Vema Seamount Research Articles

Cetacean diversity of the eastern South Atlantic Ocean and Vema Seamount detected during a visual and passive acoustic survey, 2019

Abstract Cetaceans in the eastern South Atlantic Ocean are poorly studied. We present results from a 2 week ship-based survey from Cape Town to Vema Seamount (980 km to the west) during October–November 2019, including visual and towed-hydrophone observations from the vessel, and 10 days of acoustic monitoring on the seamount. Fifty-two hours of visual surveys resulted in 39 encounters of whale groups including seven of humpback, six of fin and one sei whale, as well as four unidentified baleen whales, 18 unidentified balaenopterid whales and four unidentified odontocetes. Two humpback whales at the seamount were engaged in possible feeding behaviour. A large aggregation of mostly fin whales was observed near the continental shelf edge (22 encounters over a 70 × 50 km2 area, six fin, one sei whale, 15 not confirmed to species), an historic whaling ground for both fin and sei whales. Towed-hydrophone data (78.7 h) detected five groups of sperm whales, 45 of delphinids, one beaked whale and no Kogiids. Acoustic data from the seamount detected calls from several baleen whale species including humpback whale non-song calls, Antarctic minke ‘bioduck’ calls, sei whale down-sweep calls and a likely Bryde's whale call. Two call types could not be assigned to species, including the most detected – a simple frequency-modulated call with peak power around 130 Hz. This study contributes to an improved understanding of cetacean occurrence in the eastern South Atlantic Ocean and highlights the need for more research to improve identification of cetacean vocalizations in the region.

A HIMU volcanic belt along the SW African coast (∼83–49 Ma): New geochemical clues to deep mantle dynamics from carbonatite and silica-undersaturated complexes in Namibia

The origin of carbonatitic and highly silica-undersaturated volcanism, common along the SW coast of Africa extending from Angola through Namibia to the tip of South Africa, is still poorly understood. Here we present new geochemical data (major and trace element and Sr-Nd-Pb-Hf-O-C isotopes) from the Agate Mountain calcio- to magnesio‑carbonatites (∼83 Ma), Dicker Willem calcio‑carbonatites (49 Ma) and Swakopmund basanitic plugs (76–72 Ma) along the coast of Namibia that were emplaced after the EMI (enriched mantle one) type Etendeka flood basalts. The trace element and isotopic composition of Agate Mountain carbonatites and Swakopmund basanites indicate that they were derived from a HIMU-type (high time-integrated 238 U/ 204 Pb with radiogenic Pb isotope ratios) magma source, similar to the St. Helena global HIMU endmember in the South Atlantic. The Agate Mountain carbonatites form part of the late-stage Walvis Ridge HIMU hotspot track overlying the EM1-type Walvis Ridge basement forming part of the Tristan-Gough hotspot track. The Dicker Willem carbonatites, however, extend to higher 206 Pb/ 204 Pb than St. Helena, but have similar 206 Pb/ 204 Pb to Mangaia HIMU lavas in the Pacific. Compared to Mangaia HIMU, the Dicker Willem carbonatites with mantle-type O and C isotopes have higher 207 Pb/ 204 Pb and 87 Sr/ 86 Sr but lower 143 Nd/ 144 Nd, suggesting it may represent a new HIMU endmember flavor. The HIMU carbonatitic and silica-undersaturated rocks form a belt of age-progressive volcanic tracks, including: 1) from the Walvis Ridge, through NW Namibia to central Angola, 2) from the Vema Seamount via Dicker Willem carbonatite to Gibeon kimberlites and carbonatites, 3) from the Namaqualand to Bushmanland and to Warmbad volcanic centers in northwestern South Africa, and 4) along the older end of the Shona EMI-type volcanic track extending into South Africa. Geochemical and seismic tomographic data suggest that the EMI and HIMU mantle plumes are generated from different geochemical domains at the base of the lower mantle. The Tristan-Gough, Discovery and Shona EM1 volcanic tracks are derived from a common low-velocity anomaly (superplume-like structure with three branching arms) ascending from the outer margin, possibly lower primoridal layer, of the African large low-shear-velocity province (LLSVP). Seismic low-velocity anomalies can be traced from beneath the belt of HIMU volcanism to an internal and shallower part of the LLSVP, located ∼900–1200 km east of the outer LLSVP margin and suggest that HIMU-type (possibly subducted oceanic lithospheric) material overlies EMI-type (possibly primordial) material in the internal part of the LLSVP. • A HIMU-like volcanism belt along the southwest Africa. • The HIMU-like volcanic complexes form age-progressive volcanic tracks. • EMI and HIMU mantle plumes are from different domains in the lower mantle.

