Exposure to the toxic dinoflagellate Alexandrium catenella modulates juvenile oyster Crassostrea gigas hemocyte variables subjected to different biotic conditions

Feb 18, 2016Fish & shellfish immunology

Toxic algae exposure changes immune cell responses in young oysters under different biological conditions

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Abstract

Exposure to toxic Alexandrium catenella resulted in increased hemocyte activity in juvenile oysters.

Juvenile Pacific oysters showed higher hemocyte concentration, size, complexity, and reactive oxygen species production when exposed to A. catenella.
Cohabitation with field-exposed oysters also triggered hemocyte responses, indicating a defense mechanism against new microorganisms.
The presence of A. catenella partially inhibited hemocyte activation in response to cohabitation, suggesting possible immune strategy changes or stress impacts.
No infections from specific pathogens were found in the oysters during the study.
Challenged oysters exhibited lower accumulation of paralytic shellfish toxins, indicating potential physiological impairment affecting feeding processes.

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The Pacific oyster Crassostrea gigas is an important commercial species cultured throughout the world. Oyster production practices often include transfers of animals into new environments that can be stressful, especially at young ages. This study was undertaken to determine if a toxic Alexandrium bloom, occurring repeatedly in French oyster beds, could modulate juvenile oyster cellular immune responses (i.e. hemocyte variables). We simulated planting on commercial beds by conducting a cohabitation exposure of juvenile, "specific pathogen-free" (SPF) oysters (naïve from the environment) with previously field-exposed oysters to induce interactions with new microorganisms. Indeed, toxic Alexandrium spp. exposures have been reported to modulate bivalve interaction with specific pathogens, as well as physiological and immunological variables in bivalves. In summary, SPF oysters were subjected to an artificial bloom of Alexandrium catenella, simultaneously with a cohabitation challenge. Exposure to A. catenella, and thus to the paralytic shellfish toxins (PSTs) and extracellular bioactive compounds produced by this alga, induced higher concentration, size, complexity and reactive oxygen species (ROS) production of circulating hemocytes. Challenge by cohabitation with field-exposed oysters also activated these hemocyte responses, suggesting a defense response to new microorganism exposure. These hemocyte responses to cohabitation challenge, however, were partially inhibited by A. catenella exposure, which enhanced hemocyte mortality, suggesting either detrimental effects of the interaction of both stressors on immune capacity, or the implementation of an alternative immune strategy through apoptosis. Indeed, no infection with specific pathogens (herpesvirus OsHV-1 or Vibrio aesturianus) was detected. Additionally, lower PST accumulation in challenged oysters suggests a physiological impairment through alteration of feeding-related processes. Overall, results of this study show that a short-term exposure to A. catenella combined with an exposure to a modified microbial community inhibited some hemocyte responses, and likely compromised physiological condition of the juvenile oysters.

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Exposure to the toxic dinoflagellate Alexandrium catenella modulates juvenile oyster Crassostrea gigas hemocyte variables subjected to different biotic conditions

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