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A total of 110 specimens of the Nile catfish Chrysichlhys awalus were caught from Damietta Branch of the River Nile during spring and summer 2002 and 2003 and surveyed for monogenean gill parasites. Only a single highly host specific monogenean, namely Protoancylodiscoides mansourcnsis El-Naggar, 1987 was encountered. Out of the fish examined. 2224 monogenean worms were recovered. The parasite showed an aggregated or contagious pattern of distribution within the host population. The parasite attained high prevalence (97,27 %). abundance (20.22± 21.74) and mean intensity (20.79 ± 22.03) values. The parasite preferred larger hosts than smaller ones. Spearman Correlation Coefficient showed a positive relationship between host length or weight and intensity of/. mansourensis. The skeletal support of the holobranch is provided with a powerful sieve or a complex trapping system, being formed of interlocking epidermal semi circular folds alternated with fold-free interspaces. Several gill filaments were branched once, twice and even three times to produce additional filaments, probably to increase the gas exchange capacity in the oxygen-poor bottom of the river bed. The present model showed marked microhabitat availability as hundreds of thousands of attachment sites (each comprising two adjacent gill lamellae and an interlamellar channel) accessible for monogenean infestation were calculated on the host gills. The distribution of the parasite on the gills was non-random. The parasite showed a gradual decline in the percentage distribution from the first (outermost) to the fourth (innermost) gill arch (holobranch) as well as 332 Ahmed M. El-Naggar and Enayat S. Reda from the dorsal to the ventral gill segments. The parasite preferred the proximal half of the gill filaments over the distal. However, there was no significant difference in the percentage distribution of P. mansouremls between the left and right hand gill sets or between the anterior and posterior hemibranchs. Factors contributing to the microhabitat preference of the parasite on the gill map and those regulating its population growth among host individuals of different length or weight classes are discussed in detail.

egyptian journal of aquatic biology and fisheries

INFESTATION LEVEL AND SPATIAL DISTRIBUTION OF PROTOANCYLODISCOIDES MANSOVRENSIS EL-NAGGAR 1987, A MONOGENEAN GILL PARASITE FROM THE LONG FIN CATFISH CHRYSICHTHYSAURATUS GEOFFROY, 1809