Detection of humpback whale (Megaptera novaeangliae) non-song vocalizations around the Vema Seamount, southeast Atlantic Ocean.

Humpback whales are a cosmopolitan, highly vocal species. Investigated here are their vocalizations recorded at the Vema Seamount (31°38'S, 08°20'E) from moored hydrophones in the austral spring of 2019. During the 11-d recording period over 600 non-song calls were detected. Calls were predominantly detected at night over three consecutive days. The most common calls were low, frequency-modulated sounds (whups). An impulsive sound (gunshot) previously unknown in humpback whales was also detected. The location and timing of the calls suggests that humpback whales may be using the Vema Seamount as a temporary stop on their migration to their polar feeding grounds.

Hydroids (Cnidaria, Hydrozoa) from the Vema and Valdivia seamounts (SE Atlantic)

In this report, we analyse the benthic hydroids collected on the Vema and Valdivia seamounts during a survey conducted in 2015 in the SEAFO Convention Area, focused on mapping and analysing the occurrence and abundance of benthopelagic fish and vulnerable marine ecosystem (VMEs) indicators on selected Southeast Atlantic seamounts. A total of 27 hydroid species were identified, of which 22 belong to Leptothecata and only five to Anthoathecata. Monostaechoides gen. nov. was erected within the family Halopterididae to accommodate Plumularia providentiae Jarvis, 1922, and a new species, Monotheca bergstadi sp. nov., is also described. Campanularia africana is recorded for the first time from the Atlantic Ocean, and the Northeast Atlantic species Amphinema biscayana, Stegopoma giganteum and Clytia gigantea are also recorded from the South Atlantic. Three species were identified to the genus level only, due to the absence of their gonosomes. None of the reported species are endemic, and the hydroid community is clearly dominated by species with a wide geographical distribution in the three major oceans. Only Monotheca bergstadi sp. nov. presently has its distribution restricted to the Vema Seamount and the South African coast.

Bridging the gap: first record of sponge genus Strongylodesma in Australian waters.

Eight species of Strongylodesma Lévi, 1969 (Demospongiae Sollas, Poecilosclerida Topsent, Latrunculiidae Topsent) are recognised today (Van Soest et al. 2019): three from South Africa (S. algoaensis and S. tsitsikammaensis Samaai et al., 2003; S. aliwaliensis Samaai et al., 2004); two from the Tropical Western Atlantic (S. nigra and S. purpurea Samaai Kelly, 2009); and two from the Western Pacific Ocean (S. novaecaledoniae and S. tongaensis Samaai Kelly, 2009). The type species, S. areolata Lévi, 1969, is from Vema Seamount in the South Atlantic Ocean, about 1000 km northwest of Cape Town. Prior to this work, Strongylodesma had not been recorded in the Southwest Pacific region: here we report for the first time, a new species from temperate Australia, S. australiense sp. nov.

