Protection of deep seabed, the largest biome on the planet, is a major challenge for future generations. Covering more than 70% of the Earth's surface, the ocean encompasses ≈80% of the seawater volume, produces at least 50% of the atmospheric oxygen and absorbs ≈25% of anthropogenic CO₂. Hidden from our eyes, the deep ocean has long been considered as a vast desert of darkness, populated by sea monsters. Today it is the covetousness of developed and emerging countries for its resources in terms of biodiversity and minerals. It becomes then urgent to acquire, through holistic approaches, solid knowledge baselines with regard to the diversity of geological systems, habitats, taxonomic and functional biodiversity, at open and interconnected ocean scales.

Our consortium led a five cruises series (2014- 2023) in the framework of the French contract for exploration of Atlantic polymetallic sulphides resources (ISA). At 3600 meters depth, the TAG hydrothermal field is an area of choice for studying the geo-biodiversity of so-called inactive SMS deposits, and is, with the young and volcanic Snake Pit hydrothermal field, at the heart of our studies. Exploration of the area revealed geochemical evidences for new active vent sites, some visited such as HYDRA, a new ones to be found. These sites constitute potential relays for the organism dispersal, a key point in the overall understanding of ecosystems and their relationships along ridge segments.

LIFEDEEPER intend to develop new approaches, combining in situ, in vivo, and lab experimentations, modeling and qualitative research in social sciences to disentangle the geological, geochemical and biological natural functioning of deep ocean ecosystems. In addition to the coordination work package (WP1), 4 multidisciplinary, complementary scientific WPs and one communication WP are proposed:

WP2: Exploration and description of ecosystems associated with inactive and active massive sulphide deposits : toward integrated definitions of active and inactive vents.

WP3: Integrated 3D study coupling hydrodynamics, distribution of trace metals and numerical modeling to assess the biogeochemical impact of the hydrothermal plume to the deep ocean.

WP4: Study the connectivity and life cycle of holobiont models, capacity of adaptation and acclimation allowing the resilience of communities.

WP5: Legal and political analysis of international regulatory regimes, sociological analysis of science-technology-society, capacity building. 

A last WP6 will then aim to produce effective educational and public outreach activities targeting citizens, students, scholars, organizations and various stakeholders, through participative science, educational science and art.     

Ultimately, we want to establish a precise map of contrasting sites along the 600 km ridge segment, to provide key parameters to understand the natural functioning of these environments, both from a geological and biological point of view. Key components of the functioning of these ecosystems and associated services, along activity gradients, will permit establishing global-scale inter-comparison protocols. In a holistic and multidisciplinary approach, LIFEDEEPER aims to acquire definitions of reference ecological profiles and preservation strategies in the context of the growing interest in deep mineral resources for the carbon-free and digitized world economy. LIFEDEEPER will propose solutions to allow future informed guidelines and decision-making.

How to cite: Cambon-Bonavita, M.-A. and Pelleter, E. and the Marie Anne CAMBON: The LIFEDEEPER Project : LIving together in the Future: vulnErability of DEEP sea Ecosystems facing potential mineral Resources exploitation, One Ocean Science Congress 2025, Nice, France, 3–6 Jun 2025, OOS2025-176, https://doi.org/10.5194/oos2025-176, 2025.

Within the deep ocean, hydrothermal vent ecosystems are home to unique set of species' communities, that live nowhere else, and whose food chains have the particularity to rely on microorganisms activities through a process called chemosynthesis. Many animals living there have adopted the strategy of hosting these microorganisms in symbiosis on or within their body, including many of the most emblematic groups such as tubeworms, Pompeii worm, yeti "crabs" or scaly-foot and hairy snails. Among them, shrimps of the Alvinocarididae family constitute a major component of this endemic fauna, with a global distribution across the globe, and dominating visually vent communities of the Atlantic and Indian Ridges. Work led and carried out by Ifremer have characterized the biology of the Rimicaris shrimps inhabiting the Mid Atlantic Ridge with an emphasis on their chemosynthetic symbiosis and their lifecycle, from the egg development to the reproduction phase. Recently, studies in collaboration with other research institutes have expanded these knowledges to other shrimps of the family inhabiting different regions. They have revealed a large variation in the set of functional traits related to their reproduction, symbiosis and feeding strategy. Hence, the evolutionary history of their chemosynthetic symbiosis contrast with most of the other emblematic species, while some shrimps present a unique and intriguing case of reproductive periodicity that had never been documented before. Overall, these observations reveal the functional uniqueness of this family, that also reflect their fragility in the face of mining threats, highlighting the need of conservation measures to protect them.

