Ecology of a diplozoon parasite on the gills of the African cyprinid Barbus neumayeri

In oxygen‐deficient waters, the difficulties of oxygen uptake in gill parasites and their fish hosts may influence host and parasite densities, site selection by the parasite, and effects of the parasite on host condition. This study quantified the prevalence and intensity of the gill monogenean Neodiplozoon polycotyleus in the African cyprinid fish Barbus neumayeri from an intermittent forest stream in western Uganda. Oxygen levels were low in the stream over the 12‐month study, averaging only 2.5 mg litre^?1 (monthly range = 1.2–4.3 mg litre^?1). However, parasite prevalence was high (47.2%), suggesting high tolerance to low oxygen in N. polycotyleus. The prevalence of parasites varied with host body size, with the highest frequency of occurrence in the middle size classes. Prevalence also varied over the year; seasonal peaks of rainfall coincided with a lower frequency of N. polycotyleus. The significantly nonrandom frequency distribution of parasites among hosts suggests regulation of parasite numbers. Of the hosts infected, 37.1% harboured one N. polycotyleus parasite, and 62.9% harboured two parasites. No fish were infected with more than two diplozoons. There was evidence for strong site specificity by N. polycotyleus within hosts; 77.7% of the parasites were located on the filaments of the second gill arch, which may relate to increased oxygen availability. In addition, only one of the 178 infected fish had more than one parasite on one side of the branchial basket. Although N. polycotyleus is undoubtedly parasitic, we found no evidence of a negative parasitic effect on the condition or reproductive status of B. neumayeri.