Myxozoans are a group of diverse, spore-forming metazoan microparasites bound to aquatic environments. Sphaerospora dykovae (previously S. renicola) causes renal sphaerosporosis and acute swim bladder inflammation (SBI) in juvenile Cyprinus carpio carpio, in central Europe. A morphologically similar species with comparably low pathogenicity, S. angulata has been described from C. c. carpio, Carassius auratus auratus and Carassius gibelio. To clarify uncertainties and ambiguities in taxon identification in these hosts we decided to re-investigate differences in spore morphology using a statistical approach, in combination with SSU and LSU rDNA sequence analyses. We found that developing spores of S. angulata and S. dykovae cannot be distinguished morphologically and designed a duplex PCR assay for the cryptic species that demonstrated S. dykovae is specific to C. c. carpio, whereas S. angulata infects C. a. auratus and C. gibelio. The molecular identification of myxozoan blood stages in common carp and goldfish, which had previously been ascribed to Sphaerospora spp. showed that approximately 75% of blood stages were from non-sphaerosporid coelozoic species infecting these cyprinids and more than 10% were from an alien species, Myxobilatus gasterostei, developing in sticklebacks. We hereby report non-selective myxozoan host invasion and multi-species infections, whose role in SBI still requires clarification.

The Cyprinidae is the largest freshwater fish family in Vietnam, with over 220 recognised species, many of which play an important role in aquaculture or are harvested from the wild. Despite numerous studies on the taxonomy of this family based on traditional morphological data, the relationships between major cyprinid groups is poorly understood and the taxonomic validity of a number of these groups is under debate. While an increasing number of molecular studies on cyprinid relationships have been conducted many have used restricted sampling and none have incorporated Vietnamese species. In this study, mitochondrial 16S rRNA, D-loop and cytochrome b gene sequences from 25 species of cyprinids collected from Vietnam were obtained and combined with sequences of cyprinids available in GenBank, in order to investigate the taxonomic validity of subfamilies within Cyprinidae and their phylogenetic relationships. The molecular data supported traditional division of the Cyprinidae into two major lineages: Cyprinines and Leuciscines. The placement of the Danioninae as the sister lineage to this grouping was not supported. Many of the subfamily boundaries were questioned and doubt was raised on some of the generic level classifications. The validity of species designation in Cyprinus, Tor and Cyclocheilichthys was also questioned. This study will need to be extended with greater taxon and gene sampling to further consolidate our understanding of cyprinid relationships and classification.