Southwestern Indian Research Articles

Evidence of a pelagic nursery area for phyllosomas of the scyllarid lobster Acantharctus ornatus (Holthuis, 1960) (Decapoda: Achelata: Scyllaridae) in a coastal bight in South Africa

Abstract Lobster phyllosoma larvae collected during surveys over the continental shelf of eastern South Africa (southwestern Indian Ocean) were analysed to determine the role of the KwaZulu-Natal (KZN) Bight as a pelagic nursery area. Surface waters in the shallow bight and further offshore, between the 20- and 1,000-m depth contours, were sampled with plankton nets in 2018 to 2023. Phyllosomas were absent from 87.9% of 257 tows, with high variability in the remaining tows indicating a patchy distribution with abundance hotspots. Phyllosomas of four scyllarid and three palinurid species were found among 311 captured specimens. The scyllarid Acantharctus ornatus (Holthuis, 1960) was the most abundant, constituting 94.9% of the phyllosomas caught, with all nine larval stages (I–IX) present. Phylogenetic analysis using the COI and 16S rDNA gene markers confirmed that previously undescribed early larval stages in samples belonged to A. ornatus. Most A. ornatus phyllosomas were caught at shallow-bight stations, with catch rates declining steeply to the north and south of the bight and when bottom depth exceeded 100 m. Season and sampling station (nested within cross-shelf transect) were significant variables in explaining phyllosoma abundance. We provide evidence that the KZN Bight serves as a pelagic nursery area for A. ornatus phyllosoma larvae at all developmental stages, but not for other lobster species with known adult populations in the region.

Optimal Atmospheric Heat Sources for the Interannual Variability of South Asian Summer Monsoon

AbstractUsing a Green's function‐like approach, this study identifies optimal atmospheric heat sources for the two leading modes of South Asian Summer Monsoon (SASM) interannual variability. Optimal heating for the first mode, characterized by a lower‐level anomalous anticyclone over northern Bay of Bengal (BOB), is distributed over the Arabian Sea and tropical eastern Indian Ocean (EIO)‐Maritime Continent, with cooling over the BOB‐western North Pacific. In contrast, heating over the tropical southwestern Indian Ocean and equatorial Atlantic, along with cooling over the tropical EIO‐western Pacific, optimally drives the second mode, featuring an anomalous anticyclone over central‐northern India. El Niño/Southern Oscillation indirectly influences SASM by triggering heat sources resembling these optimal patterns. Other sea surface temperatures (SSTs), like those over equatorial Atlantic, can also generate similar heating structures, causing corresponding SASM anomalies. This suggests that the impact of SST modes on SASM depends on the similarity of induced heat sources to optimal patterns.

Diving deeper into the taxonomy of the Neoscopelus species complex (Myctophiformes: Neoscopelidae) with the description of Neoscopelus serranoi sp. nov.

Previous studies have highlighted possible cryptic biodiversity in the genus Neoscopelus. This hypothesis was tested using new morphological, molecular and biogeographical data on species of this genus caught in the north Atlantic between 2010 and 2022. The information obtained has been combined with available data in an integrative approach, including a review of morphological characters reported in the ichthyological literature and DNA-based species delimitation analyses. The main outcome of the present study is the description of Neoscopelus serranoi sp. nov. from the Atlantic and southwestern Indian Oceans. The new species is morphologically very similar to Neoscopelus microchir from which differs in having a shorter anal-fin base, a shorter pelvic fin, more dorsal and pectoral-fin rays, less anal-fin rays, fewer gillrakers and fewer isthmus and lateral photophores. They also differ in geographic distribution, with the new species occurring in the Atlantic Ocean and the near southwestern Indian Ocean, whereas N. microchir was originally described from Japanese waters of the Pacific Ocean. A literature review of available morphological data between geographic areas for Neoscopelus macrolepidotus and Neoscopelus microchir showed a large intraspecific overlap and no boundaries. However, molecular species delimitation based on the mitochondrial COI gene revealed the existence of cryptic diversity in both species, with eight to ten molecular operational taxonomic units (MOTU), compared to three valid species. Neoscopelus serranoi sp. nov. was considered an independent MOTU in all analyses performed, supporting the morphological identification as a new species. These results highlight that the taxonomy of Neoscopelus is far from settled and show that a greater sampling effort is needed to resolve the uncertainties and to describe unknown putative species. This also exemplifies the virtues of integrative taxonomy in delving into the systematics of deep-sea fishes.

