This study is the first attempt to survey the diversity of fish digeneans in the Gulf of Gabes (southern coast of Tunisia). A total of 779 fishes belonging to 32 species were sampled. 53 species of Digenea belonging to 15 families were recorded. Among these species, 24 are reported for the first time from the coast of Tunisia. We report one new host record, Lecithochirium sp. from Sardinella aurita. The Hemiuridae is the dominant family. A host-parasite list is presented with the information on the prevalence, abundance and mean intensity of each species collected. The diversity of Digenea is compared with other localities in the Mediterranean Sea and the northern east of Tunisia. The Gulf of Gabes shows the lowest diversity linked to the anthropogenic activities and impact of exotic species. The use of Digenea as indicators of the state of the ecosystem is discussed.

Oesophagotrema mediterranea n. gen., n. sp. (Digenea, Zoogonidae) is described from the needlefi sh Tylosurus acus imperialis (Beloniformes, Belonidae) off the Tunisian coast. Oesophagotrema n. gen. is distinguished from all other Lepidophyllinae genera by the combination of a prominent rounded ventral sucker, caeca reaching to the anterior edge of the posterior testis, testes arranged in tandem in the middle third of the hindbody and the vitellarium in symmetrical lateral bunches located between the ovary and anterior testis. Th is species shows a typical specifi c site of infection within the digestive tract, i.e., the oesophagus and vomer teeth. Th e position of this zoogonid digenean on the oesophagus is discussed.

Pseudodiplectanum syrticum n. sp. (Monogenea: Diplectanidae) is described from the gills of Synapturichthys kleinii (Risso) collected from the Gulf of Gabès in the Mediterranean Sea off Tunisia. The new species differs from the congeneric species by the morphology of the penis and vagina. It resembles the Mediterranean species P. kearni Vala, Lopez-Roman & Boudaoud, 1980 from Solea vulgaris Quensel and P. gibsoni Oliver, 1987 from Michrochirus variegatus (Donovan) in its tubular penis, but it differs from these two species by having a vagina with a long tube presenting two loops and a male copulatory organ ending in a curved point. It also differs in the morphology and size of the transverse bars of the haptor. In P. syrticum the ventral transverse bar and dorso-lateral bars are longer than those of P. gibsoni (114 vs 69.5-89 microm and 48 vs 33-44 microm, respectively).

Nudaciraxine imperium sp. n. (Monogenea: Axinidae) is described from the gills of the needlefish Tylosurus acus imperialis (Rafinesque) caught in the gulf of gabès, Tunisia. This new species mainly differs from both previously described species, Nudaciraxine gracilis (Linton, 1940) Price, 1962 from Strongylura marina (Walbaum) and Nudaciraxine cabosanlucensis Payne, 1990 from Ablennes sp., by having more testes (42 vs. 20-22 and 32, respectively). Nudaciraxine imperium also differs from N. gracilis by having a median vaginal pore (submedian in N. gracilis), narrower clamps (52-70 µm vs. 75-100 µm) and differently shaped haptoral lateral hamuli. From N. cabosanlucensis it can be distinguished by a J-shaped rather than U-shaped ovary, a longer cirrus pouch (125-190 µm vs. 64-70 µm), an oesophageal bifurcation slightly anterior to genital complex (immediately after pharynx in N. cabosanlucensis) and a wider guard on the lateral hamuli. In agreement with Payne (1990), the diagnosis of the genus Nudaciraxine Price, 1962 is amended as follows: vaginal pore dorsal, median to submedian, armed or not with horn-like spines. This is the first report of the genus Nudaciraxine in the Mediterranean Sea. The potential use of axinid monogeneans as biological tags for subspecies of belonid fishes is discussed. During a recent parasitological study on fishes in the gulf of gabès (Tunisia), three species of Monogenea were collected from T. a. imperialis: Aspinatrium gallieni Euzet et Ktari, 1971 (Microcotylidae) on the inner side of the operculum and also two additional previously undescribed axinid species on the gills. In this paper we describe one of these, the most abundant axinid monogenean, as a new species of Nudaciraxine. The potential use of axinid monogenean as biological tags for subspecies of belonid fishes Tylosurus is discussed. Keywords MAteRIALs AND MethoDsBetween May and July of 2004, 2006 and 2007, 74 specimens of T. a. imperialis caught in the gulf of gabès (Tunisia) by local fishermen using gill nets were examined. This fish inhabits offshore waters, but can be found in coastal waters during spring and summer. Specimens were identified using Bauchot (1987).Fish were dissected and examined a few hours after capture. The operculum and gill arches were separated and placed in Petri dishes with filtered seawater using filter paper. Monogeneans were detected using a stereomicroscope, detached from the gills and operculum and then transferred to a dish containing filtered seawater. They were studied either alive or fixed between slide and coverslip in 70% alcohol. Fixed specimens were stained with Semichon's acetic carmine. Other specimens were double-stained with light green and Semichon's acetic carmine to study the morphology of the clamps. After dehydration

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