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During colonial times, Nile tilapia Oreochromis niloticus (Linnaeus, 1758) was introduced into non-native parts of the Congo Basin (Democratic Republic of the Congo, DRC) for the first time. Currently, it is the most farmed cichlid in the DRC, and is present throughout the Congo Basin. Although Nile tilapia has been reported as an invasive species, documentation of historical introductions into this basin and its consequences are scant. Here, we study the genetic consequences of these introductions by genotyping 213 Nile tilapia from native and introduced regions, focusing on the Congo Basin. Additionally, 48 specimens from 16 other tilapia species were included to test for hybridization. Using RAD sequencing (27,611 single nucleotide polymorphisms), we discovered genetic admixture with other tilapia species in several morphologically identified Nile tilapia from the Congo Basin, reflecting their ability to interbreed and the potential threat they pose to the genetic integrity of native tilapias. Nile tilapia populations from the Upper Congo and those from the Middle-Lower Congo are strongly differentiated. The former show genetic similarity to Nile tilapia from the White Nile, while specimens from the Benue Basin and Lake Kariba are similar to Nile tilapia from the Middle-Lower Congo, suggesting independent introductions using different sources. We conclude that the presence of Nile tilapia in the Congo Basin results from independent introductions, reflecting the dynamic aquaculture history, and that their introduction probably leads to genetic interactions with native tilapias, which could lower their fitness. We therefore urge avoiding further introductions of Nile tilapia in non-native regions and to use native tilapias in future aquaculture efforts.

BackgroundMonogenea van Beneden, 1858 is a group of parasitic flatworms, commonly found infecting bony fish. Several genera, such asCichlidogyrusPaperna, 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 ofCichlidogyrusin the south of the Democratic Republic of the Congo (DRC) on three cichlid species:Orthochromissp. ‘Lomami’,Serranochromiscf.macrocephalus, andTilapia sparrmaniiSmith, 1840.MethodsSpecimens ofCichlidogyruswere 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.ResultsIn total, six species ofCichlidogyruswere found, all new to science:C. bulbophallusn. sp. andC. pseudozambezensisn. sp. onS.cf.macrocephalus,C. flagellumn. sp. andC. lobusn. sp. onT. sparrmanii,C. ranulan. sp. onS.cf.macrocephalusandOrthochromissp. ‘Lomami’, andC. maeandern. sp. found onOrthochromissp. ‘Lomami’ andT. sparrmanii. The first four species are considered to be strict specialists,C. ranulan. sp. an intermediate generalist andC. maeandern. 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.ConclusionsSerranochromiscf.macrocephalusandOrthochromissp. ‘Lomami’ can harbour respectively three and two species ofCichlidogyrus, all described in this study.Tilapia sparrmaniican 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.

This study represents the first exploration of the parasite fauna of cichlid fishes in the Mweru-Luapula subregion (Central Africa). Twelve species of cichlids and 14 species of Monogenea from three genera (Cichlidogyrus, Gyrodactylus and Scutogyrus) were collected. We present a first record of the gill parasite fauna of eight host species, Oreochromis mweruensis, Orthochromis sp. ‘Mambilima’, Sargochromis mellandi, Serranochromis angusticeps, S. stappersii, S. thumbergi and Tylochromis mylodon. The host range of 10 parasite species was expanded. The study further includes the description of Cichlidogyrus consobrini sp.n. from S. mellandi and Orthochromis sp. ‘Mambilima’. A new morphotype of C. halli is characterized and three species, C. papernastrema, C. quaestio and C. zambezensis are redescribed. Furthermore, the biodiversity and host-specificity of these parasites are compared with cichlid parasites from Lake Kariba and Cameroon. Two species, including C. consobrini sp.n. and a new morphotype of C. halli, are putative endemics. Additionally, the parasite fauna is highly similar in species composition to Lake Kariba, but in Bangweulu-Mweru the same parasite species are more host-specific, probably because of hydrogeographical differences between the two regions. This research was supported by the Belgian Federal Science Policy Office (BRAIN-be Pioneer Project BR/132/PI/TILAPIA), the University Development Cooperation of the Flemish Interuniversity Council (VLIR-UOS, South Initiative Renforcement des capacites locales pour une meilleure evaluation biologique des impacts miniers au Katanga (RDC) sur les poissons et leurs milieux aquatiques, ZRDC2014MP084) and the Mbisa Congo project, a framework agreement project of the RMCA with the Belgian Development Cooperation. M.P.M.V. was supported by a travel grant from the Research Foundation - Flanders (FWO-Vlaanderen) and Czech Science Foundation project no. P505/12/G112 (ECIP). Moise Katumbi Chapwe is cordially thanked for his kind hospitality and support. Also E. Abwe, B. Katemo Manda and C. Mukwene Mulele are cordially thanked for their help with the fish sampling, and W. Fannes, M. Kasongo Ilunga Kayaba and C. Kalombo Kabalika for their effort in parasite preparation. M. Barson, T. Dube and the technical team at the University of Zimbabwe Lake Kariba Research Station are thanked for delivering fresh gills of S. macrocephalus from Lake Kariba, Zimbabwe; J.-L. Justine (MNHN) and A.J. Phillips (Smithsonian) for the loan of type material; I. Přikrylová for sharing her expertise on Gyrodactylus; M. Van Steenberge for his valuable input regarding this ecoregion and its ichthyofauna; A. Henrard for his help with stacking images; G. Cael for his help with constructing fig. 1; and two anonymous referees for their valuable comments.

Within a comparative morphological framework, Hippopotamyrus aelsbroecki, only known from the holotype originating from Lubumbashi, most probably the Lubumbashi River, a left bank subaffluent of the Luapula River, is reallocated to the genus Cyphomyrus. This transfer is motivated by the fact that H. aelsbroecki possesses a rounded or vaulted predorsal profile, an insertion of the dorsal fin far anterior to the level of the insertion of the anal fin, and a compact, laterally compressed and deep body. In addition, a new species of Cyphomyrus is described from the Lufira basin, Cyphomyrus lufirae. Cyphomyrus lufirae was collected in large parts of the Middle Lufira, upstream of the Kyubo Falls and just downstream of these falls in the lower Lufira and its nearby left bank affluent, the Luvilombo River. The new species is distinguished from all its congeners, that is, firstly, from C. aelsbroecki, C. cubangoensis and C. discorhynchus, by a low number of dorsal fin rays, 27-32 (vs. higher, 36 (37), 34 (33-41) an 38 (38-40), respectively) and, secondly, from C. aelsbroecki, C. cubangoensis, and C. discorhynchus by a large prepelvic distance, 41.0-43.8% LS (vs. shorter, 39.7%, 38.9-39.1% and 37.0-41.0% LS , respectively). The description of yet another new species for the Upemba National Park and the Kundelungu National Park further highlights their importance for fish protection and conservation in the area. Hence, there is an urgent need for the full integration of fish into the management plans of these parks.