Microhabitats of Pseudodactylogyrus anguillae and P. bini (Monogenea: Dactylogyridae) on the gills of large-size European eel Anguilla anguilla from Lake Gaj, Poland

A study on the spatial distribution of two congeneric monogenean species Pseudodactylogyrus anguillae Yin et Sproston, 1948 and P. bini Kikuchi, 1929 on the gills of large size European eel Anguilla anguilla (L.) were conducted. Results were analysed with regard to: single-species infection, mixed infection and general occurrence of the parasites in the eel population. Statistical analysis revealed that the distribution of these species on the gill apparatus is fairly similar and the zones in which they occur more numerously coincide to a great extent. However, a number of differences were found. Both the possibility of simultaneous occurrence on the host and similar distribution of the above species on the eel’s gills indicate the reciprocal tolerance of these parasites. Data on the relations between different monogeneans inhabiting the same host and on their distribution on the gills can be obtained from several works (Llewellyn and Owen 1960, Owen 1963, Paperna 1964, Hanek and Fernando 1978, Buchmann 1988a,b, 1989a, Dzika and Szymański 1989, Bashirullah and Rodriguez 1992, Koskivaara and Valtonen 1992, Dzika 1993). The aim of this study was to determine the microhabitats of two congeneric species Pseudodactylogyrus anguillae Yin et Sproston, 1948 and Pseudodactylogyrus bini Kiknuchi, 1929 on the gills of large size of the European eel Anguilla anguilla (L.) from Lake Gaj in Poland. Both mixed and single-species infections were considered. MATERIALS AND METHODS Lake Gaj is a natural reservoir, without outflow, where maximum depth reaches 4 m and in which the density of eel population is high. There is about 15,000 eel per ha (P. Gomulka, pers. commun.). Eel gills were sampled in October 1994 and in May 1995. A total of 86 eel were examined with total body length from 32.5 to 54.0 cm (mean 38.4 cm) and weight from 50.0 to 167.0 g (mean 103.4 g). Monogeneans were collected from the gills after fixation of the gill arches from the right and the left sides in 4% formalin solution. Each gill was placed in the Petri dish with distilled water. Mucus was removed from the gills with a preparation needle (magnification 20-40 ×). Each branchial arch was divided into 6 equal parts, obtaining 24 sectors from four gill arches of one side (Fig. 1). Parasites were collected from each sector separately, fixed in Faure’s fluid, then identified to species on the basis of skeletal elements (Gusev 1985). The distribution of Pseudodactylogyrus monogeneans on particular arches and on parts of the branchial apparatus was analysed by nonparametric statisticss tests: Mann-Whitney U test and Kruskal-Wallis ANOVA, median test, both in relation to all examined fish and in relation to single and mixed infection.