How to cite: Methou, P., Cueff-Gauchard, V., Michel, L., Watanabe, H., Copley, J., Chen, C., Pradillon, F., and Cambon, M.-A.: Symbiosis, reproduction and metamorphosis: The functional uniqueness of deep-sea vent shrimps in the face of mining threat., One Ocean Science Congress 2025, Nice, France, 3–6 Jun 2025, OOS2025-392, https://doi.org/10.5194/oos2025-392, 2025.

Micronekton comprises a diverse assemblage of actively swimming aquatic animals in oceanic ecosystems, including mesopelagic fishes from 2 to 20 cm. They live in the mesopelagic zone (200-1000 meters), characterized by high environmental gradients and distinct temporal dynamics across depth layers. Depending on their vertical habitat, mesopelagic fishes have developed various adaptions and contribute to essential ecosystem processes, including nutrient cycling and carbon transport. However, the link between their functional diversity, vertical habitat use and survival strategies requires further clarification. This study, conducted during a scientific cruise in the Northeast Atlantic (APERO program), uses multi-depth trawling and acoustic observations to investigate the behaviors and habitats of mesopelagic fishes, highlighting distinct movement patterns across vertical zones. To examine the ecological roles of these species, we developed a trait matrix encompassing 80 fish species and 12 functional traits related to fitness, including morphological, physiological, life-history, and behavioral characteristics. Based on this information, we performed a factor analysis of mixed data including quantitative and qualitative traits. Mesopelagic fishes were then classified in functional groups based on clustering of the factor analysis. Traits that contributed the most to the first two axes were swimming type, teeth type, presence of swimbladder and trophic level. Traits and trade-offs were associated with survival strategies – mainly feeding and predator avoidance - differing between functional groups. We analyzed which patterns of functional diversity emerge across these different structures. This study elucidates the relationships between functional traits, habitat preferences, and feeding behaviors to deepen our understanding of the processes structuring mesopelagic fish assemblages. By exploring these relationships, we gain insights into the adaptive strategies of micronekton and their ecological roles in oceanic ecosystems.

How to cite: Thibault, H., Ménard, F., Roudaut, G., Lebourges-Dhaussy, A., Cherel, Y., Ariza, A., Eduardo, L. N., and Martini, S.: Ecological Roles of Mesopelagic Micronekton in the Northeast Atlantic: A Trait-Based Perspective, One Ocean Science Congress 2025, Nice, France, 3–6 Jun 2025, OOS2025-447, https://doi.org/10.5194/oos2025-447, 2025.

Mesopelagic ecosystems, located between 200 and 1000 meters deep are poorly studied. They are characterized by complete darkness, driving organisms to develop specialized adaptive strategies¹. One of the most remarkable of these adaptations is bioluminescence, enabling organisms to produce light for communication, camouflage, prey attraction, or defence². Mesopelagic fish, such as myctophids, dominate this zone in terms of biomass (65%) and diversity and are capable of emitting light³. They produce light either through endogenous photophores or via symbiosis with bioluminescent bacteria like Vibrionaceae. These microorganisms may also be found on their skin and in their gut or liver ^2,4. These fishes perform diel vertical migrations (DVM): they ascend to feed near the surface at night and descend to deeper waters during the day to avoid predators³. By this process, they can transport organic matter and nutrients through the water column. Indeed, they are capable of releasing consumed organic matter in the form of fecal pellets that can be glowing (as they carry bioluminescent bacteria).  This feature might play a key role in structuring deep-sea trophic networks.

This study aims to analyse prokaryotic composition across different tissues of several mesopelagic fish species and to identify correlations between the presence of bioluminescent bacteria, migratory capacity, and diet. A total of 160 samples from gut, liver, and skin were collected during the APERO cruise in July 2023 in the Northeast Atlantic. Microbial diversity was assessed via 16S metabarcoding, considering site, depth, and fish traits.

