Year

2005

Degree Name

Doctor of Philosophy (PhD)

Department

School of Biological Sciences - Faculty of Science

Abstract

Ultraviolet radiation (UVR) is an important abiotic stressor for both acquatic and terrestrial organisms. The recent anthropogenic depletion of stratospheric ozone has resulted in elevated levels of potentially damaging UV-B to which organisms are exposed, and global climate change many also herald changes in environmental conditions, particularly temperature, precipitation, and sea level. Thus, organisms may be simultaneously exposed to variable environmental stressors. In the marine environment, embryos and larvae are likely the most vulnerable to the negative effects of these stresses. Moreover, intertidal organisms are particularly vulnerable to UVR because they occur in habitats where little or no UVR is absorbed by the water column, and the effects of UVR are coupled with other potential negative stresses associated with low tides. Surprisingly, little is known about the effects of potential interactions between environmental stressors on marine larvae, particularly involving UVR. This study aims to investigate the role of UVR in the development of encapsulated intertidal embryos through a series of six independent experiments that screen a large number of taxa. In the first four experiments, I explored the direct effects of UVR in isolation and with other stressors (UVR/temperature/salinity and UVR/desiccation), as well as the indirect effects of UVR and fouling. Isolated effects of UVR were investigated on egg masses from 23 marine gastropod species collected from three intertidal habitats (full sun, partial shade, full shade) and exposed to four spectral treatments (full spectrum, no UV-B, no UVR, dark). Embryos from full shade habitats were significantly vulnerable to UVR while those from full sun habitats showed no significant mortality differences between special treatments. Multifactorial experiments were then conducted in which encapsulated embryos of three common rocky shore gastropods were exposed to simultaneous combinations (i) UVR, temperature, and salinity and (ii) UVR and desiccation. Siphonaria denticulate and Bembicium nanum embryos were expected to be tolerant to these negative interactions of stressors as they are routinely deposited on rock platforms exposed to solar radiation. In contrast, Dolabrifera brazieri embryos were predicted to be vulnerable to these stressors as they are deposited in shaded, submerged habitats. I detected species-specific synergistic effects of these stressors, and increases in mortality and retardation of development were generally associated with the most physiologically stressful conditions. Based on laboratory results, embryos of D. brazieri were the most sensitive to all the stressors. In contrast, S. denticulate and B. nanum were vulnerable to negative effects associated with synchronous spectral, thermal, and salinity stress; but they were relatively tolerant to UVR and desiccation. Nevertheless, field results indicate that embryos of these species within desiccated habitats have a significantly higher mortality than those within submerged habitats, suggesting that development on rock platform surfaces may not be optimal for these embryos. The indirect effects of UVR and fouling on encapsulated larval development were investigated on egg masses from 18 species cultured under three spectral treatments (full spectrum, no UV, dark). Algal fouling levels, protest colonization, embryonic mortality, and encapsulation period were recorded, and I found that UVR inhibited algal growth and protest colonization on egg mass surfaces. Although algal fouling was not directly related to embryonic mortality in most species, egg masses colonized by protists had a higher level of algal fouling; and overall, these egg masses had a significantly higher incidence of embryonic mortality. In the last two experiments, I examined potential behavioural and biochemical protection afforded to encapsulated intertidal embryos against UV-induced damage. I conducted surveys of intertidal egg masses in south-eastern Australia over two years to determine if spatial and temporal variation in parental site selection could reduce potential environmental stress to encapsulated embryos. I predicted that eff masses would be predominantly deposited in shaded habitats not prone to environmental extremes. Furthermore, I anticipated that egg masses deposited on rock platform surfaces would be smaller and occur less frequently in these habitats during seasons of high environmental stress. As predicted, most species spawned under boulders, thereby minimizing exposure to environmental stress. Analyses confirmed that summer had the highest UVR index, water temperature, and air temperature, as well as the lowest tides; but assemblages and abundances of egg masses on exposed rock platforms were highest during summer with no change in egg mass sizes. Thus, species spawning on rock platform surfaces do not seem to confer protection to their encapsulated offspring by avoidance of physiologically stressful times or conditions. Alternatively, one or more of these potential stressors are beneficial to embryonic development, and these benefits outweigh negative effects. For example, high temperatures associated with direct sunlight may increase developmental rate and counteract any negative effects associated with UVR. Potential biochemical protection against UV-induced damage was examine by quantifying potential chemical sunscreens, mycosporine-like amino acids (MAAs), in intertidal egg masses from 46 mollusc species, two polychaete species, and one fish species from southeastern Australia. Analyses revealed that egg mass maturity and spawning habitat did not significantly affect MAA composition with egg masses. In contrast, adult diet, phylogeny, and viability significantly affected MAA composition. Herbivores had significantly higher levels of certain MAAs than carnivores, and viable egg masses had higher levels of some MAAs than inviable egg masses. MAAs also occurred in relatively high concentrations in molluscan egg masses when compared to adult mollusks and other common intertidal organisms. Despite the complexity of factors affecting MAA composition, the prevalence of MAAs in some species is consistent with protection afforded to offspring against negative effects of UVR. Results from the experiments comprising this study indicate that encapsulated embryos use behavioural and biochemical protection against UVR and related stressors; and the presence and effectiveness of these mechanisms may be species-specific. Similarly, the species-specific effects of UVR highlight the importance of research on a range of species. Furthermore, the complex outcomes observed on applying multiple stressors could not have been predicted from examining environmental variables in isolation. Results from the single factor study here suggest that UVR does not negatively affect embryos of species that spawn in full sun, but multifactorial experiments revealed that UVR can indeed have negative effects when other stressors are considered. Hence, we may be dramatically underestimating the ecological impacts of climate change and stratospheric ozone depletion by failing to consider the complex interplay of combinations of environmental variables with organisms.

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