Characterization of new 'white grub' parasites (Posthodiplostomum spp.) (Trematoda: Diplostomidae) in Georgia, USA, with descriptions of two new species and remarks on P. nanum Dubois, 1937.

Posthodiplostomum Dubois, 1936 is a diverse, widespread genus of diplostomoideans that parasitize as adults the intestines of piscivorous birds. Larvae of Posthodiplostomum spp. commonly cause the 'white grub' disease of fishes. During July 2024, we collected Posthodiplostomum spp. from green heron Butorides virescens and great egret Ardea alba in Georgia, USA, including two new species. We provide descriptions of both new species in the present study. Partial nuclear 28S rDNA and mitochondrial cox1 gene DNA sequences were generated for all collected species; newly obtained 28S sequences were used to study the phylogenetic interrelationships among Posthodiplostomum spp. The new species have a subspherical prosoma that is characteristic in only a few other species, notably Posthodiplostomum aztlanensis and Posthodiplostomum nanum which parasitize the same avian host species. Despite minimal morphological differences among Posthodiplostomum seminolense n. sp., Posthodiplostomum dawnsherryae n. sp., P. aztlanensis and P. nanum, these morphologically similar species differ by 0.5-5.9% in partial 28S sequences and 10.9-17.4% in partial cox1 sequences. The original description of P. nanum was based on immature specimens; we provide a new description of P. nanum based on previously sequenced specimens from a great egret in Mississippi, USA to allow better comparisons.