Ichthyosporidium weissii n. sp. (Microsporidia) infecting the arrow goby (Clevelandia ios).

THE first description of an Ichthyosporidium species was by Thelohan (1895) who described in the corkwing wrasse Crenilabrus melops, the presence of large parasitic masses that occupied most of the abdominal cavity and appeared to originate from the renal connective tissue. Considering the parasite to belong to the Microsporidia, Thelohan (1895) named the parasite Glugea gigantea but did not describe developmental stages of the parasite. The genus Ichthyosporidium was later erected by Caullery and Mesnil (1905) who described two named species: Ichthyosporidium gasterophilum, which was later reassigned to the genus Ichthyophonus by Sprague (1965), and Ichthyosporidium phymogenes, with both genera causing parasitic infections in marine fish of the family Labridae. They noted the close similarity between I. phymogenes and G. gigantea described by Thelohan (1895) and observed that they were found in the same fish host, C. melops. These parasites were also considered to be haplosporidians based on the resemblance of the sporoblasts to developmental stages of haplosporidians. Numerous later authors also considered these parasites to be members of the Haplosporidia (Kudo 1966). Swellengrebel (1911) described developmental stages of a parasite from C. melops which he noted as likely being the same as that described by Thelohan (1895), but placed it within the genus Pleistophora due to the formation of a pansporoblast. Believing this organism to be a microsporidium, he was unable to germinate spores to demonstrate the presence of a polar filament. Later, Sprague and Vernick (1968) demonstrated the presence of polar filaments in spores of this microsporidium from Swellengrebel’s material by the use of electron microscopy and the periodic acid-Schiff stain by light microscopy, firmly placing these parasites within the Microsporidia. Sprague and Vernick (1968) also noted that the organism in question, Pleistophora gigantea, was synonymous with Ichthyosporidium giganteum. Infections by members of the genus Ichthyosporidium are characterized by the formation of large, multilobate xenomas in infected host tissue. These xenomas are distinguished from the cell hypertrophy-type xenoma formed by species in the genus Glugea in that they appear to be produced by the coalescence of multiple infected, hypertrophic cells, forming a syncytial-type xenoma. This type of xenoma does not have host nuclei in the periphery, is relatively devoid of host organelles, and contains a “fibro-granular layer” with no distinct inner boundary. It is limited on the outer surface by microvillus-like projections (Canning et al 1986; Lom and Dykova 2005). We found similar xenomas in the gonads of arrow gobies, Clevelandia ios, collected from one site in Morro Bay, California. These xenomas were initially thought to be gonadal neoplasms. However, we concluded that they are actually the result of infection by a novel species of the genus Ichthyosporidium. This finding represents the first description of Ichthyosporidium in a member of the family Gobiidae and also the first described from the Pacific Ocean.