LIVONECA SP. (FLABELLIFERA, CYMOTHOIDAE), AN ISOPOD PARASITE IN THE ORAL AND BRANCHIAL CAVITIES OF THE RED-SEA SILVERSIDE ATHERINOMORUS-LACUNOSUS (PERCIFORMES, ATHERINIDAE)

A parasitic isopod, Livoneca sp. (Flabellifera: Cymothoidae), is repor ted from silverside Atherinomorus lacunosus in the Red Sea. The female isopod settles on the fish's tongue. Presence of a gravid female in t he oral cavity was always associated with that of a single, smaller ma le occupying the gill chamber. Approximately 3.6 % of the fish examine d were found to be infested with the parasites in the adult phase. His tological damage to the fish appears limited to the gill rakers and fi laments upon which the female isopod rests her abdomen and tail. The g ill filaments above the site in which the male settles become similarl y dystrophic. However, none of the fish infested displayed any obvious sign of respiratory distress or stunted growth. The chitinous segment s of the isopod armoring, imbricated like roof tiles with their free m argin directed backward, seem perfectly adapted to performing the same gripping function as the fish's tongue. This host-parasite relationsh ip appears to be species-specific and has all the characteristics of a n evolutionary process of adaptive strategy, as it clearly enables the host to maintain its feeding ability while the parasite grows to sexu al maturity.