A Lankan invasion? An intriguing case of the presence of a Sri Lankan endemic skink on the southwestern Indian Coast

The biota of the Indian subcontinent is a result of Gondwanan vicariance to some extent but largely through multiple faunal exchanges between neighbouring regions. However, in recent times, humans have influenced regional biotas through the human-mediated transport of many generalist species. In this study, we report populations of Lankascincus fallax, a skink species supposedly endemic to Sri Lanka, from the west coast of Kerala in India. We conducted morphological and genetic analysis to ascertain the identity of this species and to check whether the Indian population is monophyletic. We then performed a divergence dating analysis to test whether the presence of this population is a result of ancient dispersal into India or a result of more recent events. Additionally, we used species distribution modelling to test whether the Indian L. fallax is a result of natural dispersal into India during the past (mid-Pliocene to the last glacial maximum) when there were possible land bridges between India and Sri Lanka. We find that the Indian population is nested within the Sri Lankan L. fallax and that this population diverged from the Sri Lankan population very recently, during the Pleistocene. We do not find climatic suitability between the Indian west coast and Sri Lanka under current climatic conditions, or during the past suggesting no suitable dispersal corridors between India and Sri Lanka. Overall, we hypothesize that the Indian populations of L. fallax are likely a result of human-mediated transport into India and subsequent dispersal across the west coast of Kerala.

Distribution and influencing factors of microeukaryote in different water layers of the southwestern Indian Ocean Ridge

Distribution and influencing factors of microeukaryote in different water layers of the southwestern Indian Ocean Ridge

Devils in distress: Insights into the composition and genetic population structure of devil rays landed in the Southwestern Indian Ocean

Devil ray populations in the Southwestern Indian Ocean (SWIO) are facing alarming declines due to overexploitation driven by high demand for their products in international markets. Despite local and international conservation efforts, insufficient information on their genetic population structure and uncertainties in their taxonomy complicates effective conservation strategies. This study analysed sequences of the cytochrome oxidase subunit I gene (620 base pairs) from 120 devil rays landed in Tanzania to test two hypotheses: (i) whether devil ray populations in the SWIO have lower genetic diversity compared to those in other regions, and (ii) whether Mobula mobular and M. japanica are indeed synonyms of the same species. Our findings identified four devil ray species: Mobula mobular, M. kuhlii, M. tarapacana, and M. thurstoni, all classified as endangered on the IUCN Red List but lacking national protection. The study also confirmed the taxonomic alignment of M. mobular and M. japanica as the same species, validating previous findings. Additionally, M. thurstoni dominated the landed devil rays, accounting for over 70 % of the specimens, and exhibited higher haplotype and nucleotide diversities compared to populations in the Eastern Indian Ocean (EIO). Furthermore, populations of M. thurstoni in Tanzania showed limited historical genetic connectivity (FST = 0.176, p < 0.05) with those in the EIO and Brazil. The study calls for urgent action to revisit existing fishing regulations in Tanzania to ensure the protection of the identified endangered rays. It also advocates for unified conservation measures for the species formerly considered separate, M. mobular and M. japanica. While these findings should be validated with hypervariable nuclear markers, the observed patterns in genetic connectivity highlight the need for regional-specific conservation strategies aimed at addressing local threats such as overfishing and destructive fishing practices to ensure the sustainability of M. thurstoni populations across their range.

Unveiling interconnectedness between marine protected and non‐protected areas in Tanzania: Insights from barred mudskipper (Periophthalmus argentilineatus) population genetic structure

