The genus Stauroneis (Bacillariophyceae) in the Antarctic region

Aim To provide a detailed biogeography of the diatom genus Stauroneis in the Antarctic and sub‐Antarctic regions and to establish the biogeographical relationships between the different constituent locations to test the application of a precise and refined taxonomy in generating accurate polar biogeographies. Location The Antarctic and sub‐Antarctic region comprising the islands South Georgia, Crozet, Kerguelen, Marion, Heard and the Antarctic Peninsula. Methods Diatom samples from different habitats in a large part of the sub‐Antarctic and Antarctic region were investigated for their Stauroneis content. Presence/absence data were investigated using Sørensen's similarity index. An additional 500 samples from Arctic locations were used to provide a bipolar comparison. Using reliable literature data, gaps in the distribution of the Stauroneis taxa were filled. Results The Stauroneis flora of the Antarctic and sub‐Antarctic regions is quite distinct from its Arctic equivalent, with only five species (out of 60) common to both areas. Within the (sub‐)Antarctic group, the islands of the Indian Ocean have the most diverse Stauroneis composition, which is clearly separated from the rest of the region. The South Georgia Stauroneis composition has some affinities with the Antarctic Peninsula but the latter has far fewer species. These results are in clear contrast to older data showing no biogeographical difference between the Arctic and Antarctic regions. Main conclusions Using only a single genus, a clear biogeography of the (sub‐)Antarctic region can be produced that separates the Indian Ocean islands from other sampling locations. When based on a precise taxonomy, biogeographical relationships between locations in the region become much more reliable. Contrary to previous work, there is almost no similarity in the diatom floras of the Arctic and Antarctic regions.     

Past and present distribution, densities and movements of blue whales Balaenoptera musculus in the Southern Hemisphere and northern Indian Ocean

• 1 Blue whale locations in the Southern Hemisphere and northern Indian Ocean were obtained from catches (303 239), sightings (4383 records of ≥8058 whales), strandings (103), Discovery marks (2191) and recoveries (95), and acoustic recordings.
• 2 Sighting surveys included 7 480 450 km of effort plus 14 676 days with unmeasured effort. Groups usually consisted of solitary whales (65.2%) or pairs (24.6%); larger feeding aggregations of unassociated individuals were only rarely observed. Sighting rates (groups per 1000 km from many platform types) varied by four orders of magnitude and were lowest in the waters of Brazil, South Africa, the eastern tropical Pacific, Antarctica and South Georgia; higher in the Subantarctic and Peru; and highest around Indonesia, Sri Lanka, Chile, southern Australia and south of Madagascar.
• 3 Blue whales avoid the oligotrophic central gyres of the Indian, Pacific and Atlantic Oceans, but are more common where phytoplankton densities are high, and where there are dynamic oceanographic processes like upwelling and frontal meandering.
• 4 Compared with historical catches, the Antarctic (‘true’) subspecies is exceedingly rare and usually concentrated closer to the summer pack ice. In summer they are found throughout the Antarctic; in winter they migrate to southern Africa (although recent sightings there are rare) and to other northerly locations (based on acoustics), although some overwinter in the Antarctic.
• 5 Pygmy blue whales are found around the Indian Ocean and from southern Australia to New Zealand. At least four groupings are evident: northern Indian Ocean, from Madagascar to the Subantarctic, Indonesia to western and southern Australia, and from New Zealand northwards to the equator. Sighting rates are typically much higher than for Antarctic blue whales.
• 6 South‐east Pacific blue whales have a discrete distribution and high sighting rates compared with the Antarctic. Further work is needed to clarify their subspecific status given their distinctive genetics, acoustics and length frequencies.
• 7 Antarctic blue whales numbered 1700 (95% Bayesian interval 860–2900) in 1996 (less than 1% of original levels), but are increasing at 7.3% per annum (95% Bayesian interval 1.4–11.6%). The status of other populations in the Southern Hemisphere and northern Indian Ocean is unknown because few abundance estimates are available, but higher recent sighting rates suggest that they are less depleted than Antarctic blue whales.

     

Torulopsis austromarina sp. nov. A yeast isolated from the Antarctic Ocean

A new species of yeast, Torulopsis austromarina is described from oceanic waters of the Indo-Pacific and Indian Ocean sectors of the Antarctic.     

