Patterns of diversity and host specificity in the cestodes of Neotropical freshwater stingrays

The rivers of South America are home to several animal groups of marine origin. These include freshwater stingrays (Potamotrygonidae), and their metazoan parasites such as cestodes, which were represented by 22 species prior to this project. This project was undertaken to (1) Investigate the diversity and host specificity of cestodes of freshwater stingrays, (2) Expand geographic ranges and stingray taxa examined for intestinal helminths, (3) Explore the phylogenetic relationships among species in the cestode genus Rhinebothrium. The intestinal helminth diversity of Paratrygon aiereba, Potamotrygon. motoro, and Potamotrygon cf castexi in Peru was found to include two digenean species, five nematode species, and 12 cestode species, six of which are new. Hyperparasitic larval cestodes found inside adult cestodes were identified as proteocephalideans based on histological and molecular sequence data. Sampling was conducted from ∼500 stingrays representing ∼29 (including 16 new) potamotrygonid species from multiple rivers in the Amazon and Rio de La Plata River Basins. Histological, light and scanning electron microscopic analyses revealed at least six new species of Rhinebothrium. Three were described and R. paratrygoni was redescribed. Rhinebothrium species varied in their host specificity, each parasitizing, on average, three host species. Rhinebothrium species in South America was found to be restricted to river basins, but not to individual rivers. Sequence data generated for portions of the LSU and CO I genes generally supported the species boundaries determined with discrete morphological characters. However, sequence data revealed little divergence between two pairs of species that had been distinguished with continuous morphological features. Phylogenetic analyses suggested that (1) Rhinebothrium species parasitizing potamotrygonids in South America are monophyletic, but the clade also includes the endemic genus Rhinebothroides, (2) The most basal lineage of Rhinebothrium in South America is represented by a new species that is restricted to the lower Amazon, (3) The identity of the marine sister taxon of Rhinebothrium species in South America remains unclear. Cestode diversity in potamotrygonids, and Rhinebothrium in particular, is greater than previously documented, and host specificity of Rhinebothrium species, while greater than previously documented, is more relaxed than that observed in marine tetraphyllidean cestodes. The number of cestode species parasitizing Neotropical freshwater stingrays now exceeds 30.