The results reveal significant microbial variations across the three most-sampled fish families (n = 142): Myctophidae, Stomiidae, and Sternoptychidae. Bioinformatic analyses show two distinct clusters, with skin samples clearly separated from liver and gut. Bacterial composition varied by tissue, with Pseudoalteromonadaceae dominating the skin (25% of the community) and Vibrionaceae and Enterobacteriaceae prevalent in liver and gut (52% and 24%). Within Vibrionaceae, Photobacterium and Vibrio (including many bioluminescent species⁵) comprised about half of the gut bacterial community, indicating their abundance in this tissue. Correlations emerged between bacterial composition, diet, and migratory abilities in Myctophidae and Sternoptychidae. Migratory, zooplanktivorous, and bioluminescent fish had a microbial composition largely composed of Photobacterium and Vibrio (Vibrionaceae), in contrast to other fish. These findings highlight the importance of studying microbial diversity in mesopelagic fishes and provide a strong foundation for understanding interactions between fish microbiomes and deep-sea ecological processes.

¹ Robison, B. H. Conservation of deep pelagic biodiversity. Conservation Biology 23, n°4 (2009): 847-858. https://doi.org/10.1111/j.1523-1739.2009.01219.x
² Haddock, S.H.D., M. A. Moline, and J. F. Case. Bioluminescence in the Sea. Annual Review of Marine Science 2, n^o 1 (2010):443‑93. https://doi.org/10.1146/annurev-marine-120308-081028.
³ Irigoien, X., T. A. Klevjer, A. Røstad, et al. Large Mesopelagic Fishes Biomass and Trophic Efficiency in the Open Ocean. Nature Communications 5, n^o 1 (2014):3271. https://doi.org/10.1038/ncomms4271.
⁴ Widder, Edith. Bioluminescence and the Pelagic Visual Environment. Marine and Freshwater Behaviour and Physiology 35 (2002):1‑26. https://doi.org/10.1080/10236240290025581.
⁵ Tanet, L., S. Martini, L. Casalot, and C. Tamburini. Reviews and Syntheses: Bacterial Bioluminescence – Ecology and Impact in the Biological Carbon Pump. Biogeosciences 17, n^o 14 (2020):3757‑78. https://doi.org/10.5194/bg-17-3757-2020.

How to cite: Le Cam Ligier, C., Thibault, H., Guasco, S., Valette, C., Simon, G., Casalot, L., Martini, S., and Garel, M.: Diversity and ecological dynamics of bioluminescent bacteria in the mesopelagic zone, One Ocean Science Congress 2025, Nice, France, 3–6 Jun 2025, OOS2025-472, https://doi.org/10.5194/oos2025-472, 2025.

Deep-pelagic ecosystems (below 200 meters depth) encompass thousands of species intrinsically linked to critical ecosystem processes, such as carbon sequestration and the connectivity between pelagic layers and trophic levels. However, deep-pelagic realms remain some of the least explored ecosystems globally and is increasingly imperiled by human activities. This challenge is particularly acute for developing nations, where participation in deep-sea research is hampered by limited expertise, lack of incentives, and restricted access to advanced technological infrastructure, leading to an almost complete absence of knowledge regarding their deep-pelagic biodiversity. Here, we present a case study of international cooperation that serve as a model for advancing knowledge of deep-pelagic fish diversity in historically understudied regions. The International Joint Laboratory – Tropical Atlantic Interdisciplinary Laboratory on Physical, Biogeochemical, Ecological, and Human Dynamics (IJL TAPIOCA) exemplifies this collaborative model. This consortium brings together a multidisciplinary team of researchers and students from the French Institut de Recherche pour le Développement (IRD) and several Brazilian institutions, including the Federal Rural University of Pernambuco (UFRPE) and the Federal University of Rio de Janeiro (UFRJ). One of the main goals of the consortium is to deepen understanding of the biodiversity and distribution of deep-sea fishes from off northern and northeastern coasts of Brazil, a region with limited baseline data on species diversity. Focusing on deep-pelagic fish diversity, recent expeditions have explored two different areas: the Fernando de Noronha Ridge (including the Fernando de Noronha Archipelago, Rocas Atoll, and nearby seamounts) and the oceanic waters in front of the Amazon River estuary. Through these expeditions, a remarkable diversity of deep-sea fishes was documented, with collections totaling around 15,000 fish specimens, representing approximately 250 nominal species, 60 of which represent new distribution records in Brazilian waters. Furthermore, at least 12 new species have been identified so far, including notable discoveries in the genera Eustomias, Melanosthomias, Astronesthes (Stomiiformes), Poromitra and Melamphaes (Beryciformes). The specimens and selected tissue samples were deposited at the Fish Collection of the Institute of Biodiversity and Sustainability (NUPEM/UFRJ). All data are available online at two repositories: https://specieslink.net and https://www.sibbr.gov.br. These findings not only enhance the knowledge of mesopelagic fish diversity in poorly studied areas but also provide important insights into species distribution patterns and the biodiversity of deep-sea ecosystems. This research underscores the significance of international collaboration in addressing pressing questions related to marine biodiversity and conservation, especially in underexplored regions.