Regional differences in length at sexual maturity for female blue whales based on recovered Soviet whaling data

New blue whale ovarian corpora data from illegal Soviet catches in the Southern Hemisphere and northern Indian Ocean were recovered from the original logbooks. Catches north of 52°S were assumed to be pygmy blue whales ( Balaenoptera musculus brevicauda , n = 1,272); those south of 56°S were assumed to be Antarctic (true) blue whales ( B. m. intermedia , n = 153). Three probable Antarctic blue whales north of 52°S were excluded. Lengths at which 50% and 95% of females become sexually mature ( L ₅₀ and L ₉₅) were estimated from a Bayesian logistic model. These estimates are more precise than previous Japanese estimates because Soviet catches below the legal minimum of 70 ft (21.3 m) were 32 times greater. For pygmy blue whales L ₅₀ was 19.2 m (95% interval 19.1–19.3 m) and L ₉₅ was 20.5 m (95% interval 20.4–20.7 m). Antarctic L ₅₀ (23.4 m, 95% interval 22.9–23.9 m) was much longer than L ₅₀ for pygmy blue whale regions (18.4–19.9 m). The median L ₅₀ for the northern Indian Ocean was 0.5–0.6 m shorter than for pygmy blue whales from other regions; although statistically significant, these small length differences provide little support for northern Indian Ocean blue whales being a separate subspecies, B. m. indica .     

Seasonal and Diel Vocalization Patterns of Antarctic Blue Whale (Balaenoptera musculus intermedia) in the Southern Indian Ocean: A Multi-Year and Multi-Site Study

Passive acoustic monitoring is an efficient way to provide insights on the ecology of large whales. This approach allows for long-term and species-specific monitoring over large areas. In this study, we examined six years (2010 to 2015) of continuous acoustic recordings at up to seven different locations in the Central and Southern Indian Basin to assess the peak periods of presence, seasonality and migration movements of Antarctic blue whales (Balaenoptera musculus intermedia). An automated method is used to detect the Antarctic blue whale stereotyped call, known as Z-call. Detection results are analyzed in terms of distribution, seasonal presence and diel pattern of emission at each site. Z-calls are detected year-round at each site, except for one located in the equatorial Indian Ocean, and display highly seasonal distribution. This seasonality is stable across years for every site, but varies between sites. Z-calls are mainly detected during autumn and spring at the subantarctic locations, suggesting that these sites are on the Antarctic blue whale migration routes, and mostly during winter at the subtropical sites. In addition to these seasonal trends, there is a significant diel pattern in Z-call emission, with more Z-calls in daytime than in nighttime. This diel pattern may be related to the blue whale feeding ecology.     

Cool,cold or colder? Spatial segregation of prions and blue petrels is explained by differences in preferred sea surface temperatures

The Southern Ocean provides one of the largest environmental gradients on Earth that lacks geographical barriers, and small but highly mobile petrels living there may offer fine models of evolution of diversity along environmental gradients. Using geolocation devices, we investigated the winter distribution of closely related petrel species breeding sympatrically in the southern Indian Ocean, and applied ecological niche models to compare environmental conditions in the habitat used. We show that thin-billed prions (Pachyptila belcheri), Antarctic prions (Pachyptila desolata) and blue petrels (Halobaena caerulea) from the Kerguelen archipelago in the southern Indian Ocean segregate latitudinally, sea surface temperature being the most important variable separating the distribution of the species. Antarctic prions spent the winter north of the Polar Front in temperate waters, whereas blue petrels were found south of the Polar Front in Antarctic waters. Thin-billed prions preferred intermediate latitudes and temperatures. Stable isotope values of feathers reflected this near complete niche separation across an ecological gradient that spans large scales, and suggest evolutionary isolation by environment. In pelagic seabirds that exploit large areas of ocean, spatial niche partitioning may not only facilitate coexistence among ecologically similar species, but may also have driven their evolution in the absence of geographical barriers.     

Regional genetic diversity patterns in Antarctic hairgrass (Deschampsia antarctica Desv.)

