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Diseases of Aquatic Organisms

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DAO 95:153-161 (2011)  -  DOI:

Soft tunic syndrome in the edible ascidian Halocynthia roretzi is caused by a kinetoplastid protist

Akira Kumagai1,*, Atsushi Suto1, Hiroshi Ito1, Toru Tanabe1, Jun-Young Song2, Shin-Ichi Kitamura2, Euichi Hirose3, Takashi Kamaishi4, Satoshi Miwa5

1Miyagi Prefecture Fisheries Technology Institute, Ishinomaki, Miyagi 986-2135, Japan
2Centre for Marine Environmental Studies (CMES), Ehime University, Matsuyama, Ehime 790-8577, Japan
3Department of Chemistry, Biology and Marine Science, University of the Ryukyus, Nishihara, Okinawa 903-0213, Japan
4National Research Institute of Aquaculture, Fisheries Research Agency, Minami-ise, Mie 516-0193, Japan
5Inland Station, National Research Institute of Aquaculture, Fisheries Research Agency, Tamaki, Mie 519-0423, Japan

ABSTRACT: An etiological study was conducted to clarify whether the flagellate-like cells found in histological preparations of the tunic of diseased Halocynthia roretzi (Drasche) were the causative agent of soft tunic syndrome in this ascidian. When pieces of softened diseased tunic were incubated overnight in sterile seawater, live flagellated cells, which were actively swimming in the seawater, were observed in 47 out of 61 diseased ascidians (77%), but not in moribund or abnormal individuals with normal tunics (n = 36) nor in healthy animals (n = 19). The flagellate was morphologically very similar to those observed in histological sections of the diseased tunic. By contrast, flagellates were not found in tunic pieces of healthy, moribund, and abnormal individuals that did not exhibit softening of the tunic. Light and electron microscopy revealed that the flagellate has polykinetoplastic mitochondria with discoidal cristae. The cytomorphologies of the flagellate were the same as those of the flagellate-like cells in the diseased tunic. We cultured the flagellate from the softened tunic in vitro and confirmed that the tunics of healthy ascidians, which were immersion-challenged with suspensions of the subcultured flagellates, became softened 17 d after exposure, including the final 12 d in aerated, running seawater. The occurrence of flagellates was also confirmed by incubating pieces of soft tunic from experimentally infected animals in seawater overnight. These results indicate that the flagellate is the causative agent of soft tunic syndrome.

KEY WORDS: Ascidian · Halocynthia roretzi · Soft tunic syndrome · Causative agent · Kinetoplastid protist · Flagellate

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Cite this article as: Kumagai A, Suto A, Ito H, Tanabe T and others (2011) Soft tunic syndrome in the edible ascidian Halocynthia roretzi is caused by a kinetoplastid protist. Dis Aquat Org 95:153-161.

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