Northwestern Indian Ocean Research Articles

Mitochondrial DNA-based reassessment of Antennablennius Fowler (Blenniidae: Salariini) from the north-western Indian Ocean, with resurrection of A. persicus (Regan)

Seven Operational Taxonomic Units within the blenniid genus Antennablennius are recognised from the north-western Indian Ocean, based on the Cytochrome oxidase-c subunit I gene (CO1). Five of them can be referred to nominal species with certainty. Two additional distinct lineages originate from the Persian Gulf and the Gulf of Oman, and the Socotra Archipelago. They were initially, morphologically, identified as A. adenensis Fraser-Brunner, 1951, yet could hitherto not be referred to this species with certainty, requiring further genetic data. Detailed descriptions accompanied by photographs are provided for six species, including A. adenensis. Antennablennius persicus (Regan, 1905) is resurrected from the synonymy of A. bifilum (Günther, 1861) and re-described based on specimens from the Iranian coast of the Persian Gulf and Gulf of Oman, increasing the number of valid species in the genus to eight. The former differs from the latter species in details of morphology and coloration, supported by genetic data. A revised key to all species of the genus is included. A phylogenetic investigation of the genus based on Maximum Likelihood (ML) and Bayesian Inference (BI) analyses of the CO1 data is presented for the first time.

Uptake time and enrichment mechanism of rare earth elements in deep-sea bioapatite

Bioapatite is widely recognized as the primary carrier for rare earth elements and yttrium (REY) in deep-sea REY-rich muds. The incorporation of REY into bioapatite occurs at the water-sediment interface, which has the potential to serve as a proxy for reconstructing paleoenvironmental conditions. The timing of REY uptake and the fractionation of rare earth elements (REEs) within bioapatite are crucial factors to understanding the application of these proxies. In this study, we present in-situ geochemical data for bioapatite obtained from surface sediments in the high sedimentation rate Somali Basin of the northwestern Indian Ocean (NWIO), as well as fish teeth within nodules from the low sedimentation rate in the northwestern Pacific Ocean (NWPO). Our findings indicate that the uptake time of REY occurred rapidly, with the ΣREY content reaching 7265 μg/g in bioapatite from the surface sediments in the NWIO within several thousand years. The bone fragments exhibited a high ΣREY content, which was primarily attributed to substitution processes. This led to a notably elevated proportion of middle rare earth elements (MREE) compared to fish teeth. In contrast, the adsorption and substitution mechanisms responsible for REY incorporation decreased from the root to the tip in fish teeth, resulting in a pronounced decline in ΣREY content. The adsorption mechanism was identified as the primary process responsible for REY uptake in the fish teeth within the studied nodules from the NWPO. The fractionation pattern of REEs in these teeth exhibited similarities to that of fish teeth from the NWIO. Therefore, we inferred that the fish teeth within the studied nodules may preserve the original information during late diagenesis. The variation of REY contents in the nodules was influenced by the redox environment, and there is no evidence to support the migration of REY from the nodules into the fish teeth.

Calliobothrium Narsisae n. sp. (Tetraphyllidea: Calliobothriidae) from the Triakid Shark Mustelus mosis Hemprich and Ehrenberg from the Persian Gulf, with a Note on Its Specific Proglottisation Process and a Key to the Species of the Genus.

The members of the tetraphyllidean genus Calliobothrium exhibit a high degree of host specificity in the shark genus Mustelus. In the Indian Ocean, where M. mosis occurs dominantly, there is no information on Calliobothrium. The purpose of this study was the evaluation of the Calliobothrium species in M. mosis in the Persian Gulf, northwestern Indian Ocean. Nine specimens of Mustelus mosis were caught from the Persian Gulf. The tapeworms isolated were stained, measured, and the line drawings were made. Calliobothrium narsisae n. sp. is described and differs from C. creeveyae in the intact, rather than the three-lobed, posterior bothridial margins, from C. tylotocephalum in possessing three, rather than one, suckers per bothridial muscular pad, from C. nodosum in possessing the elongated, rather than robust and thorn-like, hooks, from C. shirozame in a maximum number of four, rather than three, dorsal and four, rather than three, ventral laciniations along the strobila, and from its other congeners in possessing three, rather than two, dorsal and three, rather than two, ventral laciniations in the anteriormost immature proglottids. A key to the species of the genus is presented and the proglottisation process, by the germinative zone placed in the terminal part of the worm at least in some stages of growth to maturity, is discussed. The description of C. narsisae n. sp., which brings the total number of valid species of Calliobothrium to 10, also indicates that the germinative zone is not always at the anterior to strobila and in this new species it is placed in the terminal part of the worm at least in some stages of growth to maturity.

