Southwest Atlantic Research Articles

Temporal and spatial variability in the mesozooplankton community off the Falkland Islands (Southwest Atlantic)

Zooplankton communities vary in space and time. Their composition is strongly influenced by lower trophic levels that are dependent on the availability of light and nutrients. As all marine ecosystems are relying on zooplankton as intermediate trophic step between primary production and higher trophic levels, changes in the zooplankton community composition and biomass can cascade through the food web with important impacts on fish communities and through that on fisheries yields. An intense fisheries exist around the Falkland Islands in the SW Atlantic Ocean, around 51° S, but to the best of our knowledge, no previous study has to date investigated the seasonal variation in zooplankton community composition in these waters. We show that copepods (39.2%), the larvae of the anomurid Grimothea gregaria (33.1%) and euphausiids (10.9%) dominate the local mesozooplankton community by biomass. All species showed seasonal patterns, including ontogenetic behaviour of G. gregaria migrating to deeper waters with development, which were significantly explained by temperature (p < 0.001). While overall biomass significantly decreased with distance from shore (p < 0.001), mesozooplankton diversity was highest at 30 km from shore. The presented study is the first assessment of the mesozooplankton biomass off the Falkland Islands and provides a first baseline to aid future ecosystem studies in the context of ecosystem based fisheries management in the region.

Assessment of prokaryotic communities in Southwestern Atlantic deep-sea sediments reveals prevalent methanol-oxidising Methylomirabilales

Continental slopes can play a significant contribution to marine productivity and carbon cycling. These regions can harbour distinct geological features, such as salt diapirs and pockmarks, in which their depressions may serve as natural sediment traps where different compounds can accumulate. We investigated the prokaryotic communities in surface (0–2 cm) and subsurface (18–20 or 22–24 cm) sediments from a salt diapir and pockmark field in Santos Basin, Southwest Atlantic Ocean. Metabarcoding of 16 samples revealed that surface sediments were dominated by the archaeal class Nitrososphaeria, while the bacterial class Dehalococcoidia was the most prevalent in subsurface samples. Sediment strata were found to be a significant factor explaining 27% of the variability in community composition. However, no significant difference was observed among geomorphological features. We also performed a metagenomic analysis of three surface samples and analysed the highest quality metagenome-assembled genome retrieved, which belonged to the family CSP1–5, phylum Methylomirabilota. This non-methanotrophic methylotroph contains genes encoding for methanol oxidation and Calvin Cycle pathways, along with diverse functions that may contribute to its adaptation to deep-sea habitats and to oscillating environmental conditions. By integrating metabarcoding and metagenomic approaches, we reported that CSP1–5 is prevalent in the sediment samples from Santos Basin slope, indicating the potential importance of methanol metabolism in this region. Finally, using a phylogenetic approach integrating 16S rRNA sequences assigned to Methylomirabilota in this study with those from a public database, we argued that CSP1–5 public sequences might be misclassified as Methylomirabilaceae (the methanotrophic clade) and, therefore, the role of these organisms and the methanol cycling could also be neglected in other environments.

Geochemistry, petrology, and isotopic characteristics of the Enseada da Caieira intrusions: Insights from mineral chemistry and Nd and Sr isotopes from Fernando de Noronha, South Atlantic, Brazil

The alkaline magmatism found in the Brazilian Equatorial Margin (BEM) in the Southwest Atlantic Ocean, such as that observed in Fernando de Noronha Archipelago, poses intriguing questions about its origin and its relationship to guyots and onshore magmatism of similar alkaline compositions in northeast Brazil. The Fernando de Noronha Archipelago (FNA) exhibits a broad spectrum of alkaline magmatism, ranging from foiditic to phonolitic/trachytic compositions. At the Enseada da Caieira, a group of intrusions reflects this wide variety from FNA. This paper presents a complete description of these intrusions based on petrography, lithogeochemistry, and mineral and isotopic chemistry, as well as thermobarometry and petrogenetic modeling. The new data are compared with data compiled from the FNA, BEM onshore and offshore magmatism, and the Vitória-Trindade Ridge (VTR). The intrusions are classified as trachyte, alkali-basalt/basanite, phonotephrite, and picrobasalt. The compositional diversity of dikes outcropping in a relatively small area suggests a complex petrogenetic evolution for the feeding system of the Fernando de Noronha Island. This is supported by the presence of porphyritic clinopyroxenes (for example) with compositional zoning, indicating multistage crystallization processes. Flow, resorption, and glomeroporphyritic textures are common. Skeletal crystals also occur as phenocrysts of olivine and clinopyroxene due to supercooling. Secondary minerals such as zeolites and carbonate commonly occur as fillers in amygdules. Values for ƐNd range from +0.2 to +3.1. The Sr–Nd isotopic signatures suggest a mixture between an enriched component (EMII) and a depleted mantle component (DMM). A multi-approach thermobarometer using clinopyroxene, amphibole, and zircon mineral chemistry reveals different stages for crystallization, with pressure and temperature ranging from 320 to 1300 MPa and 750 to 1240 °C, respectively. The whole-rock lithogeochemistry modeling suggests a low degree of partial melting (around 2% or less) of an enriched source with variable proportions from spinel and garnet-lherzolite melting zones.

