Acclimatization in the bay scallop Argopecten irradians along an eutrophication gradient: Insights from heartbeat rate measurements during a simulated hypoxic event

ABSTRACT

Coastal hypoxia typically intensifies during summer months, but it is unknown if exposure of shellfish to diel-cycling hypoxia alters their ability to cope with prolonged and severe hypoxic events. In this study, adult bay scallops Argopecten irradians were acclimatized for 50 days at five sites along a eutrophication gradient, encompassing locations with frequent, occasional, and without exposure to nighttime dissolved oxygen (DO) concentrations below 2 mg O₂ L^-1. In a laboratory experiment, scallops acclimatized under these different levels of diel-cycling hypoxia were exposed to a 24-hour hypoxic event. As a proxy of whole animal metabolism, scallop heartbeat rates were recorded by non-invasive infrared sensors in normoxic and the hypoxic treatments during the initial oxygen decline, throughout 24 hours of hypoxia, and for one hour after re-establishment of normoxia. Irrespective of acclimatization site scallops increased heartbeat rates in response to extended hypoxia but this tachycardic response was less pronounced in scallops acclimatized at sites with frequent DO excursions below 2 mg O₂ L^-1. In addition, scallops that had not experienced hypoxia during acclimatization showed a rapid tachycardic response to hypoxia and rapid return to basal heartbeat rates once normoxia was re-established, while responses were delayed or less pronounced in scallops acclimatized to diel-cycling hypoxia. Correlations between cardiac response patterns and diurnal oxygen dynamics at the five acclimatization sites suggest that frequent exposure to moderate levels of hypoxia (DO below 3.0 or 4.8 mg O₂ L^-1) explained the differential responses better than occasional exposure to severe hypoxia (< 2 mg O₂ L^-1). In coastal environments where DO concentrations can vary dramatically on a daily basis, the ability to adjust metabolic activity can help to sustain aerobic metabolism during low DO periods and reduce oxidative stress when DO concentrations increase. This study suggests that the ability to adjust cardiac activity to DO changes can be affected by exposure history, which may have important ramifications for scallop ecophysiology and resilience in dynamic coastal systems.