How to cite: Maia Mincarone, M., Nole Eduardo, L., Lucena-Frédou, F., and Bertrand, A.: IJL TAPIOCA: A collaborative French-Brazilian initiative to advance biodiversity and taxonomic knowledge of deep-pelagic fishes, One Ocean Science Congress 2025, Nice, France, 3–6 Jun 2025, OOS2025-484, https://doi.org/10.5194/oos2025-484, 2025.

Archaea DPANN* superphylum harbors, to date, 12 phyla including Nanoarchaeota and Candidatus Aenigmarchaeota (previously described as DHVE-3, DSEG and VAL III). Interestingly, DPANN are detected in a wide range of anoxic or oxygen limited environments such as lakes, marine sediments and hydrothermal systems. Most known core biosynthetic pathways are also absent or incomplete in DPANN Archaea, suggesting symbiotic or parasitic lifestyles. However, much of the knowledge about their ecology, evolution, and putative metabolism mainly relies on metagenomic data. Here we repeatedly enriched two archaea distantly affiliated with Ca. Aenigmarchaeota (16S rRNA gene sequence identity <84%) and Nanoarchaeota (16S rRNA gene sequence identity <81%) in continuous cultures. These enrichment cultures were performed from hydrothermal chimney samples and fluids from Snake Pit, TAG and Lucky Strike, hydrothermal sites located on the Mid-Atlantic ridge (BICOSE, HERMINE, MoMARsat 2017, BICOSE 2 oceanographic cruises) using a gas-lift bioreactor under H₂/CO₂ at 80°C. Cell concentrations increased progressively over several weeks up to ~10⁷ cells/mL simultaneously with a relative proportion (≤75%) of these new previously uncultured phylotypes. The enigmatic group of Archaea distantly affiliated with Nanoarchaeota seems to be widespread as it was enriched from all hydrothermal sites, whereas Archaea affiliated with Ca. Aenigmarchaeota were only cultured from Snake Pit, suggesting a possible environmental selection. Based on co-occurrence of these phylotypes, several putative archaeal partners were identified. Metagenomic analyses are being performed in order to help to optimize culture conditions for pure (co-)cultures. This ongoing study should help to resolve ecology and life style of these new phylums.

*an acronym of the names of the first included phyla: Diapherotrites, Parvarchaeota, Aenigmarchaeota, Nanoarchaeota and Nanohaloarchaeota.

How to cite: Rieusset, L., Leroy, E. M., Trouche, B., Aubé, J., Baumas, C., François, D., Le Guellec, S., Rédou Creff, V., Lesongeur, F., Brizard, R., Godfroy, A., Alain, K., Jebbar, M., and Roussel, E. G.: First insights into enrichment cultures at in situ conditions of hyperthermophilic microorganisms potentially affiliated with the DPANN superphylum, One Ocean Science Congress 2025, Nice, France, 3–6 Jun 2025, OOS2025-1049, https://doi.org/10.5194/oos2025-1049, 2025.