Aim  To determine patterns in diversity of a major Antarctic plant species, including relationships of Antarctic populations with those outside the Antarctic zone.
Location  Antarctic Peninsula, Maritime Antarctica, sub-Antarctic islands, Falkland Islands and South America.
Methods  Amplified fragment length polymorphisms (AFLPs) and chloroplast sequences were used to study patterns of genetic diversity in Antarctic hairgrass ( Deschampsia antarctica Desv.) and the genetic relationships between populations over its distribution range. Thirty-eight populations were sampled from a large part of the distribution of D. antarctica , and additionally, herbarium specimens were included for areas from which we could not obtain fresh samples.
Results  A gradient in AFLP diversity was observed going from the Falklands southwards into the Antarctic. This gradient in diversity was also observed within the Antarctic Peninsula: diversity was lower further south. Diversity in the chloroplast genome of D. antarctica was low. Only three chloroplast haplotypes were found, each with a strong regional distribution.
Main conclusions  The phylogenetic construction of AFLP marker frequencies in meta-populations of D. antarctica supports a stepping-stone model of colonization, whereby gene flow mainly occurs between neighbouring populations. It is concluded that long-distance gene flow is very limited in D. antarctica . A very low diversity was found in the sub-Antarctic islands in the Indian Ocean, indicating that these populations have experienced a recent evolutionary bottleneck.     

Uvigerina proboscidea abundances and paleoceanography of the northern Indian Ocean DSDP Site 214 during the Late Neogene

This study attempts to understand the significance of Uvigerina proboscidea in paleoceanographic reconstructions at the northern (tropical) Indian Ocean DSDP Site 214 from the Late Miocene through the Pleistocene. In this interval at this site, U. proboscidea is the most abundant species of the benthic assemblage and shows abrupt frequency changes (about 1–74%). Based on relative percentages of U. proboscidea calibrated with oxygen and carbon isotope record and the sediment accumulation rates, the modern distribution of the species in the Indian Ocean, and other evidence, the peaks of abundance of U. proboscidea are inferred to represent times of high-surface productivity. This productivity is related to intensified trade winds during strong southwest (SW) Indian monsoons, causing widespread upwelling along equatorial divergence in the Indian Ocean. The sudden increase of U. proboscidea abundance at approximately 8.5–7.5 Ma reflects significant upwelling at the equatorial divergence. This event corresponds to the permanent build-up of West Antarctic ice sheets, and a major increase in SW Indian monsoons related upwelling in the northwestern Indian Ocean. The Chron-6 carbon shift at approximately 6.2 Ma is marked by another peak of abundance, reflecting widespread ocean fertility. The highest abundances of U. proboscidea and highest sediment accumulation rates occur between 5.8 and 5.1 Ma, which coincides with the greatest development of Antarctic ice sheets and strong southwest monsoons. The higher percentages at 3.2–3.1 Ma, approximately 2.4 Ma, and 1.6 Ma all represent phases of high productivity at the equatorial divergence.     

Scotia Arc Acari: antiquity and origin

The Scotia Arc comprises South Georgia, South Sandwich, South Orkney and South Shetland Islands together with the Antarctic Peninsula. Free-living Acari of the Scotia Arc contain immigrant and endemic elements. Transport of immigrant species to and within the Maritime Antarctic has been via Holocene storms and ocean currents. Most immigrants are Gamasida, Oribatida and Acaridida while Actinedida dominate the endemic element. Immigrant species on South Georgia share common 'sub-Antarctic' affinities with South Indian and, to a lesser extent, South Pacific Ocean island faunas. In contrast, immigrants to the rest of the Scotia Arc and Bouvetøya on the mid-Atlantic Ridge, form a robust 'Maritime Antarctic' Province group. The endemic component is of largely Tertiary origin and, like that of Continental Antarctica, dominated by a few cosmopolitan families and genera of Actinedida. There are no bona fide pan-Antarctic species and little evidence that Continental and Maritime Antarctic faunas have a common ancestry, indeed the Continental endemic fauna is entirely montane while that of Maritime Antarctica is coastal. The presence of common Maritime Antarctic/South Pacific island genera corroborate the Antarctic Peninsula as being derived from a South Pacific island archipelago which collided with Continental Antarctica during the Tertiary period.     

Physical and biological coupling in eddies in the lee of the South-West Indian Ridge

Eddies have some decisive functions in the dynamics of the Southern Ocean ecosystems. This is particularly true in the Indian sector of the Southern Ocean, where a region of unusually high-mesoscale variability has been observed in the vicinity of the South-West Indian Ridge. In April 2003, three eddies were studied: eddy A, a recently spawned anticyclone south of the Antarctic Polar Front (APF),; eddy B, an anticyclone north of lying between the Subantarctic Front and the APF; and eddy C, a cyclone north of the APF west of the ridge. Elevated concentrations of total Chl-a coincided with the edges of the cyclonic eddy, whereas both anticyclonic eddies A and B were characterised by low total Chl-a concentrations. Biologically, the two anticyclonic eddies A and B were distinctly different in their biogeographic origin. The zooplankton community in the larger anticyclonic eddy A was similar in composition to the Antarctic Polar Frontal Zone (APFZ) community with an addition of some Antarctic species suggesting an origin just north of the APF. In contrast, the species composition within the second anticyclonic eddy B appeared to be more typical of the transitional nature of the APFZ, comprising species of both subantarctic and subtropical origin and thus influenced by intrusions of water masses from both north and south of the Subantarctic Front. Back-tracking of these features shows that the biological composition clearly demarcates the hydrographic origin of these features.     

