Darwin’s nightmare revisited: a dual decline of cichlids and their parasites

Abstract

Lake Victoria (East Africa) earned its nickname – Darwin’s Dreampond – because of its extraordinary biodiversity and high levels of endemism, particularly among haplochromine cichlids. Since the 1980s, the lake has experienced severe environmental degradation driven by anthropogenic activities (e.g. species introductions, water eutrophication, overfishing, and climate change). This contributed to the rapid decline of haplochromines, with about half of the species becoming extinct or nearly extinct. Such environmental degradation and decline in cichlid diversity are expected to alter host-parasite interactions and, consequently, impact parasite communities. Because parasites depend both on their host(s) and on environmental conditions, they can serve as bioindicators to detect ecosystem changes. We used natural history collections to assess parasites infecting the gills of six haplochromine species (Haplochromis degeni, H. heusinkveldi, H. laparogramma, H. piceatus, H. pyrrhocephalus, H. tanaos). These zooplanktivorous fish species are demersal, living on muddy bottoms, and two of them (H. heusinkveldi, H. piceatus) are listed as threatened on the IUCN Red List. About 300 fish specimens were originally collected between 1978 and 2011 in Lake Victoria. This allowed us to reconstruct temporal changes in ectoparasite communities across three ecological periods: pristine (pre-disturbance), perturbed (during major environmental degradation), and recovery (post-2000 partial rebound). The studied fish harboured three species of parasitic flatworms (Cichlidogyrus bifurcatus, C. furu, C. nyanza, the two latter endemic to the basin), two copepods (Ergasilus lamellifer, Lamproglena monodi) and the larval stage of bivalves. We observed a decrease in parasite abundance and in the number of parasite species per host individual during the perturbed period, coinciding with peak environmental stress. Although some recovery was observed after 2000, parasite abundance did not return to pre-disturbance levels. Our results indicate that parasite communities are negatively impacted by (human-induced) pressures and environmental changes. The timing of parasite decline and partial recovery matches that of ecosystem changes in Lake Victoria, supporting the idea of parasites as indicators of ecosystem status. Our study highlights the value of historical collections for parasitology and emphasizes the need to include parasites in fish monitoring and in conservation strategies. This will enable the preservation of both parasite diversity and the ecosystem services they support (e.g. energy transfer through trophic interactions, regulation of populations, promotion od of host immune development).