Effect of Trypanoplasma borreli infection on sperm quality and reproductive success of common carp (Cyprinus carpio L.) males

Abstract A controlled fish propagation is nowadays the fundamental prerequisite for well-managed aquaculture production. However, despite practices aimed at creating equal conditions during the spawning, the share of males giving offspring is highly diversified. In traditional pond aquaculture, the fish are exposed to various environmental factors that may affect their fertility through immune activation, among which parasites are one of the most influential. It is highly feasible that in conditions of sperm overload and short sperm-egg distance, the differences in fertilization success between males are related to sperm characteristics determined by male fitness. The present study aimed to investigate if mounting an immune response through parasitic infection affects common carp (Cyrpinus carpio L) sperm quality and reproductive success. Mature carp males were experimentally infected with a dose of 1 × 109 blood parasites Trypanoplasma borreli by intraperitoneal inoculation. Control fish were PBS-injected. Parasite number was determined at weekly intervals using a Burker counting chamber. The level of parasitaemia developed within four weeks post-infection served as the criterion to classify each individual either as parasite susceptible or resistant. On day 24 post-infection, carp males were engaged in controlled spawning. Equal volumes of quantified semen, obtained by gentle abdominal massage, were used to fertilize the egg portions obtained from four females of different genetic origin. The calculated hatchability rate was used as the indicator of the males' reproductive success. The parasite-infected males demonstrated average hatchability rates of 14.18 ± 4.83%, while the control males demonstrated 11.29 ± 3.93% rates. Parasites induced a robust adaptive antibody response, but individual antibody levels were not associated with hatchability rate. A significantly higher number of spermatozoids with damaged DNA was detected in parasite-susceptible males than in the resistant ones. The 11-ketotestosterone and most CASA parameters did not differ between parasite-infected and control fish except beat cross frequency (BCF), which was higher in infected fish (57.45 ± 18 Hz in infected vs 43.64 ± 9 Hz in controls). This CASA parameter, alongside the percentage of motile spermatozoa (MOT), was positively associated with the hatchability rate. Our study shows that in artificial spawning, a mild level of parasitaemia does not negatively influence common carp males' reproductive success. The only sperm characteristic altered by immune activation is BCF, which was correlated with hatchability in infected fish. However, the mechanism by which the activated immune system can alter BCF values and how this sperm parameter affects hatchability remains unknown and requires further investigation.