Eastern oyster, Crassostrea virginica, valve gape behavior under diel-cycling hypoxia

Abstract

Hypoxia and anoxia in many estuaries worldwide can cause a wide range of negative effects on animals that are directly exposed or indirectly influenced by food web interactions. Typically, experimental studies focus on animal behavior as a function of continuous exposure to low dissolved oxygen (DO) conditions rather than short-term fluctuations. Dissolved oxygen concentrations [DO] can, however, vary throughout the day, and water can become hypoxic for minutes to hours, often during the late night/early morning hours in the summer. Valve gape of 1-year-old eastern oysters, Crassostrea virginica, from Maryland, USA, was continuously measured while exposed to diel-cycling DO in aquaria during normoxic, hypoxic, and supersaturated phases of the cycle over several 2-day periods (July–August 2012). Severe hypoxia (0.6 mg DO L−1) induced oysters to close for significantly longer times than normoxic (7.3 mg DO L−1) conditions. Oysters exposed to mild hypoxia (1.7 mg DO L−1) closed for a similar amount of time as oysters held at normoxia and severe hypoxia. At severe hypoxia, more than one-third of the oysters closed simultaneously and closed immediately when they encountered severe hypoxia while oysters at mild hypoxia often closed later in the low oxygen phase of the cycles. When normoxia was reintroduced after severe hypoxia, most oysters opened immediately and gaped throughout the period. The results indicate that while 1-year-old oysters responded negatively to diel-cycling low [DO], especially to severe hypoxia, they rapidly opened during the normoxic period that followed, potentially reducing any negative effects of a fluctuating environment.