Global patterns of modularity and narrow host use in fish-parasitic copepods (Crustacea)

Abstract

Parasitic copepods are amongst the most diverse and ecologically significant parasites of fish, yet their macroecological patterns remain poorly understood. In this study, a comprehensive dataset of fish–copepod associations was obtained and curated from the World of Copepods database. After taxonomic validation and filtering, the dataset included 2296 copepod species parasitising 2690 fish species from both marine and freshwater ecosystems. Host use was quantified at species, genus and family levels, revealing a predominance of specialists in both environments. In marine ecosystems, Caligus elongatus, a species of concern for aquaculture, was identified as the most generalist copepod. In freshwater ecosystems, Lernaea cyprinacea, considered invasive in several regions, was the most generalist species. Bipartite network analysis revealed high modularity and low nestedness in both habitats, suggesting compartmentalised host–parasite structure. Robustness simulations indicated that the loss of key fish hosts, especially several threatened elasmobranchs in marine ecosystems, could disproportionately impact parasite diversity. This study highlights the importance of host identity in shaping parasitic copepod assemblages and provides a global baseline to assess ecological vulnerability in host–parasite networks.