The persistent presence of Chrysaora fulgida (Scyphozoa; Discomedusae) in the northern Benguela ecosystem is not reflected by constant recruitment

Abstract

Abstract Unlike the situation in most temperate systems, medusae of Chrysaora (in this case, Chrysaora fulgida) are present year-round off Namibia, in a variety of size classes, implying the year-round release of ephyrae. This hypothesis was investigated by studying temporal changes in ephyrae and other gelatinous zooplankton in Walvis Bay over a 2-year period (January 2012 to December 2013), using biweekly plankton samples. All 12 of the recovered taxa were characteristically neritic and included meroplanktonic Hydrozoa and Scyphozoa as well as cydippid ctenophores and shallow water siphonophores. While ephyrae of C. fulgida were dominant overall and peaked in abundance during mid-spring, they were not present all year round, being replaced (in part) by medusae of Obelia in autumn and by the siphonophore Muggiaea atlantica in mid-summer and mid-winter. The structure of the assemblages was driven primarily by bottom water temperature and day length (explaining 27.2% of the variability in community structure). The recruitment of ephyrae of C. fulgida to the plankton off Walvis Bay is not continuous throughout the year, and this challenges our understanding of the population dynamics of this species in the region.