201 Chaetotaxy of the monogeneans Macrogyrodactylus clarii and M. congolensis from the gills and skin of the catfish Clarias gariepinus in Egypt, with a note on argentophilic elements in the nervous system Mohammed M. El-Naggar 1 , Safaa Z. Arafa 1 , Sameer A. El-Abbassy 1 and Graham C. Kearn 2 1 Zoology Department, Faculty of Sciences, Mansoura University, Mansoura, Egypt; 2 School of Biological Sciences, University of East Anglia, Norwich NR4 7TJ, UK Key words: Monogenea, Macrogyrodactylus clarii, Macrogyrodactylus congolensis, surface sensilla, nervous system, chaetotaxy, Clarias gariepinus Abstract. A comparison has been made between the chaetotaxy of the gyrodactylid monogeneans Macrogyrodactylus clarii Gussev, 1961 and M. congolensis (Prudhoe, 1957) Yamaguti, 1963 from the gills and skin, respectively, of the catfish Clarias gariepinus (Burchell) from the river Nile in Egypt. Bilaterally arranged argentophilic structures on the surface of these parasites are presumed to be sensilla and are more abundant in M. clarii than in M. congolensis especially on the ventral surface (124 vs. 66). In both species these sensilla are concentrated on the head lobes and in the pharyngeal region, but there are features of the sensilla patterns that can be used to distinguish the two species. Comparison is made with sensilla patterns of other gyrodactylids. A system of cells and dendritic processes, most probably part of the nervous system, also has an affinity for silver in the two species. There are no previous records of extensive argentophilic elements in the nervous systems of monogeneans. The silver nitrate staining technique of Lynch (1933) is an effective way of visualising cell junctions on the surface of monogeneans. These animals are covered by a syncytial tegument, which is not argentophilic. How- ever, wherever cellular structures breach the tegument, the intercellular material between the tegumental limiting membrane and the cell membrane of the intruding structure appear to have strong affinities for silver. Intruding cellular structures include ciliated or unciliated nerve endings, which are presumed to be sensory (sensilla), gland duct terminations and openings of the excretory system, reproductive system and gut. Most previous studies have concentrated on the distribution of sensilla in monogenean larvae (onco- miracidia) (see, for example, Combes and Lambert 1972, Lambert 1977a, b, 1978a, b, Tinsley 1978). Studies of the distribution patterns of surface sensilla in adult monogeneans are few (Lambert 1979, El-Naggar et al. 1993, Khidr and El-Naggar 1996, Shinn et al. 1997, 1998a, b). Chaetotaxy maps have been used by some authors (Maeder 1973, Shinn et al. 1997, 1998a, b) as a diagnostic tool for the discrimination of different species of monogeneans. The finding in Egypt of Macrogyrodactylus clarii Gussev, 1961 on the gills and M. congolensis (Prudhoe, 1957) Yamaguti, 1963 on the skin of the catfish, Clarias gariepinus (Burchell), provided an opportunity to compare the chaetotaxy of two related gyrodactylids occupying different micro- habitats on the same host. MATERIALS AND METHODS Specimens of the catfish Clarias gariepinus were caught from the Demietta branch of the river Nile near Mansoura, Egypt. Fishes were kept alive until required in a tank containing aerated river water. The head of each fish was cut using a knife blade and the whole body of the fish including the head was placed in dissecting dish containing river water. The gills were removed and searched for Macrogyrodactylus clarii and the fins and body surface were also examined for M. congolensis. The silver nitrate staining technique of Lynch (1933) was applied to about 20 living, unflattened specimens of each species. Living parasites were washed several times in river water and expelled from a pipette into a shallow dish containing hot (70-80°C) 0.5-1% silver nitrate solution. The parasites were then placed beneath a tungsten lamp for about 15 min. When argentophilic features of the parasites were sufficiently well defined, the parasites were removed and washed in several changes of distilled water. They were dehydrated in an ascending series of ethanol, cleared in xylol, mounted in Canada balsam and examined using the light microscope. RESULTS After exposure of silver-nitrate-treated parasites to light, small dark brown rings, assumed to be sensilla, appear on their surfaces. In each species, the distribution of these sensilla is bilaterally symmetrical. The most common distribution patterns in M. clarii and in M. con- FOLIA PARASITOLOGICA 48: 201-208, 2001 Address for correspondence: S.Z. Arafa, Zoology Department, Mansoura University, Mansoura, Egypt. Phone: ++20 50 2231 306; Fax: ++20 50 2246 781; E-mail: safaa19@hotmail.com