Ichthyodinium is a monotypic genus of dinoflagellates in the family Dinophysaceae. Ichthyodinium chabelardi (/ɪkθioʊˈdɪniəm/) is currently the sole described species of the genus. Ichthyodinium chabelardi is a generalist parasite with a very broad geographic range. It has been found in locations such as the coasts of Indonesia and Vietnam, the Mediterranean Sea, and off the coast of Portugal. The full extent of its distribution is not known. While it can be found throughout most of the year, it has been observed to infect sardine eggs with the highest rates in the winter and early spring, from December to March. It is of economic significance to the fish industry, where it depletes the numbers of many species of fin fish by acting as an endoparasite in host embryos. It was originally described in 1952 by Hollande and Cachon, who described it as an agent of infection amongst sardines off the coast of Algeria, in the Mediterranean Sea. In 2006, Gestal et al. proposed a reclassification of I. chabelardi into the genus Perkinsoide based on its ultrastructure and RNA sequence, but several papers have opposed this reclassification based on its zoospore ultrastructure, as well as its small subunit rRNA and large subunit rRNA sequences. Recently, there has been evidence brought forth that there may be at least two distinct species of Ichthyodinium, since slight genetic differences have been detected between the European and Asian populations.
Taxonomy The genus Ichthyodinium was originally described in 1952 by Hollande and Cachon, who named the novel species they found Ichthyodinium chabelardi, and described it as an agent of infection amongst sardines off the coast of Algeria, in the Mediterranean Sea. They published a second paper with additional life cycle observations a year later, in 1953. Phylogenetic analysis of I. chabelardi conducted in 2009 found evidence to support the inclusion of Ichthyodinium in Marine Alveolate Group I (MAGI). With this addition, MAGI will consist of the genera Ichthyodinium and Dubosquella, both of which are parasitic alveolates, thus maintaining MAGI as an exclusively parasitic group. When looking at the flagella of Ichthyodinium in cross section, they are spherical and thus lack the typical striated strand of a dinoflagellate transverse flagellum. The striated strand is also lacking on the flagella of zoospores of Duboscquella, suggesting this may be a synapomorphy of MAGI. In 2006, Gestal et al. published a controversial paper calling for the reclassification of I. chabelardi. Gestal et al. asserted that I. chabelardi had been wrongly placed in the order Syndiniales, and should instead be placed in a new genus, Perkinsoide, and thus be renamed Perkinsoide chabelardi. They claimed that evidence for this reclassification could be found by looking at the life cycle, schizogonic divisions, structure of schizonts inside the host, nuclei without the typical dinoflagellate appearance, presence of rhoptry-like structures, and the presence of possible pseudo-conoid and biflaglleated spores. Gestal et al. went on to conduct a phylogenetic analysis of the small and large subunit ribosomal RNA genes in the hope that this would provide additional evidence to support the reclassification. However, the results they found did not provide clear answers. Based on their phylogenetic analysis, they were merely able to determine that the parasite of interest should be positioned somewhere within the group consisting of dinoflagellates, perkinsids, and syndiniales. Several papers have since come out against this reclassification, arguing that I. chabelardi is the rightful name. One argument for this is that the life cycle of the parasite described by Gestal et al. does not match those of the genus Perkinsus, as it was described by Perkins in 1996. The parasite in question is reported to have a different pattern of cell division from members of Perkinsus and lacks a discharge tube of zoospores. It has been suggested that perhaps the early exogenous phase that Gestal et al. observed was indeed I. chabelardi, but that the observations made of the later stages, including the zoosporangia and zoospores, were of a different parasitic protist, one that was quite possibly not a dinoflagellate. There is currently only one identified species in the genus Ichthyodinium, named Ichthyodinium chabelardi, which was first identified in the Mediterranean Sea, off the coast of Algeria by Hollande and Cachon in 1952. However, there is growing evidence that at least two distinct species may belong in the genus (Mori et al., 2007; Skovgaard et al., 2009;). In 2007, the mode of infection, development, and SSU rDNA sequence of an unknown parasite infecting leopard coral grouper (Plectropomus leopardus) off Japan were analyzed and compared to I. chabelardi (Mori et al., 2007). Although many similarities were drawn between the two, several differences were noted, causing the unknown parasite to be tentatively classified as Ichthyodinium sp. PL (PL being the initials of the scientific name for Leopard Coral Grouper) (Mori et al., 2007). Polymerase chain reaction (PCR) analysis using specific primers created from the SSU rDNA sequence of Icthyodinium sp. over several different years revealed the same species of parasite to be infecting the fish year after year (Mori et al., 2007). In 2009, the sequences of the SSU rDNA, ITS1 (Internal transcribed spacer 1), ITS2, and 5.8S rDNA of I. chabelardi isolated from Bogue (Boops boops) and European pilchard (Sardina pilchardus) off the coast of Portugal were analyzed and found to be identical to each other (Skovgaard et al., 2009). This suggests that the parasites infecting both species of fish were from a single population (Skovgaard et al., 2009). However, when the SSU rDNA sequence from the Portuguese population was compared to sequences gathered from yellowfin tuna (Thunnas albacares) and leopard coral grouper (Plectropomus leopardus) in Asia, there was only a 97% similarity between them (Skovgaard et al., 2009). No morphological differences between the European and Asian populations have been identified yet (Skovgaard et al., 2009).
Life cycle I. chabelardi exists mainly in its parasitic form, during which time it lives within and obtains energy from a host fish egg. Later, it is released from its host and enters the free-living phase of its life cycle.
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