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

Abstract

a b s t r a c tNorthern quahogs, Mercenaria mercenaria (L.), frequently are infected with the parasite Quahog Parasite Unknown (QPX, Labyrintohomorpha, Thraustochytriales), which can cause morbidity and mortality of the quahogs. Possible interactions between this parasitic disease and exposure to the harmful dinoflagellate Prorocentrum minimum in M. mercenaria were studied experimentally. Quahogs from Massachusetts with variable intensity of QPX infection were exposed, under controlled laboratory conditions, to cultured P. minimum added to the natural plankton at a cell density equivalent to a natural bloom. After 5 days of exposure, individual clams were diagnosed histologically to assess prevalence and intensity of parasitic infection, as well as other pathological conditions. Further, cellular defense status of clams was evaluated by analyzing hemocyte parameters (morphological and functional) using flow-cytometry. Exposure of quahogs to P. minimum resulted in: a lower percentage of phagocytic hemocytes, higher production of reactive oxygen species (ROS), larger hemocyte size, more-numerous hemocytic aggregates, and increased numbers of hemocytes in gills accompanied by vacuolation and hyperplasia of the water-tubular epithelial cells of the gills. Quahogs had a low prevalence of QPX; by chance, the parasite was present only in quahogs exposed to P. minimum. Thus, the effect of QPX alone on the hemocyte parameters of quahogs could not be assessed in this experiment, but it was possible to assess different responses of infected versus non-infected quahogs to P. minimum. QPX-infected quahogs exposed to P. minimum had repressed percentage of phagocytic hemocytes, consistent with immuno-modulating effect of P. minimum upon several molluscan species, as well as smaller hemocytes and increased hemocyte infiltration throughout the soft tissues. This experiment demonstrates the importance of considering interactive effects of different factors on the immunology and histopathology of bivalve shellfish, and highlights the importance of considering the presence of parasites when bivalves are subjected to harmful-algal blooms.

In vitro interactions between several species of harmful algae and haemocytes of bivalve molluscs

“…Quahog haemocytes exposed in vitro to P. minimum form aggregates surrounding algal cells, confirming previous observations of , probably to isolate P. minimum cells from the host tissues. Similarly, in vivo exposures of quahogs, Manila clams, mussels and bay scallops exposed to P. minimum showed depression of phagocytosis, induced aggregation of haemocytes surrounding the harmful algae and a large inflammatory response characterised by a massive infiltration of haemocytes into the intestine (Wikfors and Smolowitz 1993;Hégaret and Wikfors 2005a;Galimany et al 2008a;Hégaret et al 2009Hégaret et al , 2010. Present in vitro results support the hypothesis that this massive migration of haemocytes through the intestinal epithelium occurred to surround and isolate (via encapsulation) the cells of P. minimum.…”

“…Our results indicated that in vitro interactions between haemocytes and P. minimum are good proxies for in vivo experiments and show consistently different haemocyte responses, according to the bivalve tested, showing no measurable effects on fundamental haemocyte functions in oysters, a decrease in phagocytosis associated to the formation of large haemocyte aggregates surrounding the algal cells in quahogs, Manila clams and blue mussels (Galimany et al 2008a;Hégaret et al 2009Hégaret et al , 2010. In addition, previous studies (Hégaret and Wikfors 2005a, b) also demonstrated that laboratory, in vivo experiments exposing bivalves to cultures of P. minimum were good proxies for field HAB exposures.…”

“…The effects of P. minimum were previously tested in various in vivo bivalve-HAB interactions with eastern oysters C. virginica (Hégaret and Wikfors 2005a, b), bay scallops Argopecten irradians (Hégaret and Wikfors 2005a), blue mussels Mytilus edulis (Galimany et al 2008a), quahogs M. mercenaria (Hégaret et al 2010) and Manila clams R. philippinarum (Hégaret et al 2009). Our results indicated that in vitro interactions between haemocytes and P. minimum are good proxies for in vivo experiments and show consistently different haemocyte responses, according to the bivalve tested, showing no measurable effects on fundamental haemocyte functions in oysters, a decrease in phagocytosis associated to the formation of large haemocyte aggregates surrounding the algal cells in quahogs, Manila clams and blue mussels (Galimany et al 2008a;Hégaret et al 2009Hégaret et al , 2010.…”

Effects of the toxic dinoflagellate Alexandrium catenella on histopathogical and escape responses of the Northern scallop Argopecten purpuratus

“…This suggests that a bloom of PSP producers can weaken bivalves enough that they are much more susceptible to any additional stress. Previous studies demonstrated that HAB exposure often depressed phagocytosis and enhanced aggregation and circulating hemocyte concentration (Hé garet and Wikfors, 2005a,b;Hé garet et al, 2007bHé garet et al, , 2009Hé garet et al, , 2010. Hemocytes represent the first line of defense (Janeway, 1994) against parasites, pathogens and foreign organisms which can be either engulfed by phagocytosis or encapsulated.…”

Common European harmful algal blooms affect the viability and innate immune responses of Mytilus edulis larvae

“…Harmful algae have been shown to temper as well as aggravate the host-pathogen interactions in both adult and juvenile bivalves [18,33,38]. The implications of an elevated larval PO activity are, however, largely unknown as the immunological role of PO is still poorly understood.…”