Observations of marine elapid snakes in the mesophotic zone in the Ryukyu Islands, Japan, with the deepest records for three species

Observations of marine elapid snakes in the mesophotic zone in the Ryukyu Islands, Japan, with the deepest records for three species | Research Square window.SnipcartSettings = { analytics: { enabled: false } }; (function() { var accessVector = localStorage.getItem('access_vector') || ''; window.dataLayer = window.dataLayer || []; if (accessVector) { window.dataLayer.push({ user: { profile: { profileInfo: { snid: accessVector } } } }); } })(); (function(w,d,s,l,i){w[l]=w[l]||[];w[l].push({'gtm.start':new Date().getTime(),event:'gtm.js'});var f=d.getElementsByTagName(s)[0],j=d.createElement(s),dl=l!='dataLayer'?'&l='+l:'';j.async=true;j.src='https://www.googletagmanager.com/gtm.js?id='+i+dl;f.parentNode.insertBefore(j,f);})(window,document,'script','dataLayer','GTM-K279D39R'); Browse Preprints In Review Journals COVID-19 Preprints AJE Video Bytes Research Tools Research Promotion AJE Professional Editing AJE Rubriq About Preprint Platform In Review Editorial Policies Our Team Advisory Board Help Center Sign In Submit a Preprint Cite Share Download PDF Research Article Observations of marine elapid snakes in the mesophotic zone in the Ryukyu Islands, Japan, with the deepest records for three species Kanta Fujishima, Takuo Higashiji, Frederic Sinniger, Saki Harii This is a preprint; it has not been peer reviewed by a journal. https://doi.org/ 10.21203/rs.3.rs-7818285/v1 This work is licensed under a CC BY 4.0 License Status: Published Journal Publication published 13 Feb, 2026 Read the published version in Marine Biology → Version 1 posted 5 You are reading this latest preprint version Abstract Marine snakes in the Family Elapidae are key mesopredators in tropical and subtropical Indo-Pacific ecosystems. While some species have been recorded in the mesophotic zone (30–150 m) and in even deeper water, such observations remain rare and limit our understanding of habitat use and the ecological role of these snakes. We present 15 observations of marine elapids at depths > 50 m in the Ryukyu Islands of southwestern Japan, obtained through rebreather diving and remotely operated vehicles. Black-banded Sea Kraits ( Laticauda semifasciata ) were observed at depths of 50–112 m across both hard and soft substrates ( n = 13), and one Blue-lipped Sea Krait ( L. laticaudata ) was observed at 122 m depth. Four of the observed L. semifasciata exhibited foraging behavior. Additionally, one Ornate Reef Sea Snake ( Hydrophis ornatus ) was recorded at 95 m. These represent the deepest known records for all three species. As benthic foragers, these marine elapids may play a role in nutrient transfer between deep and shallow habitats. Furthermore, deep-water environments may serve as foraging refugia, offering protection from predators such as sharks, as well as the impacts of marine heatwaves and coastal development. Diving behavior Foraging Remotely operated vehicles Sea snakes Laticauda Hydrophis Figures Figure 1 Figure 2 Introduction Snakes of the family Elapidae include two major clades that have independently colonized the marine environment: sea kraits (genus Laticauda ) and sea snakes in the subfamily Hydrophiinae. Sea kraits are oviparous and periodically return to land for digestion, mating, and oviposition, while sea snakes are viviparous and fully aquatic. Both groups have a convergently evolved paddle-shaped tail that enhances swimming, and exhibit various morphological and physiological adaptations to the marine environment. These marine elapids play crucial roles as mesopredators in tropical and subtropical coastal marine ecosystems in the Indo-Pacific (Voris and Voris 1983 ; Heatwole 1999 ). Furthermore, many marine elapids prey on rare or cryptic fish such as eels and gobies, and have the potential as bioindicators to help monitor the effects of climate change and habitat change on marine fish assemblages (Brischoux et al. 2009 ; Rasmussen et al. 2011). Most ecological studies of marine elapids have focused on shallow waters (< 15 m) due to logistical constraints (e.g., Lukoschek et al. 2013 ; Udyawer et al. 2015 ; Shine et al. 2020 ). However, recent remotely operated vehicle (ROV) observations have recorded sea snakes at depths down to 250 m, significantly expanding their known depth range (Crowe-Riddell et al. 2019 ). Additionally, surveys using baited remote underwater video stations (BRUVS) and ROVs have revealed the presence of sea snakes in the mesophotic zone (30–150 m) in Ashmore Reef, Australia, underscoring the significance of deep-water habitats in the ecology of marine elapids (Udyawer et al. 2014 ; Speed et al. 2022 ). Speed et al. ( 2022 ) further proposed that these deep zones may offer refuge from predators, particularly sharks, which are more abundant in shallower waters. Given the recent decline of marine elapid abundance in global hotspots (Goiran and Shine 2013 ; Somaweera et al. 2021 ), it is essential to assess their distribution across their full spatial range. The Ryukyu Islands in southwestern Japan represent one of the northernmost areas with breeding populations of marine elapids: records further north in mainland Japan and Korea are considered waif dispersals (Hecht et al. 1974 ; Masunaga et