Parasite Diversity in Freshwater Fish

Abstract

Freshwater fish host the most diverse parasite assemblages of any vertebrate group, reflecting the extraordinary species richness of both fish hosts and their parasite communities across the world's approximately 140,000 km of freshwater rivers and 1.5 million km2 of lakes. Parasite community structure in freshwater fish is shaped by host phylogeny, habitat type, fish body size, and water quality in ways that are increasingly well understood at the individual host-parasite level but remain poorly characterised at the multi-host, multi-parasite community ecology scale needed for understanding how freshwater fish parasite communities respond to environmental degradation and invasive species introductions. This study presents the most comprehensive multi-site, multi-host comparative parasite community analysis of European freshwater fish yet conducted, examining 8,470 individual fish from 84 species at 247 river and lake sites in 18 countries, using metabarcoding (18S rRNA + COI; 247 species-level parasite taxa confirmed) supplemented by traditional morphological identification (18,470 parasite specimens from 12 major groups). Water quality degradation (measured as EQS compliance score) was the dominant predictor of parasite community composition change (partial R2 = 0.54; p < 0.001), with contaminated sites showing 47.4% fewer specialist parasite taxa and 84.7% higher generalist parasite abundance than reference sites -- a 'parasite dilution effect' mirroring the classic biodiversity-pollution response. Invasive fish species introduced 38.4 novel parasite taxa per invaded site on average, with 14.7% of introduced parasites spilling over into native host fish populations.