Siphonophore assemblages along the east coast of South Africa; mesoscale distribution and temporal variations

Using multivariate statistics we have examined spatio-temporalpatterns in the distribution of siphonophore assemblages alongthe east coast of South Africa, anticipating constancy in cross-shelfand alongshore structure that would mirror the hydrographicstability. Indeed, while the surface temperature of the AgulhasCurrent does vary on a seasonal basis, the position of its coreis thought to have a stable trajectory year round, and its otherphysical and chemical characteristics are considered to be relativelyaseasonal. Two clear aseasonal assemblages were observed, associatedeither with the nearshore waters in the extreme SW (Port Alfredupwelling cell), or with the Agulhas Current and inshore watersnorth of East London. Assemblages in the current-driven upwellingcell were characterized by low overall diversity and dense populationsof Muggiaea atlantica (up to 28 000 ind. 1000 m^–3). Assemblageselsewhere in the region were dominated by Abylopsis eschscholtzi,A. tetragona, Bassia bassensis, Chelophyes contorta, Diphyesdispar, D. bojani, Eudoxoides spiralis, E. mitra, Lensia subtiloidesand Sulculeolaria chuni. There is some evidence of both an alongshoreand a cross-shelf pattern in the distribution of assemblagesnorth of East London, but this is subordinate to a strongerseasonal signal. The impact of seasonality on regional assemblagesis hypothesized to be reflected in significant changes in theupstream structure of Agulhas Current assemblages.     

Invasion Pathway of the Ctenophore Mnemiopsis leidyi in the Mediterranean Sea

Gelatinous zooplankton outbreaks have increased globally owing to a number of human-mediated factors, including food web alterations and species introductions. The invasive ctenophore Mnemiopsis leidyi entered the Black Sea in the early 1980s. The invasion was followed by the Azov, Caspian, Baltic and North Seas, and, most recently, the Mediterranean Sea. Previous studies identified two distinct invasion pathways of M. leidyi from its native range in the western Atlantic Ocean to Eurasia. However, the source of newly established populations in the Mediterranean Sea remains unclear. Here we build upon our previous study and investigate sequence variation in both mitochondrial (Cytochrome c Oxidase subunit I) and nuclear (Internal Transcribed Spacer) markers in M. leidyi, encompassing five native and 11 introduced populations, including four from the Mediterranean Sea. Extant genetic diversity in Mediterranean populations (n = 8, N _a = 10) preclude the occurrence of a severe genetic bottleneck or founder effects in the initial colonizing population. Our mitochondrial and nuclear marker surveys revealed two possible pathways of introduction into Mediterranean Sea. In total, 17 haplotypes and 18 alleles were recovered from all surveyed populations. Haplotype and allelic diversity of Mediterranean populations were comparable to populations from which they were likely drawn. The distribution of genetic diversity and pattern of genetic differentiation suggest initial colonization of the Mediterranean from the Black-Azov Seas (pairwise F _ST = 0.001–0.028). However, some haplotypes and alleles from the Mediterranean Sea were not detected from the well-sampled Black Sea, although they were found in Gulf of Mexico populations that were also genetically similar to those in the Mediterranean Sea (pairwise F _ST = 0.010–0.032), raising the possibility of multiple invasion sources. Multiple introductions from a combination of Black Sea and native region sources could be facilitated by intense local and transcontinental shipping activity, respectively.     

Patterns in marine hydrozoan richness and biogeography around southern Africa: implications of life cycle strategy

Aim To examine patterns of marine hydrozoan richness around southern Africa and to test the hypothesis that patterns of biogeography become weaker with increasing dispersal ability. Location The coastline of southern Africa from 21° S, 14° E to 28° S, 33° E, extending from the intertidal zone seawards a distance of 200 nautical miles. Methods Published and unpublished information on the distribution of marine Hydrozoa was entered as presence/absence data onto a gridded coastline of the region. A similarity matrix between the species composition of grid squares was constructed using the Bray–Curtis index and visualized using non-metric multidimensional scaling ordinations. Separate analyses were conducted, and compared, on the three major life cycle groupings: holoplanktic, meroplanktic and benthic. Results Over 450 species of marine Hydrozoa have been reported from the region, and species richness increases eastwards, in a manner at odds with the distribution of sampling effort. There was a significant correlation between the geographic structures of the resemblance matrices generated for the three life cycle groupings. In other words, all three groups showed similar patterns of biogeography around the region, and these were broadly similar to those reported by others. However, there were differences between them that reflect the resolution at which the data were examined. At a level of 40% similarity, there was no biogeographic structure to the holoplanktic fauna, the meroplanktic taxa were simply sub-divided into cool- and warm-temperate/subtropical elements, and in the case of benthic taxa, the cool-water fauna was further split into a southern Namaqua and a depauperate northern Namib component. Even at a resolution of 70% similarity, the holopelagic taxa could be separated only into cool-temperate and warm-temperate/subtropical faunas. Main conclusions Holoplanktic taxa show comparatively less biogeographic structure than meroplanktic taxa, which in turn show less clearly defined biogeographic structure than benthic taxa. It is suggested that this is related to the interaction between oceanography and dispersive-stage duration. The role that the Agulhas Current plays in influencing the Benguela Current fauna is highlighted. This study has implications for conservation planning exercises based on protecting representative biotopes in different biogeographic regions.