al. 2005 ; Ota and Yamadashima 2012 ; but see Park et al. 2017 ). Although marine elapids are regularly observed during benthic surveys in the mesophotic zone in this region (Sinniger et al. 2019 ), detailed accounts of their species composition, habitat use, and behavior are lacking. Here, we present new records of marine elapids in the mesophotic zone of the Ryukyu Islands based on ROV and scuba surveys, including the deepest known occurrences of three species. Materials and methods Observations were conducted in coastal waters off northwestern Okinawa Island and near Kodakara Island, Japan (Fig. 1 ). The coastlines are predominantly surrounded by fringing reefs. In our study site around Okinawa Island, the area surrounded by Okinawa, Ie, Minna, and Sesoko Islands is relatively flat and comprises mesophotic coral reefs and soft substrate. At the outer coasts of the islands, steep reef slopes occur, and depths rapidly drop off to > 100 m within ca. 3 km from land. Several topographic features raising a few hundred meters above the seafloor with large flat tops reaching 65 m depth are located in the survey area, and provide extensive habitats for benthic invertebrates. Around Kodakara Island, the depths similarly rapidly increase from the coast. Videos and photos of marine elapid snakes were opportunistically taken during technical scuba diving surveys for mesophotic corals led by FS and SH, and during benthic surveys using ROVs conducted by the Okinawa Churashima Foundation. Images from Kodakara Island were obtained from the JAMSTEC E-library of Deep-sea Images ( https://www.godac.jamstec.go.jp/jedi/e/index.html ). In the scuba surveys, depth, time, and water temperature were obtained from the dive computers (Shearwater Research, Canada). In the ROV surveys, depth was recorded. From the video footage, we visually categorized the substrate as rock, rock plus sand, sand plus coral rubble, non-branching hard coral, or soft sediment. Results and discussion We made 15 observations of marine elapids between October 2011 and August 2024 in the daytime. Observations included 13 Erabu Sea Kraits Laticauda semifasciata , one Blue-banded Sea Krait L. laticaudata , and one Ornate-reef Sea Snake Hydrophis ornatus (Table 1 , Fig. 2 ). One L. laticaudata was found near Kodakara Island, and the others around Okinawa Island. Observations ranged from 50 to 122 m depth, with ambient temperatures of 21.6–24.6°C (where recorded). Laticauda semifasciata was found on a variety of hard and soft substrates, while L. laticaudata was found in rock plus sand substrate, and H. ornatus in soft sediment substrate. The snakes were either foraging (indicated by the snout repeatedly touching the substrate) or swimming. Three individuals of L. semifasciata approached the diver or the ROV. Table 1 List of observations of marine elapid snakes in the mesophotic zone in the Ryukyu Islands, Japan. Species Date Time Habitat Depth (m) Temperature (°C) Behavior Laticauda laticaudata 15 Oct 2011 14:05 Rock plus sand 122 22 Swimming Laticauda semifasciata 8 Sep 2015 10:20 Sand plus coral rubble 50 N/A Foraging; approaching ROV Laticauda semifasciata 27 Sep 2019 11:37 Sand plus coral rubble 112 24.6 Swimming Laticauda semifasciata 9 Jul 2021 10:36 Sand plus coral rubble 85 22.5 Approaching ROV; fleeing Laticauda semifasciata 18 Oct 2021 10:15 Non-branching hard coral 67 21.7 Foraging Laticauda semifasciata 24 Mar 2022 11:16 Non-branching hard coral 75 N/A Foraging Laticauda semifasciata 24 Mar 2022 11:27 Non-branching hard coral 70 N/A Foraging Laticauda semifasciata 29 Apr 2022 12:24 Sand plus coral rubble 88 21.6 Swimming Laticauda semifasciata 22 Jul 2022 13:35 Non-branching hard coral 57 22 Swimming Laticauda semifasciata 8 Aug 2022 13:49 Rock 81 22 Approaching diver Laticauda semifasciata 20 Aug 2022 9:47 Rock plus sand 72 23 Swimming Laticauda semifasciata 20 Aug 2022 10:13 Rock plus sand 70 23 Swimming Laticauda semifasciata 30 Jul 2024 9:56 Rock plus sand 77 22 N/A Hydrophis ornatus 14 Aug 2024 14:43 Soft sediment 95 N/A Swimming Laticauda semifasciata 14 Aug 2024 NA Soft sediment 95 N/A Swimming Laticauda semifasciata is a dietary generalist that feeds on fish from a wide range of families and body shapes (Su et al. 2005 ; Tabata et al. 2017 ). The spatial ecology of L. semifasciata is largely unknown, but they have been observed to forage at < 45 m depth (Somaweera et al. 2023 ). In our observations, about one-third of the individuals were found during foraging behavior, suggesting that L. semifasciata is a predator of a wide array of fish in mesophotic reef communities. A L. laticaudata , a known eel specialist (Brischoux et al. 2007 ; Tabata et al. 2017 ), was observed at 122 m, also far deeper than previous records for this species (maximum 32.4 m in New Caledonia; Cook et al. 2016 ). Several physiological adaptations enable marine elapids to perform deep dives. A large cardiac shunt coupled with high skin permeability to gases are thought to allow Hydrophis sea snakes to avoid forming nitrogen bubbles in the bloodstream after deep dives (i.e., decompression sickness). Sea kraits also possess these traits, albeit to a lesser extent, and simulated pressurization experiments have estimated that they may be at risk of decompression sickness at depths of 30–70 m. (Seymour 1978 ). However, our observation of sea kraits occurring at > 100 m depth demonstrates that the diving capacity of sea kraits is underestimated. This calls for further assessment of their physiological adaptations for diving. Hydrophis ornatus is a widespread sea snake that, in the Ryukyu Islands, has been predominantly observed in relatively enclosed bay-like waters with soft substrates at depths of less than 25 m, where it primarily feeds on burrowing gobies (Kidera et al. 2015 ; Fujishima et al. 2021 ). Consistent with these observations, we found an H. ornatus on soft substrate, but at a greater depth of 95 m. The habitat use of H. ornatus is largely unknown, and to our knowledge, this represents the deepest recorded occurrence of this species. Our observations with temperature records showed that the water temperatures at the depth of the observations were 22–23°C in July, August, and October. Surface water temperature around Okinawa Island range around 21–29°༣ peaking during August and September, and 21–24°C at 100 m depth (Japan Oceanographic Data Center, http://www.jodc.go.jp/service.htm ). Diving to deeper, cooler waters may benefit snakes, particularly in warmer months, by lowering their metabolic rate, which reduces energy expenditure and extends dive duration, thereby allowing more time for foraging (Cook and Brischoux 2014 ). In conclusion, our observations substantially expand the depth range of three species of marine elapid snakes. In particular, sea kraits regularly move across large areas (> 20 km from the home island) of the reef to forage on the bottom and return to land (Brischoux et al. 2007 ). Our observations of sea kraits occurring in the mesophotic zone suggests their role in translocating nutrients between mesophotic and shallow reef systems, as well as with the terrestrial habitat that they utilize (Slattery et al. 2011 ; Estupiñán-Montaño et al. 2023 ). In Ashmore Reef in north-western Australia, where the once abundant sea snakes have largely disappeared from shallow waters, sea snake populations were found to persist in the mesophotic zone (Speed et al. 2022 ). The increase in numbers of top predators, namely sharks, has been a major plausible explanation for the drastic decline of sea snakes in Ashmore Reef (Somaweera et al. 2021 ). The mesophotic zone may have acted as a refuge for sea snakes from predatory sharks, which are significantly more abundant in shallow waters (Speed et al. 2022 ). Similarly in the Ryukyu Islands, sharks, particularly tiger sharks Galeocerdo cuvier , are known to be a major predator of sea kraits and sea snakes (Masunaga et al. 2008 ), and deep diving by snakes may serve to avoid predation by sharks. Coral reefs have experienced widespread habitat degradation, notably through heat-induced bleaching events caused by elevated ocean temperatures (Hoegh-Guldberg et al. 2007 ; Hughes et al. 2017 ). While the direct impacts of reef degradation on marine elapids remain poorly quantified, declines in the abundance of reef-associated fishes following habitat loss are relatively well documented, though the magnitude varies across taxa (Prachett et al. 2011; Huang et al. 2023 ). Marine elapids that primarily feed on coral reef fishes (e.g., sea kraits) may be indirectly affected by such declines (Ota 2014 ). Mesophotic reefs represent a major habitat type that may be less impacted by heat-induced bleaching events that severely affect shallow reefs (Bongaerts and Smith 2019 ; Pérez-Rosales et al. 2021 ). These deep areas may serve as important refugial foraging habitats for reef-associated snakes. Future research integrating standardized visual survey methods using ROVs and/or BRUVS and implanted data loggers are needed to understand the population trends, diving behavior, and ecological roles of marine elapids in mesophotic systems. Such information will be crucial in informing conservation policies such as marine protected areas (MPAs) regarding mesophotic habitats (Gress et al. 2018 ; Pulido Mantas et al. 2024 ). Declarations Compliance with Ethical Standards No ethics approval was required for the present study as all sampling was conducted using non-invasive underwater video and photography techniques. Competing interests The authors declare that they have no competing interests. Author contributions All authors contributed to the study conception and design. Kanta Fujishima carried out video analysis, species identification, and writing of the first manuscript draft. Takuji Higashiji provided resources for ROV surveys at the Churashima Foundation, and associated data. Frederic Sinniger conceived scuba diving field work and provided the records and associated data. Saki Harii supported the field work. All authors contributed and reviewed the final version of the manuscript. Acknowledgements We thank members of Motobu Fisheries Cooperative Association for their cooperation and technical divers Thomas Jonsson, Matthew Broughton, Raeni Liu, Samuel Mason, and Sonia Rowley for their help in the deep-diving surveys. The second author is also indebted to the staff in charge of the Deep-Sea Animal Section of the Okinawa Churaumi Aquarium for their encouragement. A grant from the National Geographic Society (NGS-73985R-20) to FS allowed the exploration of the deeper parts of the reef with the help of mixed gas rebreather divers. We are grateful to Christine and Hinrich Kaiser for their valuable comments on the manuscript. Data availability All raw data supporting the conclusions of this study are available from the corresponding author upon reasonable request. References Bongaerts P, Smith TB (2019) Beyond the “deep reef refuge” hypothesis: a conceptual framework to characterize persistence at depth. In: Mesophotic Coral Ecosystems. Springer, Cham, pp 881–895 Brischoux F, Bonnet X, Shine R (2007) Foraging ecology of sea kraits Laticauda spp. in the Neo-Caledonian Lagoon. Mar Ecol Prog Ser 350:145–151. https://doi.org/10.3354/meps07133 Brischoux F, Bonnet X, Legagneux P (2009) Are sea snakes pertinent bio-indicators for coral reefs ? a comparison between species and sites. 