Date of Award

January 2021

Document Type


Degree Name

Doctor of Philosophy (PhD)



First Advisor

Vasyl V. Tkach


The Diplostomoidea is a large superfamily of digeneans which possess a unique holdfast organ. Members of the superfamily are distributed worldwide and are known to parasitize a wide variety of animals, both invertebrates and vertebrates. In some cases, diplostomoideans have been associated with diseases such as ocular diplostomiasis and ‘black spot’ disease in fishes. The taxonomic and systematic history of diplostomoideans is complex and includes numerous revisions based on morphology and host associations. Prior to this study, the Diplostomoidea included 6 families and 16 subfamilies. Recent molecular phylogenetic analyses have revealed the Diplostomidae and the Strigeidae to be non-monophyletic and demonstrated a need for re-evaluation of the group. In the present study, diplostomoideans were collected from a diversity of intermediate and definitive hosts from around the world which resulted in the most comprehensive sample set to date. Digenean specimens were studied using morphological and molecular tools (primarily molecular phylogenies of the 28S rRNA and cytochrome c oxidase mtDNA genes) to study the interrelationships of diplostomoidean taxa, host-parasite relationships and diversity of taxa. Our results clearly demonstrate the non-monophyly of the Cyathocotylidae, Diplostomidae and Strigeidae and support the monophyletic status of the Proterodiplostomidae. Based on re-evaluation of morphological characters and results of phylogenetic analysis of partial 28S sequence, the Brauninidae is considered a junior synonym of the Cyathocotylidae. Further, molecular phylogenies were used to re-evaluate the system of the Proterodiplostomidae. Among other findings, the current subfamily system of the Proterodiplostomidae was rejected. The results of morphological and molecular study clearly demonstrates that the diversity of diplostomoidean taxa has been underestimated, including species likely associated with ‘black spot’ disease in fish. In total, we described 1 new subfamily, 3 new genera and 5 new species of diplostomoideans with descriptions of many additional new taxa pending. Molecular phylogenetic analyses demonstrated numerous host-switching events during the evolutionary history of the Diplostomoidea along with evidence of multiple dispersal events between biogeographic realms.