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Background: Monogenea van Beneden, 1858 is a group of parasitic fatworms, commonly found infecting bony fsh. Several genera, such as Cichlidogyrus Paperna, 1960, are reported to include potential pathogenic species that can negatively impact aquaculture fsh stocks. They can switch from introduced to native fsh and vice versa. In Africa (and all over the world), fsh species belonging to Cichlidae are often kept in aquaculture and represent a major source of food. Thus, research on the biodiversity and occurrence of monogenean species on these fsh is of importance for aquaculture and conservation. The present study is a survey of the diversity of species of Cichlidogyrus in the south of the Democratic Republic of the Congo (DRC) on three cichlid species: Orthochromis sp. 'Lomami', Serranochromis cf. macrocephalus, and Tilapia sparrmanii Smith, 1840. Methods: Specimens of Cichlidogyrus were isolated from the gills and mounted on glass slides with Hoyer's medium. The genital and haptoral hard parts were measured and drawn using interference contrast. Results: In total, six species of Cichlidogyrus were found, all new to science: C. bulbophallus n. sp. and C. pseudozambezensis n. sp. on S. cf. macrocephalus, C. fagellum n. sp. and C. lobus n. sp. on T. sparrmanii, C. ranula n. sp. on S. cf. macrocephalus and Orthochromis sp. 'Lomami', and C. maeander n. sp. found on Orthochromis sp. 'Lomami' and T. sparrmanii. The frst four species are considered to be strict specialists, C. ranula n. sp. an intermediate generalist and C. maeander n. sp. a generalist. These parasite species show morphological similarities to species found in the Lower Guinea and Zambezi ichthyofaunal provinces, which might be explained by past river capture events between river systems of the Congo Province and both these regions. Conclusions: Serranochromis cf. macrocephalus and Orthochromis sp. 'Lomami' can harbour respectively three and two species of Cichlidogyrus, all described in this study. Tilapia sparrmanii can harbour seven species, of which three are described in the present study. These results highlight the species diversity of this parasite genus in the Congo Basin. ; The research leading to the results presented in this publication was partly carried out with infrastructure funded by EMBRC Belgium - FWO project GOH3817N. This study was also supported by the Belgian Federal Science Policy Ofce (BRAIN-be Pioneer Project BR/132/PI/TILAPIA), Czech Science Foundation Project No. P505/12/G112 (ECIP) and the "MbiSa Congo" project (2013–2018), a framework agreement project of the RMCA with the Belgian Development Cooperation. MG is supported by the Bijzonder Onderzoeksfonds (BOF-Project 7NI02) of Hasselt University for the analysis, interpretation of data and the writing of the manuscript

Background Monogenea van Beneden, 1858 is a group of parasitic flatworms, commonly found infecting bony fish. Several genera, such as Cichlidogyrus Paperna, 1960, are reported to include potential pathogenic species that can negatively impact aquaculture fish stocks. They can switch from introduced to native fish and vice versa. In Africa (and all over the world), fish species belonging to Cichlidae are often kept in aquaculture and represent a major source of food. Thus, research on the biodiversity and occurrence of monogenean species on these fish is of importance for aquaculture and conservation. The present study is a survey of the diversity of species of Cichlidogyrus in the south of the Democratic Republic of the Congo (DRC) on three cichlid species: Orthochromis sp. 'Lomami', Serranochromis cf. macrocephalus, and Tilapia sparrmanii Smith, 1840. Methods Specimens of Cichlidogyrus were isolated from the gills and mounted on glass slides with Hoyer's medium. The genital and haptoral hard parts were measured and drawn using interference contrast. Results In total, six species of Cichlidogyrus were found, all new to science: C. bulbophallus n. sp. and C. pseudozambezensis n. sp. on S. cf. macrocephalus, C. flagellum n. sp. and C. lobus n. sp. on T. sparrmanii, C. ranula n. sp. on S. cf. macrocephalus and Orthochromis sp. 'Lomami', and C. maeander n. sp. found on Orthochromis sp. 'Lomami' and T. sparrmanii. The first four species are considered to be strict specialists, C. ranula n. sp. an intermediate generalist and C. maeander n. sp. a generalist. These parasite species show morphological similarities to species found in the Lower Guinea and Zambezi ichthyofaunal provinces, which might be explained by past river capture events between river systems of the Congo Province and both these regions. Conclusions Serranochromis cf. macrocephalus and Orthochromis sp. 'Lomami' can harbour respectively three and two species of Cichlidogyrus, all described in this study. Tilapia sparrmanii can harbour seven species, of which three are described in the present study. These results highlight the species diversity of this parasite genus in the Congo Basin. ; Peer reviewed

In most tropical regions, the Nile tilapia (Oreochromis niloticus, Cichlidae) has been introduced for aquaculture purposes, making it one of the most widely-introduced species. Adversely, it has escaped aquaculture and became invasive in many places with detrimental effects on the local fauna. It is a fast grower and aggressive species that can hybridize with indigenous cichlids and indigenous cichlid species decline after introduction of Nile tilapia. However, little is known of the detrimental effects of co-introduction of parasites from Nile tilapia and possible parasite spillover and spillback to and from indigenous cichlids. Parasites can strongly influence the fitness and competitiveness of infected individuals and species. Therefore, it is important to monitor host switching between native and invasive fish species. The most prevalent gill parasites on Nile tilapia are Monogenea (Platyhelminthes). These are obligate parasites which have a direct lifecycle (no intermediate hosts), are species rich and generally host-specific. They mainly infect gills or skin of fish. These aspects make Monogenea ideal candidates for host switching, because they only require a single host species in the invasive area and are easily transferred. Also, because monogeneans are host-specific each host species hosts its own unique fauna of parasites, thus providing a distinguishable character between host species. The goal of this research is to trace co-introduction of Nile tilapia parasites and subsequent host switching to indigenous cichlids. To trace this, the pre-introduction (or native) parasite fauna is reconstructed by sampling hosts stored in museum collections (baseline data). Contemporary samples from the same localities are compared with the collected baseline data, which allows for a reliable tracing of host switching events. We present results from two ecoregions, Bas-Congo and Bangwuelu-Mweru, both within the Democratic Republic of the Congo. ; Belspo