Long-term responses of phytoplankton biomass to the ocean surface variables in the central parts of the Arabian Sea and the Bay of Bengal

The northern Indian Ocean has been warming steadily for over a half-century, especially the north-western Indian Ocean. It is widely reported that the increasing sea surface temperature in the global oceans decreases phytoplankton biomass and productivity. The impacts of long-term variations in the sea surface properties on the phytoplankton biomass (chlorophyll a) are least studied in the northern Indian Ocean. In this study, we have retrieved satellite, model, and ARGO float data sets to investigate the long-term variations in the distributions and trends of major oceanic variables for a better understanding of the respective changes that occurred in chlorophyll a concentration in the central regions of the Arabian Sea (AS) and the Bay of Bengal (BB). We have selected variables such as sea surface temperature (SST), sea surface salinity (SSS), photosynthetically available radiation (PAR), euphotic depth (ZEU), mixed layer depth (MLD), wind speed, mean sea level anomaly (MSLA), surface currents, etc., to relate with chlorophyll a. We found significant increasing trends in SST and positive-MSLA in both basins, and the chlorophyll a was decreased in the AS but contrastingly increased in the BB. Further data analysis revealed the possible reasons, such as seasonal changes in mean sea level anomaly and meridional currents, for the increasing trend of chlorophyll a in the central Bay of Bengal. The northward flow of the meridional currents during the southwest monsoon (SWM), transports the nutrient-rich water from the coastal upwelling zone of the southwest coast of India to the southern and central BB, and enhances chlorophyll a. Contrastingly, the southward flow of low-saline and nutrient-depleted Bay of Bengal water reduces the chlorophyll a. In addition, the large area of cold-core eddies found during the NEM enhanced the chlorophyll a in the central BB. Though contrasting trends between both basins in chlorophyll a distribution were found, the mean concentration of chlorophyll a in the northern Indian Ocean decreases. The present study signifies the importance of monsoon currents and eddies in regulating the chlorophyll a biomass and primary productivity in the AS and BB.

A new species of xanthid crab, Palapedia schubarti n. sp. (Decapoda, Brachyura, Xanthoidea), from Socotra, northwestern Indian Ocean

Abstract Palapedia schubarti n. sp. is the third species of the genus Palapedia, described from the northwestern Indian Ocean. Two species, P. persica and P. apeli, had already been recorded from the Persian Gulf, and the new species is described from farther south in Socotra Island. Palapedia persica has appendages markedly ornamented with large denticles, while P. apeli has distinctly smooth appendages. Compared to the two above-mentioned species, the new species has an intermediate position regarding the spination, with small denticles on the upper margin of the palm, fingers and anterior margin of walking legs. The morphology of the first male gonopod of P. schubarti n. sp. is very close to P. rastripes, described from Ceylon, and P. truncatifrons described from Japan. The new species, however, is easily distinguishable from those two congeners in having a spade form ending and several other characteristics of chelipeds and carapace.