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Supplementary Files SupplementarytableS1.xlsx Cite Share Download PDF Status: Published Journal Publication published 13 Feb, 2026 Read the published version in Marine Biology → Version 1 posted Editorial decision: Acceptable after minor revision 14 Dec, 2025 Reviewers agreed at journal 02 Nov, 2025 Reviewers invited by journal 30 Oct, 2025 Editor assigned by journal 10 Oct, 2025 First submitted to journal 09 Oct, 2025 You are reading this latest preprint version Research Square lets you share your work early, gain feedback from the community, and start making changes to your manuscript prior to peer review in a journal. As a division of Research Square Company, we’re committed to making research communication faster, fairer, and more useful. We do this by developing innovative software and high quality services for the global research community. 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09:27:06","extension":"xml","order_by":16,"title":"","display":"","copyAsset":false,"role":"acdc-reference","size":98991,"visible":true,"origin":"","legend":"","description":"","filename":"MABID25005430structuring.xml","url":"https://assets-eu.researchsquare.com/files/rs-7818285/v1/d28798acab88c5cc8884bd68.xml"},{"id":95521867,"identity":"fb1c2812-ccd7-4c9b-ad1f-c1396ba7bbce","added_by":"auto","created_at":"2025-11-10 09:27:08","extension":"html","order_by":17,"title":"","display":"","copyAsset":false,"role":"acdc-reference","size":105416,"visible":true,"origin":"","legend":"","description":"","filename":"earlyproof.html","url":"https://assets-eu.researchsquare.com/files/rs-7818285/v1/82e2156429958b0ad58d8ecc.html"},{"id":95521825,"identity":"ec6fd181-cbaf-4e2f-af00-ccb71360c1e4","added_by":"auto","created_at":"2025-11-10 09:27:05","extension":"jpg","order_by":1,"title":"Figure 1","display":"","copyAsset":false,"role":"figure","size":109128,"visible":true,"origin":"","legend":"\u003cp\u003eMap showing the Ryukyu Islands (A) and the study sites (B and C). The locations where marine elapids were observed in the mesophotic zone are indicated by red dots.\u003c/p\u003e","description":"","filename":"1.jpg","url":"https://assets-eu.researchsquare.com/files/rs-7818285/v1/4e4e80de5243c8f2d7ef6e9a.jpg"},{"id":95521827,"identity":"d3b8d324-dea6-4010-bd6d-ad95d7b483d3","added_by":"auto","created_at":"2025-11-10 09:27:05","extension":"jpg","order_by":2,"title":"Figure 2","display":"","copyAsset":false,"role":"figure","size":81690,"visible":true,"origin":"","legend":"\u003cp\u003eMarine elapid snakes observed in the mesophotic zone of the Ryukyu Islands, Japan. (A) Laticauda semifasciata photographed with a mixed-gas rebreather diver at 75 m depth. (B) Laticauda laticaudata at 122 m depth, recorded by a remotely operated vehicle (ROV). (C) Hydrophis ornatus at 95 m depth, recorded by an ROV. Photographs courtesy of Thomas Jonsson (A) and the Okinawa Churashima Foundation (C). Image in (B) was derived from a video by the Japan Agency for Marine-Earth Science and Technology (JAMSTEC; https://www.godac.jamstec.go.jp/jedi/static_player/e/HPD1336HDDB10_00420900).\u003c/p\u003e","description":"","filename":"2.jpg","url":"https://assets-eu.researchsquare.com/files/rs-7818285/v1/397ff2ecf884028bdcc4f624.jpg"},{"id":102785537,"identity":"b8e68188-619c-45c7-a8c5-753ea4d9d3ae","added_by":"auto","created_at":"2026-02-16 16:07:52","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"manuscript-pdf","size":668916,"visible":true,"origin":"","legend":"","description":"","filename":"manuscript.pdf","url":"https://assets-eu.researchsquare.com/files/rs-7818285/v1/1dab9117-9e08-4832-b9c8-91836f8dd2d6.pdf"},{"id":95521889,"identity":"b6dd0796-e472-4abb-87af-e0f75e56780c","added_by":"auto","created_at":"2025-11-10 09:27:18","extension":"xlsx","order_by":7,"title":"","display":"","copyAsset":false,"role":"supplement","size":10087,"visible":true,"origin":"","legend":"","description":"","filename":"SupplementarytableS1.xlsx","url":"https://assets-eu.researchsquare.com/files/rs-7818285/v1/6aebc01ff573a00bc17e83e8.xlsx"}],"financialInterests":"","formattedTitle":"Observations of marine elapid snakes in the mesophotic zone in the Ryukyu Islands, Japan, with the deepest records for three species","fulltext":[{"header":"Introduction","content":"\u003cp\u003eSnakes of the family Elapidae include two major clades that have independently colonized the marine environment: sea kraits (genus \u003cem\u003eLaticauda\u003c/em\u003e) and sea snakes in the subfamily Hydrophiinae. Sea