AbstractMarine protected areas (MPAs) are globally recognised for their role in safeguarding marine fisheries from the detrimental impacts of human activities and climate change. Tanzania, in alignment with global conservation efforts, has designated 18 MPAs and plans to establish four more in the Southwestern Indian Ocean. However, a critical challenge arises from the lack of a thorough understanding of genetic connectivity between populations in MPAs and non‐protected areas (NPAs). To address this, 151 mitochondrial D‐loop sequences (357 base pairs) of Barred mudskipper (Periophthalmus argentilineatus) were analysed to assess genetic connectivity between MPAs and NPAs in Tanzania. The findings revealed a low and insignificant index of genetic differentiation (FST = −0.0046, p &gt; 0.05) between populations in MPAs and NPAs. Furthermore, haplotypes from both MPAs and NPAs did not cluster based on their geographical origins, with over 9 haplotypes shared between these areas. Additionally, the measured haplotype and nucleotide diversities did not differ significantly between MPAs and NPAs (p &gt; 0.05). However, MPAs showed a significantly higher number of private haplotypes compared to NPAs (p &lt; 0.05). These findings suggest high genetic interconnectedness between populations in MPAs and NPAs, underlining the importance of existing MPAs in preserving local genetic diversity and their potential to replenish depleted NPAs. However, realising the full potential of this interconnectedness necessitates stringent enforcement of sustainable fishing practices to alleviate pressure on fish stocks. It also requires the protection of critical habitats that serve as corridors for fish dispersal, facilitating interconnectedness between MPAs and NPAs.

Description of a Remarkable New Skate Species of Leucoraja Malm, 1877 (Rajiformes, Rajidae) from the Southwestern Indian Ocean: Introducing 3D Modeling as an Innovative Tool for the Visualization of Clasper Characters.

A remarkable new deep-water skate, Leucoraja longirostris n. sp., is described based on eight specimens caught during different expeditions to the southern Madagascar Ridge in the southwestern Indian Ocean. The new species differs from all congeners by its remarkably long and acutely angled snout (horizontal preorbital length 17.2-22.6% TL vs. 8.5-11.9% TL and 4.2-6.1 vs. 1.7-3.5 times orbit length, snout angle 65-85° vs. 90-150°). Furthermore, it is apparently endemic to the Madagascar Ridge, distant from the known distribution areas of all congeners. In addition to L. fullonica and L. pristispina, L. longirostris n. sp. is also the only species with plain dorsal coloration. Furthermore, the new species is the only Leucoraja species with an external clasper component dike and, besides L. wallacei, the only one with four dorsal terminal (dt) cartilages. The shape of the accessory terminal 1 (at1) cartilage with four tips is also unique within the genus. A new approach for the visualization of the clasper characters is introduced based on 3D models of all skeletal and external features. This enables a much easier and much more precise interpretation of every single clasper component, of the entire structure, and, in particular, the relationship between external features and skeletal cartilages. A new English translation of the first diagnosis of Leucoraja is provided, along with a revised generic diagnosis and a key to the species of Leucoraja in the Indian Ocean.

Genetic diversity of endosymbiotic bacteria Wolbachia infecting two mosquito species of the genus Eretmapodites occurring in sympatry in the Comoros archipelago.

The influence of Wolbachia on mosquito reproduction and vector competence has led to renewed interest in studying the genetic diversity of these bacteria and the phenotypes they induced in mosquito vectors. In this study, we focused on two species of Eretmapodites, namely Eretmapodites quinquevittatus and Eretmapodites subsimplicipes, from three islands in the Comoros archipelago (in the Southwestern Indian Ocean). Using the COI gene, we examined the mitochondrial genetic diversity of 879 Eretmapodites individuals from 54 sites. Additionally, we investigated the presence and genetic diversity of Wolbachia using the wsp marker and the diversity of five housekeeping genes commonly used for genotyping through Multiple Locus Sequence Typing (MLST). Overall, Er. quinquevittatus was the most abundant species in the three surveyed islands and both mosquito species occurred in sympatry in most of the investigated sites. We detected a higher mitochondrial genetic diversity in Er. quinquevittatus with 35 reported haplotypes (N = 615 specimens, Hd = 0.481 and π = 0.002) while 13 haplotypes were found in Er. subsimplicipes (N = 205 specimens, Hd = 0.338 and π = 0.001), this difference is likely due to the bias in sampling size between the two species. We report for the first time the presence of Wolbachia in these two Eretmapodites species. The prevalence of Wolbachia infection varied significantly between species, with a low prevalence recorded in Er. quinquevittatus (0.8%, N = 5/627) while infection was close to fixation in Er. subsimplicipes (87.7%, N = 221/252). Both male and female individuals of the two mosquito species appeared to be infected. The analysis of MLST genes revealed the presence of two Wolbachia strains corresponding to two new strain types (STs) within the supergroups A and B, which have been named wEretA and wEretB. These strains were found as mono-infections and are closely related, phylogenetically, to Wolbachia strains previously reported in Drosophila species. Finally, we demonstrate that maternal transmission of Wolbachia is imperfect in Er. subsimplicipes, which could explain the presence of a minority of uninfected individuals in the field.

