Geoduck clam (Panopea generosa): Anatomy, Histology, Development, Pathology, Parasites and Symbionts
Intracellular Prokaryotes of Geoduck Clams
Clusters (microcolonies) of intracellular prokaryotes believed to be species of Rickettsea or Chlamydia were observed in epithelial cells of the gill filaments (Figs. 1a and 1b) and occasionally in the epithelium of the palps. Because of their appearance they are often referred to as basophilic inclusion bodies. Although the prevalence of infection in geoduck clams is high (almost all adult clams infected), the intensity of infection is usually very low which has hindered the specific identification of these parasites.
Figures 1a and b. Inclusion bodies (arrows) in histological sections of gill epithelial cells with no indication of an adverse response by the geoduck clam. Hematoxylin and eosin stain.
Inclusion bodies have been observed in other bivalves of British Columbia and the causative agent(s) are not known to be pathogenic in most cases. Dorfmeier et al. (2015) also reported inclusion bodies (intracytoplasmic Rickettsia-like colonies) in the gill epithelial cells of P. generosa from Washington State. “Rickettsia-like” inclusions were only detected inside the epithelial cells of the secondary digestive diverticula and not the gills (ctenidial epithelial cells) of a few P. generosa surveyed from the Pacific coast of Baja California, Mexico (Cáceres-Martínez et al. 2015). In all cases there was no evidence of significant pathology associated with infection.
Intracellular prokaryotes belonging to the Rickettsiales have also been reported in the gills and digestive gland epithelial cells of many species of clam around the world including Cerastoderma edule, Donax trunculus, Mercenaria mercenaria, Mya arenaria, Venerupis (=Tapes) philippinarum, Tapes (=Ruditapes) decussatus, Siliqua patula, Tapes pullastra, Tellina tenuis, Tridacna crocea and Hippopus hippopus as well as a wide variety of marine molluscs including oysters, mussels and scallops. Infections are usually light and usually not associated with disease. However, extensive inflammation of gill tissue and mortalities in cultured giant clams (H. Hippopus) were associated with heavy gill infections of Rickettsiales-like organisms in the Philippines and Micronesia (Norton et al. 1993). Also, a similar organism, 'Candidatus Xenohaliotis californiensis', that inhabits cells of the digestive gland of abalone has been accused of causing foot withering syndrome and mass mortalities of black abalone in California (Gardner et al. 1995, Friedman et al. 2000).
Methods of control
No known methods of prevention or control. It was suggested that overcrowding and low exchange rates of water in land based culture tanks predisposed Hippopus hippopus to increased intensity of infection, clinical disease, and mortalities. Experiments with 'Candidatus Xenohaliotis californiensis' indicated that this pathogen could be transmitted via the water column and did not require direct contact between infected and uninfected abalone and above normal temperatures seem to have a synergistic effect on the disease (Martínez et al. 2000, Moore et al. 2000).
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Bower, S.M. and Blackbourn, J. (2003): Geoduck clam (Panopea generosa): Anatomy, Histology, Development, Pathology, Parasites and Symbionts: Intracellular Prokaryotes of Geoduck Clams.
Date last revised: August 2020
Comments to: Susan Bower
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