The use of the shape and chemistry of fish otoliths as a subpopulational discrimination tool for Eugerres brasilianus in lagoon systems in the Southwest Atlantic Ocean

The Brazilian mojarra, Eugerres brasilianus, is an economically important species for the artisanal fisheries usually found in the estuarine waters along the southwest Atlantic Ocean. Despite this, knowledge about its population structure is scarce, and no management strategies have been applied to ensure the fisheries sustainability in Brazil. Thus, the present study intended to understand the population dynamics of E. brasilianus in three costal lagoons located in Rio de Janeiro, Brasil. A total of 90 individuals were collected in the lagoon systems of Itaipu, Saquarema and Araruama, between December 2019 and March 2020. A pre-selection of 30 individuals per location from the same age group (2 years old), following age estimation by counting the annual growth increments, were used. The contour of the sagittal otoliths was evaluated using elliptical Fourier descriptors (EFD), and the multi-elemental signatures (MES) of the whole otoliths were obtained using solution-based inductively coupled plasma mass spectrometry. Data were analyzed using univariate and multivariate statistics to assess the degree of separation between individuals from different lagoons. EFD revealed significant differences among individuals from the different sampling regions. MES exhibited distinct regional patterns, mainly driven by differences in Cu/Ca, Li/Ca, Mg/Ca, Mn/Ca, and Sr/Ca ratios. Reclassification accuracy rates obtained from linear discriminant function analyses using both EFD and MES of otoliths were 100% (Itaipu), 97% (Araruama) and 90% (Saquarema). Therefore, a clear distinction was observed among these groups, which was related to the inherent characteristics of each lagoon system, their semi-restricted connectivity with the adjacent coastal zone, as well as the estuarine-opportunistic behavior of the species. Thus, the results suggest that these fisheries should be managed as different subpopulation-units. However, more studies should be carried out about the fish movements and life history events of this species in southeastern Brazil.

NEW RECORD OF THE BLUNTNOSE SIXGILL SHARK, Hexanchus griseus (CHONDRICHTHYES, HEXANCHIDAE) IN THE SOUTH WEST ATLANTIC OCEAN

A new record of Hexanchus griseus is reported for the coast of Argentina in Patagonia (45° 36´32.73´´S - 67° 16´ 33.68 W) at a depth of 19 m and with water temperature at 15.4°C. The specimen was a female captured, tagged and released inside a Marine Protected Area by local recreational fishermen as part of a national citizen science project. The specimen was a juvenile of 284.1 cm total length and it also constitutes the first tagged individual in the Southwest Atlantic Ocean for this species.&#x0D; Keywords: Hexanchus griseus, sixgill shark, Argentina, recreational fishery, citizen science, tagging, juvenile.

Macro-, meso- and microplastic abundance in sandy beaches and factors influencing their distribution in an SW Atlantic resort

The present study recorded the abundance of plastic debris in marine sediments of tourist beaches in the Southwest Atlantic (Argentina). We aimed to determine the abundance of macro-, meso- and microplastics in different beaches, considering different anthropogenic and natural factors. Generalized Linear Mixed Models (GLMMs) were performed and the explanatory factors were: Beach Morphology; Grain Size; Recreational Use Level; Continental Water Discharge, Location, Distance Urban Centre and Season. The Continental Water Discharge was the factor responsible for the highest abundance of plastic debris on the beach surface. Beaches with fine granulometry, between groins, with high to very high intensity of recreational use, tend to accumulate and/or retain greater amounts of plastic debris. The seasonal factor influences the abundance of plastic waste in the central zone between the pre-summer and post-summer seasons, despite the cleaning effect of the city goverment. In beaches with greater anthropogenic pressure, the influence of this factor on the abundance of litter is altered.