Origin,diversity, and biogeography of Antarctic scale worms (Polychaeta: Polynoidae): a wide‐scale barcoding approach

The Antarctic marine environment hosts diversified and highly endemic benthos owing to its unique geologic and climatic history. Current warming trends have increased the urgency of understanding Antarctic species history to predict how environmental changes will impact ecosystem functioning. Antarctic benthic lineages have traditionally been examined under three hypotheses: (1) high endemism and local radiation, (2) emergence of deep‐sea taxa through thermohaline circulation, and (3) species migrations across the Polar Front. In this study, we investigated which hypotheses best describe benthic invertebrate origins by examining Antarctic scale worms (Polynoidae). We amassed 691 polynoid sequences from the Southern Ocean and neighboring areas: the Kerguelen and Tierra del Fuego (South America) archipelagos, the Indian Ocean, and waters around New Zealand. We performed phylogenetic reconstructions to identify lineages across geographic regions, aided by mitochondrial markers cytochrome c oxidase subunit I (Cox1) and 16S ribosomal RNA (16S). Additionally, we produced haplotype networks at the species scale to examine genetic diversity, biogeographic separations, and past demography. The Cox1 dataset provided the most illuminating insights into the evolution of polynoids, with a total of 36 lineages identified. Eunoe sp. was present at Tierra del Fuego and Kerguelen, in favor of the latter acting as a migration crossroads. Harmothoe fuligineum, widespread around the Antarctic continent, was also present but isolated at Kerguelen, possibly resulting from historical freeze–thaw cycles. The genus Polyeunoa appears to have diversified prior to colonizing the continent, leading to the co‐occurrence of at least three cryptic species around the Southern and Indian Oceans. Analyses identified that nearly all populations are presently expanding following a bottleneck event, possibly caused by habitat reduction from the last glacial episodes. Findings support multiple origins for contemporary Antarctic polynoids, and some species investigated here provide information on ancestral scenarios of (re)colonization. First, it is apparent that species collected from the Antarctic continent are endemic, as the absence of closely related species in the Kerguelen and Tierra del Fuego datasets for most lineages argues in favor of Hypothesis 1 of local origin. Next, Eunoe sp. and H. fuligineum, however, support the possibility of Kerguelen and other sub‐Antarctic islands acting as a crossroads for larvae of some species, in support of Hypothesis 3. Finally, the genus Polyeunoa, conversely, is found at depths greater than 150 m and may have a deep origin, in line with Hypothesis 2. These “non endemic” groups, nevertheless, have a distribution that is either north or south of the Antarctic Polar Front, indicating that there is still a barrier to dispersal, even in the deep sea.     

Plankton community structure in the physical environment surrounding the Prince Edward Islands (Southern Ocean)

The results are presented of a macroscale physical and biological oceanographic survey conducted during the second Marion Island Offshore Study in the upstream and downstream regions of the Prince Edward Islands in the austral autumn (April/May) 1997. Upstream of the islands, the Sub-Antarctic Front appeared to combine with the Antarctic Polar Front to form an intensive frontal feature. Closer to the islands, the fronts appeared to separate. Influenced by the shallow topography of the southwest Indian ridge, the Sub-Antarctic Front was steered northwards around the islands while the Antarctic polar front appeared to meander eastwards, where it was again encountered in the southeastern corner of the survey grid. Downstream of the islands, an intensive cold-core eddy within the Polar Frontal Zone was observed. Its exact genesis is unknown but it is possibly generated by instabilities within the meandering Antarctic polar front as its surface signature was characteristic of Antarctic surface water masses found south of the Antarctic polar front. The cold-core eddy appeared to displace the sub-Antarctic front northwards. South of the eddy, a warm patch of sub-Antarctic surface water was observed; its position appeared to be controlled by the meandering Antarctic Polar Front which lay on either side of this feature. No distinct microphytoplankton groupings could be distinguished by numerical analyses, although four distinct zooplankton groupings were identified. These corresponded to the sub-Antarctic surface waters, Antarctic surface waters and the polar frontal zone waters. The fourth grouping comprised those stations where the lowest zooplankton abundances during the entire investigation were recorded and, as a consequence, does not reflect any spatial patterns. These results suggest that the species composition and distribution of plankton in the vicinity of the islands were consistent with the prevailing oceanographic regime. Accepted: 15 March 1999     

