Ichthyophonus irregularis sp. nov. from the yellowtail flounder Limanda ferruginea from the Nova Scotia shelf

Abstract

A previously described unusual form of the protistan parasite Ichthyophonus, differing in morphological and developmental features from I. hoferi sensu Plehn & Mulsow, was recovered from yellowtail flounder Limanda ferruginea Storer from the Brown's Bank area of the Nova Scotia shelf. The nuclear gene encoding the rRNA of the small ribosomal subunit was amplified from this unusual form of Ichthyophonus using the polymerase chain reaction, sequenced and aligned with other eukaryote small subunit (ssu)-rDNAs. Inferred phylogenetic trees clearly show that its ssu-rDNA is distinct from those of 2 isolates of I. hofen sensu Plehn & Mulsow from different hosts and geographical locations (herring in the North Sea, and yellowtail flounder from the Nova Scotia shelf). We consider the unusual form to be a separate species, I. irregularis. The occurrence of a second, distinct type of Ichthyophonus within a single host species raises the possibility that ichthyophoniasis could be produced by different (although related) pathogens, and in some cases, by concurrent infections of the two.

Dynamics of viral hemorrhagic septicemia, viral erythrocytic necrosis and ichthyophoniasis in confined juvenile Pacific herring Clupea pallasii

“…Currently I. hoferi (reviewed in McVicar 1999) and I. irregularis (Rand et al 2000) are the only 2 recognized species in the genus, but other species have probably been grouped with I. hoferi based on the plasticity of morphological characteristics (McVicar 1999). Additional molecular phylogenetic studies are necessary to better understand the relatedness of I. hoferi isolates (Criscione et al 2002, Halos et al 2005; therefore, the organism will be referred to generically as Ichthyophonus hereafter in this manuscript.…”

Ichthyophonus hoferi Plehn & Mulsow in British Columbia stocks of Pacific herring, Clupea pallasi Valenciennes, and its infectivity to chinook salmon, Oncorhynchus tshawytscha (Walbaum)

“…While previously considered to be a fungus, genetic evidence now supports the inclusion of this organism within a novel protozoan clade along with Dermocystidium , Psorospermium and the choanoflagellates (Ragan, Goggins, Cawthorn, Cerenius, Jamieson, Plourde, Rand, Söderhäll & Gutell 1996; Spanggaard, Skouboe, Rossen & Taylor 1996). The wide host range and recent evidence of genetic heterogeneity suggests that I. hoferi may be an assemblage of related species (Rand, White, Cannone, Gutell, Murphy & Ragan 2000). This organism will hereinafter be referred to as Ichthyophonus .…”

Differential Survival of Ichthyophonus Isolates Indicates Parasite Adaptation to its Host Environment

“…Alternatively, differences in Ichthyophonus tolerances to water types may result from the effects of parasite speciation. Currently, only 2 species of Ichthyophonus are recognized, I. hoferi and I. irregularis (Rand et al, 2000), and the majority of reports in the scientific literature have been ascribed to the former species. Based on reports of extremely broad host and geographical distributions of I. hoferi that include Ͼ80 species of freshwater and marine fishes throughout the world (Spanggaard et al, 1994), the likelihood is minimal that all reports and isolations are representatives of the same species (McVicar, 1999).…”

“…Previous taxonomic groupings incorrectly positioned the organism with fungi, based on observed life history stages that superficially resemble fungal spores and hyphae (reviewed in McVicar, 1999); however, recent molecular phylogenetic assessments resulted in reclassification of the organism within the Mesomycetozoea, a novel clade of protozoans positioned near the animal-fungal divergence (Mendoza et al, 2002). Although its geographical distribution is extremely broad (reviewed in McVicar, 1999), only 2 species are currently recognized (Rand et al, 2000); presumably, other species have been incorrectly grouped with I. hoferi based on similar gross morphologies (Fish, 1934;McVicar, 1999). Therefore, the organism will hereafter be referred to generically as Ichthyophonus. Ichthyophonus currently occurs in high prevalences and intensities among populations of wild Pacific herring (Clupea pallasii) throughout the northeastern Pacific (Marty et al, 1998;Hershberger et al, 2002;Jones and Dawe, 2002;Marty et al, 2003) and among populations of cultured, freshwater rainbow trout in southern Idaho.…”