explained by changes in the lake or in densities of snail intermediate or bird definitive hosts. New data revealed that the suspensory ligaments of the eye were the preferred site of all three species and that the eye was partitioned out between them. The data from the post-crash period do not refute but rather confirm earlier conclusions that inter-specific competition is responsible for the decline inD. gasterosteiand this remains the preferred hypothesis.