Branchial monogenean parasites (Monogenea: Dactylogyridae) of characin fishes from the Okavango river and Delta, Botswana

English: The fishes of the Okavango System, Botswana, are of considerable economical and ecological importance. Recently, scientists and local fishermen have reported a dramatic decline in the fish stocks of the Okavango River and Delta. There are many reasons for this decline to which fish parasites ~may at least be a contributing factor. In recent surveys to the Okavango, monogeneans have been found to be of the most prominent parasites infesting the Okavango fishes. The tigerfish, Hydrocynus vittatus Castelnau 1861, especially, show high infestations of the branchial monogeneans, Annulotrema pikei (Price, Peebles & Bamford 1969). The genera Annulotrema Paperna & Thurston 1969 and Characidotrema Paperna & Thurston 1968, are exclusively parasites of characiform fish in Africa. In the Okavango the characiforms are represented by five species, Brycinus lateralis ( Boulenger 1900), Hepsetus odoe (Bloch 1794), Hydrocynus vittatus, Mieralestes acutidens (Peters 1852) en Rhabdalestes maunensis (Fowler 1935), all of which, are infested by species of the genus Annulotrema and only one, Brycinus lateralis (Boulenger 1900), is infested by a species of the genus Charcidotrema. During two field trips to the Okavango System, five Annulotrema species were collected; three of which are known species and two new species. This is the first record of monogeneans from Botswana and represents the southern most distribution recorded of A. curvipenis Paperna 1969 & A. hepseti Paperna & Thurston 1969. Comparative descriptions of the three known species are given and the two new species are described as A. micralesti sp. n. and A. rhabdalesti sp. n. Mixed infestations of A. curvipenis and Characidotrema nursei Ergens 1973 were also recorded from the gills of Brycinus lateralis. This also represents the southernmost, recorded distribution of C. nursei. Investigation of histological sections revealed that although these monogeneans occurred in relatively high numbers on the gills of the fish, the pathology they caused was limited. Examination of the infestation statistics showed that these parasites exhibit site preferences to the gills on which they occurred. This preference is influenced by the number of monogeneans of the same or other species as well as the number of other parasites occurring on the gills of the fish hosts. When considering the results, these monogenean parasites do not seem to cause severe harm to the host fish. Any changes in the environment could alter the face of this parasite-host relationship dramatically and hence the potential pathogenicity of these parasites should, however, not be ignored.