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.