Megabenthos and benthopelagic fishes on Southeast Atlantic seamounts

In a descriptive study of megafauna of several Southeast Atlantic seamounts, multiple video-transects on upper slopes and summits documented the occurrence of benthic invertebrate taxa, primarily corals, regarded as indicators of vulnerable marine ecosystems (VMEs) as defined in international guidelines. At Schmitt-Ott Seamount there was a pronounced dominance of gorgonian corals (seafans, Alcyonacea). In all other study areas the diversity was greater, and more scleractinians (stony corals, Scleractinia) were observed. Scleractinian corals were mainly dead, and much of the coral framework and rubble may have been ancient. In the Valdivia complex and on Ewing Seamount, which are open to fisheries, scleractinians seemed restricted to some slopes of knolls, and on Valdivia Bank and the subarea denoted Valdivia West the summit substrate was mostly bare rock. Pelagic armourhead Pseudopentaceros richardsoni and splendid alfonsino Beryx splendens (two targets of commercial fisheries in the area) were observed at a few sites, but did not appear to be abundant in the main former fishing areas of the Valdivia area. Orange roughy Hoplostethus atlanticus was common in video records around the summit at Ewing Seamount. The deep-sea red crab Chaceon erytheiae was abundant in the Valdivia area and at Ewing Seamount, and crabs were distributed across a more extensive depth range than the fishes. In areas with high densities of live coral, the video records suggested that the benthic communities were intact and not impacted by fishing. Evidence of past fishing activities included observations of lost pots and rope at Vema Seamount and in the Valdivia area.

The Recent Cypraeidae of Northern New Zealand from the Kermadec Islands to the Poor Knights Islands, Southwest Pacific Ocean (Mollusca: Gastropoda: Cypraeidae)

For researchers, isolated regions at the periphery of species’ distributions hold a peculiar fascination. The causes of their remoteness vary based on: distance (e.g. the Tropical Eastern Pacific), distance and countervailing currents (e.g. the Marquesas), location in a present day gyre (e.g. the Pitcairn Group) or the absence of present day means of veliger transport (e.g. the Vema Seamount). (Daughenbaugh & Beals 2013; Daughenbaugh 2015a & b, 2017). The northern New Zealand Region from the Kermadec Islands (Kermadecs) to the coastal and shelf areas in the northernmost part of New Zealand’s North Island (Northland), including the Poor Knights Islands (PKI), constitute the distributional boundaries for a number of Cypraeidae species. The boundaries are the result of the absence of coastal shelves along the east side of the Kermadec Ridge (Ridge) and precipitous drops to abyssal depths along Northland’s east coast continental shelf. Tropical waters, with their potential to transport Cypraeidae larvae, flow eastward from southern Queensland, Australia, entrained in the Tasman Front which terminates when reaching North Cape, the northernmost tip of Northland. There, the North Cape Eddy captures most of this flow while the remainder, the East Auckland Current (EAUC), flows intermittently southward along the eastern coastal, shelf and offshore areas of Northland into waters incapable of supporting Cypraeidae populations.

The Cypraeidae of the Southeast Atlantic Vema Seamount (Mollusca: Gastropoda: Cypraeidae)

The Vema Seamount, discovered in 1957, is located in the southeast Atlantic Ocean, 480 km west off the Namibian coast and 1,000 km west northwest of Cape Town, South Africa. To say this is an unusual location and habitat for Indo-Pacific Cypraeidae species is an obvious understatement. Yet, in the late 1980s and early 1990s, at least two species and possibly a third were reported from that location. Are these migrants or from a population at the seamount? The acquisition of one such species from Werner Massier, a prominent Namibian dealer and author, prompted the research which led to this article.

High connectivity and lack of mtDNA differentiation among two previously recognized spiny lobster species in the southern Atlantic and Indian Oceans

Adult spiny lobsters Jasus paulensis and J. tristani inhabit non-emergent seamounts and remote islands in the southern mid-latitude (30–45°S) Indian and Atlantic Oceans. Populations are often thousands of kilometres apart, and metapopulation genetic structure relies on successful exchange of long-lived drifting larvae. Genetic population structure and connectivity were investigated by sequencing the mitochondrial cytochrome oxidase I gene from lobsters collected at two Indian Ocean sites (Seamount 150 and St Paul/Amsterdam Islands) and two Atlantic sites (Vema Seamount and Gough/Inaccessible Islands) between 2006 and 2011. Jasus paulensis and J. tristani individuals share identical haplotypes across the distribution range and Bayesian phylogenetic analyses strongly support the monophyly of all individuals sampled, to the exclusion of all other recognized Jasus species. Analyses of molecular variance revealed no significant population genetic differentiation between sites, and Fu's F s and a mismatch distribution suggested demographic expansion, which was estimated to have occurred between 14,000 and 118,000 years ago. The results show an apparent lack of barriers to dispersal and gene flow over thousands of kilometres of the southern Indian and Atlantic Oceans, and suggest that J. paulensis and J. tristani should be synonymized as J. paulensis (Heller, 1862).

