Observations on the ecology and life-history of Chrysaora fulgida (Reynaud 1830) (Scyphozoa: Semaeostomeae) and other pelagic cnidarians in the inshore waters off central Namibia
MetadataShow full item record
Although jellyfish are recognised recently as key components that can influence ecosystem functioning and trophic flows in the northern Benguela upwelling ecosystem, the number of published studies on their abundance, seasonality, life history and ecological roles off Namibia is strictly limited. Chrysaora fulgida is one of the most common and conspicuous medusae in the plankton off Namibia, and has flourished in the region, following the decline of the pilchard fishery at the end of the 1960s. It is said that their biomass (together with Aequorea forskalea) exceed that of the commercially important fish stocks off Namibia. In addition, this species is also capable of forming large swarms in northern Benguela where they are a nuisance to fisheries operations. The objective of this study is to try and fill gaps regarding our knowledge of the biology and ecology of Chrysaora fulgida off Namibia, with a view to improve our understanding of its success in the northern Benguela ecosystem. In the Chapter 1, a general overview on the current knowledge and population dynamics of jellyish blooms and their ecology is compiled. Other key topics of the thesis such as jellyfish life cycles and their reproduction are also introduced. Chapter 2 investigates the temporal changes in the jellyfish community in Walvis Bay over a 23-month period from biweekly plankton samples. All twelve of the recovered taxa were characteristically neritic, and included meroplanktonic Hydrozoa and Scyphozoa, as well as cydippid ctenophores and shallow water siphonophores. Whilst, ephyrae of Chrysaora fulgida were dominant overall, and peaked in abundance during mid-spring (Year 2012: 168 933 ind. 100 m-3) and late winter (Year 2013: 23 389 ind. 100 m-3), they were not present all year round, being replaced (in part) by Obelia in summer and autumn, Bougainvillia in spring and summer, and Muggiaea atlantica in summer. Seasonal changes in the composition and structure of the community were driven primarily by bottom water temperature and day length (explaining 24% of the variability in community structure), with wind speed and moon illumination playing a secondary role. The recruitment of ephyrae of C. fulgida to the plankton off Walvis Bay is confirmed not to be continuous throughout the year. Chapter 3 present the first detailed investigation on the identification, morphological development and growth of wild caught ―ephyrae‖ of the scyphozoan Chrysaora fulgida and Chrysaora africana in Walvis Bay, off Namibia. Concrete morphological dissimilarities are documented to distinguish C. africana from C. fulgida, despite the limited sample size of C. africana: coloration differences and the presence/absence of branched canals on the periphery of velar and rhopalial canal tips. In the case of C. fulgida the morphological development from an ephyra (Stage 0) to a juvenile medusa could be described successfully in six stages, whilst missing stages were noted for C. africana. In general, the development of ephyrae described here agrees with patterns described for other species in the genus from elsewhere. The ephyrae stages of C. fulgida illustrated a low overall growth rate (4.33 and 3.45% d-1, respectively) and longer ontogenic development (~164 days), respectively, than most other jellyfish species. Through the histological examination of medusa gonads, Chapter 4 investigates the sexual reproduction and maturation of both Chrysaora species, collected off Walvis Bay, Namibia. Both species were non-brooding, gonochoristic, displayed a 1:1 sex ratio and exhibited no clear sexual dimorphism features. Gametogenesis in both species was similar to that displayed by other Discomedusae, whilst some differences in gonad maturity were evident between them – Chrysaora fulgida displayed aseasonal, reproductive heterogeneity (maturing at ~300 mm diameter) and individuals were semelparous, whilst C. africana appeared strongly seasonal but iteroparous. Through stable isotope analysis (𝛿13C, 𝛿15N and C:N ratios), Chapter 5 examines the presence of tissue, ontogenetic, seasonal, spatial and interspecific variability in medusae of Chrysaora fulgida and Chrysaora africana off Walvis Bay, in the northern Benguela, Namibia. This study did not only illustrate size-associated shifts in trophic ecology, but also revealed spatial, inter-species and some tissue differences in the northern Benguela upwelling system. Size would appear to be the over-riding factor that influences the isotope signatures of Chrysaora fulgida; size being linked in turn to space. A clear negative relationship is illustratred between 𝛿15N and individual size for two scyphozoans (C. fulgida and C. africana) off central Namibia, indicating that larger jellyfish feed lower down the food chain than smaller ones in both species. This is explained by the need and ability of ephyrae and small medusae to access the microbial food web which consists of many trophic steps and hence numerous opportunities for enrichment of nitrogen isotopes, resulting in higher 𝛿15N values of smaller individuals. Chapter 6 provides a synthesis of the main findings of the thesis, and makes recommendations on ways that the research can be carried forward.