A new species of short unpatterned moray eel (Anguilliformes: Muraenidae) from the northwestern Indian Ocean, including the Socotra Archipelago, with a redescription of Gymnothorax pseudoherrei Böhlke

A new species of short brown unpatterned moray eel, Gymnothorax. arabicus sp. nov., is described based on specimens collected from the northwestern Indian Ocean, including the Red Sea, Socotra Archipelago, and Arabian Gulf. The new species is characterized by having the dorsal-fin origin before the gill opening, two branchial pores, one or two median intermaxillary teeth, biserial maxillary teeth, total vertebrae 114–120, and a plain brown head and body with dark yellow color at the tip of the tail. The new species formerly has been confused with G. pseudoherrei but is distinguished from it by having slightly more total vertebrae (114–120 vs. 111–116), dark stripes on the throat and side of the head, and a larger maximum size (ca. 300 mm TL vs. ca. 200 mm TL). Gymnothorax pseudoherrei, currently known from the Maldives and Sri Lanka to the Philippines, Solomon Islands, and northern Australia, is redescribed in the present study. A comparison of the COI mtDNA in BOLD, in association with published phylogenetic evidence, confirms that G. arabicus and G. pseudoherrei form two separate genetic lineages, corroborating the morphological data.

Taxonomic characterization of five species of emperor fishes (Actinopterygii: Eupercaria: Lethrinidae) based on external morphology, morphometry, and geographic distribution in the northwestern Indian Ocean

Species of the genus Lethrinus, commonly referred to as emperor fishes, mainly distributed in the Indo–West Pacific, including the Persian Gulf and Gulf of Oman are poorly known. It is difficult to characterize them morphologically, as there are no distinguishing meristic features for the majority of species. We studied the morphological features of 65 specimens representing the genus Lethrinus to define and identify the species involved. Among the morphological characters commonly considered in this type of study, we also included new, previously undescribed characters. The results of Tukey’s post hoc HSD test for metric and meristic variables, which showed significant differences between species in ANOVA, indicate that most metric traits and the number of scale rows above the lateral line differ significantly among the five morphotypes distinguished. Principal component analysis (PCA) and discriminant function analysis (DFA) effectively showed the morphometric variation between these morphotypes. The characteristics of body depth, head length, pectoral fin length, and caudal fin length are the most distinguishing diagnostic at the species level. We determined that the collected fish specimens represented: Lethrinus borbonicus Valenciennes, 1830, Lethrinus crocineus Smith, 1959, Lethrinus lentjan (Lacepède, 1802), Lethrinus microdon Valenciennes, 1830, and Lethrinus nebulosus (Forsskål, 1775). We also confirmed their validity as distinct species. Notably, L. crocineus, previously known only from the western Indian Ocean (Africa and Socotra), is now reported for the first time from the Persian Gulf and the Gulf of Oman. Lastly, a distribution map for the Lethrinus species is included.

Hydroclimate variability over the past 4730 years based on multi-proxy stalagmite records from southwest China

Southwest China has experienced a series of record-breaking droughts over the last two decades, threatening regional ecosystems and exerting socioeconomic effects. Recent studies have focused on the drivers of specific drought events that seasonally vary in occurrence and duration, and a better understanding of regional paleo-records may be key to understanding their underlying mechanisms. Here, we present records of dry and wet alterations from 4730 yr B.P. based on multi-proxy stalagmite records. In particular, we report δ13C records at a five-year-resolution, and trace element (Mg, Sr, and Ba) to Ca ratios at a 25-year-resolution for stalagmite HD12 from Dark Cave, southwest China. Statistically significant correlations between δ13C and Mg/Ca suggest that they are controlled by a common factor, possibly related to local hydrological variations associated with the monsoon circulation. Long-term trends in the Sr/Ca and Ba/Ca ratios may be related to temperature- and/or precipitation-controlled growth rates. The first principal component (multi-proxy PC1) of δ13C and Mg/Ca can be used as the hydrological index in southwest China, indicating high-frequency oscillations on the millennial to multidecadal scales, with four prominent dry periods at 4255 ± 21 to 3430 ± 11, 2605 ± 12 to 2105 ± 9, 1755 ± 11 to 1255 ± 13, and 705 ± 8 to 405 ± 7 yr B.P. This is in general agreement with the monsoon index reconstructed from the detrended and z-scored cave δ18O PC1 in southwest China, indicating the sensitivity of regional hydrological response to monsoon intensity. The dry conditions of southwest China on the millennial-centennial scale may be related to solar forcing and the North Atlantic Oscillation owing to their influence on the strength and position of the westerlies and the Intertropical Convergence Zone. Additionally, the El Niño Southern Oscillation could also affect water vapor transport to southwest China by influencing the strength and position of the Walker circulation and Western Pacific Subtropical High. Moreover, the dry periods of southwest China over the past 4730 years are closely related to the warming of the sea surface temperature of the tropical warm pool on multidecadal scales. Such warming may be detrimental to the transport of water vapor from the North Western Pacific and Indian Ocean to southwest China, resulting in a dry regional environment.