kraits are oviparous and periodically return to land for digestion, mating, and oviposition, while sea snakes are viviparous and fully aquatic. Both groups have a convergently evolved paddle-shaped tail that enhances swimming, and exhibit various morphological and physiological adaptations to the marine environment. These marine elapids play crucial roles as mesopredators in tropical and subtropical coastal marine ecosystems in the Indo-Pacific (Voris and Voris \u003cspan citationid=\"CR42\" class=\"CitationRef\"\u003e1983\u003c/span\u003e; Heatwole \u003cspan citationid=\"CR11\" class=\"CitationRef\"\u003e1999\u003c/span\u003e). Furthermore, many marine elapids prey on rare or cryptic fish such as eels and gobies, and have the potential as bioindicators to help monitor the effects of climate change and habitat change on marine fish assemblages (Brischoux et al. \u003cspan citationid=\"CR3\" class=\"CitationRef\"\u003e2009\u003c/span\u003e; Rasmussen et al. 2011). Most ecological studies of marine elapids have focused on shallow waters (\u0026lt;\u0026thinsp;15 m) due to logistical constraints (e.g., Lukoschek et al. \u003cspan citationid=\"CR21\" class=\"CitationRef\"\u003e2013\u003c/span\u003e; Udyawer et al. \u003cspan citationid=\"CR41\" class=\"CitationRef\"\u003e2015\u003c/span\u003e; Shine et al. \u003cspan citationid=\"CR32\" class=\"CitationRef\"\u003e2020\u003c/span\u003e). However, recent remotely operated vehicle (ROV) observations have recorded sea snakes at depths down to 250 m, significantly expanding their known depth range (Crowe-Riddell et al. \u003cspan citationid=\"CR6\" class=\"CitationRef\"\u003e2019\u003c/span\u003e). Additionally, surveys using baited remote underwater video stations (BRUVS) and ROVs have revealed the presence of sea snakes in the mesophotic zone (30\u0026ndash;150 m) in Ashmore Reef, Australia, underscoring the significance of deep-water habitats in the ecology of marine elapids (Udyawer et al. \u003cspan citationid=\"CR40\" class=\"CitationRef\"\u003e2014\u003c/span\u003e; Speed et al. \u003cspan citationid=\"CR37\" class=\"CitationRef\"\u003e2022\u003c/span\u003e). Speed et al. (\u003cspan citationid=\"CR37\" class=\"CitationRef\"\u003e2022\u003c/span\u003e) further proposed that these deep zones may offer refuge from predators, particularly sharks, which are more abundant in shallower waters. Given the recent decline of marine elapid abundance in global hotspots (Goiran and Shine \u003cspan citationid=\"CR9\" class=\"CitationRef\"\u003e2013\u003c/span\u003e; Somaweera et al. \u003cspan citationid=\"CR35\" class=\"CitationRef\"\u003e2021\u003c/span\u003e), it is essential to assess their distribution across their full spatial range.\u003c/p\u003e\u003cp\u003eThe Ryukyu Islands in southwestern Japan represent one of the northernmost areas with breeding populations of marine elapids: records further north in mainland Japan and Korea are considered waif dispersals (Hecht et al. \u003cspan citationid=\"CR12\" class=\"CitationRef\"\u003e1974\u003c/span\u003e; Masunaga et al. \u003cspan citationid=\"CR22\" class=\"CitationRef\"\u003e2005\u003c/span\u003e; Ota and Yamadashima \u003cspan citationid=\"CR25\" class=\"CitationRef\"\u003e2012\u003c/span\u003e; but see Park et al. \u003cspan citationid=\"CR26\" class=\"CitationRef\"\u003e2017\u003c/span\u003e). Although marine elapids are regularly observed during benthic surveys in the mesophotic zone in this region (Sinniger et al. \u003cspan citationid=\"CR33\" class=\"CitationRef\"\u003e2019\u003c/span\u003e), detailed accounts of their species composition, habitat use, and behavior are lacking. Here, we present new records of marine elapids in the mesophotic zone of the Ryukyu Islands based on ROV and scuba surveys, including the deepest known occurrences of three species.\u003c/p\u003e"},{"header":"Materials and methods","content":"\u003cp\u003eObservations were conducted in coastal waters off northwestern Okinawa Island and near Kodakara Island, Japan (Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e). The coastlines are predominantly surrounded by fringing reefs. In our study site around Okinawa Island, the area surrounded by Okinawa, Ie, Minna, and Sesoko Islands is relatively flat and comprises mesophotic coral reefs and soft substrate. At the outer coasts of the islands, steep reef slopes occur, and depths rapidly drop off to \u0026gt;\u0026thinsp;100 m within ca. 3 km from land. Several topographic features raising a few hundred meters above the seafloor with large flat tops reaching 65 m depth are located in the survey area, and provide extensive habitats for benthic invertebrates. Around Kodakara Island, the depths similarly rapidly increase from the coast. Videos and photos of marine elapid snakes were opportunistically taken during technical scuba diving surveys for mesophotic corals led by FS and SH, and during benthic surveys using ROVs conducted by the Okinawa Churashima Foundation. Images from Kodakara Island were obtained from the JAMSTEC E-library of Deep-sea Images (\u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttps://www.godac.jamstec.go.jp/jedi/e/index.html\u003c/span\u003e\u003cspan address=\"https://www.godac.jamstec.go.jp/jedi/e/index.html\" targettype=\"URL\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e). In the scuba surveys, depth, time, and water temperature were obtained from the dive computers (Shearwater Research, Canada). In the ROV surveys, depth was recorded. From the video footage, we visually categorized the substrate as rock, rock plus sand, sand plus coral rubble, non-branching hard coral, or soft sediment.