Diversity, phylogeny, and bathymetric zonation of Sirsoe (Annelida: Hesionidae) from colonization experiments in the South China Sea, with the description of three new species.

The South China Sea (SCS) basin is hypothesized to host distinct and bathymetrically differentiated fauna due to its semi-enclosed basin and three-layer circulation system. To test this hypothesis, three cow falls are artificially deployed at separate depths (655, 1604, and 3402 m) on Zhongnan seamount in the middle SCS, and the associated worms, Sirsoe spp. are selected as targets to explore their diversity, phylogeny, and zonation pattern. Analyses of collected specimens reveal three new Sirsoe species, which were then nominally described and named as S. polita sp. nov. (655 m), S. nanhaiensis sp. nov. (1604 and 3402 m), and S. feitiana sp. nov. (3402 m), and one known species (S. balaenophila lineage II). Metabarcoding analyses on cow-fall sediments reveal seven additional Operated Taxonomic Units (OTUs) assigned to Sirsoe, increasing the Sirsoe diversity to 10 species/OTUs in the middle SCS. Their distribution along depth shows increasing diversity toward the deeper sites. Phylogenetic inferences recover S. polita closely related to S. alucia from the Southwest Atlantic, forming a lineage deeply divergent from others. The nine deep-water species/OTUs are scattered in three distinct lineages showing closer phylogenetic relationships between 1604- and 3402-m counterparts. The lineage formed by S. naihaiensis and S. feitiana is distinct from other non-SCS congeners both morphologically and genetically. These results suggest multiple independent invasions of Sirsoe to the SCS, a new lineage potentially endemic to the SCS, and a strong zonation pattern related to depth, especially between the shallow (655 m) and the deep (1604 and 3402 m) sites. The semi-enclosed feature combined with the physical structure of the SCS may contribute to such a pattern. This work is registered in ZooBank under: urn:lsid:zoobank.org:pub:317771C8-42D717-4765-A168-B3BE99B09FBF.

Coastal upwelling influences population structure of dusky grouper Epinephelus marginatus: An integrative approach based on otolith chemistry and muscle stable isotopes

The dusky grouper (Epinephelus marginatus) is an overfished and threatened fish species with coastal distribution. In the Southwestern Atlantic, it occurs across a broad region influenced by two major oceanographic features: the Cabo Frio (23°S) and the Cabo Santa Marta (28°S) upwelling systems. Along the Brazilian coast, the species may present continuous or discrete populations, depending on the methodological approach used. In this study we combined otolith chemistry and muscle stable isotope analyses to examine the population structure of dusky groupers and its association with the two upwelling systems. Fish were collected in shallow coastal waters of the Southwest Atlantic Ocean, covering the southeastern and southern Brazilian coasts, among Macaé (22°S), Santos (24°S), Florianópolis (27°S), and in Rio Grande (32°S). The results show three statistically well-separated population groups along the region. We named these population groups as North (north of Cabo Frio); Center (between upwelling regions); and South (south of the Cabo Santa Marta system). Our findings allow to suggest that the upwelling systems may influence the distribution of E. marginatus stocks along the Brazilian south-western coast, even though a causal effect may not be attributed at this point. Overall, this combined approach, leveraging information from distinct natural tags, and reflecting variability of water chemistry and food webs with latitude, allowed us to enhance our understanding on how major upwelling systems influence the structuring of fish populations along the southwestern Atlantic Ocean.

An estuarine system as a possible nursery habitat for the broadnose sevengill shark: San Antonio cape—Southwest Atlantic

An estuarine system as a possible nursery habitat for the broadnose sevengill shark: San Antonio cape—Southwest Atlantic

Assessment of the stock status of blackfin tuna Thunnus atlanticus in the Southwest Atlantic Ocean: a length-based approach