Probabilistic tsunami hazard assessment for the makran subduction zone using logic tree and stochastic rupture sources

ABSTRACT The Makran Subduction Zone (MSZ) in the northwestern Indian Ocean can generate large tsunamigenic thrust earthquakes affecting the coastal regions of Pakistan, Iran, Oman, and western India. In this paper, a probabilistic tsunami hazard assessment is conducted for the MSZ using stochastic tsunami simulations of moment magnitude (Mw) of 7.7–9.1 earthquake scenarios. This study investigates uncertainties associated with earthquake occurrence rate, single-segment (eastern and western MSZ) or two-segment (full MSZ) rupture scenarios, source geometry, and slip heterogeneity. The total number of simulated source models is 15,000. This study presents two major categories of results: stochastic source models and ranges of 475, 975, and 2475-year tsunami heights. For instance, tsunami heights generated by Mw 8.5‒8.7 stochastic sources of western MSZ vary between 1 m and 10 m with a mean of ~ 4.5 m in the affected areas. The tsunami heights are sensitive to the source models’ characteristics, such as location of the large slip areas, bathymetry of the nearshore area, and the location of bays. Considering different occurrence rates results in significant variability in the estimated 475, 975, and 2475-year tsunami heights. For example, 2475-year tsunami height in Chabahar is in the range of 3‒7.4 m at 10 m water depth.

Phylogeography of the intertidal barnacle Chthamalus barnesi in the northwestern Indian Ocean

The phylogeography of intertidal invertebrates in the northwestern Indian Ocean is poorly known due to difficulties in crossing diverse political regions for sampling. This study, based on mitochondrial cytochrome c oxidase subunit 1 (cox1) and nuclear sodium-potassium ATPase alpha-subunit (NaK) sequences, is the first assessment of the phylogeography of the intertidal barnacle Chthamalus barnesi in the Indian Ocean and includes all peripheral seas of the northwestern region. Our phylogenetic analyses defined 1 monophyletic clade, suggesting that C. barnesi does not comprise any cryptic species. Phylogeographic analyses showed no clear differentiation among populations of C. barnesi, except in the Red Sea. This isolated population apparently did not leave the Red Sea during post-Pleistocene glaciation. High ΦST values between the Red Sea and other populations is further indication of isolation. Low values of ΦST among the Persian Gulf, Gulf of Oman and the northeastern Arabian Sea indicate gene flow among these populations. Analysis of molecular variance revealed no significant genetic differentiation among these 3 areas. Phylogeographic analyses suggested that there may be 2 major populations (Red Sea and northwestern Indian Ocean) of C. barnesi in the Indian Ocean resulting from interactions between the Pleistocene glaciation and present-day ocean currents. Among these populations, the Persian Gulf seems to have a higher probability of being the ancestral population. After the Red Sea, the Persian Gulf has the highest genetic diversity of the cox1 gene. This evidence suggests that the Arabian Sea population may have originated from the expansion of the Persian Gulf population after the Pleistocene glaciation.

A New Species of the Shrimp Genus Salmoneus Holthuis, 1955 (Malacostraca: Decapoda: Alpheidae) from the Exposed Shores of Eastern and Southern Oman

Salmoneus sultanus sp. nov. is described based on several specimens collected on the exposed shores of Masirah Island and Dhofar, Oman, a region characterised by seasonal upwellings cooling water temperature to less than 23 °C. The new species is compared with the morphologically most similar congeners present in the north-western Indian Ocean, e.g., S. latirostris (Coutière, 1897), S. cristatus (Coutière, 1897), S. serratidigitus (Coutière, 1897), and S. chadwickae Ďuriš and Horká, 2016; the latter species is recorded for the first time from Oman. The conspicuous colour pattern of S. sultanus sp. nov. appears to be diagnostic, differing greatly from that of S. latirostris, S. cristatus, S. serratidigitus, and S. chadwickae.