Investigating the potential roles of intra-colonial genetic variability in Pocillopora corals using genomics

Intra-colonial genetic variability (IGV), the presence of more than one genotype in a single colony, has been increasingly studied in scleractinians, revealing its high prevalence. Several studies hypothesised that IGV brings benefits, but few have investigated its roles from a genetic perspective. Here, using genomic data (SNPs), we investigated these potential benefits in populations of the coral Pocillopora acuta from Reunion Island (southwestern Indian Ocean). As the detection of IGV depends on sequencing and bioinformatics errors, we first explored the impact of the bioinformatics pipeline on its detection. Then, SNPs and genes variable within colonies were characterised. While most of the tested bioinformatics parameters did not significantly impact the detection of IGV, filtering on genotype depth of coverage strongly improved its detection by reducing genotyping errors. Mosaicism and chimerism, the two processes leading to IGV (the first through somatic mutations, the second through fusion of distinct organisms), were found in 7% and 12% of the colonies, respectively. Both processes led to several intra-colonial allelic differences, but most were non-coding or silent. However, 7% of the differences were non-silent and found in genes involved in a high diversity of biological processes, some of which were directly linked to responses to environmental stresses. IGV, therefore, appears as a source of genetic diversity and genetic plasticity, increasing the adaptive potential of colonies. Such benefits undoubtedly play an important role in the maintenance and the evolution of scleractinian populations and appear crucial for the future of coral reefs in the context of ongoing global changes.

The Vetigastropoda (Mollusca) of Walters Shoal, with descriptions of two new genera and thirty new species

The vetigastropod material collected on Walters Shoal during Cruise MD208 of the Tropical Deep-Sea Benthos programme is documented. In total, 50 species were obtained, 30 of which are new and apparently endemic to the seamount. Of the other 20 species, eight are regionally endemic to the south-western Indian Ocean, 11 are more widely distributed in the Indo-West Pacific and one is possibly of deep-water Atlantic origin. The primary affinities of the fauna are with warm temperate South Africa and the tropical western Indian Ocean, but one species is potentially a seamount endemic of southern affinity. A new pseudococculinid genus living on decomposing bird feathers is described, a biogenic substrate association previously unknown in the Mollusca. The following new genera are described: Imbricoscelis gen. nov. and Pterodacna gen. nov. The following new species are described: Akritogyra crenulata sp. nov., Bathymophila williamsae sp. nov., Benthobrookula araneum sp. nov., Be. galeneae sp. nov., Be. laticostata sp. nov., Be. scalaroides sp. nov., Be. semisculpta sp. nov., Bruceina areneformis sp. nov., Calliostoma pantopunctatum sp. nov., Cantrainea herosae sp. nov., Carinastele achrosta sp. nov., Cornisepta marshalli sp. nov., Emarginula lentiginosa sp. nov., E. nodulicostata sp. nov., E. retrogyra sp. nov., E. salebrosa sp. nov., Fluxinella dufresneae sp. nov., Gibbula roseosticta sp. nov., Hadroconus scobina sp. nov., Kaiparathina monticola sp. nov., Lissotesta wareni sp. nov., Microcollonia miniata sp. nov., Mikro crassus sp. nov., Parviturbo cicatricosus sp. nov., Phragmomphalina candida sp. nov., Pterodacna boucheti gen. et sp. nov., Solariella asaphea sp. nov., Spinicalliotropis lepidota sp. nov., Stomatella multilirata sp. nov. and Trenchia mcleani sp. nov. The following new combinations are proposed: Brookula coronis Barnard, 1963 is transferred to Imbricoscelis gen. nov., Cantharidus nolfi Poppe, Tagaro &amp; H. Dekker, 2006 is transferred to Kaiparathina Laws, 1941 and Solariella incisura Melvill, 1909 is transferred to Phragmomphalina Herbert &amp; Williams, 2020. The following new synonyms are proposed: Carinastele wareni Vilvens, 2014 is a synonym of Bruceina cognata (Marshall, 1988); Fluxinella stellaris Bozzetti, 2008 is a synonym of Agagus stellamaris Herbert, 1991.