Mesopelagic fish communities remain under studied in the highly dynamic Canary Current Ecosystem. We analyzed mesopelagic fish assemblage structure based on 2,153 specimens representing 98 species across two distinct marine zones: Mauritania, characterized by high oxygen and chlorophyll-a concentrations, and Senegal, distinguished by high temperatures and low salinities. Our findings reveal clear biogeographic patterns, with Diaphus vanhoeffeni, Diaphus dumerilii, Argyropelecus gigas, and Sternoptyx diaphana showing high abundance in Senegalese waters but limited presence in Mauritania waters. Conversely, Diaphius rafinesquii, Benthosema glaciale, Argyropelecus hemigymnus, Lobiancha dofleini, and Hygophum taaningi exhibited higher abundances in Mauritania waters. Statistical analyses revealed significant correlations between species abundance patterns and physicochemical parameters specific to each marine zone. This pioneering study of mesopelagic fish community composition and structure in relation to environmental factors in the Canary Current Ecosystem provides valuable insights into a marine fauna, which holds potential for sustainable exploitation in aquaculture feed production and nutritional supplements, beyond its crucial role in the carbon cycle and marine food web.

Keywords: Mesopelagic fish, Canary Current Ecosystem, Upwelling, Environmental factors, West Africa.

How to cite: Ndiaye, M., Ndour, I., Duncan, S., Ba, K., Sarr, A., and Ndiaye, I.: Spatial variation in mesopelagic fish assemblage structure in the upwelling of the Canary Current Ecosystem, One Ocean Science Congress 2025, Nice, France, 3–6 Jun 2025, OOS2025-1054, https://doi.org/10.5194/oos2025-1054, 2025.

Today, deep-sea research is expensive, largely limiting this science to countries and institutes with the greatest resourcing. Such efforts have been critical to expanding our knowledge of the deep, yet we still know too little to make time-sensitive decisions about resource use, protection and responsible interactions with the deep.

We need more people doing deep-sea research, and we need to do it efficiently and cost-effectively. Our ocean, our efforts to preserve our planet and the well-being of our coastal communities depend on it. 

How do we do this? We believe one of the answers lies around a small island chain in the North Atlantic.

The Madeira Archipelago, an autonomous region of Portugal, is a series of volcanic islands with steep slopes diving straight into the deep sea. Within a few kilometers from shore, ocean depths already reach 500-1,000m. Another dozen kilometers and depths reach over 3,000m. Thanks to Madeira’s relatively calm seas and mild climate, these depths are also accessible to researchers year-round by small boats. 

Madeira does not have the economic might to finance state-of-the-art deep-sea research equipment and expeditions as do the world’s leading research institutions. Only through creativity and collaboration can we take advantage of our island’s special access and begin to discover and democratize the deep.  

MARE-Madeira, the largest non-profit aquatic research institute in Madeira, is working with partners at the Helmholtz Center for Ocean Research Kiel (GEOMAR) and the Norwegian University of Science and Technology (NTNU) to do just that. Funded by the European Commission Horizon Europe program and within the project TWILIGHTED (TWInning Laboratory for an Innovative, Global Hub To Explore the Deep), we are working to strengthen deep-sea research capacity in Madeira, to develop lower-cost methods for exploring and monitoring deeper waters and, ultimately, to create a global hub for deep-sea research on the island. 

Whether you have deep-sea experience or not, and especially those from small islands and institutes that have been financially restricted from participating in deep-sea research in the past, we invite you to join our capacity-building journey. Follow us on social media, use the how-to guides for low-cost tools we’ll share on our website and sign up to our Impossible Things Workshops or International Twilighted Conference. We hope to engage more fully in this important and globally underfunded area of research in the coming years and together help build a future where deep-sea research is done more efficiently, more quickly and even more collaboratively.

Funding acknowledgement: The TWILIGHTED project is funded by the European Commission’s Horizon EuropeTwinning program.

How to cite: Esson, D., Monteiro, J., Radeta, M., Hoving, H.-J., Dierking, J., Širović, A., Aberle-Malzahn, N., Ludvigsen, M., and Canning-Clode, J.: Madeira: A Global Deep-Sea Research Hub in the Atlantic, One Ocean Science Congress 2025, Nice, France, 3–6 Jun 2025, OOS2025-1133, https://doi.org/10.5194/oos2025-1133, 2025.

