From Bilharz’s twig to thriving tree: revealing the diversity of blood flukes

Abstract Bilharz’s discovery in 1851 of the first fluke formally known to live within blood vessels initiated a very much ongoing process of discovery involving many investigators that today recognizes a blood fluke Superfamily Schistosomatoidea containing 13 families and about 95 genera. Feeding on vertebrate blood is a trait they likely inherited from common ancestors with polyopisthocotylean monogeneans, recently recognized as the sister group to the trematodes. A case has been made for considering the blood flukes to be the primordial digeneans, not only because of pre-adaptations acquired from polyopisthocotylean-like ancestors, but for other reasons as well, including their 2-host life cycles and broad patterns of host usage. Blood fluke adults are found in an unparalleled spectrum of definitive hosts, ranging from chondrichthyans, acipenseriforms, teleosts, turtles, crocodilians, and birds and mammals. Eggs produced by adult blood flukes are striking in their diverse shapes and collectively vary over an order of magnitude in size. Blood flukes also infect a greater diversity of first intermediate hosts – marine polychaete annelids, marine bivalves and a broad variety of gastropods - than any other digenean group. Their larval stages are also diverse, ranging from small, ovoid daughter sporocysts or rediae producing few cercariae, to amorphous daughter sporocysts that produce prodigious numbers of cercariae. Blood fluke cercariae range from nearly tailless non-swimming forms to forms with partially forked tails to cercariae with long forked tails well-adapted to penetrate host skin. Additionally, members of the Schistosomatidae exhibit dioecy, and have subsequently evolved a variety of alternative egg-laying strategies, bringing us back to Bilharz’s seminal discovery, of a schistosome unique or nearly so among the entire superfamily for predictably ovipositing in the bladder wall. Additional discoveries and surprises await.