Sphaerospora oncorhynchi n.sp. (Myxosporea: Sphaerosporidae) from the kidney of sockeye salmon (Oncorhynchus nerka) in British Columbia and its possible relationship to the myxosporean causing proliferative kidney disease in salmonid fishes

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Abstract
Sphaerospora oncorhynchi n.sp. is described from sockeye salmon (Oncorhynchus nerka) from Great Central Lake, Vancouver Island, British Columbia, Canada. Subspherical spores and monosporous pseudoplasmodia were observed in the lumen of renal tubules. Pseudoplasmodia are oval or elongate and range from 5 to 15 μm in the greatest dimension. Spores are 8.2–10.2 μm (length) × 7.8–9.2 μm (width, in sutural plane) × 10.2–11.2 μm (thickness, perpendicular to sutural plane), with two spherical polar capsules of equal size measuring 2.9–3.3 μm in diameter and containing polar filaments with 4–5 coils. The spore contains two uninucleate sporoplasms and is usually surrounded by a degenerated pseudoplasmodium. The type locality for S. oncorhynchi is enzootic for proliferative kidney disease of salmonid fishes. This disease is caused by the PKX organisms, the extrasporogonic stage of an unidentified myxosporean similar to species of Sphaerospora. Sphaerospora oncorhynchi and the PKX organism are similar in that monosporous sporogenesis occurs within the lumen of renal tubules, and the trophozoites and pseudoplasmodia in early sporogenesis of the two parasites are indistinguishable. Furthermore, Sphaerospora spores very similar to those of S. oncorhynchi have been observed in salmonids from two localities in the U.S.A. that are enzootic for proliferative kidney disease. Therefore, the PKX myxosporean in North America may be an extrasporogonic form of S. oncorhynchi n.sp.

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