Zoogeography of the southern New Zealand, Tasmanian and southern African ascidian fauna

We studied the relationships among the ascidian fauna of southern New Zealand, Tasmania, and southern Africa, considering main currents and surface water temperature. We constructed a presence/absence matrix of 494 ascidian species from these regions (which were also divided into provinces) for 28 biogeographical worldwide regions using published records. Biogeographical areas were classified using cluster analysis combined with non‐metric multidimensional scaling ordination. The region classification separated Vema Seamount because of the low number of citations and the high number of ascidian endemism owing to an island effect. The Tasmanian, southern African and southern New Zealand regions formed separate groups. The ascidian fauna was dominated by endemic species in southern New Zealand (43%) and southern Africa (45%), whereas southern Australian species dominated the Tasmanian fauna (41%).

Long current to nowhere? — Genetic connectivity of Jasus tristani populations in the southern Atlantic Ocean

The commercially exploited spiny lobster Jasus tristani has a disjunct distribution in the southern Atlantic Ocean, with populations occurring at the Tristan da Cunha Archipelago, Gough Island and on Vema Seamount. A distance of 2 000km separates Vema and the Tristan Archipelago. In order to determine genetic connectivity of lobster between these locations, a region of the cytochrome oxidase II (COII) gene was sequenced for 193 individuals of J. tristani from five sampling sites (Tristan da Cunha, Nightingale, Inaccessible, Gough islands and Vema Seamount). Our results show that J. tristani in the southern Atlantic share a most recent common ancestry that dates back at least one million years. Analyses of molecular variance and pairwise Φst analyses reveal shallow but significant genetic partitioning between Vema Seamount and all other locations. No population differentiation was detected among any of the remaining sampling sites. Coalescent analyses show limited gene flow between Vema and the archipelago (including Gough Island), possibly mediated by larval dispersal in the South Atlantic gyre system. Population divergence between Vema and the other populations, using MDIV, was estimated to be approx. 270 000 years ago. Mismatch distributions, a haplotype network, and Fu's Fs test, all suggest that the J. tristani population underwent population expansion between 12 000 and 99 000 years ago.

Meganthias carpenteri, new species of fish from the eastern Atlantic Ocean, with a key to eastern Atlantic Anthiinae (Perciformes: Serranidae)

ABSTRACT Meganthias carpenteri is described from two specimens collected off Nigeria. An additional specimen of Meganthias, caught in the eastern South Atlantic over Vema Seamount, is considered, as is M. natalensis, the type species of the genus. The three Atlantic specimens of Meganthias are the first of the genus recorded from that ocean. Previously described species of Meganthias are M. natalensis (western Indian Ocean) and M. kingyo (Ryukyu Islands, western Pacific). Meganthias carpenteri differs from them in the morphology of the lips and maxilla.

