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Diseases of Aquatic Organisms

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DAO 76:241-249 (2007)  -  doi:10.3354/dao076241

Intestinal pH profile in rainbow trout Oncorhynchus mykiss and microhabitat preference of the flagellate Spironucleus salmonis (Diplomonadida)

M. Reza Saghari Fard1,2,*, Claudia Weisheit1,2, Sarah L. Poynton2,3

1College of Agriculture and Horticulture, Humboldt University of Berlin, Invalidenstrasse 42, 10115 Berlin, Germany
2Department of Inland Fisheries, Leibniz-Institute of Freshwater Ecology and Inland Fisheries, Müggelseedamm 310, 12587 Berlin, Germany
3Department of Molecular and Comparative Pathobiology, Johns Hopkins University School of Medicine, 733 North Broadway, Room 807, Baltimore, Maryland 21205-2196, USA

ABSTRACT: In farmed rainbow trout Oncorhynchus mykiss, the flagellate Spironucleus salmonis (Diplomonadida) is often found in the pyloric region of the intestine. While previous in vitro studies report a pH of 7.5 to 8.0 as optimal for presumed S. salmonis, no previous in vivo studies have investigated the relationship between pH and microhabitat preference. Therefore, in 698 rainbow trout (75% were 5 to 6 mo old juveniles, 10 to 20 cm total length), we recorded occurrence and density of S. salmonis, and pH, in the pyloric, anterior, middle, and posterior intestine. There were no significant differences in total length or weight between infected and uninfected fish. S. salmonis preferred the pyloric region, with occurrence and density decreasing significantly from pyloric to posterior regions. In infected fish, pH in pyloric (6.8 to 7.9, mean 7.3) and posterior regions (6.5 to 8.0, mean 7.1) was significantly lower than in anterior (6.5 to 8.5, mean 7.7) and middle (6.8 to 8.2, mean 7.7) regions; in uninfected fish, the pH profile was similar. At the individual level, 90% of infected fish and 79% of uninfected fish showed this pH profile. In the pyloric region, pH was not significantly different among uninfected fish, and fish with light, moderate, or heavy infections. Our in vivo study suggests the optimal pH for S. salmonis is between 7.1 and 7.5, possibly close to 7.3 (the mean in pyloric region of infected fish). We conclude that while the presence of S. salmonis reflected tolerable pH, density of infection was not correlated with pH, and thus a causal relationship between microhabitat preference and pH is unlikely.

KEY WORDS: Diplomonad flagellate · Intestinal tract · Microhabitat preference · Oncoryhnchus mykiss · pH profile · Rainbow trout · Spironucleus salmonis

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