Evidence of the American was introduced along with its host in Europe: Molecular and histological data.

The American pumpkinseed , was introduced to Europe more than one hundred years ago. Currently it is a common fish in European freshwaters but relatively few specific parasites infect this fish in this new habitat. In Europe only a single species, seems to represent the American myxosporean fauna of centrarchid fishes. was found in both Portugal and Hungary. This species forms plasmodia with elongated shape inside the cartilaginous rays of gill filaments. In the advanced stage of infection, after disruption of plasmodia, small groups of myxospores remain enclosed in the cartilaginous gill rays causing distortions in the filaments. Myxospores were ellipsoidal in frontal view and lemon-shape in sutural, length 12.5 ± 0.46 (12-13.4) μm, width 10 ± 0.37 (9.6-10.4) μm, and thickness 7.4 ± 0.37 (7-8) μm; the polar capsules were pyriform, equal in size, length 5.6 ± 0.21 (5.3-6) μm, width 3.2 ± 0.16 (3-3.6) μm; Seven to eight polar tube coils were arranged perpendicularly to the capsule length. There was a small, round, 0.4 ± 0.1 (0.3-05) (N = 50) intercapsular appendix in the spores. The small subunit ribosomal DNA (ssrDNA) of differed from other myxozoans sequenced to date. Phylogenetic analysis of the ssrDNA gene sequence placed this species in a clade including actinospores and species: Raabeia type1, Triactinomyxon sp., and infecting the same host fish. The focus of our study was to prove that the pumpkinseed, a fish originated from North-America introduced one of its myxosporean parasite to Europe. Emphasis was put on to demonstrate the unique feature of this parasite causing infection in the cartilaginous gill rays.