Zoogeography of the southern African ascidian fauna

AbstractAim To describe the biogeography of the ascidian fauna of southern Africa, to compare the results obtained with those reported for other fauna and flora of the same region, and to speculate about the origin of ascidians in the region.Location Southern Africa extending over 4000 km from Mossâmedes (15° S–12° E) to Inhaca Island (26°30′ S–33° E), including Vema Seamount (31°40′ S–8 °20′ E), Amsterdam‐Saint Paul Islands (38° S–77°30′ E) and the Tristan‐Gough Islands (38° S–12°20′ W).Methods We constructed a presence/absence matrix of 168 species for 26 biogeographical divisions, 21 classical biogeographical regions described by Briggs (Marine zoogeography, McGraw‐Hill, New York, 1974) and five provinces within the southern African region. We considered the following limits and divisions into provinces for the southern African region: Namibia, Namaqua, Agulhas and Natal as proposed by Branch et al. (Two oceans. A guide to the marine life of southern Africa, David Philip Publishers, 1994), and the West Wind Drift Islands province (WWD) according to Briggs (Global biogeography, Elsevier Health Sciences, Amsterdam, 1995). To examine the biogeographical structure, species and divisions were classified using cluster analysis (based on UPGMA as the aggregation algorithm) with the Bray–Curtis index of similarity. This classification was combined with MDS ordination.Main conclusions Four main groups were obtained from the analysis of affinities among species: (1) species present in the WWD, separated by a high percentage of endemisms and a low number of species with a southern African distribution. Moreover, in the light of the species distribution and the results of further analysis, which revealed that they are completely separated and not at all related to the southern African region, it appears that there are no close relationships among the different islands and seamounts of the West Wind Drift Island province. This province was therefore removed from the remaining analyses; (2) species with a wide distribution; (3) species of colder waters present in Namaqua and Agulhas provinces, a transitional temperate area in which gradual mixing and replacement of species negate previous hypotheses on the existence of a marked distributional break at Cape of Good Hope; (4) species of warmer waters related to Natal province. The classification into biogeographical components was dominated by the endemic (47%), Indo‐Pacific (25%) and cosmopolitan (13%) components. The analysis of affinities among biogeographical areas separated Namibia from the rest of the southern African provinces and showed that it was related to some extent to the Antarctic region because of the cold‐temperate character of the province and the low sampling effort; Namaqua, Agulhas and Natal were grouped together and found to be closely related to the Indo‐West Pacific region. In general, our results were consistent with those obtained for other southern African marine invertebrates. The frequency distribution of solitary/colonial strategies among provinces confirmed the domination of colonial organisms in tropical regions and solitary organisms in colder regions. Finally, we speculate that the southern African ascidian fauna mainly comprises Indo‐Pacific, Antarctic and eastern Atlantic ascidians.

First record of Kentrocapros rosapinto (Tetraodontiformes: Ostraciidae) from Vema Seamount (south-east Atlantic)

The capture of a specimen of Kentrocapros rosapinto at Vema Seamount in the south-east Atlantic Ocean is reported as the first record of this species in the Atlantic Ocean. This species was previously known only from the western Indian Ocean.

A kinematic analysis of the Indian/Atlantic interocean exchange

We analyze the Indian/Atlantic interocean exchanges in an eddy-permitting numerical simulation focusing on the Cape Basin, a region of the southeastern Atlantic bounded on the northwest by the Walvis Ridge and on the south by the Agulhas Ridge (near 40°S). To quantify the relative importance of the different dynamical mechanisms involved in the interocean exchange, we separated the climatological mean circulation from the transients. The analysis indicates that Agulhas eddies influence not only the transient but also fluxes associated with the mean circulation (eddy fluxes, for example, supply most of the energy of the Benguela Current). A distinctive characteristic of the eddy variability within the Cape Basin is the co-existence of cyclonic and anticyclonic vortices in dipole structures that resemble the Heton model of Hogg and Stommel (Proc. R. Soc. London. A 397 (1985) 1). Anticyclones are surface-intensified vortices that, in spite of their baroclinic structure, reach to deep layers. Cyclones are bottom-intensified vortices with dominant barotropic structure that projects into the upper layer. The propagation of both cyclones and anticyclones is strongly affected by the bottom topography. Our analysis shows that the Walvis Ridge and the Vema Seamount block the passage of bottom-intensified cyclones and changes the trajectories of the upper-intensified anticyclones. Although most anticyclones are able to escape the basin, the deep compensation generated by the ridge generates an energy loss of approximately 30%, and a change of the eddy trajectory to a more westward direction.

