Co-existence of nine gill ectoparasites (Dactylogyrus : Monogenea) parasitising the roach (Rutilus rutilus L.): history and present ecology

Co-existence among potentially competing species can be favoured by niche s pecialisation and/or by reducing the overall intensity of competition via a ggregated utilisation of fragmented resources. We investigated the respecti ve roles of niche specialisation and aggregation in the case of nine congen eric monogenean parasites on the gills of Roach (Rutilus rutilus L.) belong ing to the genus Dactylogyrus. The position of each individual parasite of the nine Dactylogyrus species was recorded. Niche breadth and niche overlap of parasite species were estimated. Comparative methods, which take into a ccount phylogenetic information of the analysed species, were used. We reco nstructed a phylogeny of the nine Dactylogyrus species based on morphologic al characters. We used the 'aggregation model of co-existence' in the model to test if species co-existence is facilitated when intraspecific aggregat ion exceeds interspecific aggregation. We observed a lack of negative corre lation in abundance between pairs of parasites, and a negative correlation between niche size and parasite aggregation, for both intraspecific and int erspecific aggregation. Our comparative analysis showed that parasite abund ance is positively correlated with niche breadth. Then parasite abundance, and not interactions between Dactylogyrus species, seems to be the most imp ortant factor determining niche size This result gives some support to nich e segregation by specialisation. Niche size was negatively correlated with both intraspecific and interspecific aggregation No relationship was found between an increase of interspecific aggregation with an increase of niche overlapping, which suggests that competition may play little role. A lack o f competition could be also confirmed by the lack of negative correlation i n abundance between species pairs. A parsimony analysis of the evolution of gill distribution indicates a change in one parameter of the niche (arch, segment and/or area) at each branching event. (C) 2000 Australian Society f or Parasitology Inc. Published by Elsevier Science Ltd. All rights reserved .