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Ligophorus mugilinus was originally described on the flathead mullet Mugil cephalus from the Gulf of Mexico. The species was later recorded in the Mediterranean Sea on the flathead mullet and in the Caribbean Sea on the white mullet Mugil curema. After examination of the type material and additional specimens of Ligophorus from the northwest Atlantic coast, the Caribbean Sea, and the Mediterranean and Black Sea, we observed morphological differences concerning haptoral and male copulatory structures between L. mugilinus from the northwest Atlantic and the forms from the Mediterranean and Black Seas. Accordingly, the latter specimens are described as a new species, Ligophorus mediterraneus n. sp. A redescription of L. mugilinus, including new morphometric data, is also given. The morphometric analysis of the Caribbean specimens suggested that their ascription to L. mugilinus is uncertain, but additional material is needed to clarify their specific identity. The results of this study, together with previous zoogeographical evidence, indicate that flathead mullets from different seas harbor different species complexes of Ligophorus. Given the coastal habits of the host, it is suggested that open oceans could act as geographical barriers favoring speciation within disjointed flathead mullet populations.

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A comparative morphological study of specimens of Ligophorus spp. from Mugil cephalus in western Mediterranean and the Black Sea localities has been carried out, indicating the presence of 2 distinct forms, i.e., Ligophorus chabaudi and Ligophorus cephali n. sp. A detailed description of the latter and an up-to-date redescription of L. chabaudi are provided. The existence of these 2 morphological species was additionally supported by principal component analysis based on 19 metric characters of 87 specimens arranged in samples defined by parasite species and geographical locality. The analysis indicated consistent differences between species but not between localities; this pattern was well supported by jackknife procedures. Linear discriminant analyses showed that the main metric differences between the 2 species were attributable to the lengths of the dorsal and ventral bars and to the uncinulus length; use of the first 2 variables was enough to allocate all specimens studied to either form. The erection of L. cephali n. sp. raises the number of sympatric Mediterranean species of Ligophorus on M. cephalus to 3. The known geographical ranges of these species are compared.

Dicrogaster contracta and D. perpusilla were originally described about 100 years ago from the thicklip grey mullet Chelon labrosus. Subsequent studies have reported D. contracta in several mullet species in the Mediterranean, Azov and Caspian Seas, but D. perpusilla has never been reported again. Since previous evidence suggests that the distinguishing features between these two species are uncertain, this study provides new and additional data on the variability of morphometric and meristic characters of D. contracta in order to assess their usefulness for differentiating this species from D. perpusilla. Body measurements, egg counts and shape statistics of three sets of specimens of D. contracta from different hosts and localities were compared with similar information from previous studies, including the original descriptions of D. contracta and D. perpusilla. The results showed consistent differences between the three sets in metric measurements, egg counts and shape statistics. In addition, the values of morphometric and meristic characters of D. perpusilla fell within the range of variation recorded for D. contracta. Neither conventional body measurements, eggs counts, nor shape statistics seemed useful to distinguish D. contracta from D. perpusilla, suggesting that they represent the same species. Accordingly, the valid designation of the species should be D. perpusilla because it is the type-species of the genus. This study proposes a neotype for D. perpusilla, since no type-specimens seem to be available, and provides additional morphologic data that might be useful for diagnosis of the genus Dicrogaster.