RESULTS: The morphologies of the myxospores from Icelandic eels were very similar but the overall dimensions were significantly different from the various tissue locations. Myxospores from the kidney of the Malaysian tarpon, Megalops cyprinoides (Broussonet), were noticeably smaller. However, the SSU rDNA sequences from the different tissues locations in eels, were all very distinct, with percentage similarities ranging from 92.93% to as low as 89.8%, with the sequence from Malaysia being even more dissimilar. Molecular phylogenies consistently placed these sequences together in a clade that we refer to as the Paramyxidium clade that is strongly associated with the Myxidium clade (sensu stricto). We erect the genus Paramyxidium n. g. (Myxidiidae) to accommodate these histozoic taxa, and transfer Myxidium giardi as Paramyxidium giardi Cépède, 1906 n. comb. as the type-species.
CONCLUSIONS: There is not a single species of Myxidium (M. giardi) causing systemic infections in eels in Iceland. There are three species, confirmed with a robust phylogeny, one of which represents Paramyxidium giardi n. comb. Additional species probably exist that infect different tissues in the eel and the site of infection in the host fish is an important diagnostic feature for this group (Paramyxidium n. g. clade). Myxospore morphology is generally conserved in the Paramyxidium clade, although actual spore dimensions can vary between some species. Paramyxidium spp. are currently only known to infect fishes from the Elopomorpha.
PURPOSE: The purpose is to describe the new species morphologically and molecularly and provide new information of its evolutionally relationships with other species of the subgenus.
METHODS: Standard methods of collection and examination of marine hosts, processing and illustrating of specimens, and taxonomic identification of parasites using the extensive collection of the lead author were used. Specimens were further studied using energy-dispersive X-ray analysis and ion sectioning of hooks, SEM analysis, and molecular sequencing. Type specimens were deposited at the Harold W. Manter Lab. collection, Lincoln, Nebraska.
RESULTS: Acanthogyrus (Acanthosentis) fusiformis n. sp. is described from the catfish, Arius sp. (Ariidae: Siluriformes) off the Pacific Coast of Vietnam at Bac Lieu in the Gulf of Thailand. The three other marine Indian species include A. (A.) arii Bilqees, 1971 which is also described from a similar catfish, Arius serratus Day off the Karachi coast in the Arabian Sea, Indian Ocean. Our new species from Vietnam is distinguished from the other 46 species by a combination of characters including a small fusiform trunk, complete circles of small hollow spines covering the entire trunk, prominent double apical organs often extending posteriorly past posterior hooks, middle and posterior hooks of equal size slightly smaller than anterior hooks, large neck continuous with the outline of the proboscis without distinct separation, big drop-shaped cephalic ganglion, extension of the proboscis receptacle anteriorly past the base of the proboscis up to the insertion point of the posterior hooks, presence of two para-receptacle structures (PRSs), free unattached thick lemnisci, short female reproductive system with filamentous attachment of the distal end of the uterine bell to the ventral body wall, and small narrowly ellipsoid eggs with thickened polar ends. Partial sequences of the 18S and internal transcribed spacers (ITS1-5.8S-ITS2) of ribosomal RNA were generated and used for phylogenetic analyses to confirm the taxonomic identity of Acanthogyrus (Acanthosentis) fusiformis n. sp.
CONCLUSIONS: We describe unique morphological features of A. fusiformis never before known in the subgenus Acanthosentis. The uniqueness of A. fusiformis is further demonstrated by its EDXA fingerprint characterized by high levels of calcium and phosphorous in hooks. The zoogeography of species of Acanthosentis is elucidated in the Indian subcontinent, the Caribbean, China, and Africa. Molecular data have been available only in few species of Acanthogyrus (Acanthosentis) to date on GenBank database. For 18S, only two sequences from unknown Acanthosentis sp. from India are available, while for the ITS1-5.8S-ITS2 region, only sequences of A. cheni from China and of two unidentified species from Malaysia are available. Additional studies of species of Acanthosentis based on morphological and molecular genetic data will be needed to reconstruct the evolutionary history and phylogenetic affinities of this group of acanthocephalans.