Parasite inventories and DNA-based taxonomy: lessons from helminths of freshwater fishes in a megadiverse country.

Freshwater fish helminth parasites are undoubtedly the most well-known group among vertebrate parasites in Mexico; these parasites include 37 adult and 43 larval (metacercariae) species of trematodes, 62 monogeneans, 15 adult and 18 larval (metacestode) cestodes, 6 adult and 4 larval (cystacanth) acanthocephalans, and 54 adult and 15 larval nematodes (L3). Here, we evaluate the extent of the freshwater fish helminth inventory of Mexico using species accumulation curves and discuss the implications of DNA-based taxonomic methods in our understanding of the helminth biodiversity in freshwater fishes. Future directions for the study of the helminth parasites of freshwater fishes are proposed, particularly the role of DNA-based species delimitation criteria in recognizing parasite species, but also in discovering cryptic species. Species accumulation curves indicate that even though the asymptote has not been reached, a tendency toward stabilization is observed in all taxonomic groups except monogeneans. We suggest, therefore, that the inventory, as conventionally understood, is nearing completion for most groups. We suggest that future survey work aimed at enhancing the biodiversity inventory should be strategic, i.e., it should combine the need to target missing components of the host spectrum with the choice of appropriate drainages based on biogeographic, faunistic, and hydrologic data. We also posit that the future belongs to DNA-based taxonomic approaches that aim to uncover previously unrecognized biodiversity.