This is the first assessment of blackfin tuna Thunnus atlanticus stock status. The aim of the present study was to estimate the status of its Southwestern stock using length-based models. A total of 1929 individuals was gathered in four years of length data (1998–2019). Two length-based models were applied, Length-Based Spawning Potential Ratio (LBSPR) and Stock Synthesis Data-limited tool (SS-DL) and five scenarios combining a set of growth parameters (asymptotic length-L∞, and growth constant-K) were built to accommodate different life histories available in the literature. The natural mortality (M) was estimated through four empirical methods. Given that the fishery only selects mature fish, length at selectivity were larger than the length at first maturity and high relative fishing mortality were found at most scenarios, even with high spawning potential ratio (SPR) values. The LBSPR estimated higher values of SPR, it presented a steeper decline than SS-DL between 1998 and 2019. In addition, growth parameters influence the estimation of the virginal stock, and intermediate M/K values seems to represent better the estimated length composition, given the low historical catch of the species. A declining pattern in SPR was observed over time with a reduction of 12% according to LBSPR and 7% decrease based on SS-DL methods. However, the overall evaluation of the Southwestern Atlantic stock of blackfin tuna in 2019 revealed an SPR range of 0.41 to 0.63, suggesting sustainable exploitation. Even with the limited data, we could have a proxy of stock status estimation by using length-based models which highlights the importance of such data. However, due to the high uncertainty of the results, better collection of catch, effort and length data should be considered. Additionally, SS-DL seems to estimate better SPR values when variability of growth parameters are placed for a given species.

The Response of Southwest Atlantic Storm Tracks to Climate Change in the Brazilian Earth System Model

The Earth’s weather and climate are strongly influenced by synoptic-scale systems such as extratropical cyclones. From this point of view, extratropical cyclones are very important for Equator–Pole heat exchange, and their positions are relevant to the understanding of the behavior of this system under current conditions and in the context of climate change. Baroclinic instability (BI), meridional heat flux (MHF), and kinetic energy (KE) are among the ways of calculating storm tracks (the regions in which extratropical cyclones most often occur). Forecasting is important for predicting the evolution of these phenomena and preparing future political decisions. In this study, we used ERA5 reanalysis data and BESM model forecasts to calculate BI, MHF, and KE. Overestimation of the BESM BI at lower and higher latitudes and underestimation of BI at medium latitudes were observed. In general, KE and MHF were underestimated and were displaced southward in the BESM. The analyses show a tendency towards poleward displacement of these tracks for all variables studied in in this paper. The scenarios show the same bias, with RCP8.5 having more extreme changes in all situations.

Characterizing Atlantic interhemispheric teleconnection established by South American monsoon in austral summer

This study aims to characterize the interhemispheric teleconnection pattern, which is established by the South America (SA) summer monsoon over the Atlantic Ocean during January and February, and referred it as the Atlantic symmetric pattern (ASP). The ASP is characterized using the leading mode of interannual sea surface temperature (SST) variability of the Southwest Atlantic Ocean, where strong convection-SST coupling occurs. The pattern is manifested as two anomalous cyclonic-anticyclonic-cyclonic circulation trains aligned meridionally over the Atlantic Ocean, with a distinct SST dipole of the Southwest Atlantic Ocean and a tripole of the North Atlantic Ocean. The interhemispheric wave trains of the ASP are excited as a Gill-type response to convective activity in the SA summer monsoon, as confirmed in linear baroclinic model. Complementing previous studies on observed interhemispheric connection in the Atlantic, our findings highlight the importance of characterizing the ASP and its role in linking the South Atlantic SST, the SA summer monsoon, and North Atlantic climate. Further research is warranted to explore the impacts of the ASP on the Northern Hemisphere and its interactions with other climatic modes.

Taxonomy and abundance of epibenthic Prorocentrum (Dinophyceae) species from the tropical and subtropical Southwest Atlantic Ocean including a review of their global diversity and distribution

In the tropical and subtropical South Atlantic Ocean, studies on the taxonomy and abundance of benthic harmful algae are scarce and the region has been largely under investigated. In this study, morphological descriptions, molecular (LSU rDNA and ITS region) and abundance data of benthic Prorocentrum species from the tropical and subtropical Southwest Atlantic and three oceanic islands are presented. Moreover, a review of global benthic Prorocentrum species richness and distribution is presented. Eleven benthic Prorocentrum species were found in Brazil. Morphological and molecular data on P. borbonicum, P. hoffmannianum, P. lima species complex and P. rhathymum were provided. Prorocentrum panamense, P. cf. caipirignum, P. cf. concavum, P. cf. norrisianum, P. emarginatum/fukuyoi/sculptile complex and two not identified species were observed using scanning electron and/or light microscopy, and morphological descriptions are presented. Prorocentrum lima species complex was found at all investigated sites, in abundances up to 2 × 104 cells g−1 FW at the Northeast Brazil, while maximum abundance of all the remaining species did not exceed 1 × 103 cells g−1 FW. The Fernando de Noronha archipelago can be considered a hotspot of benthic Prorocentrum species diversity, with ten species registered. Data compiled in the literature review shows a clear latitudinal gradient with higher species richness in tropical and subtropical regions relative to temperate areas. It is also evident that there is a bias caused by taxonomic impediment and an uneven sampling effort, with many regions still to be investigated using a combined morphological and molecular effort. Therefore, the current knowledge on the global distribution of benthic Prorocentrum species is likely underestimated.