The deep ocean, one of Earth's largest and least explored biomes, spans depths with extreme conditions, including high hydrostatic pressure, low temperatures, and minimal organic carbon. These characteristics significantly influence microbial life, requiring unique adaptations for survival and metabolic function. Prokaryotes thriving in high-pressure environments are known as piezophiles (Tamburini et al. 2013 and references therein). Those deep-sea micro-organisms adapt to high-pressure environments through cellular and genetic changes, constitute ‘bathytypes’  specific  to ocean depths (Oger and Jebbar 2010; Peoples and Bartlett 2017).

Due to technical challenges in sampling under in situ conditions, much of the data about deep-sea microbes has historically been gathered under decompressed conditions. This decompression can result in significant changes in community composition (La Cono et al. 2009; Edgcomb et al. 2016; Garel et al. 2019) and activity (Tamburini et al. 2013; Garel et al. 2019), which generally leads to underestimated assessments of their ecological roles. Innovations, such as pressure-retaining samplers (Peoples and Bartlett 2017; Garel et al. 2019) or in situ microbial incubator (Amano et al. 2022a), now allow for the collection or incubation of samples at in situ pressures, preserving the natural state of these microorganisms for more accurate study .

Deep-sea microbes play a critical role in the global carbon cycle, particularly through their participation in the biological carbon pump, which involves the decomposition and mineralization of organic material as it sinks through the water column (Arıstegui et al. 2009). Research suggests that deep-sea microbes are dynamically responsive to organic matter input, with a substantial portion of global heterotrophic production occurring below the photic zone. According to Aristegui et al. (2009) the integrated prokaryotic heterotrophic production (PHP) in the dark ocean represents around 50% of the total water column. Understanding the pressure effect on prokaryotic activities is crucial to better estimate their role in the remineralization in the dark ocean. However, to date, the effect of decompression is controversial, depending on the methodology used and the authors (Amano et al. 2022b versus Tamburini et al. 2013 and reference therein). 

Continued advancements in sampling technology and high-throughput sequencing will likely reveal more about these resilient organisms' ecological roles and physiological capacities, enhancing our comprehension of the deep ocean's contributions to Earth's biogeochemical processes​.

Over the last few years, pressure-retaining samplers were deployed during different oceanographic cruises in North Atlantic Ocean and in the Mediterranean Sea, allowing the constitution of a unique dataset of PHP rates (using 3H-Leucine) and MICROFISH, under in situ conditions. Among this cruise, a specific one dedicated to the study of deep convection and re-stratification phenomena brings new insights in the biological and physical coupling of this study area, and the necessity to well understand this context when talking about organisms adapted to pressure or not. Pressure effects on other activities (dark dissolved inorganic carbon fixation, ectoenzymatic activities, and high molecular organic compounds' degradation) will also be presented. Finally, we will also show how surface prokaryotes attached to gravitationally sinking particles cope with the increase in pressure with depth.

How to cite: Le coq, P., Garel, M., and Tamburini, C.: Life under pressure in prokaryotes and the impact on the biological carbon pump, One Ocean Science Congress 2025, Nice, France, 3–6 Jun 2025, OOS2025-1291, https://doi.org/10.5194/oos2025-1291, 2025.

The Latin American Deep Sea Exploration Network (LADEN) is an initiative aimed at building a collaborative, interdisciplinary, and sustainable network across Latin America to explore, protect, and sustainably manage deep-sea ecosystems. The concept of LADEN is based on the recent Deep Blue Initiative that forms a global platform for deep-sea research cooperation, mobilization, funding and equitable capacity sharing. This network will leverage the region’s local expertise, international partnerships, and philanthropic support to ensure that Latin American countries, particularly those with less developed infrastructure, can contribute to and benefit from cutting-edge marine science and technology. Through this collective effort, LADEN will prioritize scientific research, technological innovation, early-career scientists’ development, best practices and standardize research methods and data analysis and capacity building and sharing to address critical environmental challenges in the deep ocean, aiming for biodiversity conservation and ocean literacy.  This initiative brings together experts, organizations, and governments from multiple disciplines and regions to identify key information gaps and resolve bottlenecks in deep-sea exploration. LADEN will build upon this collaborative foundation to expand its efforts in Latin America, working across national borders to share resources, knowledge, and expertise.

