Parasite Diversity in Freshwater Fish

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.