Translation, decay and splitting of Agulhas rings in the southeastern Atlantic Ocean

All Agulhas rings that were spawned at the Agulhas retroflection between 1993 and 1996 (a total of 21 rings) have been monitored using TOPEX/Poseidon satellite altimetry and followed as they moved through the southeastern Atlantic Ocean, decayed, interacted with bottom topography and each other, or dissipated completely. Rings preferentially crossed the Walvis Ridge at its deepest parts. After having crossed this ridge they have lower translational speeds, and their decay rate decreases markedly. Half the decay of long‐lived rings takes place in the first 5 months of their lifetimes. In addition to the strong decay of rings in the Cape Basin, about one third of the observed rings do not seem to leave this region at all but totally disintegrate here. The interaction of rings with bottom topography, in particular with the Vema Seamount, is shown frequently to cause splitting of rings. This will enhance mixing of the rings' Indian Ocean water into that of the southern Atlantic. This localized mixing may well provide a considerable source of warm and salty Indian Ocean water into the Atlantic overturning circulation.

An agulhas ring in the South Atlantic ocean and its interaction with the Benguela upwelling frontal system

An Agulhas ring was detected using XBT probes on a cruise between Cape Town and Vema Seamount in the south-east Atlantic Ocean in April 1989. CTD and nutrient data, collected on a second cruise in May 1989, GEOSAT altimeter data for February–April 1989 and cloud-free NOAA-11 satellite imagery from June 1989 were used to characterize the ring. The ring was elliptical (330 km E-W and 165 km N-S, relative to the 16°C isotherm at 200 m depth), evident to at least 1200 m, and centred on 30.5°S, 9.2°E in May, about 700 km west of the Orange River. Its drift velocity was 6.4 ± 1 cm s −1 to the NW. Maximum anticyclonic geostrophic surface currents near its edge were 55 cm s −1 relative to 1150 db. The available potential energy was estimated to be 38.8 × 10 15 J and the kinetic energy 2.3 × 10 15 J using a two layer model of the ring. A cool filament extending 450 km offshore from the Benguela upwelling front was identified in the hydrography and the NOAA-11 imagery. Entrainment velocities (maximum of 75 cm s −1) of mature upwelled water from the Benguela frontal region were inferred from feature tracking.

The rock-lobster Jasus tristani on Vema Seamount: Drifting buoys suggest a possible recruiting mechanism

The recruitment mechanism for rock-lobster larvae to the isolated location of Vema Seamount in the South Atlantic Ocean could occur by the drift of larvae from the distant Tristan da Cunha Islands. An analysis of the drifts of free-floating weather bouys in the area presents the first support for this long-standing hypothesis by showing that the drift directions, the travel times, and the sea-surface temperatures experienced on route are those required for successful recruitment.

Ocean circulation studies in the vicinity of Southern Africa: Preliminary results using FGGE drifters and remote sensing

The three highly variable ocean circulation systems which meet south of South Africa have been studied by combining the use of satellite thermal infra-red imagery, hydrographic measurements and drifting buoys. We present a few preliminary examples where this method has been successful. These include the sources of the Agulhas Current, the influence of the Agulhas Current on the Cape upwelling region as well as the interaction of the Agulhas Current with the Subtropical Convergence. An example is also given of how a biological hypothesis concerning the recruitment of a rock lobster species at Vema Seamount has been supported by the analysis of the drifts of FGGE buoys in the South Atlantic Ocean.

BIOLOGY AND ZOOGEOGRAPHY OF THE VEMA SEAMOUNT: A REPORT ON THE FIRST BIOLOGICAL COLLECTION MADE ON THE SUMMIT

Summary A collection of marine organisms, made in 1964 on the summit region of the volcanic Vema Seamount, situated some 450 miles off the west coast of South Africa, is discussed. The general terrain of the summit was rocky, and kelp, lesser algae and encrusting animals were dominant. The collection consisted of some 22 species of algae and over 105 species of benthic invertebrates. The three largest zoogeographical groups of animals on the Seamount were (a) the endemic species (28%, of which the majority belonged to the little known Porifera and Ascidiacea), (b) the cosmopolitan, and those species with a scattered distribution (27%) and (c) those species found only on the Seamount and in South Africa (25%). Species found on the Seamount whose distribution was limited to the Indo-Pacific region, and to West Africa and Europe with some overlapping into South Africa, amounted each to about 10% of the fauna. Only one species, the rock lobster, Jasus tristani, is confined to the Seamount and the relatively close Tristan da Cunha group of islands.