School of BioSciences - Theses
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ItemEffects of temporally heterogeneous stress on individual marine invertebrates( 2021)Some environmental conditions may be stressful, adversely affecting the growth and reproduction of organisms. The impact of stress, however, may vary with its (1) timing, relative to an individual’s stage of life, (2) duration, or the total length of stress, and (3) frequency, or the number of times an individual is exposed. I used the colonial bryozoan Watersipora subatra and the solitary ascidian Styela plicata to study the response of individuals to stress that varies in timing, duration and frequency. I exposed these invertebrates to stress using elevated concentrations of trace metals, W. subatra to copper in the field, and S. plicata to cadmium and zinc via seawater or food in the laboratory. To expose W. subatra to variable stress, and assess their growth and reproduction, I applied stress (1) at different times within a single life-history stage (early reproductive maturity), (2) that varied in duration at different life-history stages (juvenile, young adult or mature adult), and (3) that differed in duration and frequency for adults at two field locations. To assess the uptake and loss of metals with repeated exposure, I exposed S. plicata twice to cadmium and/or zinc in seawater. Individuals performed differently within a life-history stage. When stressed at the onset of embryo production, fewer adult colonies reproduced, but when stressed as adults in the later stages of brooding or producing subsequent bouts of offspring, colonies decreased in size (fragmented) and released fewer larvae compared with unstressed colonies. Generally, juveniles, young adults and mature adults responded according to life-history stage, not the nature of the stress. Young adults produced more embryos than mature adults, whilst mature adults produced smaller larvae with greater settlement success than young adults. Once reproductive, juveniles produced fewer embryos and were less likely to release larvae than both young and mature adults. The duration of stress affected individuals’ reproduction and, sometimes, growth. Regardless of life-history stage, colonies grew little or fragmented when stressed for longer (2 weeks). Adult colonies at one field location produced fewer larvae when stressed for a shorter period (1, 2 weeks), whilst colonies at a second field location were less likely to release offspring and produced fewer larvae when stressed for longer (2, 3 weeks). When the frequency of exposure differed, colonies altered the quality of larvae. Colonies stressed once (cf. multiply) for two weeks, produced smaller larvae with lower settlement success at one field location, whilst similarly sized, yet better settling, larvae at a second field location. S. plicata accumulated more metal after the second (cf. first) dissolved exposure, and more Cd in the presence of Zn than when exposed singly via sea water. This thesis demonstrates that the temporal nature of stress is important for determining an individual’s response – their growth and reproduction, and future success of their offspring. However, the effects of stress for a given individual may depend on the stress’ temporality – timing, duration, frequency – and conditions at a specific location and time.
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ItemThe evolution of life-history, dispersal, and plasticity: expanding the Daphnia model( 2021)It is often noted that, in the face of accelerating environmental change, species must adapt or disperse in order to avoid extinction. Increasingly, however, it is recognised that we might need to refrain from seeing these concepts as independent. The integration of dispersal as a plastic, evolving trait within the framework of life-history is critical to our understanding of evolution in changing environments. Through dispersal an individual’s environment can change, but so too does the environment change an individual’s dispersal decisions. While the role of dispersal has gained appreciation, the drivers and genetic underpinnings of dynamic dispersal have only begun to be explored. Currently, there are few models which explore the spatial facet of life-history evolution. This thesis details the key first steps in the development of a spatially explicit model for life-history evolution using the Australian water flea, Daphnia carinata. This process began with the collection of Daphnia clones from sites across south-eastern Australia to establish a laboratory population for experimentation, phenotyped for life-history traits. The effect of natural selection on traits is heavily influenced by the genetic and environmental relationships of correlated traits and this is reflected in locally adapted life-history strategies. I therefore use a lens of habitat permanency to explore the life-history strategies of Daphnia from temporary and permanent habitats to test for evidence of local adaptation. Using classic life-table experimentation and analysis I find that D. carinata clearly demonstrate life-history strategies that covary with features of habitat permanence. I analyse the environmental robustness of these life-history strategies by using a multivariate reaction norm approach and phenotypic trajectory analysis of plastic responses to two different stressors: food restriction and predation threat. The plasticity employed by organisms to navigate the variation in their local habitat may play a critical role in further evolution of adaptive responses to novel or extreme environments. When plasticity is in the direction of selection this may have consequences for the rate and trajectory of evolution, but identifying if plasticity is adaptive is not straightforward. With a multi-environment, multivariate approach I assess whether plasticity in this population shows evidence of a stressor specific response, and whether the populations demonstrate genetic variation in multivariate plasticity. I find that this population does not demonstrate a general response to the two stressors and that nature of plasticity also varies between habitat groups. Finally, I establish methods for measuring passive and active dispersal in Daphnia. I calculate clonal repeatability as a measure of broad-sense heritability to determine the suitability of a suite of dispersal measures to the relationship of dispersal with other life-history traits. Daphnia display high levels of intraclonal variation in measures of active dispersal, indicative of the diverse environmental drivers of this complex trait. I find that the effect of the environment on aspects of dispersal, from decision to disperse to distance travelled, often leads to low repeatability estimates. However, this suggests that the tools I have developed in measuring adaptive plasticity will be useful in studying the spatial aspect of life-history in this system.