Mercury and selenium concentrations and their toxicological implications in silky sharks Carcharhinus falciformis (Elasmobranchii: Chondrichthyes) in the northwestern Indian Ocean

Mercury (Hg) and selenium (Se) were assessed in silky sharks, Carcharhinus falciformis, a carnivorous apex predator and an important resource in the Indian Ocean. Muscle from the caudal peduncle was collected from individual sharks and analyzed using ICP-MS. Results showed that 40% of the specimens exceeded the allowed limits (1.00μg/g ww) of Hg. The mean Hg concentration (1.39 ±1.44μg/g ww) was also higher than the limits. The mean Se concentration in shark muscle was 0.324 ±1.44μg/g ww. The mean Se/Hg molar ratio was 2.23 (range: 0.17–24.5), with 30% of individuals exceeding 1.0, showing a protective effect of Se against Hg. However, for individual sharks’ mean Se health benefit value (Se-HBV) was −3.83. The risks of consuming silky sharks by male and female adults were also calculated. Results showed that the estimated weekly intake (EWI) was 2.9 to 3.5 times higher than the provisional tolerable weekly intake (PTWI). Consumption risks based on hazard quotient (HQ), estimated daily intakes (EDI), and EWI exceeded the threshold for fish consumption by adults, posing immediate health concerns.

Response of the Northwest Indian Ocean purpleback flying squid (Sthenoteuthis oualaniensis) fishing grounds to marine environmental changes and its prediction model construction based on multi-models and multi-spatial and temporal scales

Climate change has posed a great challenge to global fisheries harvesting. Purpleback flying squid (Sthenoteuthis oualaniensis) is a major economic cephalopod in the northwestern Indian Ocean waters, but how to choose the optimal spatiotemporal scales and models for constructing a prediction model of its fishery remains to be researched. Reasonable modeling is essential for predicting and managing the fishery of purpleback flying squid. Based on the statistical data of purpleback flying squid fishery and multi-variable marine environment data in the northwest Indian Ocean from 2015 to 2021, this paper used the gravity center of fishing grounds method, GAM, GBT, 2DCNN, and 3DCNN models to analyze. Results: 1) The catches of purpleback flying squid reached their peak in 2019, and the higher catch areas were concentrated at 15° ∼ 19°N and 61° ∼ 65°E. The annual gravity center of the fishing grounds as a whole moved northeast. 2) Different models have advantages and disadvantages on different scales; Overall, the optimal models were GBT and 3DCNN models; The Optimal spatiotemporal scale was three days (0.25° × 0.25°). 3)In the GBT model, the most critical variables were SST, Chla, 97 m-DO, and SSS; In the 3DCNN model, the relative importance ranking of variables depended on the training data set. The results of the study help us to select the optimal model and optimal spatial and temporal scales to construct a prediction model for the Northwest Indian Ocean purpleback flying squid fishing grounds, which will provide a scientific basis for the sustainable development and management of the purpleback flying squid fishery in the region.

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.

Pilumnidae Samouelle, 1819 (Crustacea: Decapoda: Brachyura: Pilumnidae) of the Persian Gulf and Gulf of Oman, with a note on their phylogeny.

Members of the family Pilumnidae are common on intertidal and subtidal tropical and subtropical rocky, coral rubble and mud habitats. In the current study we collected 17 species belonging to 10 genera from the Iranian coast of the Persian Gulf and Gulf of ​​Oman. The species were examined using detailed morphological characteristics and any variation was recorded and compared with material from other geographical regions. Further, we compared 1171 base pairs of two mitochondrial genes (COI and 16S rRNA) and one nuclear gene (H3), and present reference sequences for the regional pilumnids, some of them being the first, as a useful basis for comparison with pilumnids from elsewhere. Two species, Glabropilumnus levimanus and Pilumnopeus africanus, are recorded for the first time from northwestern Indian Ocean, and a possibly undescribed species of Pilumnus was found that requires more detailed study. The results confirmed that the two previously recorded species, Pilumnus minutus De Haan, 1835 and P. vespertilio (Fabricius, 1793) do not occur in the Persian Gulf and Gulf of Oman. The current diversity of Pilumnidae in the region is increased to 28 species in 14 genera. Monophyly of the given genera and species in the study area is well corroborated.