LADEN is scientifically based on ongoing national initiatives in deep-sea research across Latin America, such as cold-water coral studies, seabed habitat mapping, geohazards assessment, biodiversity conservation efforts, among others. One of LADEN’s core objectives is to provide the essential baseline data needed to better understand the often- overlooked deep-sea areas within the region. This data will support the effective management and designation of new Marine Protected Areas (MPAs), help achieve the global 30x30 conservation goals and complement the provisions of the Biodiversity Beyond National Jurisdiction (BBNJ) treaty. Additionally, LADEN will foster collaboration by connecting diverse deep-sea research communities and across and governmental actors accros Latin America and linking them with global counterparts and established networks, such as the Deep-Ocean Stewardship Initiative (DOSI) and Challenger 150, an African-based network focused on deep-sea exploration. Through these united efforts, LADEN will position Latin America as a decisive contributor to global marine conservation and sustainable deep-ocean management.

How to cite: Zapata-Ramirez, P. A., Bastos, A., Delance, J., Dueñas, L., Escobar, E., Cortés, J., Sánchez, J. A., Gómez, C. E., Benavides, M., Rojas, X., Pérez, A., Díaz, M., and Visbeck, M.: Towards the implementation of the Latin American Deep Sea Exploration Network (LADEN), One Ocean Science Congress 2025, Nice, France, 3–6 Jun 2025, OOS2025-1501, https://doi.org/10.5194/oos2025-1501, 2025.

The twilight or mesopelagic zone is host to a wealth of ocean biodiversity. A critical component of the ocean food web,these species are important prey species. Due to daily mass migrations, mesopelagic species transfer 2-6 gigatons of carbon per year to the deep sea, an amount equivalent to twice the emissions produced by cars worldwide annually. Recognizing the significant abundance of biomass in the twilight zone, efforts have been made over time to commercially extract these resources. While none of those past fisheries have resulted in fishing at scale, interest remains and pressure to find new sources of protein is rising as current stocks of small pelagic species, critical inputs to fishmeal and fish oil production, are declining or shifting their distributions due to climate change. Other potential activities associated with deep-sea mining and marine carbon dioxide reduction may impact the natural carbon sequestration pathways facilitated by the mesopelagic. Given what we know about the importance of the twilight zone, it is imperative that these critical ecological and climate services are protected. A number of published papers have identified pathways for large-scale protections. They include actions within country EEZs based on existing governance frameworks, expert guidance from the UN Food and Agriculture Organization, the IPCC, and the wider scientific community, actions by regional fishery management organizations, expansion of the role of the London Protocol on regulating marine carbon dioxide reduction research; and the pending ratification of the BBNJ agreement. This talk will present the work of a group of actors spanning academia, NGOs, and government entities to leverage the existing base of science and knowledge about the ocean twilight zone in support of sound policy to protect these critical resources.

How to cite: Kleisner, K., Dorsett, C., and Ramakrishna, K.: Protecting the ocean twilight zone: Building from Science to Action, One Ocean Science Congress 2025, Nice, France, 3–6 Jun 2025, OOS2025-1532, https://doi.org/10.5194/oos2025-1532, 2025.

The objective of this study is to describe the associated fauna of the two cold water coral (CWC) ecosystems occurring off Mauritania; the chain of mounds and the live reefs.

The fauna inhabiting the coral mound has been described through a comparative analysis of trawl catches from coral mounds (400-600m) and catches from adjacent off-mound areas (200-350m) conducted from 1982 to 2022 by four research vessels, while the fauna associated with CWC reef habitats was described through the analysis of 11 video dives undertaken by using ROV as part of two seafloor habitat mapping surveys conducted by the R/V Dr. Fridtjof Nansen off Mauritania in 2020 and 2021.

