Browsing by Author "Bell, Geoffrey Weszely"

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    Behavioral Response of Free-Ranging Blue Crabs to Episodic Hypoxia
    (2002-07-02) Bell, Geoffrey Weszely; David B. Eggleston, Committee Chair
    Hypoxia is increasing in frequency and magnitude in estuarine and coastal systems throughout the world. Very little is known about how periodic hypoxic intrusions into shallow, nearshore habitats influence local migration patterns and trophic dynamics of mobile species such as the blue crab, Callinectes sapidus. Studying these behavioral responses is important because hypoxic events may cause direct and indirect mortality of crabs and alter key trophic interactions. Moreover, when crabs recolonize deeper water habitats during the relaxation of hypoxic events they may increase consumption rates by feeding on slow-recovering infaunal prey, thus, altering higher level trophic dynamics. We used 1) biotelemetry techniques with concurrent water quality measurements to monitor movement and feeding responses of free-ranging crabs to spatiotemporal dynamics of water quality, and 2) a trawl survey to determine how periodic hypoxic upwelling events alter distribution and abundance patterns of blue crabs in nearshore habitats. Free-ranging blue crabs were moderately successful at avoiding drops in DO concentrations to hypoxic levels. They generally moved to higher DO concentrations and shallower depths but sometimes remained within hypoxic water for hours. Similarly, from our trawling study, most blue crabs were collected in relatively shallow water during hypoxic upwelling events, however, some crabs remained within near-anoxic mid-depth zones during these events. Although crabs fed within hypoxic water, most did not feed when DO concentrations dropped to or from hypoxic levels. The frequency of feeding did not increase when DO concentrations increased as was originally hypothesized, and is likely due to: 1) crabs foraging on prey other than sessile benthic infauna or 2) the duration of upwelling events which may not last long enough for infauna to migrate close enough to the sediment surface to be vulnerable to predation from blue crabs. One telemetered crab died after only a few hours of exposure to near-anoxic water during a hypoxic upwelling event. Thus, hypoxic upwelling events can kill even highly mobile species if they are not successful at avoiding rapidly dropping DO levels. Understanding the direct and indirect impacts of episodic hypoxic disturbance on free-ranging blue crabs will help to predict how poor water quality impacts blue crab population and trophic dynamics.
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    The impact of episodic hypoxia on blue crabs (Callinectes sapidus): from molecules to populations)
    (2008-10-31) Bell, Geoffrey Weszely; James Rice, Committee Member; Thomas Wolcott, Committee Member; Edward Noga, Committee Member; David Eggleston, Committee Chair
    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.