Pathology and immune response of the blue mussel (Mytilus edulis L.) after an exposure to the harmful dinoflagellate Prorocentrum minimum

Abstract

The harmful dinoflagellate Prorocentrum minimum has different effects upon various species of grazing bivalves, and these effects also vary with life-history stage.Possible effects of this dinoflagellate upon mussels have not been reported; therefore, experiments exposing adult blue mussels, Mytilus edulis, to P. minimum were conducted. Mussels were exposed to cultures of toxic P. minimum or benign Rhodomonas sp. in glass aquaria. After a short period of acclimation, samples were collected on day 0 (before the exposure) and after 3, 6, and 9 days of continuousexposure experiment. Hemolymph was extracted for flow-cytometric analyses of hemocyte, immune-response functions, and soft tissues were excised for histopathology.Mussels responded to P. minimum exposure with diapedesis of hemocytes into the intestine, presumably to isolate P. minimum cells within the gut, thereby minimizing damage to other tissues. This immune response appeared to have been sustained throughout the 9-day exposure period, as circulating hemocytes retained hematological and functional properties. Bacteria proliferated in the intestines of the P. minimumexposed mussels. Hemocytes within the intestine appeared to be either overwhelmed by the large number of bacteria or fully occupied in the encapsulating response to P. minimum cells; when hemocytes reached the intestine lumina, they underwent apoptosis and bacterial degradation. This experiment demonstrated that M. edulis is affected by ingestion of toxic P. minimum; however, the specific responses observed in the blue mussel differed from those reported for other bivalve species. This finding highlights the need to study effects of HABs on different bivalve species, rather than inferring that results from one species reflect the exposure responses of all bivalves.

Effects of Alexandrium minutum exposure upon physiological and hematological variables of diploid and triploid oysters, Crassostrea gigas

“…Hégaret et al (2009) suggested that presence of large amounts of bacteria and hemocytes in the intestinal lumena of R. philippinarum reflected a failed attempt of the immune system to suppress opportunistic bacterial pathogens. Furthermore, Galimany et al (2008b) observed that hemocytes engulfed bacteria by phagocytosis in the alimentary canal lumina of mussels exposed to P. minimum.…”

Combined effects of a parasite, QPX, and the harmful-alga, Prorocentrum minimum on northern quahogs, Mercenaria mercenaria

“…Hemocyte infiltration and granuloma in the tissues have also been observed in other bivalves, such as bay scallops A. irradians, Pacific oysters Crassostrea gigas, blue mussels Mytilus edulis and Manila clams R. philippinarum exposed to Prorocentrum spp. (Wikfors and Smolowitz, 1993;Pearce et al, 2005;Galimany et al, 2008a;Hégaret et al, 2009). Wikfors and Smolowitz (1993) also reported degeneration of the digestive gland and presence of hemocyte aggregates (granulomas) in several tissues of bay scallops exposed to P. minimum.…”

“…Toxic effects of P. minimum have been demonstrated several times with scallop bioassays (Hégaret and Wikfors, 2005a;Galimany et al, 2008a), even though the toxic agent is not clearly identified at this time. The dinoflagellate P. minimum may produce some toxic compound responsible for epithelial cell necrosis (Grzebyk et al, 1997) generating hemocyte infiltration or aggregation to accomplish tissue repair or to isolate the tissues from the algal cells.…”

“…As inconsistent responses of bivalves to P. minimum have been observed in nature and in experiments (Wikfors, 2005), the strain JA-98-01 was chosen for its toxicity to juvenile bay scallops, Argopecten irradians, used as a bioassay to test algal toxicity; mortality of juvenile bay scallops occurs after 24 h exposure to stationary-phase cells, but log-phase cells are less toxic (Hégaret and Wikfors, 2005a;Galimany et al, 2008a). Cultures of P. minimum were grown in EDL7 medium, a modified version of the enriched-seawater E-medium (Ukeles, 1973) that contains L-1 trace metals, double the EDTA of the standard E formulation, KNO 3 rather than NaNO 3 , and soil extract.…”

“…Several studies have highlighted effects of harmful-algal species upon bivalve hemocytes (Hégaret and Wikfors, 2005a, b;Hégaret et al, 2007a, b;da Silva et al, 2008;Ford et al, 2008;Galimany et al, 2008a, b). Specifically, the dinoflagellate Prorocentrum minimum has been shown to cause morphological and functional changes in hemocytes of several bivalve species (Hégaret and Wikfors, 2005a, b;Galimany et al, 2008a;Hégaret et al, 2008bHégaret et al, , 2009). This dinoflagellate is present throughout the world (Heil et al, 2005) and has been reported to affect filtration, growth, survival, or organ and tissue development of northern quahogs, bay scallops and juvenile eastern oysters (Leibovitz et al, 1984;Shumway et al, 1985;Luckenbach et al, 1993;Wikfors and Smolowitz, 1993, reviewed in Wikfors (2005).…”