Browsing by Author "Gibbons, Mark J"

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    Cross-shelf movement of chrysaora fulgida (scyphozoa; discomedusae) off namibia inferred from stable isotopes (δ15n and δ13c)
    (Taylor and Francis Group, 2021) Skrypzeck, Heidi; van der Lingen, Carl David; Gibbons, Mark J
    Large and small specimens of two species of metagenic Scyphozoa (true jellyfishes) can be found in nearshore waters off central Namibia throughout the year. Whereas populations of Chrysaora africana are largely restricted to inshore waters, C. fulgida occurs across the shelf, with small individuals found inshore and large individuals primarily found offshore. We examined stable isotopes δ15N and δ13C of both species in Walvis Bay and found that large-sized C. fulgida have lower δ15N values than small individuals and C. africana throughout the year. These differences are interpreted to reflect cross-shelf changes in δ15N baseline levels, with greater nitrogen recycling (and hence lower δ15N values) occurring offshore. The occasional/seasonal nearshore appearance of large C. fulgida with low δ15N values therefore implies routine, onshore advection. The values of δ13C did not show cross-shelf differences, which suggests that jellyfish populations across the shelf are supported by phytoplankton-based food chains. This study emphasises the value of using stable isotopes to examine the mesoscale structuring of jellyfish populations.
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    Dynamics and biological implications of the Port St Johns cyclonic lee eddy
    (University of the Western Cape, 2016) Jacobs, Lee-Ann; Gibbons, Mark J
    This project describes the current dynamics of the cyclonic lee eddy off Rame Head, Port St Johns and Waterfall Bluff, and how its physical forcing may influence the biological communities. Three research surveys were conducted during July 2013, December 2013 and July 2014 during which environmental data (temperature, oxygen, salinity and chlorophyll) and biological samples (zooplankton) were collected from 18 stations spanning latitudes 31.4 S - 32 S and longitudes 29.3 E - 30 E. A 300 kHz Acoustic Doppler Conductivity Profiler (ADCP) mooring deployed off Port St Johns revealed maximum current velocities 1.5 m s-1, associated with the Agulhas Current. Forty-one current reversals were observed over a 12 month period, with most of these reversals noted on the bottom boundary layer. Warming and cooling events were associated with these current reversals. Ship-ADCP measurements also revealed inshore counter-currents and cyclonic flow along the surface layers. There may have been a spin-up of a cyclonic eddy between Rame Head and Port St Johns during July (2013 and 2014), which could have influenced the productivity on the shelf during the winter months. Satellite altimetry also revealed the presence of both cyclonic and anticyclonic eddies over and offshore of the study area during the cruises.
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    Towards an unravelling of the taxonomy of Chrysaora (Scyphozoa; Semaeostomeae; Pelagiidae) from around South Africa
    (University of the Western Cape, 2017) Ras, Verena; Gibbons, Mark J; Engelbrecht, Adriaan
    Historically, two species of Chrysaora are known from the Benguela Current Ecosystem: C.fulgida (Reynaud 1830) and C. africana (Vanhöffen 1902). However a third morphotype is now seen, which bears a resemblance to both. Thus a complete qualitative and quantitative analysis of the morphometric and meristic data of these three species was conducted, along with an in depth study into the cnidome as a potential tool of identification. These findings are supplemented by a genetic analysis using cytochrome c oxidase subunit I and internal transcribed spacer 1 gene markers. Three species were unambiguously identified. The genetics and morphology showed considerable divergence, with some of the features used to separate them including: tentacle number and shape, colouration and shape of the oral arm, shape of the gastrovascular pouches and the number and shape of the marginal lappets. Although the mtDNA indicated clear separation of the three Chrysaora, the nucDNA displayed some ambiguity. The cnidome showed considerable divergence and succeeded in separating these species, while the rhopalia of the three species also showed distinct differences in the lengths of the rhopalal canals and basal stems. Much of the confusion surrounding jellyfish taxonomy has been the result of observations made on predominantly preserved specimens that are in subpar quality, a hinderence which this study endeavored to overcome.