Macrobrachium hirtimanus (Olivier, 1811) is a species of freshwater shrimp endemic to the Mascarene Islands (Réunion, Mauritius and Rodrigues). The last documented occurrences of this species date from 1980 on Réunion Island and it is thought to be now extinct. In order to determine its taxonomic distinctiveness and conservation status, historical specimens preserved in the collections of the Muséum national d’Histoire naturelle of Paris have been sequenced using a shotgun Next Generation Sequencing (NGS) method, allowing to get mitochondrial DNA barcode sequences (16S, COI) from 4 specimens collected between 1818 and 1926 on Réunion and Mauritius Islands. These sequences will be useful to build reference libraries for future eDNA surveys in the rivers of the Mascarenes that will help ascertain the conservation status of the species by guiding environmental surveys and updating the IUCN assessment. A discussion is provided on reasons leading to the possible extinction of M. hirtimanus as well as a list of specimens extant in museum collections worldwide.

Freshwater ecosystems on tropical oceanic islands host unique biodiversity. On Réunion Island, freshwater decapod crustaceans are key indicators of ecosystem health, yet their distribution and conservation status remain poorly documented. This study aimed to investigate the presence of the endemic prawn Macrobrachium hirtimanus, historically reported but unconfirmed for decades, while producing the first island-wide molecular inventory of freshwater decapods using environmental DNA (eDNA) metabarcoding. Water samples (4 L) were collected from 19 riverine sites, and eDNA was extracted and amplified using mitochondrial 16S rRNA primers validated for crustacean detection, followed by high-throughput sequencing. Bioinformatics analyses yielded 250,545 reads and 198 operational taxonomic units (OTUs), representing diverse amphidromous freshwater, coastal marine, and terrestrial decapods. Species richness was largely uniform across sites, though Rivière des Roches, Sainte-Marie, and Saint-Denis hosted higher diversity. Beta diversity showed strong spatial structuring among watersheds, driven primarily by species turnover. Widespread taxa included Atyoida serrata, Caridina typus, and Macrobrachium australe, while C. henriettae and Halocaridinides sp. were more localized. M. hirtimanus was not detected, consistent with its current absence, though continued targeted surveys are required before any formal reassessment. Overall, this study demonstrates the utility of eDNA metabarcoding for monitoring freshwater biodiversity and informing conservation planning on tropical islands.

Freshwater habitats in the Indo-Pacific Islands are colonised by amphidromous fish, which undergo a marine larval phase while the rest of the life cycle takes place in the riverine habitat. These fish are mainly represented by Gobiiformes. Among these, Stenogobius genivittatus (Valenciennes, 1837) is widely distributed in the Indo-Pacific Region, from South Africa to French Polynesia. We studied elements of the life history of S. genivittatus to better understand the processes contributing to its vast distribution: we analysed population genetic structure based on partial mitochondrial DNA sequences ( COI and ND6 ) and we used otolith microstructural analysis to estimate the duration of the marine pelagic larval phase (PLD). The analysis indicates the presence of a unique population broadly distributed across the Indo-Pacific Ocean, from South Africa to French Polynesia, while individuals from Hawaii appear to constitute a potentially distinct population. Also, our results show a homogeneous PLD in the West Indo-Pacific Region, except for Hawaii where it is longer and the Central Pacific Ocean (French Polynesia and the Marquesas) where it is shorter.

