Vol. 44: 203-216, 2001

DISEASES OF AQUATIC ORGANISMS Dis Aquat Org

Published April 10

Characterization of intracytoplasmic prokaryote infections in Dreissena sp. (Bivalvia: Dreissenidae)

Daniel P. Molloy¹*, Laure Giamberini², J. Frank Morado³, Sergei I. Fokin⁴, Franck Laruelle⁵

¹New York State Museum, The State Education Department, Cultural Education Center, Albany, New York 12230, USA

²Equipe de Production des Ecosystemes et Ecotoxicologie, Laboratoire EBSE, Universite de Metz, Campus Bridoux, rue du Gal Delestraint, 57070 Metz Cedex, France

³National Oceanic & Atmospheric Administration, National Marine Fisheries Service, Alaska Fisheries Science Center, Resource Assessment & Conservation Engineering Division, 7600 Sand Point Way NE, Seattle, Washington 98115-0070, USA

⁴Biological Research Institute, St. Petersburg State University, St. Petersburg 198904, Russia

⁵UMR CNRS 6539, Institut Universitaire Europeen de la Mer, UBO, Place Nicolas Copernic, Technopole Brest-Iroise, 29280 Plouzane, France

ABSTRACT: This study characterizes intracytoplasmic infections with prokaryote microorganisms in Dreissena sp. (near Dreissena polymorpha) from northeastern Greece and represents the first report of such infections in freshwater bivalves. Light microscope observations of stained tissues revealed basophilic, cytoplasmic inclusion bodies in 87.5% (28/32) of the mussels sectioned. Inclusions in epithelial cells and connective tissues were noted, respectively, in 34.4 and 71.9% of the sample, with 5 mussels (15.6%) having both tissue types infected. Epithelial cell infections were observed in histological sections only in digestive gland tubules and ducts; within tubules, inclusions were present more often in secretory than digestive cells. Connective tissue infections, however, were systemic; among the 32 mussels sectioned, inclusions were found in the gills (65.6%), foot (12.5%), mantle (9.4%), labial palps (6.3%), digestive gland (6.3%), stomach (6.3%), and gonads (3.1%). Cytoplasmic inclusions (maximum dimension, 138 pm) were prominent enough in the gills to be visible in 17.0% of the 247 mussels dissected. Ultrastructurally, prokaryote cells in gill connective tissues were clearly characteristic of Chlamydiales-like organisms, with each intracytoplasmic inclusion containing a loosely packed mixture of elementary, reticulate, intermediate bodies, and blebs. Prokaryote colonies in digestive gland epithelial cells exclusively contained 1 of 4 morphological cell types and were considered Rickettsiales-like. Hexagonal, virus-like particles were present in the cytoplasm of the largest of these Rickettsiales-like prokaryotes. Although host stress was evident from localized cell necrosis and dense hemocyte infiltration, overall infection was fairly benign, with no major, adverse impact on body condition evident among sectioned or dissected mussels. A possible negative effect was partial constriction of gill water tubes, but at the infection intensity observed (typical range 1 to 7 inclusion bodies per section), significant interference with respiration and other metabolic functions of the gills was highly unlikely.

KEY WORDS: Zebra mussels, Intracytoplasmic prokaryote, Epithelium, Connective tissue, Virus-like particles, Rickettsiales-like, Chlamydiales-like