Biogeography and ecology of freshwater diatoms in Subantarctica: a review

Subantarctica is situated between the Antarctic and the Subtropical Convergence and consists of the islands in the southern Atlantic, Indian and Pacific Oceans. Diatoms are an important component of all Subantarctic aquatic, moss and soil habitats. Taxonomic studies reveal a high diversity and species richness in both the present-day and the fossil diatom flora. Planktonic diatoms are almost completely absent. A similarity analysis of the diatom composition from different localities in the southern zone (below 40°S) resulted in a biogeographical zonation. Three regions could be formed, based on their diatom composition: Subantarctica, Maritime Antarctica and the Antarctic Continent. The diatom communities in the different regions are all characterized by a high proportion of cosmopolitan species. A second feature of the southern diatom floras is the decreasing diversity when moving southwards.     

Sapayoa aenigma: a New World representative of 'Old World suboscines'

Passerine birds are very plastic in their adaptations, which has made it difficult to define phylogenetic lineages and correctly allocate all species to these. Sapayoa aenigma, a member of the large group of New World flycatchers, has been difficult to place, and DNA-DNA hybridization experiments have indicated that it may have been misplaced. This is confirmed here, as base sequencing of two nuclear genes places it as a deep branch in the group of broadbills and pittas of the Old World tropics. The peculiar distribution of this lineage may be best explained in terms of a Gondwanic and Late Cretaceous origin of the passerine birds, as this particular lineage dispersed from the Antarctic landmass, reaching the Old World tropics via the drifting Indian plate, and South America via the West Antarctic Peninsula.     

Speciation chronology of rockhopper penguins inferred from molecular, geological and palaeoceanographic data

Aim The Southern Ocean is split into several biogeographical provinces between convergence zones that separate watermasses of different temperatures. Recent molecular phylogenies have uncovered a strong phylogeographic structure among rockhopper penguin populations, Eudyptes chrysocome sensu lato, from different biogeographical provinces. These studies suggested a reclassification as three species in two major clades, corresponding, respectively, to warm, subtropical and cold sub‐Antarctic watermasses rather than to geographic proximity. Such a phylogeographic pattern, also observed in plants, invertebrates and fishes of the Southern Ocean, suggests that past changes in the positions of watermasses may have affected the evolutionary history of penguins. We calculated divergence times among various rockhopper penguin clades and calibrated these data with palaeomagmatic and palaeoceanographic events to generate a speciation chronology in rockhopper penguins. Location Southern Ocean. Methods Divergence times between populations were calculated using five distinct mitochondrial DNA loci, and assuming a molecular clock model as implemented in mdiv . The molecular evolution rate of rockhopper penguins was calibrated using the radiochronological age of St Paul Island and Amsterdam Island in the southern Indian Ocean. Separations within other clades were correlated with palaeoceanographic data using this calibrated rate. Results The split between the Atlantic and Indian populations of rockhopper penguins was dated as 0.25 Ma, using the date of emergence of St Paul and Amsterdam islands, and the divergence between sub‐Antarctic and subtropical rockhopper penguins was dated as c. 0.9 Ma (i.e. during the mid‐Pleistocene transition, a major change in the Earth’s climate cycles). Main conclusions The mid‐Pleistocene transition is known to have caused a major southward shift in watermasses in the Southern Ocean, thus changing the environment around the northernmost rockhopper penguin breeding sites. This ecological isolation of northernmost populations may have caused vicariant speciation, splitting the species into two major clades. After the emergence of St Paul and Amsterdam islands in the subtropical Indian Ocean 0.25 Ma, these islands were colonized by penguins from the subtropical Atlantic, 6000 km away, rather than by penguins from the sub‐Antarctic Indian Ocean, 5000 km closer.     

