Systematics, biogeography and character evolution in the tapeworm genus Acanthobothrium van Beneden, 1850

Acanthobothrium is a specious and ubiquitous genus of elasmobranch tapeworm. It is particularly intriguing because of its widespread geographic distribution and its host range, the breadth of which is unparalleled in any other tetraphyllidean genus. Prior to this project, 150 species of Acanthobothrium had been described from regions across the globe. The number of described species is, however, likely a gross underestimate of the true diversity because there exist many elasmobranch hosts and geographic regions that have never been sampled for Acanthobothrium. This project was undertaken to (1) Investigate the geographic distribution and overall host range of Acanthobothrium, (2) Discover and describe a suite of Acanthobothrium species new to science, and (3) Generate a molecular phylogenetic hypothesis for a representative subset of species in the genus. Sampling was conducted from 33 elasmobranch species from 26 geographic localities around the globe. Species were prepared for histological, light and scanning electron microscopic analyses. This work revealed at least 42 new species of Acanthobothrium, 11 of which were formally described. Sequence data were generated for 49 Acanthobothrium species for portions of the 28S rDNA gene and 16S rDNA gene. Results of the phylogenetic analyses suggest that (1) The genus Acanthobothrium is essentially monophyletic, (2) Morphologically determined species are generally corroborated by molecular data, (3) Synhospitalic congeners are more commonly not found to be each other's closest relatives, (4) Acanthobothrium species parasitizing the freshwater stingrays of South America appear to be the result of a single colonization event, (5) Co-speciation has not been the predominant mechanism of evolution in Acanthobothrium, (6) The presence of Acanthobothrium in sharks is the result of multiple colonization events, (7) Acanthobothrium species parasitizing elasmobranchs in a given geographic region are closely related to one another despite their disparate host associations, and (8) Morphological characters used in species determinations are extremely homoplasious. A number of new character states for Acanthobothrium species were discovered during the morphological analyses of this study. The results of this study indicate that many Acanthobothrium species have yet to be discovered, the majority of which likely parasitize batoids in the tropical regions of the world. ^