Increased reproductive investment in an exploited African cyprinid fish following invasion by a castrating parasite in Lake Nyasa, Tanzania

Abstract

Evolutionary theory predicts that a host infected with a parasite that reduces its future survival and/or fecundity should be selected towards an increased investment in current reproduction. In this study we use the cestode Ligula intestinalis and its intermediate fish host Engraulicypris sardella in Wissman Bay, Lake Nyasa (Tanzania) as a model system. Using data about infestation of E. sardella fish hosts by L. intestinalis collected for a period of 10 years, we explored (i) whether parasite infection affects the fecundity of the fish host E. sardella and (ii) whether host reproductive investment has increased at the expense of growth at maturity. We found that L. intestinalis had a strong negative effect on the fecundity of its intermediate fish host. For the non-infected fish we observed an increase in relative gonadal weight over the study period, while somatic growth at maturity decreased during the study period. Our findings suggest that the cestode L. intestinalis might have selected for increased investment in current reproduction in its fish host E. sardella in Lake Nyasa. This calls for further studies to assess whether these changes reflect plastic or evolutionary responses. We also discuss the synergy between parasite and fishery-mediated selections as a possible explanation for the decline of E. sardella stock in the lake. KEYWORDS Engraulicypris sardella; Ligula intestinalis; life history evolution; selective agents; parasitism; fishing.