Gymnurahemecus bulbosus gen. et sp. nov. (Digenea: Aporocotylidae) infecting smooth butterfly rays, Gymnura micrura (Myliobatiformes: Gymnuridae) in the northern Gulf of Mexico, with a taxonomic key and further evidence for monophyly of chondrichthyan blood flukes.

Gymnurahemecus bulbosus gen. et sp. nov. infects the heart of smooth butterfly rays, Gymnura micrura in the Gulf of Mexico. Gymnurahemecus differs from all other accepted aporocotylid genera by having one column of C-shaped lateral tegumental spines, a medial oesophageal bulb anterior to a diverticulate region of the oesophagus, inverse U-shaped intestinal caeca, a non-looped testis, an oviducal ampulla, a Laurer's canal, and a post-caecal common genital pore. The new species, the shark blood flukes (Selachohemecus spp. and Hyperandrotrema spp.), and the chimaera blood fluke Chimaerohemecus trondheimensis are unique by having C-shaped lateral tegumental spines. Selachohemecus spp. and the new species have a single column of lateral tegumental spines, whereas Hyperandrotrema spp. and C. trondheimensis have 2-7 columns of lateral tegumental spines. The new species differs from Selachohemecus spp. most notably by having an inverse U-shaped intestine. The other ray blood flukes (Orchispirium heterovitellatum, Myliobaticola richardheardi, and Ogawaia glaucostegi) differ from the new species by lacking lateral tegumental spines, a medial oesophageal bulb, and a Laurer's canal and by having a looped testis. Phylogenetic analysis using large subunit ribosomal DNA (28S) indicated that the new species is sister to the clade that includes the other sequenced adult blood fluke (O. glaucostegi), which infects a ray in Australia. These results agree with and extend previous morphology- and nucleotide-based phylogenetic assertions that the blood flukes of early-branching jawed craniates (Chondrichthyes) are monophyletic and phylogenetically separated from the blood flukes of later-branching ray-finned fishes (Actinopterygii: Euteleostei).