Understanding how tropical rivers change in response to human-induced disturbances represents a major challenge for management and conservation. However, monitoring networks based on ecological indicators remain scarce in oceanic islands, where freshwater indigenous fish and macro-crustacean assemblages are dominated by diadromous species. We investigated the relevance of these taxa for assessing the ecological status of rivers in two tropical islands, Reunion and Mayotte, Indian Ocean, to fulfill objectives of the European Water Framework Directive in overseas regions. Beyond providing insights on ecological responses of fish and macro-crustacean assemblages, we proposed a methodological framework to designed stressors-specific multimetric index by selecting primarily shared metrics between and within islands to improve the robustness and interpretation of this index. Numerous candidate metrics were tested to reflect the alterations induced by three stressors categories (i.e., continuity alteration, agricultural and urbanisation stresses) on the diversity, abundance, or size-structure of assemblages. Our results demonstrated that fish and macro-crustacean assemblages were sensitive to multiple stressors, but the ecological responses were more congruent when facing continuity alterations, compared to land use changes associated with agriculture or urbanization. These migratory species are understandably vulnerable to river fragmentation, but their oceanic dispersive stage favor exchanges between watersheds, which in turn can promote the resilience and persistence of local populations in degraded areas. In such insular context, we thus suggested using fish and macro-crustaceans to firstly assess the state of ecological continuity, whereas other taxa, such as diatom or macroinvertebrates, can be complementary to reflect the alteration of water quality.

The taxonomy of the genus Macrobrachium , a group of mostly amphidromous shrimps inhabiting unstable streams on Indo-Pacific tropical islands, is based on several characters repeatedly associated with specific environments and stream velocities. This allows the identification of morphological traits that improve an organism's stability in given flow conditions and an assessment of their monophyly. This study investigates whether the altitude-dependent distribution of certain Macrobrachium species - characterised by specific morphological traits - is phylogenetically constrained or a result of phenotypic plasticity. We first use an integrative taxonomy approach to assess species hypotheses based on ~200 individuals collected across the Indo-Pacific. We then construct multi-gene phylogenetic trees (65 individuals; 3-5 concatenated genes; 2711bp) to explore the evolutionary origins of morphological similarities among closely related species. Our results support the species status of 30 Macrobrachium species, with 16 species exhibiting distinct morphologies and 14 species forming 5 species complexes. Phylogenetic and distribution patterns of closely related species suggest habitat-driven speciation, with species differentiating in similar environments across vast distances. This indicates that diversification can occur over large distances, yet still in sympatry within a given environment. Lastly, our findings suggest that the Macrobrachium body shape is derived from evolutionarily conserved adaptations to flow velocity.

We provide an updated checklist and an identification key of the species of Caridina living in freshwater habitats of the Western Indian Ocean islands, including the Seychelles, the Comoros, Mayotte, La Réunion and Mauritius. We list six species including one new for science: Caridina mauritii, Caridina spathulirostris, Caridina similis, Caridina natalensis, Caridina typus and Caridina henriettae sp. nov. Molecular and morphological evidence led us to consider Caridina richtersii as a junior synonym of C. spathulirostris. Due to different life histories, some species are endemic to their island or archipelago, like C. mauritii and C. spathulirostris, endemic to Mauritius, or C. similis endemic to the Seychelles, whereas others have a wider distribution in the Western Indian Ocean like C. natalensis or C. henriettae sp. nov. or have an Indo-West Pacific distribution like C. typus.

Analytical imaging via synchrotron analysis was used to study sagittal otoliths of four freshwater pipefish species exhibiting amphidromous or freshwater resident lifestyles. We undertook a hyperfine elemental scanning of a 25 μm diameter length area centred on the core, namely the pre-hatch zone, corresponding to the sagitta formed during embryogenesis. We analysed calcium (Ca), strontium (Sr), manganese (Mn), chromium (Cr), cobalt (Co), nickel (Ni) and sulphur (S), with the latter serving to count growth increments. Whatever the species, a Ca pit consistently marked the core; the amphidromous species exhibited Mn enrichment at the core and the sagitta formed a week before hatching. These results illustrate common otolith formation mechanistic processes. Sr and heavy metals’ signals in the pre-hatch zone were compared to their signal in the edge of the otolith, an area encompassing the adult freshwater (adFW) environmental signature. We found that Co and Ni otolith signals had no relation to environmental conditions, but it is worth looking into Cr as a marker of early-life environment. While we caution against the use of a Sr:Ca ratio for life history interpretations at the core because of the Ca pit, the Sr signal analysis uncovered possible variations in the parental behaviour that potentially trigger newly-hatched juvenile migration to sea or residency. Finally, our results demonstrate that the microchemistry of otolith formation during early development is a melting of maternal signal during egg yolk elaboration, of the environmental signal during egg incubation, and of individual physiology during early growth.