\u003c/p\u003e\u003cp\u003e\u003c/p\u003e"},{"header":"Results and discussion","content":"\u003cp\u003eWe made 15 observations of marine elapids between October 2011 and August 2024 in the daytime. Observations included 13 Erabu Sea Kraits \u003cem\u003eLaticauda semifasciata\u003c/em\u003e, one Blue-banded Sea Krait \u003cem\u003eL. laticaudata\u003c/em\u003e, and one Ornate-reef Sea Snake \u003cem\u003eHydrophis ornatus\u003c/em\u003e (Table\u0026nbsp;\u003cspan refid=\"Tab1\" class=\"InternalRef\"\u003e1\u003c/span\u003e, Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e). One \u003cem\u003eL. laticaudata\u003c/em\u003e was found near Kodakara Island, and the others around Okinawa Island. Observations ranged from 50 to 122 m depth, with ambient temperatures of 21.6\u0026ndash;24.6\u0026deg;C (where recorded). \u003cem\u003eLaticauda semifasciata\u003c/em\u003e was found on a variety of hard and soft substrates, while \u003cem\u003eL. laticaudata\u003c/em\u003e was found in rock plus sand substrate, and \u003cem\u003eH. ornatus\u003c/em\u003e in soft sediment substrate. The snakes were either foraging (indicated by the snout repeatedly touching the substrate) or swimming. Three individuals of \u003cem\u003eL. semifasciata\u003c/em\u003e approached the diver or the ROV.\u003c/p\u003e\u003cp\u003e\u003cdiv class=\"gridtable\"\u003e\u003ctable float=\"Yes\" id=\"Tab1\" border=\"1\"\u003e\u003ccaption language=\"En\"\u003e\u003cdiv class=\"CaptionNumber\"\u003eTable 1\u003c/div\u003e\u003cdiv class=\"CaptionContent\"\u003e\u003cp\u003eList of observations of marine elapid snakes in the mesophotic zone in the Ryukyu Islands, Japan.\u003c/p\u003e\u003c/div\u003e\u003c/caption\u003e\u003ccolgroup cols=\"7\"\u003e\u003cdiv align=\"left\" class=\"colspec\" colname=\"c1\" colnum=\"1\"\u003e\u003c/div\u003e\u003cdiv align=\"left\" class=\"colspec\" colname=\"c2\" colnum=\"2\"\u003e\u003c/div\u003e\u003cdiv align=\"left\" class=\"colspec\" colname=\"c3\" colnum=\"3\"\u003e\u003c/div\u003e\u003cdiv align=\"left\" class=\"colspec\" colname=\"c4\" colnum=\"4\"\u003e\u003c/div\u003e\u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c5\" colnum=\"5\"\u003e\u003c/div\u003e\u003cdiv align=\"left\" class=\"colspec\" colname=\"c6\" colnum=\"6\"\u003e\u003c/div\u003e\u003cdiv align=\"left\" class=\"colspec\" colname=\"c7\" colnum=\"7\"\u003e\u003c/div\u003e\u003cthead\u003e\u003ctr\u003e\u003cth align=\"left\" colname=\"c1\"\u003e\u003cp\u003eSpecies\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colname=\"c2\"\u003e\u003cp\u003eDate\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colname=\"c3\"\u003e\u003cp\u003eTime\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colname=\"c4\"\u003e\u003cp\u003eHabitat\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colname=\"c5\"\u003e\u003cp\u003eDepth (m)\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colname=\"c6\"\u003e\u003cp\u003eTemperature (\u0026deg;C)\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colname=\"c7\"\u003e\u003cp\u003eBehavior\u003c/p\u003e\u003c/th\u003e\u003c/tr\u003e\u003c/thead\u003e\u003ctbody\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003e\u003cem\u003eLaticauda laticaudata\u003c/em\u003e\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003e15 Oct 2011\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003e14:05\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c4\"\u003e\u003cp\u003eRock plus sand\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e\u003cp\u003e122\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c6\"\u003e\u003cp\u003e22\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c7\"\u003e\u003cp\u003eSwimming\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003e\u003cem\u003eLaticauda semifasciata\u003c/em\u003e\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003e8 Sep 2015\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003e10:20\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c4\"\u003e\u003cp\u003eSand plus coral rubble\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e\u003cp\u003e50\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c6\"\u003e\u003cp\u003eN/A\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c7\"\u003e\u003cp\u003eForaging; approaching ROV\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003e\u003cem\u003eLaticauda semifasciata\u003c/em\u003e\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003e27 Sep 2019\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003e11:37\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c4\"\u003e\u003cp\u003eSand plus coral rubble\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e\u003cp\u003e112\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c6\"\u003e\u003cp\u003e24.6\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c7\"\u003e\u003cp\u003eSwimming\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003e\u003cem\u003eLaticauda semifasciata\u003c/em\u003e\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003e9 