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Diseases of Aquatic Organisms

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DAO 80:157-165 (2008)  -  DOI:

Seasonality in the infection and invasion of Marteilioides chungmuensis in the Pacific oyster Crassostrea gigas

Kay Lwin Tun1, Yasuko Shimizu2, Hideo Yamanoi2, Tomoyoshi Yoshinaga1, Kazuo Ogawa1,*

1Laboratory of Fish Diseases, Department of Aquatic Bioscience, Graduate School of Agricultural and Life Sciences, University of Tokyo, Yayoi 1-1-1, Bunkyo, Tokyo 113-8657, Japan
2Fisheries Experiment Station of Okayama Prefecture, Ushimado, Setouchi 701-4303, Japan
*Corresponding author. Email:

ABSTRACT: The protozoan parasite Marteilioides chungmuensis causes irregular enlargement of the ovary in the Pacific oyster Crassostrea gigas. The parasite invades the oyster through the epithelial tissue of the labial palp, replicates in the connective tissue, and then moves to the gonad, producing spores inside the oocytes. In this study the seasonality and invasion period of the parasite into the host was investigated over a 1 yr cycle. Uninfected 1 and 0 yr old (spat) oysters were placed in an epizootic area every month from July 2004 to July 2005 and September 2005 to March 2006, respectively, and left for 1 mo. Labial palps and gonad were sampled monthly and examined for infection by nested PCR and histological observations. Prevalence of infection detected by PCR was 70% or higher from August to October, but declined sharply in November and reached 7% or lower from February to April. To explain the low detection rate in winter, 1 yr old uninfected oysters were placed in an epizootic area in winter (water temperature: 8 to 10°C) for 2 wk and then transferred to M. chungmuensis-free seawater at 24°C. Although prevalence of infection was ca. 7% before transfer to heated seawater, levels of 87% were detected after 1 wk. After a 3 wk exposure to heated seawater, parasites were found in host oocytes by histological observation. It was concluded that the low prevalence in winter was due to insufficient replication of M. chungmuensis at low seawater temperatures, resulting in levels not detectable by nested PCR, and not to the absence of invasion.

KEY WORDS: Crassostrea gigas · Gonad development · Marteilioides chungmuensis · Invasion period · Parasite

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Cite this article as: Tun KL, Shimizu Y, Yamanoi H, Yoshinaga T, Ogawa K (2008) Seasonality in the infection and invasion of Marteilioides chungmuensis in the Pacific oyster Crassostrea gigas. Dis Aquat Org 80:157-165.

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