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Diseases of Aquatic Organisms

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DAO 60:165-171 (2004)  -  doi:10.3354/dao060165

Occurrence of Perkinsus sp. in undulated surf clams Paphia undulata from the Gulf of Thailand

Supannee Leethochavalit1,*, Kashane Chalermwat2, E. Suchart Upatham3, Kwang-Sik Choi4, Pichan Sawangwong2, Maleeya Kruatrachue5

1Institute of Marine Science, 2Faculty of Science, Department of Aquatic Science and 3Faculty of Science,Department of Medical Science, Burapha University, Bangsaen, Chonburi 20131, Thailand
4School of Applied Marine Sciences, Cheju National University, Jeju 690-756, Korea
5Faculty of Science, Department of Biology, Mahidol University, Phayatai, Bangkok 10900, Thailand

ABSTRACT: The undulated surf clam Paphia undulata supports Thailand’s largest shellfishery in the Gulf of Thailand, with landings in 1999 recorded at 70000 t (metric tonnes) yr–1. We report, for the first time, the prevalence of Perkinsus sp. in clams in the Gulf. A monthly survey from January to December 2001 utilizing the fluid thioglycollate medium (FTM) method showed that average monthly prevalence was 84.7% (n = 360). The monthly percentage of infected clams was generally 100%, with low prevalence in May (66.7%) and no infection in September. The monthly mean infection intensity in terms of Perkinsus sp. cells g–1 tissue varied from 0 in September to 187759 ± 18970 (x ± SE) in October. No obvious annual variation in intensity and prevalence was observed. Prezoosporangia that developed in FTM were 25 to 75 µm in diameter. A few days after incubation in aerated seawater, the prezoosporangia underwent successive binary cell division and formed motile zoospores (2 to 5 µm long). The zoospores were released into the seawater through a discharge tube formed during the 2- and 4-cell stages. Serial semi-thin sections (1 to 4 µm thickness) of clam tissue (n = 120 clams) showed developing trophozoites 3 to 6 µm in diameter within gills, connective tissue, gonads and, especially, the digestive glands. Microscopic features of different life stages indicated that Perkinsus sp. in Thailand closely resembled P. olseni (= P. atlanticus) reported in Australia, New Zealand, Korea, Japan, Spain and Portugal.

KEY WORDS: Clam · Perkinsus sp. · Paphia undulata · Gulf of Thailand · Zoosporulation · Infection intensity · Prevalence

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