Characterization of an induced morphological defense in the Eastern Oyster Crassostrea virginica

A dissertation Submitted in Partial Fulfillment of the Requirements for the Degree of DOCTOR OF PHILOSOPHY in MARINE BIOLOGY from Texas A&M University-Corpus Christi in Corpus Christi, Texas.To reduce their risk of consumption, many prey species use plastic defenses that are employed in situations where predators pose high risk of injury or death. By modifying prey traits, predators often exert strong influence on community structure and function. These nonconsumptive effects can be stronger than effects from prey consumption, but have only become the focus of study in recent decades. Further, they are often more complicated than consumptive effects because they can be influenced by characteristics of the predator, the prey, the response type, and the environment. In order to fully understand the effects of predator species on prey organisms, prey responses must be fully characterized and understood in an ecological context. Eastern oysters Crassostrea virginica were selected as a model organism to investigate the costs, benefits, and mechanisms of predator-induced changes in prey traits. Oysters are an ecologically and economically valuable species, known to increase their shell weight and strength in response to crab predators. In this study, oyster responses were investigated by 1) fully characterizing the mechanism and cost of oyster responses to predation risk, and determining the effect of 2) risk cue type (predator versus prey), 3) an ecological prey characteristic (size), 4) an ecological predator characteristic (diet), and 5) an environmental characteristic (food availability) on defense induction. In response to risk, oysters increased calcium carbonate production to rapidly reach a size refuge from predators. This led to an inverse relationship between shell thickness and shell density. Although not significant, there was a trend for reduced gonad investment in oysters which had thick, low density shells. As this study provides a conservative estimate of defense costs, this suggests oysters may experience reductions in reproduction under conditions of high predation risk which may be exacerbated under conditions of ocean acidification. Oysters responded to all tested cues indicative of risk, but responses were less intense when oysters were exposed to cues from injured prey organisms or to predators which were food deprived or fed aged oyster tissue. In addition, oyster responses were inversely related to oyster size. Oysters responded to predators regardless of resource availability, despite reduced shell metrics in response to limited food under control conditions. There is a diverse array of factors which influence oyster defenses and which may be altered by anthropogenic disturbance. If oyster defenses reduce investment in reproduction, this could have important long-term consequences for oyster populations and oyster reef communities. Therefore, understanding factors which influence prey responses allows us to understand conditions which promote or attenuate nonconsumptive predator effects. And understanding the costs of these effects for organisms allows us to predict the effect of disturbances on community structure and function.Life SciencesCollege of Science and Engineerin

Reproductive Habitat Requirements of the Federally Threatened Blackside Dace (Chrosomus cumberlandensis)

The blackside dace, Chrosomus cumberlandensis, is a federally threatened fish species endemic to the Cumberland River system. Previous work done on this species has consisted of population surveys and one attempt at captive breeding with limited success. As a result, general stream characteristics for blackside dace habitat are well understood. Reproductive habitat, however, has been assessed only qualitatively and characteristics necessary for successful reproduction within these habitats are not known. Within streams, these fish spawn in gravel pits we observed to be generally clustered in certain areas suggesting there are specific reproductive microhabitat requirements for which no quantitative characteristics are known. Thus, we assessed the physical habitat of these reproductive nests and the immediate surrounding areas by measuring depth, stream flow, and nest temperature. The physical structure of these gravel nests was also assessed as this is likely important in egg development. Number of males utilizing nests, assumed to represent nest quality, was correlated with nest characteristics. These data suggest that, amongst the factors measured, flow is most important in determining nest location while depth is most important in determining nest success. Overall, these data are a critical component to current habitat restoration that will not only support reproduction but also enable survival, as well as aid any future attempts at captive breeding

Predatory blue crabs induce stronger nonconsumptive effects in eastern oysters Crassostrea virginica than scavenging blue crabs

By influencing critical prey traits such as foraging or habitat selection, predators can affect entire ecosystems, but the nature of cues that trigger prey reactions to predators are not well understood. Predators may scavenge to supplement their energetic needs and scavenging frequency may vary among individuals within a species due to preferences and prey availability. Yet prey reactions to consumers that are primarily scavengers versus those that are active foragers have not been investigated, even though variation in prey reactions to scavengers or predators might influence cascading nonconsumptive effects in food webs. Oysters Crassostrea virginica react to crab predators by growing stronger shells. We exposed oysters to exudates from crabs fed live oysters or fed aged oyster tissue to simulate scavenging, and to controls without crab cues. Oysters grew stronger shells when exposed to either crab exudate, but their shells were significantly stronger when crabs were fed live oysters. The stronger response to predators than scavengers could be due to inherent differences in diet cues representative of reduced risk in the presence of scavengers or to degradation of conspecific alarm cues in aged treatments, which may mask risk from potential predators subsisting by scavenging

Shell (weight, density, thickness, crushing force, amino acid content) and soft tissue metrics (somatic tissue weight, gonad index) for eastern oysters Crassostrea virginica reared under variable predation conditions (S. Texas, 2013-14)

Inducing defenses to deter predators is a necessary process theorized to incur costs. Although studies have investigated defense trade-offs, quantifying trade-offs is challenging and costs are often inferred. Additionally, prey employ strategies to reduce costs, making costs difficult to predict. Our purpose was to investigate induced defense costs by characterizing the defense mechanisms and costs in eastern oysters (Crassostrea virginica). In the field (summer 2014; 28.13°N, 96.98°W), newly-settled oysters reared under field conditions and assigned to control or predator (blue crab, Callinectes sapidus) conditions, were tested for shell weight and crushing force (a proxy for shell strength; both metrics are known to increase in response to exudates from crab predators), and amino acid content. Amino acid content indicates the quantity and type of organic material present in shells and informs our understanding of the physiological mechanism of defense in oysters. Oysters exposed to blue crab exudates grew stronger shells containing less percent organic material than oysters in controls. In oysters collected from natural populations (spring 2014 and 2017; 28°N, 97°W), we tested the correlation between shell density and shell thickness to determine natural patterns of oyster shell morphology. We also performed regression analyses of soft tissue mass and gonad investment (gonad index, calculated as gonad tissue mass/soft tissue mass) to assess the relationship between shell morphology and other biologically valuable processes (growth and reproduction respectively). Shell density was negatively correlated with shell thickness, further suggesting oysters thicken their shells by increasing low-density calcium carbonate. Reproductive investment showed an increasingly negative relationship with thickness as density decreased (and induction increased). In a lab experiment (Texas A&M University-Corpus Christi; summer 2013), oysters were exposed to a temporal gradient in risk and tested for shell weight and strength to indirectly test hypotheses regarding the mechanism and costs of oyster defenses suggested by the above experiments. Oysters grew heavier shells in all crab treatments, but only grew stronger shells under constant exposure. Collectively, these results suggest oysters initially react to predators by adding inexpensive calcium carbonate to their shells to quickly outgrow risk. However, in high risk environments, oysters may increase production of costly organic material to increase shell strength. Thus, oysters demonstrate a two-tier mechanism allowing them to cheaply escape predation at lower risk but to build stronger shells at greater expense when warranted. These results illuminate the complex strategies prey deploy to balance predation risk and defense costs and the importance of understanding these strategies to accurately predict predator effects