Syngnathidae are a charismatic family of teleost fishes, represented by seahorses, seadragons, and pipefishes. Syngnathidae are mainly composed of marine species, but about 30 species of pipefishes inhabit freshwater insular environments of the Indo-Pacific realm. Recent research has shown that some freshwater pipefish species are amphidromous and exhibit high intraspecific divergences across their distribution range, like Microphis brachyurus (Bleeker, 1854) distributed from Sri Lanka to French Polynesia and Microphis retzii (Bleeker, 1856) distributed from Taiwan to Indonesia. In this study, we used the mitochondrial cytochrome oxidase I (COI) partial gene of 91 specimens of M. brachyurus and 30 specimens of M. retzii from localities representative of their respective distribution area to improve knowledge on the population structure of these two widespread species. Genetic species delimitation and phylogeographic analyses were combined to explore spatial patterns of genetic diversity across the distribution ranges of the two species. We have highlighted deep genetic structuring within the two species and relate these results to various biotic and abiotic factors. For M. brachyurus, the population in Polynesia is distinct from those in the West Pacific, suggesting its distinctiveness and recognition as an evolutionary significant unit (ESU). For M. retzii, three lineages are delimited in its range distribution, suggesting the existence of two distinct species in Southeast Asia (Bali/Java/Lombok and China/Taiwan). Pipefish species are particularly vulnerable to anthropogenic pressures (inherent to Syngnathidae and insular environments). The present results, revising species delimitation and geographic distribution, will help implement effective conservation and management measures.

In the present contribution, five species of Ptychognathus Stimpson, 1858 are reported from Polynesia: P. crassimanus Finnegan, 1931, P. easteranus Rathbun, 1907, P. riedelii (A. Milne-Edwards, 1868) and two new species. The first new species named P. ngankeeae sp. nov. is close to P. barbatus (A. Milne-Edwards, 1873) from New Caledonia, which is here redescribed for comparison purposes. The second new species named P. similis sp. nov. is close to P. hachijoensis Sakai, 1955.

Tomini Bay in Gorontalo offers significant potential for amphidromous goby-fry, a group of fish found in at least five estuaries in the area, namely Bone-Bolango, Paguyaman, Bilungala, Tombulilato, and Taludaa Estuaries. Preliminary results were limited to only two locations in estuarine waters, namely the Bone-Bolango and Paguyaman rivers. Further exploration of goby-fry species in various locations within Tomini Bay is essential to enhance knowledge about the abundance and aid conservation efforts. Therefore, this study identified the goby-fry species and created their distribution map in the waters of Tomini Bay Gorontalo. The samples used were obtained from daily catches of fishermen in the five estuaries during the recruitment period from February to September 2022. These samples were categorized based on their morphological similarities and specific melanophore patterns. Distinct groups with different melanophore patterns from those previously reported were identified as newly recorded species, photographed, and described in terms of their morphology. Furthermore, two specimens from each newly recorded species underwent molecular identification using the cytochrome oxidase subunit 1 (COI) gene for DNA amplification and were analyzed through the Basic Local Alignment Search Tool (BLAST) method. The phylogenetic tree was constructed using the Maximum Likelihood Method. The results showed the existence of nongoby-fry species caught together with goby fry school. A total of 75,881 goby-fry and 1,687 nongoby-fry were successfully collected. Among the goby-fry species, 13 were identified, including three new records, namely Eleotris fusca (Forster, 1801), Sicyopterus microcephalus (Bleeker, 1855), and Sicyopus zosterophorus (Bleeker, 1856). This study also documented the existence of nongoby-fry species, namely Anguilla celebesensis (Kaup, 1856), Moringua microchir (Bleeker, 1853), and Microphis leiaspis (Bleeker, 1854). It significantly contributed to the understanding of fish biodiversity in Tomini Bay.

