The impact of episodic hypoxia on blue crabs (Callinectes sapidus): from molecules to populations)
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2008-10-31
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Abstract
Episodic hypoxia impacts mobile aquatic animals directly when animals die from exposure to low dissolved oxygen (DO) and indirectly when they avoid intrusions of hypoxic water and aggregate in shallow nearshore habitats where increased competition for resources and spatial overlap between predators and prey reduce growth and increase predation/cannibalism. It is difficult to assess the impact of episodic hypoxia on population dynamics because episodic hypoxic events differ in their severity, duration, and hydrodynamics (i.e., current velocity and strength of the DO frontal boundary). Moreover, some individuals within a population can become acclimated to hypoxia, which affects their behavioral responses to and survival of hypoxia. Therefore, a comprehensive approach examining the behavioral and physiological responses of mobile animals to hypoxia can help predict the impact of hypoxia on population dynamics.
I used a series of laboratory studies, coupled with molecular techniques, to test whether two potential molecular biomarkers (structure and concentration of the hemocyanin respiratory protein) would indicate blue crabs’ (Callinectes sapidus) degree of physiological acclimation to low DO and influence their behavioral responses to and survival of hypoxia. Only hemocyanin (Hcy) structure correlated with blue crab behavior and survival, suggesting that Hcy “quality†is more important for survival than Hcy “quantity†. Blue crabs with hypoxia-tolerant Hcy structures were acclimated to hypoxia, survived longer, and were more active under chronic hypoxic conditions than conspecifics with hypoxia-sensitive Hcy structures
Laboratory flume studies also identified the specific hydrodynamic and hydrographic cues blue crabs use to avoid hypoxia and how their physiology influences these behavioral avoidance responses. Drops in DO stimulated increased movement rates, regardless of whether the change resulted in hypoxia, suggesting that blue crabs may anticipate the onset of episodic hypoxic events. Moreover, faster rates of declining DO stimulated faster movement rates under hypoxic conditions, indicating that the hydrodynamic conditions under which crabs are exposed to hypoxia are important for structuring their behavioral responses. Blue crabs also tended to move down-current with declining DO, suggesting they may use current direction to orient away from hypoxia. Lastly, blue crabs with hypoxia-tolerant Hcy structures were less active under hypoxic conditions than conspecifics with hypoxia-sensitive Hcy structures. Therefore, physiological state affects blue crab survival and behavioral responses to hypoxia.
The final study used the functional relationships between physiology, behavior, and survival generated from the laboratory studies to develop a spatially-explicit, individual-based, population simulation model. This initial model: (i) identified which hydrodynamic factors exert the greatest effect on blue crab population escape responses and mortality rates during episodic hypoxic events, (ii) tested whether physiological acclimation (i.e., hypoxia-tolerant vs. -sensitive populations) can alter these population responses, and (iii) provided a preliminary assessment of the consequences of hypoxia-induced mortality to blue crab population dynamics and the blue crab fishery. The model predicts that: (i) direct mortality from hypoxia is low for blue crabs, (ii) blue crab distribution and abundance patterns are most strongly influenced by the duration of episodic hypoxic events and the strength of the frontal DO gradient, (iii) episodic hypoxic events do not have a substantial impact on blue crab population dynamics, and (iv) the economic loss to the fishery is minor. Lastly, mortality rates during hypoxic events for blue crab populations with hypoxia-tolerant individuals were 35% lower than for populations with hypoxia sensitive crabs; therefore the physiological state of individuals in a population may influence population-level responses to stressors.
This is the first study documenting a physiological mechanism underlying individual-based differences in the behavioral responses to and survival of an invertebrate hypoxia. Our findings highlight the importance of understanding how physiological stress responses influence behavior and survival because these responses have implications at the population level. Moreover, quantifying the functional relationships between physiology, behavior, and survival is critical for developing mechanistic models that can predict how changes in the severity, duration, and frequency of disturbances over time in coastal ecosystems will impact ecological processes, particularly in the context of global climate change.
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Keywords
survival, hypoxia, behavior, population dynamics, blue crab, ecophysiology, physiology
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Degree
PhD
Discipline
Marine, Earth and Atmospheric Sciences