Different Configurations of Cross-Equatorial Flows over the Indian Ocean–Western Maritime Continent and Their Implications for Improving Regional Climate Predictability

Abstract Low-level cross-equatorial flows (CEFs) over the Indian Ocean–western Maritime Continent (IO-wMC) play a crucial role in transporting energy, mass, and water vapor into the Northern Hemisphere during the Asian summer monsoon season (May–September). Utilizing ECMWF reanalysis data (ERA5), we investigate three CEFs over the IO-wMC: the Somali-CEF, Bay of Bengal CEF (BoB-CEF), and South China Sea CEF (SCS-CEF). The statistical independence of the Somali-CEF and BoB-CEF implies distinct formation processes on the interannual time scale. To examine the interannual variability of CEFs, we perform an empirical orthogonal function (EOF) analysis of vertically integrated meridional winds from surface pressure to 850 hPa over the equatorial IO-wMC. The first EOF mode reveals a weakening of the Somali-CEF and strengthening of the BoB-CEF and SCS-CEF, which is associated with the concurrent summer Indian Ocean dipole and quasi-biennial phase transition of El Niño events. The second EOF is related to the Indo-western Pacific Ocean capacitor (IPOC) mode that often emerges in post–El Niño summers. Despite the IPOC’s influence on meridional winds over the tropical southwestern Indian Ocean, three CEFs are uncorrelated with the EOF2. On the other hand, the Somali-CEF and BoB-CEF are significantly weakened in the EOF3. The EOF3 is an ENSO-unrelated internal IPOC mode. We further use a multilinear regression model based on preceding sea surface temperature (SST) over the Indo-Pacific and EOF3 to predict regional climate anomalies and compare the prediction skill with the SST-based models. Our results suggest that adding EOF2- and EOF3-related variability to the prediction model can improve Asian summer monsoon predictability. Significance Statement This study examines the interannual variability of low-level cross-equatorial flows (CEFs) over the Indian Ocean and western Maritime Continent, crucial components of the Asian summer monsoon. We identify three CEFs and investigate their relationships with interannual sea surface temperature modes in the Indo-Pacific region. The leading empirical orthogonal function (EOF) modes of equatorial meridional wind reveal diverse CEF configurations and post–El Niño summer wind variability. Also, the EOFs distinguishes between El Niño and El Niño–unrelated equatorial meridional wind variability. Utilizing a multilinear regression model based on both preceding Indo-Pacific SST modes and configurations of CEFs, we demonstrate the potential to enhance regional climate predictions, contributing to a more comprehensive understanding and forecasting of the Asian summer monsoon and its associated climate impacts.

Surveillance of Fleas and Their Small Mammal Hosts for Plague Risks in Some Main Seaports of the Islands of the Southwestern Indian Ocean.

Since ancient times, seaports have been the hot spots for plague introduction into free countries. Infected ship rats reached new areas, and epizootics occurred prior to human infection via flea bites. Beginning in the 1920s/1930s, rodent and flea surveillance was carried out as part of plague hazard management in seaports of the world. Nowadays, such activity is not done regularly. In the southwestern Indian Ocean (SWIO) region, plague surveillance is of great importance given plague endemicity in Madagascar and thus the incurred risk for neighboring islands. This study reports animal-based surveillance aimed at identifying fleas and their small mammal hosts in SWIO seaports as well as Yersinia pestis detection. Small mammal trappings were performed in five main seaports of Madagascar (Toamasina and Mahajanga), Mauritius (Port Louis), and the Union of Comoros (Moroni and Mutsamudu). Mammals were euthanized and their fleas collected and morphologically identified before Y. pestis detection. In total, 145 mammals were trapped: the brown rat Rattus norvegicus (76.5%), the black rat Rattus rattus (8.3%), and the Asian house shrew Suncus murinus (15.2%). Fur brushing allowed collection of 1,596 fleas exclusively identified as Xenopsylla cheopis. All tested fleas were negative for Y. pestis DNA. This study shows that both well-known plague mammal hosts and flea vectors occur in SWIO seaports. It also highlights the necessity of carrying out regular animal-based surveillance for plague hazard management in this region.