Morphology, molecular systematics and phylogeography of the Spine-cheek Gudgeon, Eleotris acanthopomus (Teleostei: Eleotridae) from the north-western Indian Ocean

General morphology, cephalic sensory papillae pattern, postcranial osteology, phylogenetic placement, phylogeographic pattern, and distribution of Eleotris acanthopomus Bleeker, 1853 (Teleostei: Gobiiformes: Eleotridae) from the north-western Indian Ocean (Arabian Sea) are described. The cephalic papillae pattern follows the ‘open’ opercular pattern, and the cheek pattern ‘2.4’. The vertebral column includes 10 abdominal and 14–15 caudal vertebrae, for a total vertebral count of 24–25. Corroborating the morphological results, DNA barcoding based on mitochondrial COI confirmed that the specimens collected from the Arabian Sea coastal area are conspecific with E. acanthopomus from other Indo-Pacific localities. Eleotris acanthopomus from the Arabian Sea shows the closest phylogenetic relationship to E. pellegrini (11.9% K2P genetic distance). The presence of E. acanthopomus in the coastal area of the Arabian Sea represents the first record of this genus and species for the northern Indian Ocean and markedly expands its known geographical distribution range. A preliminary phylogeographic analysis of the newly collected E. acanthopomus haplotype from the Arabian Sea and archived haplotypes from other Indo-Pacific localities suggests that extensive dispersal occurs among regional populations, most probably during the pelagic larval stage.

Gene flow with the help of sea turtles: phylogeography of the epibiont barnacleChelonibia testudinaria(Linnaeus, 1758) (Crustacea: Cirripedia: Chelonibiidae) from the Persian Gulf

AbstractChelonibia testudinaria (Linnaeus, 1758) is a globally distributed epibiont barnacle using different hosts, including sea turtles and portunid crabs. Previous phylogenetic analyses have recovered three distinct clades for this species corresponding to the Atlantic, Eastern Pacific, and Indo-West Pacific regions. The present study of the genetic diversity of this species in the Persian Gulf (northwestern Indian Ocean) using the COX1 mitochondrial marker estimates the genetic connectivity of the barnacle between the Persian Gulf and two western Pacific regions. No significant genetic differentiation is evident between the Persian Gulf and the western Pacific, but the moderate level of genetic connectivity found between these areas across the Indian Ocean is likely mediated by migratory sea turtles. Based on the present analysis, the Persian Gulf displays a lower genetic diversity than the diversity of the western Pacific populations but shares some haplotypes with the latter.

Acoustic detections and sightings of blue whales Balaenoptera musculus in the Seychelles, western tropical Indian Ocean (2020-2022)

Historically, the Seychelles archipelago was an opportunistic whaling ground for fleets en route to and from the Antarctic. Soviet whalers illegally killed 500 blue whales near the Seychelles in the 1960s. Since then, no dedicated research has occurred to understand the ecological importance of this region for blue whales. Based on opportunistic sightings, we undertook 2 expeditions to assess the occurrence of blue whales. The overall goals were to determine blue whale distribution, obtain photo-identification data and collect the first acoustic data on this species in this region using a hydrophone deployed for a year. The expeditions consisted of vessel-based visual surveys that focused on the slope habitat (500-2000 m) off the northern portion of the Mahé Plateau. Over the 2 expeditions, a total of 5 sightings of up to 10 animals were seen. The results of our acoustic monitoring off Seychelles demonstrate that blue whales occur there regularly, primarily from December to April, and that the acoustic population identity matches that from near Sri Lanka. Published records and the results of our work suggest that blue whales from the northwestern Indian Ocean are seasonally present in the western equatorial Indian Ocean.