Machine learning sheds light on physical-chemical and biological parameters leading to Abrolhos coral reef microbialization

Microbes play a central role in coral reef health. However, the relative importance of physical-chemical and biological processes in the control of microbial biomass are unknown. Here, we applied machine learning to analyze a large dataset of biological, physical, and chemical parameters (N = 665 coral reef seawater samples) to understand the factors that modulate microbial abundance in the water of Abrolhos reefs, the largest and richest coral reefs of the Southwest Atlantic. Random Forest (RF) and Boosted Regression Tree (BRT) models indicated that hydrodynamic forcing, Dissolved Organic Carbon (DOC), and Total Nitrogen (TN) were the most important predictors of microbial abundance. The possible cumulative effects of higher temperatures, longer seawater residence time, higher nutrient concentration, and lower coral and fish biomass observed in coastal reefs resulted in higher microbial abundance, potentially impacting coral resilience against stressors.

Tapeworms (Platyhelminthes, Cestoda) from marine chondrichthyans of the Southwestern Atlantic Ocean, and the sub-Antarctic and Antarctic islands: a checklist.

A parasite-host list of cestodes parasitizing chondrichthyans in the Southwest Atlantic off Argentina and surrounding waters of Antarctica is compiled based on the available literature. The list is based on published descriptions and redescriptions of species, and newly collected worms during the current study. A total of 57 valid species belonging to 28 genera of the orders Cathetocephalidea, Diphyllidea, Gyrocotylidea, Lecanicephalidea, Onchoproteocephalidea, Phyllobothriidea, Rhinebothriidea, "Tetraphyllidea", and Trypanorhyncha is listed. Information on hosts, localities, specimens in collections and comments on tapeworms are also included. A host-parasite list including chimaeras (1 order, 1 genus), batoids (4 orders, 10 genera), and sharks (3 orders, 5 genera) is provided. Tapeworm diversity, distribution range, and host associations are discussed. The cestodes orders Phyllobothriidea and Rhinebothriidea exhibit the highest species richness, with 13 and 12 species, respectively. Onchoproteocephalideans and rhinebothriideans have the broadest geographic distribution in the study area. Regarding hosts, arhynchobatid skates are the group most frequently associated with cestodes. However, further collecting efforts are necessary to understand whether this data reflect the real diversity and host association of these parasites or is a result of a bias in sampling.

Going deeper into the molecular ecology of the Southwest Atlantic Caretta caretta (Testudinata: Cheloniidae), what do microsatellites reveal to us?

Going deeper into the molecular ecology of the Southwest Atlantic Caretta caretta (Testudinata: Cheloniidae), what do microsatellites reveal to us?

The role of oceanic currents in the dispersal and connectivity of the mangrove Rhizophora mangle on the Southwest Atlantic region.

Dispersal is a crucial mechanism to living beings, allowing them to reach new resources such that populations and species can occupy new environments. However, directly observing the dispersal mechanisms of widespread species can be costly or even impractical, which is the case for mangrove trees. The influence of ocean currents on mangrove dispersal is increasingly evident; however, few studies mechanistically relate the patterns of population distribution with the dispersal by oceanic currents under an integrated framework. Here, we evaluate the role of oceanic currents on connectivity of Rhizophora mangle along the Southwest Atlantic. We inferred population genetic structure and migration rates, simulated the displacement of propagules and tested our hypotheses with Mantel tests and redundancy analysis. We observed populations structured in two major groups, north and south, which is corroborated by other studies with Rhizophora and other coastal plants. Inferred recent migration rates do not indicate ongoing gene flow between sites. Conversely, long-term migration rates were low across groups and contrasting dispersal patterns within each one, which is consistent with long-distance dispersal events. Our hypothesis tests suggest that both isolation by distance and isolation by oceanography (derived from the oceanic currents) can explain the neutral genetic variation of R. mangle in the region. Our findings expand current knowledge of mangrove connectivity and highlight how the association of molecular methods with oceanographic simulations improve the interpretation of the dispersal process. This integrative approach is a cost- and time-efficient strategy to include dispersal and connectivity data into marine protected areas planning and management.