Transcending borders: Remarkable genetic homogeneity among Barred mudskipper (Periophthalmus argentilineatus) populations in the southwestern Indian Ocean

AbstractIn the past 24 years, the Southwestern Indian Ocean (SWIO) region has lost about 4% of its mangroves due to unsustainable extraction, land clearance for agriculture and climate change impacts. Since this loss risks fragmenting mangrove fauna, this study analysed 179 D‐loop sequences (329 base pairs) of Barred mudskipper (Periophthalmus argentilineatus) sampled from estuarine mangroves in the SWIO to test two hypotheses: (1) whether Barred mudskipper populations in the region display genetic connectivity along the fragmented mangroves and (2) whether these populations have experienced demographic declines in recent history. The populations showed low and insignificant indices of genetic differentiation (FST = −0.00068, p &gt; 0.05; ΦST = 0.005, p &gt; 0.05), and haplotypes from different localities did not cluster according to their geographical origins. Similarly, Bayesian phylogenetic analysis grouped all SWIO haplotypes into one cluster, suggesting the hypothesis of genetic interconnectedness could not be rejected. Furthermore, the populations showed high haplotype diversity (0.96 ± 0.007) combined with low nucleotide diversity (1.09 ± 0.61%). They also showed significant negative Tajima's D values (−1.71, p &lt; 0.02) and a unimodal mismatch distribution, indicating the hypothesis of demographic expansion post a historical bottleneck could not be rejected. Nevertheless, the Bayesian skyline plot indicated an increase in population size post‐bottleneck, suggesting an ongoing recovery. These findings indicate that, despite varying management approaches in the SWIO, Barred mudskipper populations exhibit genetic exchange that transcends geographical and administrative borders. This implies that their populations should be considered a single stock within the region. These findings underscore the need for coordinated management approaches among the SWIO countries. This is crucial because inconsistencies in conservation measures across borders could disrupt the population's interconnectedness, potentially impacting their genetic integrity and long‐term survival.

Diversity of the Austral Summer Quasi-Biweekly Oscillation in the Southwestern Indian Ocean

Abstract The 10–20-day quasi-biweekly oscillation (QBWO) is active in the southwestern Indian Ocean (SWIO) during austral summer. Compared with comprehensive analyses of the QBWO in the Asian monsoon regions during boreal summer, studies focusing on the austral summer QBWO in the SWIO are relatively scarce. In this study, the diversity of the austral summer QBWO in the SWIO is examined based on K-means cluster analysis, which objectively classifies two distinct modes: an eastward-propagating mode (EM) and a poleward-propagating mode (PM). For the EM (PM), an active convection center originates from the subtropical ocean (tropical ocean) and exhibits an eastward (poleward) propagation path. Moisture budget analysis reveals that positive moisture time tendency anomalies show a phase-leading relationship relative to both QBWO convection centers. This phase leading in moisture tendency anomalies is mainly due to horizontal moisture advection. Further analysis demonstrates that meridional moisture transport (i.e., the summer mean moisture advected by the meridional quasi-biweekly wind) is fundamentally responsible for moisture phase leading in both QBWO modes in their mature phases. The combined scale interaction among low frequency, quasi-biweekly, and high frequency contributes to the initial movement for both modes in the growing phases. Although the two modes in the SWIO are initiated in different regions and exhibit distinct evolutionary features, they are regulated by similar moisture dynamics: the northerlies (northeasterlies) of the cyclonic wind response bring higher mean moisture levels east (south) of the convective center, which leads to the eastward (southward) movement of the EM (PM). Significance Statement The quasi-biweekly oscillation (QBWO), which can affect extreme weather events, such as extreme precipitation and heat waves, is active in the southwestern Indian Ocean (SWIO) during austral summer. Compared with previous studies of the QBWO in the Asian monsoon regions during boreal summer, studies focusing on the austral summer QBWO in the SWIO are relatively scarce. Specifically, we objectively classify the austral summer QBWO in the SWIO into two distinct modes: an eastward-propagating mode (EM) and a poleward-propagating mode (PM). Through moisture tendency diagnosis, we find that the two QBWO modes are regulated by similar moisture dynamics, although they are initiated in different regions and exhibit distinct evolutionary features. This improved understanding may provide insights into the monitoring and prediction of the QBWO.

