Biogeographic patterns of endolithic cyanobacteria and their negative impacts on mussels along the South African coast
- Authors: Ndhlovu, Aldwin
- Date: 2020
- Subjects: Cyanobacteria -- South Africa , Cyanobacteria -- Geographical distribution , Prokaryotes -- South Africa , Mexilhao mussel -- South Africa , Mytilus galloprovincialis -- South Africa , Coastal biology -- South Africa , Coastal ecology -- South Africa , Mussels -- Geographical distribution , Mussels -- Predators of , Mussels -- Mortality -- South Africa , Mussels -- Ecology -- South Africa , Mussels -- Growth -- South Africa , Mussels -- Fertility -- South Africa
- Language: English
- Type: Thesis , Doctoral , PhD
- Identifier: http://hdl.handle.net/10962/144656 , vital:38367
- Description: Endolithic cyanobacterial species occur in a wide range of environments including cold and hot deserts as well as marine systems where they attack biological material such as corals and the shells of molluscs including limpets, mussels and abalone. Endoliths live as parasites in mussel shells, where they erode and extract calcium carbonate leading to shell weakening, creating fracture holes that lead to shell collapse and death, but they also have positive effects when they lead to discolouration of mussel shells hence giving them the ability to reduce stressful heat gain during periods of extreme heat stress. Mussels are ecological engineers on which the abundance and diversity of associated species assemblages depend. Understanding how endolithic cyanobacteria affect mussels will not only help in predicting future patterns of mussel abundances, but also future patterns of the infauna that depend on them. Firstly, I identified endolithic species infesting mussels and assessed the prevalence of endolithic parasitism in two intertidal mussel species in South Africa, the native Perna perna and the invasive Mytilus galloprovincialis. Large-scale surveys of endolithic infestation of mussels were conducted along 2500 km of the South African coast, covering three biogeographic regions: the subtropical east coast, dominated by P. perna, the warm temperate south coast where the indigenous species coexists with M. galloprovincialis, and the cool temperate west coast which is dominated by M. galloprovincialis. The prevalence of endolithic infestation was higher in the cool temperate bioregion than in the warm temperate and subtropical bioregions which did not differ and for P. perna endolithic species assemblages revealed clear groupings by bioregion. Results for endolithic induced mortality followed the same trend, with no significant difference between the two mussel species where they coexist and these results attribute biogeography of endoliths to environmental factors rather than host identity. Secondly, I assessed energy budgets of infested and clean mussels, to evaluate the energetic cost of infestation. This involved measuring energy acquisition, expenditure, calculating scope for growth and lethal temperatures (LT50s). The results revealed that endolithic cyanobacteria have a negative effect on scope for growth due to increased metabolic rates for infested mussels, with no effect of endoliths on the rates or efficiency of energy acquisition through filtration and no effect on lethal temperatures. The effects of infestation were then examined in more detail through a qualitative and quantitative analysis of mussel gonads and byssal attachment strength to the substratum. Endolithic infestation was found to affect reproduction by affecting the size (mass) of gonads, but not the density of eggs within them. Attachment strength was affected by endolithic infestation with very infested mussels requiring much less force to detach them from the substratum compared to mussels with low or no infestation. These results show that endolithic infestation affects mussel fitness by directly affecting attachment strength and by reducing their reproductive output. Thirdly, endolithic succession within mussel shells was examined by assessing endolithic species composition in different regions of the shell and as a function of time. The results on the spatial distribution of endolith species within a shell supported those for temporal succession in shells deployed in the field. Endolithic species that were early colonists of clean shells were similar to those that were found in the distal edge, the new and growing region of the shell and species that arrived late in succession were similar to endolithic species found near the umbo, the oldest region of the shell. Overall, the study shows that endolithic cyanobacteria show the effects of biogeography on species distribution and clear patterns of succession within mussel shells. Cyanobacteria affect mussels negatively; they lead to low scope for growth and hence low growth rates, low reproductive output and reduced attachment strength for infested mussels. This, in turn is expected to have indirect consequences for other species that rely on mussels as ecological engineers for their survival.
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- Date Issued: 2020
Ecological engineering on rocky shores: grazing, predation, nutrient availability and their influence on epifaunal communities
- Authors: Ndhlovu, Aldwin
- Date: 2016
- Language: English
- Type: Thesis , Masters , MSc
- Identifier: http://hdl.handle.net/10962/3453 , vital:20500
- Description: “Ecosystem engineers modify, create/destroy habitat and directly/indirectly modulate availability of resources to other species by causing physical state changes to biotic and abiotic materials” (Jones et al, 1994, page 1). Previous studies have analysed top-down and bottom-up relationships to determine which form of regulation is key in controlling community composition. This study assessed the direct and indirect effects of top down and bottom up factors on the epifaunal communities of macro-algae. Epifauna are subject to the direct top-down effects of predation and bottom up effects of habitat availability. Habitat availability in turn experiences the direct top down effects of grazing and bottom up effects of nutrient availability due to upwelling. Grazing and upwelling may therefore have indirect effects on macro-algal epifauna. Four treatments (Procedural controls, Controls, Predator or Grazer exclusion) set out in a block design (n = 5) were monitored monthly for algal cover of the substratum for 12 months with the surface area of algal plants and epifaunal species composition and abundances assessed at the end of the experiment. The red alga Gelidium pristoides was selected as the ecosystem engineer as it is common, supports a diverse community of epiphytic animals and acts as a nursery for small epifaunal organisms. The experiment was run at two upwelling sites interspersed with two non-upwelling sites. Sites were separated on scales of 100km along approximately 500km of coastline. Dipping whole algae in dish washing liquid provided a strong relationship between their surface area and the weight of the film of dish washing liquid covering them. Surface area was strongly correlated to dry weight but neither surface area nor dry weight was correlated to algal cover of the substratum. Algal cover was influenced by the interactions of treatment with site (nested in upwelling) and upwelling. At all sites, treatments that allowed access to grazers, Grazer + and Control treatments, showed no significant differences and these two treatments had lower algal cover than Predator + and Closed treatments which did not differ from one another 3 [Grazer + = Control < Predator + = Closed]. A total of 44 epifaunal species were identified, with the predominant orders being Amphipoda and Isopoda. Primer results showed that only site had a significant effect on species composition, with sites that were further apart being more different. Site (nested in upwelling) had an effect on total epifaunal abundances when data were non-normalised. When total epifaunal abundances were normalised for algal cover of the substratum or algal surface area to provide density data, predation had no significant effect. Grazing did have a significant effect, but only when data were normalised to algal surface area, not cover, leading to the conclusion that indirect top-down factors through grazing of the sea weed are important in structuring epifaunal communities depending on how habitat availability is measured.
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- Date Issued: 2016