Browsing by Author "Eager, Rosemary"

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    Documenting the association between a non-geniculate coralline red alga and its molluscan host
    (University of the Western Cape, 2010) Eager, Rosemary; Maneveldt, Gavin W.; Faculty of Science
    To further investigate the strength of the association and the relative advantages of the association to both organisms, several manipulation experiments were set up. A cage experiment set up in the shallow subtidal zone showed that the coralline survived equally well without the winkle and did therefore not require the winkle or its empty shell for survival. A second controlled laboratory aquarium experiment was designed under both fluorescent (rich in blue light) and incandescent light (rich in red light) to ascertain whether the coralline had a preference for O. sinensis over the similar O. tigrina. This experiment was inconclusive as no recruitment was obtained under either of the light regimes. A third laboratory experiment was designed to determine whether the extra coralline weight had any possible advantage to the winkle, particularly against predation from the rock lobster Jasus lalandii. Results suggested that there were no apparent advantages to the winkle bearing the extra coralline load as adult O. sinensis bearing the coralline alga (3.7 ± 2.2 winkles 24hr-1) were equally prone to predation than those lacking the coralline (2.3 ± 1.9 winkles 24hr-1) (p = 0.184). Observations suggested instead that the convoluted nature of the coralline may indeed have promoted predation. We ultimately deduced that the high occurrence of the coralline on the shells of O. sinensis was probably due to the substantial overlap in the niches of the two organisms. This conclusion was supported by the high densities of juvenile O. sinensis combined with the high percent cover abundance of the coralline in intertidal rockpools. Understanding sexual reproduction in coralline algae as well as the life cycle of the winkle, ultimately provided insight into the postulated life cycle of this coralline-winkle association.
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    The effects of long-term exclusion of the limpet Cymbula oculus (Born) on the distribution of intertidal organisms on a rocky shore
    (NISC (Pty) Ltd and Taylor & Francis, 2009) Maneveldt, Gavin; Eager, Rosemary; Bassier, Aghmat
    Zonation patterns on rocky shores are typically as a result of both physical factors and biological interactions. Physical factors generally set the upper limits of species distributions, while biological interactions generally set their lower limits. Recent research has shown, however, that biological factors often can also influence the upward recruitment and colonisation by species. While such evidence exists in the international literature, very little experimental evidence exists for South Africa. This study provided experimental evidence for the biological effects of long-term exclusion (2003-2008) of the South African herbivorous limpet Cymbula oculus (Born), on the community structure of the Kalk Bay rocky intertidal. To demonstrate this, an herbivore exclusion experiment was set up in the mid-Eulittoral zone in May 2003. Initially, all herbivores were removed from the exclusion plots; after one year, 2 only C. oculus individuals recruiting into the plots were continually removed. Algal recruitment (percent cover abundance) was determined monthly for the first 12 months and then annually thereafter, while invertebrate recruitment (density) was monitored only annually. The results firstly show that the natural density of C. oculus had increased dramatically during the first year (from 4.87 ± 1.09 individuals m-2 to 12.35 ± 1.70 individuals m-2, p = 0.001). Secondly, grazing by C. oculus is the primary biological factor preventing the recruitment and colonization of macroalgae and some invertebrates onto the Kalk Bay intertidal. Not only does herbivory by C. oculus prevent recruitment and colonization, but it also decreases algal diversity and prevents algal succession. Within the mid-Eulittoral zone of the Kalk Bay rocky intertidal, herbivory by C. oculus is thus more important in shaping this marine community than physical factors associated with desiccation stress.