Indo-Pacific tropical island streams are home to freshwater pipefish (Microphis spp., Syngnathidae). Otoliths were used to uncover life history traits in four species, including a New Caledonian endemic. All four species present the same methodological challenge: their otoliths are small, fragile and mute for growth marks using basic observation tools. Strontium (Sr) is calcium substituent in the mineral lattice, driven by salinity conditions, and thus useful to study diadromous migrations. Synchrotron-based scanning X-ray fluorescence 2D high-resolution mapping allowed us to tackle the global and hyperfine strontium (Sr) distribution. We developed analytical imaging processes to retrieve biological information from otoliths from the data generated via synchrotron analysis. We uncovered plasticity in the life cycle: all species were amphidromous, apart from some freshwater residents from New Caledonia. Understanding life cycle modalities is crucial to categorize species distribution limits and to implement adapted conservation measures, especially when endemic species are at stake. 2D fine-scale images outlined the heterogeneity of Sr distribution: in addition to the trivial Sr incorporation driven by environmental ionic conditions, there is an unusual mosaic arrangement of Sr distribution and we hypothesize that biological control, especially growth during the early life stages, may sometimes overrule stoichiometry. This shows that it is worth studying otolith formation and element integration at imbricated scales, and our methods and results provide a strong basis for future works and prospects in otolith science.

Twelve species of atyid shrimps are reported from three Micronesian islands (Babeldaob, Pohnpei, and Guam) and studied using a combined morphological and molecular approach. Among them, three are new records for the area (Caridina appendiculata, Caridina lobocensis, and Caridina rubella), while three new species are here described: Atyoida chacei sp. nov., Caridina ponapensis sp. nov., and Caridina rintelenorum sp. nov. Descriptions for these new species, diagnoses for poorly known species, and taxonomic notes are provided herein and their biogeography is discussed.

Insular biodiversity hotspots of Southeast Asia are remarkable for their biodiverse faunas. With a marine larval phase lasting up to several months, the freshwater fish subfamily Sicydiinae has colonized most islands of these hotspots. However, Sicydiinae diversity is still poorly understood in Southeast Asia. With the objective of estimating intraspecific genetic diversity and inferring past demography, we conducted the molecular inventory of Sicydiinae species in Sundaland and Wallacea using 652 bp of the mitochondrial cytochrome oxidase I gene, species delimitation methods, and Bayesian Skyline plot reconstructions. In total, 24 Molecular Operational Taxonomic Units are delimited among the 603 sequences belonging to 27 species and 5 genera. Two cases of discordance between morphology and mitochondrial sequence are observed, suggesting ongoing speciation and/or introgression in 2 genera. Multiple new occurrences are reported, either for a single biodiversity hotspot or both, some of which correspond to observations of a few individuals far from the range distribution of their conspecifics. Among the 10 species or species groups whose intraspecific diversity was examined, high levels of genetic diversity and past population expansion are revealed by Tajima’s D tests and Bayesian Skyline Plot reconstructions. Together, these results indicate that long-distance dispersal is common and suggest that most endemic species originated through founder events followed by population expansion. Patterns of sexual dimorphism and males’ coloration among diverging species pairs seem to point to sexual selection as an important mechanism contributing to speciation in the Sicydiinae of Sundaland and Wallacea.