Encountering the morphological and molecular complexity in the bramble shark Echinorhinus cf. E. brucus (Bonnaterre 1788) from the Oman Sea

Abstract This is the first simultaneous morphological and barcoding characterization with the cytochrome c oxidase subunit I (COI) of the bramble shark Echinorhinus from the coast of Oman. The morphology of the specimen was consistent with previous records of Echinorhinus from the Gulf of Oman and the Arabian Sea (Northwestern Indian Ocean). However, the new COI haplotype clustered together with homologous sequences of specimens from India. The specimen from Oman distinguished morphologically and genetically from an E. brucus from the Western Atlantic Ocean on the shape and size of the dermal denticles, the proportions of twelve morphometric measurements (differences ⩾3%) and the genetic p-distance = 3.8% of the COI fragment. The haplotype reported here increases the genetic diversity in genus Echinorhinus in the Northwest Indian Ocean, demonstrates conspecificity between specimens from Oman and Echinorhinus cf. E. brucus distributed in India and extends its range of distribution. The limited morphological and molecular data available constrained assigning our specimen to other than Echinorhinus cf. E. brucus (Bonnaterrez, 1788). Our findings highlight the urgent need of morphological review, redescription and the assignment of a neotype in order to guarantee accurate species identification and thus effective conservation measures for these deep-sea sharks. The existence of a third living species in the genus is briefly discussed.

Disproportionation of Inorganic Sulfur Compounds by Mesophilic Chemolithoautotrophic Campylobacterota.

The disproportionation of inorganic sulfur compounds could be widespread in natural habitats, and microorganisms could produce energy to support primary productivity through this catabolism. However, the microorganisms that carry this process out and the catabolic pathways at work remain relatively unstudied. Here, we investigated the bacterial diversity involved in sulfur disproportionation in hydrothermal plumes from Carlsberg Ridge in the northwestern Indian Ocean by enrichment cultures. A bacterial community analysis revealed that bacteria of the genera Sulfurimonas and Sulfurovum, belonging to the phylum Campylobacterota and previously having been characterized as chemolithoautotrophic sulfur oxidizers, were the most dominant members in six enrichment cultures. Subsequent bacterial isolation and physiological studies confirmed that five Sulfurimonas and Sulfurovum isolates could disproportionate thiosulfate and elemental sulfur. The ability to disproportionate sulfur was also demonstrated in several strains of Sulfurimonas and Sulfurovum that were isolated from hydrothermal vents or other natural environments. Dialysis membrane experiments showed that S0 disproportionation did not require the direct contact of cells with bulk sulfur. A comparative genomic analysis showed that Campylobacterota strains did not contain some genes of the Dsr and rDSR pathways (aprAB, dsrAB, dsrC, dsrMKJOP, and qmoABC) that are involved in sulfur disproportionation in some other taxa, suggesting the existence of an unrevealed catabolic pathway for sulfur disproportionation. These findings provide evidence for the catabolic versatility of these Campylobacterota genera, which are widely distributed in chemosynthetic environments, and expand our knowledge of the microbial taxa involved in this reaction of the biogeochemical sulfur cycle in hydrothermal vent environments. IMPORTANCE The phylum Campylobacterota, notably represented by the genera Sulfurimonas and Sulfurovum, is ubiquitous and predominant in deep-sea hydrothermal systems. It is well-known to be the major chemolithoautotrophic sulfur-oxidizing group in these habitats. Herein, we show that the mesophilic predominant chemolithoautotrophs of the genera Sulfurimonas and Sulfurovum could grow via sulfur disproportionation to gain energy. This is the first report of the chemolithoautotrophic disproportionation of thiosulfate and elemental sulfur within the genera Sulfurimonas and Sulfurovum, and this comes in addition to their already known role in the chemolithoautotrophic oxidation of sulfur compounds. Sulfur disproportionation via chemolithoautotrophic Campylobacterota may represent a previously unrecognized primary production process in hydrothermal vent ecosystems.