Analysis of 47 trawling station data carried out on coral mounds off Mauritania revealed significant diversity. Overall, 282 taxa were identified, these species belong to the 5 taxonomic group, Fish (187), Crustacea (56), Mollusca (31), Cnidaria (5) and Echinodermata (3). The most ten species inhabiting coral mound was: Helicolenus dactylopterus, Laemonema laureysi, Merluccius polli, Coelorinchus caelorhincus, Hoplostethus cadenati, Malacocephalus occidentalis, Gephyroberyx darwinii, Hoplostethus mediterraneus, Galeus polli and Malacocephalus laevis. Results showed also that the values ​​of the diversity indices (richness, Shannon and Simpson index, accumulation curve) were higher on-mound than off-mound.

The analysis of 11 ROV dives allowed to identify eighth species of CWC to species level, the most abundant was the gorgonian coral Acanthogorgia cf. hirsuta followed by Desmophyllum pertusum, Swiftia phaeton (a new octocoral species of Swiftia), the sea fan Thesea talismani, Madrepora oculata, the black coral Tanacetipathes cf. spinescens, Anthomastus cf. grandifloras and Clavularia borealis.

The results showed that the reef offers habitat for many species. In effect, 120 taxa belonging to 68 families have been identified divided into 11 groups, the group of fish was the most represented with 47 taxa, followed by the group of cnidaria (18 taxa), crustacea (17 taxa), Mollusca and Echinodermata with 11 taxa for each group and Porifera with 7 taxa. In terms of occurrence, H. dactylopterus was the most observed for the fish group, with over 371 individuals. The cnidarians are dominated by the lesser cylinder anemone Synarachnactis cf. lloydii with more than 13000 individuals. For the crustaceans, the African spider shrimp Nematocarcinus africanus is the most common with more than 900 individuals. As far as the group of mollusks is concerned, the bivalve Gigantidas mauritanicus is the most observed with more than 1800 individuals. The sponge Cladorhiza corallophila was common for the group of Porifera. Several of these species are known to be associated with CWC from other studies even though the reefs are in an oxygen minimum zone they offer habitats for many of the same associates.

The results of our study provide insight to the importance of these CWC habitats to the Mauritanian deep-sea ecosystem, information that is essential for the development of a national management plan for use of marine resources and their ecosystems, especially in the face of ongoing environmental challenges (e.g. oil and gas activities, deep sea fishing etc.).

How to cite: El Vadhel, H.: Cold-water coral reefs and mounds off Mauritania and associated fauna, One Ocean Science Congress 2025, Nice, France, 3–6 Jun 2025, OOS2025-70, https://doi.org/10.5194/oos2025-70, 2025.

The fixed-point profiler project, called PROLIXE, aims to develop a measurement and observation system capable of moving between the seabed (up to 6,000 m) and the sub-surface, along an anchored mooring line, so as to characterize the entire water column at a given geographical position, with adaptative sampling intervals. The system must be capable of recharging its batteries at regular intervals on a docking station to ensure long autonomy. This station is designed to be used either on a standalone mode or connected to a cabled node. This project is part of the « technological innovation » activities of the IFREMER ScInObs project (science and innovation for subsea observatories).

How to cite: Lagadec, J.-R.: PROLIXE, a versatile long-term profiling mooring, One Ocean Science Congress 2025, Nice, France, 3–6 Jun 2025, OOS2025-1117, https://doi.org/10.5194/oos2025-1117, 2025.

The resident AUV (Autonomous Underwater Vehicle) project aims at developing an observation vehicle based on and returning from a cabled observation infrastructure on the seabed, so as to extend its observation radius without recourse to a ship. It is the mobile extension of a permanent fixed-point underwater observation system, designed for reducing the cost of in situ observational data, decreasing their environmental footprint and broadening their spatial range. The project has started in May 2023. It is currently focusing on de-risking the main critical building blocks associated to resident systems (docking maneuver, contactless data/power transmission, secured positioning) before launching the design of the vehicle itself. This project is part of the « technological innovation » activities of the IFREMER ScInObs project (science and innovation for subsea observatories).

How to cite: Blandin, J.: Development of a resident AUV for scientific observation, One Ocean Science Congress 2025, Nice, France, 3–6 Jun 2025, OOS2025-1156, https://doi.org/10.5194/oos2025-1156, 2025.