Whale falls as chemosynthetic refugia: a perspective from free-living deep-sea nematodes

Whale carcasses create habitats in the deep sea which are colonized by organisms related to other chemosynthetic environments suggesting that whale falls may act as intermediate refugia for the dispersal and evolution of deep-sea organisms. Such evidence comes mainly from macrofaunal organisms whereas for the smaller meiofauna, data on whalebone assemblages is lacking. In this study, we investigated nematode colonization of whalebones experimentally deployed at 1500 and 3300 m depth and bones from a natural whale carcass found at 4204 m, in the Southwest Atlantic Ocean, off Brazil. By comparing whalebone assemblages to other deep-sea environments, we tested the hypothesis that whale fall nematode assemblages are more similar to those from other cognate chemosynthetic-based habitats, rather than those from sediments surrounding the carcass or other deep-sea habitats. Our results showed that whalebone nematode assemblages resemble those of hydrothermal vents and cold seeps and suggest that organic falls may act as intermediate refugia for meiofauna from chemosynthetic environments. It also showed that oceanographic conditions and the age of the carcasses on the ocean floor could influence nematode assemblage composition and richness. Such findings highlight the importance of organic falls for understanding the connectivity and phylogeny of benthic organisms, including representatives of the meiofauna.

Natal Origin and Spatiotemporal Distribution of Leatherback Turtle (Dermochelys coriacea) Strandings at a Foraging Hotspot in Temperate Waters of the Southwest Atlantic Ocean.

Leatherback turtles migrate long distances between nesting beaches and distant foraging areas worldwide. This study analyzes the genetic diversity, life history stage, spatiotemporal distribution, and associated threats of a foraging aggregation in the Southwest Atlantic Ocean. A total of 242 leatherbacks stranded or bycaught by artisanal fisheries were recorded from 1997 to 2021 in Uruguay, with sizes ranging from 110.0 to 170.0 cm carapace lengths, indicating that the aggregation is composed of large juveniles and adults. Results of Bayesian mixed-stock analysis show that leatherbacks come primarily from the West African rookeries, based on mitochondrial DNA sequences obtained from 59 of the turtles representing seven haplotypes, including a novel one (Dc1.7). The main threat identified in the area is the fisheries bycatch but most of the carcasses observed were badly decomposed. There was significant seasonal and interannual variability in strandings that is likely associated with the availability of prey and the intensity of the fishing effort. Taken together, these findings reinforce the importance of these South American foraging areas for leatherbacks and the need to determine regional habitat use and migratory routes across the broader Atlantic region, in order to develop effective conservation measures to mitigate threats both at nesting beaches and foraging areas.

The ichthyofaunistic colonization and complex biogeographic history of the southern portion of the Southwest Atlantic Ocean

AbstractHere, we highlight the geological processes that resulted in the current conformation of the southern Southwest Atlantic Ocean, and explore the heterogeneous composition of the marine ichthyofauna found between 33° and 56° from a paleobiological perspective. During the early Cretaceous (140 Mya), the South Atlantic was still not formed, and Gondwana was probably a set of united plates with shallow continental waters. In the middle Cretaceous (112 Mya), the major Gondwanan plates started diverging from each other, allowing shallow marine intrusions and the diversification of an endemic fish fauna. By the end of the Mesozoic (66 Mya), the proto‐South Atlantic connected with the North Atlantic, which was still part of the Tethys Sea, allowing its ichthyofauna to colonize the south and reach the Antarctic region. The opening of the Drake Passage in the Oligocene (33 Mya) enabled the cold waters of the Pacific Ocean to reach the South Atlantic, causing drastic effects on the thermophilic fauna and favoring the dispersion of cold‐water species. Successive glaciations during the Quaternary (2 Myr to 10,000 years ago) resulted in the prevalence of Antarctic climatic conditions in the southern Southwest Atlantic. The long history of changing scenarios in the constitution of the southern Southwest Atlantic is reflected in the heterogeneous composition of the marine ichthyofauna between 33° and 56°S, which is characterized by a mixture of cosmopolitan, Tethyan, Pacific, Gondwanan, Antarctic, and endemic origins.