Siluriformes are considered as primarily freshwater and have frequently been a model for the study of historical biogeography. Among catfishes, the most diverse clade is the Loricarioidei, a Neotropical group for which the fossil record extends back to the Palaeocene of Argentina. Here we describe a fossil from the early Late Cretaceous of Morocco, exhibiting typical morphological traits of the Loricariidae. A phylogenetic analysis integrating morphological characters with a multigene database for the main loricarioid lineages and outgroups highly supports inclusion of the fossil within the Loricariidae. A time-calibrated analysis corroborates the origin of loricarioids at about 112 MYA. The presence of this loricariid in Africa provides evidence that loricarioids have diversified before the separation of Africa and South America. The Moroccan loricariid shows an ancient evolutionary history that, in Africa, ended in the Late Cretaceous but persisted in South America, later surviving the K/Pg extinction.

Freshwater habitats in Indo-Pacific tropical islands are mainly inhabited by amphidromous fish, which display a marine larval phase while the rest of their life cycle takes place in rivers. Gobiiformes are the most diversified fish component in these ecosystems. In this study, we focused on the status of the widespread Stenogobius genus (Gobioidei). This genus is distributed from the Indian Ocean to French Polynesia. Recently, a first revision of Stenogobius was undertaken leading to the redescription of S. genivittatus (Valenciennes, 1837) and the placement of 12 species names as synonyms of this species. This paper focuses on the status of nine Stenogobius species supposedly present in Indonesia, a country well known as one of the freshwater hotspots of biodiversity. The present study is based on 95 specimens, and includes morphomeristic data and partial mito-chondrial COI sequences. Our results show that, at least four species occur in Indonesia, including the wide-spread species S. genivittat

Twelve species of atyid shrimps are reported from three Micronesian islands (Babeldaob, Pohnpei, and Guam) and studied using a combined morphological and molecular approach. Among them, three are new records for the area (Caridina appendiculata, Caridina lobocensis, and Caridina rubella), while three new species are here described: Atyoida chacei sp. nov., Caridina ponapensis sp. nov., and Caridina rintelenorum sp. nov. Descriptions for these new species, diagnoses for poorly known species, and taxonomic notes are provided herein and their biogeography is discussed.

A new species of freshwater pipefish, Microphis arrakisae sp. nov., is described from the West Indonesian Islands (Java, Bali and Lombok). This species is morphologically very close to Microphis retzii (Bleeker, 1856), which is found in the eastern Indonesian Islands (Sulawesi, Ceram, Ambon and Papua). However, it can be distinguished by its in vivo coloration. Furthermore, genetic analysis of the partial COI gene (barcoding) indicates that it represents a distinct genetic lineage in the Indonesian region.

To tackle the question of the reliability of otoliths as recorders of individual life events, we compared the information enclosed in otolith pairs: the sagittae pair and the sagitta/lapillus pair. We used the synchrotron XRF scanning imaging method, which enabled the comparison of this information at both global and hyperfine scales. Using otoliths of diadromous pipefish, we compared element incorporation in each pair with a focus on (i) environment and transition between water bodies with strontium (Sr) and heavy metals, (ii) temporal information and age estimation based on sulphur (S) incorporation, and (iii) otolith growth and biomineralization processes with zinc (Zn). Results show that the global information in terms of Sr and heavy metals given by both otoliths of a pair is the same and that any otolith may be used to retrieve such global data. In terms of S-based growth increment counts, the numbers are the same between two otoliths of the same kind, but the sagitta/lapillus pairs show a significant difference. Hyperfine-scale analysis of element distribution reveals that a given otolith is under the control of specific growth mechanisms, which can lead to heterogeneous elemental incorporation. The present results lead us to consider otolith growth dynamics and biomineralization processes in the context of a fluid mosaic perspective.

Twelve species of atyid shrimps are reported from three Micronesian islands (Babeldaob, Pohnpei, and Guam) and studied using a combined morphological and molecular approach. Among them, three are new records for the area (Caridina appendiculata, Caridina lobocensis, and Caridina rubella), while three new species are here described: Atyoida chacei sp. nov., Caridina ponapensis sp. nov., and Caridina rintelenorum sp. nov. Descriptions for these new species, diagnoses for poorly known species, and taxonomic notes are provided herein and their biogeography is discussed.