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Diseases of Aquatic Organisms

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DAO 63:25-32 (2005)  -  doi:10.3354/dao063025

Picoeucaryot alga infecting blue mussel Mytilus edulis in southern Norway

Stein Mortensen1,*, Lisbeth S. Harkestad1, Rolf-Olav Stene2, Tristan Renault3

1Institute of Marine Research, PO Box 1870, Nordnes, 5817 Bergen, Norway
2County Governor of Vest-Agder, Department of Environment, PO Box 513, 4605 Kristiansand, Norway
3IFREMER, Laboratoire de Génétique et Pathologie, 17390 La Tremblade, France

ABSTRACT: During summer 2001, blue mussels Mytilus edulis with abnormal shell growth were collected near Kragerø, southern Norway. The mussels had green spots in their mantle tissues, mainly posteriorly and ventrally, and in the adductor muscle. Mussels from 4 sites had a prevalence of green spots varying from 2 to 71% that correlated well with shell deformities. Histological examination revealed the presence of round or ovoid algae, 0.9 to 1.5 × 1.2 to 2.4 µm, free within haemocytes and in the lesions, characterised by an inflammatory response and the presence of cellular debris. The alga contain a relatively large nucleus, 1 chloroplast and 1 mitochondrion. Size and morphology suggest that the alga might be a picoeucaryot green alga. Infection of mussel tissues appears to start in the posterior mantle edge, near the siphons, and spread anterior-ventrally in the mantle connective and storage tissues—occasionally spots were also found in the gonad follicles. Large infected areas were also observed in sinuses within the adductor muscle. Only mussels that were 3 yr old or more were infected. Deformations apparently resulted from years of continuous shell formation by a contracted, partly deformed mantle. Most deformed mussels had eroded shells, allowing some light penetration through the exposed, thin nacre. Young, thin-shelled mussels were not infected. The present work suggests that the alga has, at least partially, a parasitic relationship with the mussels, and is associated with pathological alterations in mussel tissues.

KEY WORDS: Blue mussel · Mytilus edulis · Picoeucaryot · Algae · Infections · Green pustules · Inflammation · Shell deformities

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