Light and scanning electron microscopy observations on Gnathia vorax (Isopoda: Gnathiidae) larvae

A large population of gnathid isopods (Crustacea) has been observed on sea water teleost fishes coming from the Civic Aquarium of Messina (Italy). Gnathiidae are parasite of fishes only at the stage of praniza, while adults are free living. Several teleosts, reared in captivity at the Civic Aquarium of Messina were sent for parasitological examination over a period of three years. Numerous parasites, identified as pranizae of Gnathiidae, were found on dorsum, on dorsal, pectoral and caudal fins, as well as in oro-pharyngeal cavity and gills of the affected fishes. The cranial part of an exuvie of adult male belonging to Gnathia sp. was also found on the floor of the tank. Because adult parasites are rarely detected in the sand bottom, the taxonomic identification is based on the morphological characteristics of larvae, even if it is very hard to show interspecific differences among gnathiids. For this reason, the morphological study was performed also by SEM to better show the surface details with taxonomic meaning. One to five mm long pranizae had a flattened body characterised by a head followed by 5 thoracic and 6 abdominal segments and a caudal plate with two biarticulated uropods. In the oral part of the head there was a ventrally oriented and smooth trapezoidal rostrum. Oral appendages, not clearly visible by light microscopy, were: a) two jaws with denticulate internal sides longer than the rostrum; b) a single ipostoma apically divided into two long appendixes; c) long and thin maxillae, with denticulate distal margins; d) two paired maxillipedes showing ciliate internal side that reaches the extremity of buccal cone; e) two big gnatopods with apical nail. Flagellum of antennulae had 7 articles. Telson was triangularly shaped, longer than large, with truncated apex having two apical bristles. Uropods were short with rounded internal angle and were provided by smooth and plumose setae; endopod was larger than exopod. The jaw of the adult gnathiid male, studied only by light microscopy, was solid, with oblique and denticulated blade; long and thin apex; rounded internal lobe; prominent external spur. Morphological characteristics of the jaw of the adult male and the shape of larvae permitted us to identify our specimens as Gnathia vorax (Lucas, 1850).