Abstract
A microsporidian species which invades the trunk muscle of yellowtail juveniles (Seriola quinqueradiata) and causes “Beko” disease was studied. The parasites appeared as multiform masses in cross sections of the muscle and usually elongated masses in longitudinal sections. The masses were bounded externally by a host-produced fibrous membrane. Almost all the parasites clearly represented stages in the sporulation sequence. Sporogony was believed to occur by multiple fission of sporogonial plasmodia. Sporocyst, sporogony vacuole and pansporoblast membrane were absent. The diplocaryon was not confirmed in any sporulation stage. Fresh spores were ovoid and measured 2.9-3.7×1.9-2.4 μm and had a coiled polar tube within the spore membrane. Extruded polar tubes were 44-52 μm in length. This species could not be classified in any established genus and therefore it seemed appropriate to place it provisionally in the collecting group Microsporidium. The name M. seriolae was proposed. The specific name refers to the genus name of the host.
