الفهرس 
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Abstract
I- Taxonomical studies In the present study, detailed observations have been done to identify the monogenean fauna of the gills, skin and fins of the Egyptian catfish Clarias gariepinus inhabiting three differing water quality sites namely :Manzala Lake (freshwater), Nawasa El-Gheit drain (agricultural drainage water) and Demietta branch of the River Nile (freshwater), Nile Delta, Dakahlia Governorate, Egypt. Eight monogeneans belonging to three genera were identified from these localities. These monogeneans are Quadriacanthus clariadis Paperna 1961, Quadriacanthus kearni El-Naggar and Serag 1985, Quadriacanthus aegypticus El-Naggar and Serag 1986, Macrogyrodactylus congolensis Prudhoe, 1957, Macrogyrodactylus clarii Gussev, 1961, Gyrodactylus rysavyi Ergens 1973, Gyrodactylus groschafti Ergens 1973 and Gyrodactylus sp. II- Ecological studiesA total of 36 surface water samples were collected from three sources with different water qualities in Nile Delta, Dakahlia Governorate, during the period from March, 2017 to February, 2018. Water samples were analyzed for some physical and chemical environmental parameters as well as some heavy metals. The physical parameters include water temperature (oC), Hydrogen Ion concentration (pH), Electrical Conductivity (EC) and Dissolved Oxygen (DO) while the chemical parameters include Bicarbonates (HCO3−), Chlorides (Cl−), Sulphate (SO4+2), Total Dissolved Solids (TDS), minerals (Sodium Na+, Potassium K+, Calcium Ca+2 and Magnesium Mg+2), Nitrogen (N) and Phosphorus (P) content. The heavy metals include Cadmium (Cd), Lead (Pb), Copper (Cu), Iron (Fe) and Zinc (Zn). Collectively, it appears that Manzala Lake harbored the highest number (1577) of monogenean parasites followed by Nawasa El-Gheit drain (1201) and finally Demietta branch of the River Nile (810).In pooled data, all Quadriacanthus species are eudominant (D<10%) and other monogeneans are subrecedent (>1%). However, Q. aegypticus was the most eudominant followed by Q. kearni and lastly Q. clariadis. The dominance of both Q. aegypticus and Q. kearni over Q. clariadis could be attributed to the morphological features of the reproductive organs of these monogeneans particularly the copulatory apparatus and vaginal tube.
III- Mode of attachment and local histopathological impacts of Quadriacanthus spp. In the present study, a detailed account has been given of the normal structure of non-infested gills of the catfish Clarias gariepinus, mode of attachment of Quadriacanthus species, their local histopathological impacts and responses of the host gill tissues. This is the first study to be done on the mode of attachment and histopathological impacts of Quadriacanthus monogenean worms. The majority of adult worms insert their haptor between two adjacent gill lamellae, others to the gill filaments particularly in the interlamellar spaces and a few attach to the top (distal) part of the gill lamella. In all cases, the dorsal and ventral hamuli, dorsal and ventral bars, and marginal hooklets share in securing attachment. The posterior glandular system of Quadriacanthus species may play a significant role in attachment. The mode of attachment of Quadriacanthus species has been discussed with respect to that of other monogeneans possessing similar attachment system.