Assessment of thermocline depth bias in the Seychelles-Chagos Thermocline Ridge of the Southwestern Indian Ocean simulated by the CMIP6 models

The Seychelles-Chagos Thermocline Ridge (SCTR, 5°S-10°S, 50°E-80°E) is a unique open-ocean upwelling region in the southwestern Indian Ocean. Due to the negative wind stress curl between the equatorial westerlies and southeasterly trade winds, SCTR is known as a strong upwelling region with high biological productivity, providing a primary fishing zone for the surrounding countries. Given its importance in shaping the variability of the Indian Ocean climate by understanding the sea-air interaction and its dynamics, the simulation of SCTR is evaluated using outputs from the Coupled Model Intercomparison Project Phase Sixth (CMIP6). Compared to observations, 23 out of 27 CMIP6 models tend to simulate considerably deeper SCTR thermocline depth (defined as the 20°C isotherm depth (D20))– a common bias in climate models. The deep bias is related to the easterly wind bias in the equatorial to southern Indian Ocean, which is prominent in boreal summer and fall. This easterly wind bias produces a weak annual mean Ekman pumping, especially in the boreal fall. Throughout the year, the observed Ekman pumping is positive and is driven by two components: the curl term, is associated with the wind stress curl, leads to upwelling during boreal summer to fall; the beta term, is linked to planetary beta and zonal wind stress, contributes to downwelling during boreal spring to fall. However, the easterly wind bias in the CMIP6 increases both the positive curl and negative beta terms. The beta term bias offsets the curl term bias and reduces the upwelling velocity. Furthermore, the easterly wind bias is likely caused by the reduced east-west sea surface temperature (SST) difference associated with a pronounced warm bias in the western equatorial Indian Ocean, accompanied by the east-west mean sea level pressure gradient over the Indian Ocean. Furthermore, this study finds local wind-induced Ekman pumping to be a more dominant factor in thermocline depth bias than Rossby waves, despite CMIP6 models replicating Rossby wave propagation. This study highlights the importance of the beta term in the Ekman pumping simulation. Thus, reducing the boreal summer-to-fall easterly wind bias over the Indian Ocean region may improve the thermocline depth simulation over the SCTR region.

An overview of the specific diversity in the genus Cystiscus Stimpson, 1865 and the description of 27 new Cystiscus species (Muricoidea: Cystiscidae) from the Walters Shoal, southwestern Indian Ocean

The Cystiscus fauna collected on the Walters Shoal by the 2017 MNHN/IRD Expedition is revised on the ground of shell morphology. Twenty-nine different species are recognized, 27 of them being formally described as Cystiscus subcylindricus, sp. nov., C. triplicatus sp. nov., C. herberti sp. nov., C. inflatus sp. nov., C. stimpsoni sp. nov., C. pyriformis sp. nov., C. harpaeformis sp. nov., C. laurae sp. nov., C. robustus sp. nov., C. minimus sp. nov., C. ovoides sp. nov., C. mamillatus sp. nov., C. angustus sp. nov., C. profundus sp. nov., C. granum sp. nov., C. dentiferus sp. nov., C. alatus sp. nov., C. apertus sp. nov., C. contortus sp. nov., C. sinuosus sp. nov., C. pupoides sp. nov., C. lussii sp. nov., C. smithi sp. nov., C. serratus sp. nov., C. lachrymaformis sp. nov., and C. quinqueplicatus sp. nov. Among the 27 described species, 25 of them are only known from their holotype, one paratype from the type station is reported for C. subcylindricus, and two conspecific specimens collected from a non-type station are reported for C. lussii. Eight morphs represented by one or two shells are tentatively ascribed to some of the 27 recognized species. Among the 29 recognized species, 28 of them are found at bathyal levels, and only one of them (C. quinqueplicatus) was collected at a reef level, so the Cystiscus fauna of the Walters Shoal is characterized as bathyal fauna. From the fact that any other Cystiscus assemblage was ever studied from the bathyal of the Indo-Pacific Province, no direct comparison is made with external items, and the Cystiscus species of the Walters Shoal are only compared to each other.

First Records of a Hydrolagus Species (Holocephali: Chimaeridae) from Reunion Island and Mayotte (Southwestern Indian Ocean)

Two specimens of large blackish chimaeras of the genus Hydrolagus were caught, one off Reunion Island and the other off Mayotte in the southwestern Indian Ocean. The specimens, an adult male of 710 mm BDL and a female of 870 m BDL, are described, compared to similar species (i.e., having a large size, over 110 cm TL, dark blackish colour, and rather long conical snout), and tentatively identified to the small-eyed rabbitfish Hydrolagus affinis, pending a taxonomic revision of the large blackish chimaeras is completed.