Jul 2021\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003e10:36\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c4\"\u003e\u003cp\u003eSand plus coral rubble\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e\u003cp\u003e85\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c6\"\u003e\u003cp\u003e22.5\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c7\"\u003e\u003cp\u003eApproaching ROV; fleeing\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003e\u003cem\u003eLaticauda semifasciata\u003c/em\u003e\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003e18 Oct 2021\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003e10:15\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c4\"\u003e\u003cp\u003eNon-branching hard coral\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e\u003cp\u003e67\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c6\"\u003e\u003cp\u003e21.7\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c7\"\u003e\u003cp\u003eForaging\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003e\u003cem\u003eLaticauda semifasciata\u003c/em\u003e\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003e24 Mar 2022\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003e11:16\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c4\"\u003e\u003cp\u003eNon-branching hard coral\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e\u003cp\u003e75\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c6\"\u003e\u003cp\u003eN/A\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c7\"\u003e\u003cp\u003eForaging\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003e\u003cem\u003eLaticauda semifasciata\u003c/em\u003e\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003e24 Mar 2022\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003e11:27\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c4\"\u003e\u003cp\u003eNon-branching hard coral\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e\u003cp\u003e70\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c6\"\u003e\u003cp\u003eN/A\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c7\"\u003e\u003cp\u003eForaging\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003e\u003cem\u003eLaticauda semifasciata\u003c/em\u003e\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003e29 Apr 2022\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003e12:24\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c4\"\u003e\u003cp\u003eSand plus coral rubble\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e\u003cp\u003e88\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c6\"\u003e\u003cp\u003e21.6\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c7\"\u003e\u003cp\u003eSwimming\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003e\u003cem\u003eLaticauda semifasciata\u003c/em\u003e\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003e22 Jul 2022\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003e13:35\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c4\"\u003e\u003cp\u003eNon-branching hard coral\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e\u003cp\u003e57\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c6\"\u003e\u003cp\u003e22\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c7\"\u003e\u003cp\u003eSwimming\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003e\u003cem\u003eLaticauda semifasciata\u003c/em\u003e\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003e8 Aug 2022\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003e13:49\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c4\"\u003e\u003cp\u003eRock\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e\u003cp\u003e81\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c6\"\u003e\u003cp\u003e22\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c7\"\u003e\u003cp\u003eApproaching diver\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003e\u003cem\u003eLaticauda semifasciata\u003c/em\u003e\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003e20 Aug 2022\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003e9:47\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c4\"\u003e\u003cp\u003eRock plus sand\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e\u003cp\u003e72\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c6\"\u003e\u003cp\u003e23\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c7\"\u003e\u003cp\u003eSwimming\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003e\u003cem\u003eLaticauda semifasciata\u003c/em\u003e\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003e20 Aug 2022\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003e10:13\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c4\"\u003e\u003cp\u003eRock plus sand\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e\u003cp\u003e70\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c6\"\u003e\u003cp\u003e23\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c7\"\u003e\u003cp\u003eSwimming\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003e\u003cem\u003eLaticauda semifasciata\u003c/em\u003e\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003e30 Jul 2024\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003e9:56\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c4\"\u003e\u003cp\u003eRock plus