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

The changes in prevalence and abundance of the three species of metacercari ae in the humour of the eyes of perch Perca fluviatilis in Slapton Ley, Dev on, have been monitored over a period of 29 years. Earlier studies had reve aled that Diplostomum gasterostei was originally the sole occupant of this niche, but Tylodelphis clavata colonised in 1073 and T. podicipina in 1976. A decline in the number of perch with heavy infections of D. gastrostri wa s significantly negatively correlated with population abundance of T. clava ta and a decline in recruitment rate of D. gasterostei coincided with the p opulation increase in T. podicipina over the period 1976 - 1979. It was sug gested that the decline in population size of D. gasterostei was due to int er-specific competition, but this hypothesis could not be tested experiment ally. Subsequent investigations, reported here, confirmed the decline when the data set was extended to 1085. A severe decline in the perch population over the winter of 1984-1985 resulted in the disappearance of D. gasterost ei and T. podicipina and this was followed by a slow recovery from 1900 onw ards. This natural experiment provided an opportunity to test the hypothesi s. Only T. clavata survived throughout the perch crash and the population c ontinued at pre-crash levels up to 1999. Its congener T. podicipina did not re-appear until 1991 land was probably a re-introduction: it did not attai n pre-crash levels until 1999. It is likely that D. gasterostei survived th e crash as it re-appeared in 1991, but was confined to young of the year fi sh and barely approached pre-crash levels even in 1999. Its continual low l evels cannot be explained by changes in the lake or in densities of snail i ntermediate 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 per iod do not refute but rather confirm earlier conclusions that inter-specifi c competition is responsible for the decline in D. gasterostei and this rem ains the preferred hypothesis.