Gut evacuation rates of Antarctic copepods during austral spring

The results of gut evacuation experiments performed on Antarctic copepods during the austral spring are presented and discussed. Four species of large copepods commonly occurring in the Indian sector of the Antarctic Ocean were studied: Calanus propinquus, Calanoides acutus, Rhincalanus gigas and Pleuromamma robusta. For each species two experiments were carried out, one in daytime and one in night-time, except for Calanoides acutus, which was only studied at night. P. robusta showed pigment gut retention in all experiments. The results showed that all species studied had a longer gut passage time than that previously recorded and that gut evacuation rate appears to decrease during daytime. Accepted: 1 October 1998     

Foraging routes of Antarctic fur seals (Arctocephalus gazella) investigated by the concurrent use of satellite tracking and time-depth recorders

Little is known about movement behaviour in terms of route choice in Antarctic fur seals (Arctocephalus gazella). We deployed satellite transmitters and time-depth recorders simultaneously on 11 animals, and time-depth recorders with a speed recorder on 10 animals, to investigate the foraging routes of Antarctic fur seals belonging to a colony located at Iles Kerguelen (Southern Indian Ocean). The study took place during the 1997/1998 austral summer, and results indicate a preferred foraging area, with two main strategies in route choice apparent during foraging trips. In one strategy seals tended to reach an apparently known foraging ground and stopped there to feed. In the other strategy, animals performed looped trips, foraging en route and probably searching for a food patch better than the one previously exploited. Accepted: 13 September 1999     

Photosynthetic oxygen production and community respiration in the Indian sector of the Antarctic Ocean during austral summer

Photosynthetic oxygen production by phytoplankton and community respiration in the Indian sector of the Antarctic Ocean were estimated from changes in oxygen concentrations in light and dark bottles. Gross production varied between 0.1 and 5.1 µmol O₂ l^-1 day^-1. In the same water, community respiration (the sum of oxygen consumption by heterotrophs and phytoplankton) was 0.4-3.6 µmol O₂ l^-1 day^-1, which accounted for 47-343% of the gross production. Algal and heterotrophic respirations were distinguished using some assumptions. These estimates showed that heterotrophic respiration accounted for most of the community respiration (70-91% depending upon the assumptions), indicating that heterotrophic respiration plays an important role in the mineralization of phytoplankton production in the surveyed sea area. Gross production rate correlated with chlorophyll a concentration, showing that the photosynthetic production rate of oxygen depends on the abundance of phytoplankton. Moreover, there was a significant relationship between gross production and community respiration rates. These regression equations suggested that negative net production occurred under the usually low concentration of chlorophyll observed in the Indian sector of the Antarctic Ocean. Hence, the net exchange of carbon dioxide due to biological processes through the sea surface seemed to be not as large as expected in the Antarctic Ocean, although the number of data were limited at this stage.     

Systematic notes on some Antarctic Ascophora (Bryozoa, Cheilostomata)

Nine new species of ascophoran cheilostome Bryozoa are described from Antarctic localities. Five species are considered to be new to science ( Smittina favulosa, Smittina diffidentia, Smittoidea pugiuncula, Escharella mamillata, Fenestrulina antarctica ), while four others have been previously recorded under other names ( Smittoidea malleata, Escharella watersi, Lacerna watersi, Hippothoa belgica ). Escharella crozetensis Waters, formerly recorded from Antarctica, is considered to be limited to the Kerguelen region of the South Indian Ocean.     

Seasonal and Geographic Variation of Southern Blue Whale Subspecies in the Indian Ocean

Understanding the seasonal movements and distribution patterns of migratory species over ocean basin scales is vital for appropriate conservation and management measures. However, assessing populations over remote regions is challenging, particularly if they are rare. Blue whales (Balaenoptera musculus spp) are an endangered species found in the Southern and Indian Oceans. Here two recognized subspecies of blue whales and, based on passive acoustic monitoring, four “acoustic populations” occur. Three of these are pygmy blue whale (B.m. brevicauda) populations while the fourth is the Antarctic blue whale (B.m. intermedia). Past whaling catches have dramatically reduced their numbers but recent acoustic recordings show that these oceans are still important habitat for blue whales. Presently little is known about the seasonal movements and degree of overlap of these four populations, particularly in the central Indian Ocean. We examined the geographic and seasonal occurrence of different blue whale acoustic populations using one year of passive acoustic recording from three sites located at different latitudes in the Indian Ocean. The vocalizations of the different blue whale subspecies and acoustic populations were recorded seasonally in different regions. For some call types and locations, there was spatial and temporal overlap, particularly between Antarctic and different pygmy blue whale acoustic populations. Except on the southernmost hydrophone, all three pygmy blue whale acoustic populations were found at different sites or during different seasons, which further suggests that these populations are generally geographically distinct. This unusual blue whale diversity in sub-Antarctic and sub-tropical waters indicates the importance of the area for blue whales in these former whaling grounds.