Glacial expansion of carbon-rich deep waters into the Southwestern Indian Ocean over the last 630 kyr

Oceanic carbon storage is one of the main sinks for atmospheric CO2, and thought to be the major contributing factor for CO2 drawdown during past glacial periods. Both physical and biogeochemical processes control the capacity of carbon storage in the ocean. During glacial periods of the Pleistocene the larger volume of deep-water masses of Southern Hemisphere origin in the Atlantic has been shown to promote carbon storage in the Southern Ocean. However, the latitudinal extension of this water mass in the Indian Ocean has been scarcely studied. In this study, we combine foraminiferal εNd and benthic δ13C of two sediment cores in the southwest Indian Ocean (MD96–2077, 33°S, 3781 m water depth; MD96–2052, 19°S, 2627 m water depth), to reconstruct the spatial and temporal evolution of glacial carbon-rich deep waters in the SW Indian over the last 630 kyr. The combined use of foraminiferal εNd and benthic δ13C allows to distinguish δ13C changes related to water mass mixing and from respired carbon accumulation within the water masses. Nutrient-rich deep waters, which cannot be explained by the enhanced proportion of southern-sourced waters, were present at core sites deeper than 2700 m during glacial periods and extended at least until 33°S into the SW Indian Ocean. From Marine Isotope Stage (MIS) 14 to MIS 10, glacial carbon storage increased gradually until reaching its highest capacity during the extreme glacial periods MIS 12 and 10. Orbital forcing (100-kyr eccentricity, 41-kyr obliquity), restricted air-sea exchange and enhanced ocean stratification, fostered higher carbon storage during periods of relatively lower eccentricity and obliquity. Furthermore, after MIS 10, a progressive transition was observed from 100-kyr eccentricity to 41-kyr obliquity cycles in benthic δ13C and δ18O records of core MD96–2077 and sea-ice cover changes derived from ice-rafted debris of the Agulhas Plateau composite core site.

Loggerhead turtle oceanic-neritic habitat shift reveals key foraging areas in the Western Indian Ocean

Loggerhead sea turtles (Caretta caretta) use both oceanic and neritic habitats depending on their life stage, eventually undertaking an ontogenetic shift. Juveniles likely start foraging in a purely opportunistic manner and later seek resources more actively. In the Indian Ocean, it is still unclear where oceanic-stage individuals go, what they do, and importantly where they forage. Yet, such information is crucial to protect this endangered species from anthropogenic threats such as bycatch in fisheries. To address this, 67 individuals (66 late juveniles and one adult) bycaught in the open ocean were equipped with satellite tags and released in the Southwestern Indian Ocean between 2008 and 2021. Most individuals traveled to the Northwestern Indian Ocean where they used neritic habitats of the continental shelf (i.e., largely between 0 and 200-m depth). Using hidden Markov models, we identified three types of movements likely associated with traveling, wandering, and foraging behaviors. We found that the movement characteristics of these behaviors differ depending on turtles’ target destination and habitat (oceanic vs neritic), highlighting different strategies of habitat use among individuals of presumably the same life stage (late juveniles). The turtles that traveled to the Northwestern Indian Ocean encountered warmer waters (mean = 27.6°C, min. = 20.6°C, max. = 33.1°C) than their counterparts remaining in the Southern Hemisphere (mean = 22.5°C, min. = 14.6°C, max. = 29.7°C) but were found foraging at locations with comparable biomass of potential prey (mean = 2.5 g C m-2, min. = 0.5 g C m-2, max. = 10.4 g C m-2) once in the Northern Hemisphere. It remains obscure why these individuals undertook a trans-equatorial migration. Once in neritic habitats, the proportion of time spent traveling was considerably reduced (from 33% to 19%) and allocated to foraging instead. In light of this, it is very likely that the individuals migrated to the Northwestern Indian Ocean to undergo an oceanic-to-neritic ontogenetic shift. Our study sheds light on the behavioral ecology of loggerhead turtles and identifies important foraging areas in the Western Indian Ocean, with the top-three most densely used ones being the Gulf of Oman, the Central Somali Coast, and the Western Arabian Sea.