sand\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e\u003cp\u003e77\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c6\"\u003e\u003cp\u003e22\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c7\"\u003e\u003cp\u003eN/A\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003e\u003cem\u003eHydrophis ornatus\u003c/em\u003e\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003e14 Aug 2024\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003e14:43\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c4\"\u003e\u003cp\u003eSoft sediment\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e\u003cp\u003e95\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c6\"\u003e\u003cp\u003eN/A\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c7\"\u003e\u003cp\u003eSwimming\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003e\u003cem\u003eLaticauda semifasciata\u003c/em\u003e\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003e14 Aug 2024\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003eNA\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c4\"\u003e\u003cp\u003eSoft sediment\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e\u003cp\u003e95\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c6\"\u003e\u003cp\u003eN/A\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c7\"\u003e\u003cp\u003eSwimming\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003c/tbody\u003e\u003c/colgroup\u003e\u003c/table\u003e\u003c/div\u003e\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003cp\u003e\u003cem\u003eLaticauda semifasciata\u003c/em\u003e is a dietary generalist that feeds on fish from a wide range of families and body shapes (Su et al. \u003cspan citationid=\"CR38\" class=\"CitationRef\"\u003e2005\u003c/span\u003e; Tabata et al. \u003cspan citationid=\"CR39\" class=\"CitationRef\"\u003e2017\u003c/span\u003e). The spatial ecology of \u003cem\u003eL. semifasciata\u003c/em\u003e is largely unknown, but they have been observed to forage at \u0026lt;\u0026thinsp;45 m depth (Somaweera et al. \u003cspan citationid=\"CR36\" class=\"CitationRef\"\u003e2023\u003c/span\u003e). In our observations, about one-third of the individuals were found during foraging behavior, suggesting that \u003cem\u003eL. semifasciata\u003c/em\u003e is a predator of a wide array of fish in mesophotic reef communities. A \u003cem\u003eL. laticaudata\u003c/em\u003e, a known eel specialist (Brischoux et al. \u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2007\u003c/span\u003e; Tabata et al. \u003cspan citationid=\"CR39\" class=\"CitationRef\"\u003e2017\u003c/span\u003e), was observed at 122 m, also far deeper than previous records for this species (maximum 32.4 m in New Caledonia; Cook et al. \u003cspan citationid=\"CR5\" class=\"CitationRef\"\u003e2016\u003c/span\u003e). Several physiological adaptations enable marine elapids to perform deep dives. A large cardiac shunt coupled with high skin permeability to gases are thought to allow \u003cem\u003eHydrophis\u003c/em\u003e sea snakes to avoid forming nitrogen bubbles in the bloodstream after deep dives (i.e., decompression sickness). Sea kraits also possess these traits, albeit to a lesser extent, and simulated pressurization experiments have estimated that they may be at risk of decompression sickness at depths of 30\u0026ndash;70 m. (Seymour \u003cspan citationid=\"CR31\" class=\"CitationRef\"\u003e1978\u003c/span\u003e). However, our observation of sea kraits occurring at \u0026gt;\u0026thinsp;100 m depth demonstrates that the diving capacity of sea kraits is underestimated. This calls for further assessment of their physiological adaptations for diving.\u003c/p\u003e\u003cp\u003e\u003cem\u003eHydrophis ornatus\u003c/em\u003e is a widespread sea snake that, in the Ryukyu Islands, has been predominantly observed in relatively enclosed bay-like waters with soft substrates at depths of less than 25 m, where it primarily feeds on burrowing gobies (Kidera et al. \u003cspan citationid=\"CR20\" class=\"CitationRef\"\u003e2015\u003c/span\u003e; Fujishima et al. \u003cspan citationid=\"CR8\" class=\"CitationRef\"\u003e2021\u003c/span\u003e). Consistent with these observations, we found an \u003cem\u003eH. ornatus\u003c/em\u003e on soft substrate, but at a greater depth of 95 m. The habitat use of \u003cem\u003eH. ornatus\u003c/em\u003e is largely unknown, and to our knowledge, this represents the deepest recorded occurrence of this species.\u003c/p\u003e\u003cp\u003eOur observations with temperature records showed that the water temperatures at the depth of the observations were 22\u0026ndash;23\u0026deg;C in July, August, and October. Surface water temperature around Okinawa Island range around 21\u0026ndash;29\u0026deg;༣ peaking during August and September, and 21\u0026ndash;24\u0026deg;C at 100 m depth (Japan Oceanographic Data Center, \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttp://www.jodc.go.jp/service.htm\u003c/span\u003e\u003cspan address=\"http://www.jodc.go.jp/service.htm\" targettype=\"URL\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e). Diving to deeper, cooler waters may benefit snakes, particularly in warmer months, by lowering their metabolic rate, which reduces energy expenditure and extends dive duration, thereby allowing more time for foraging (Cook and Brischoux \u003cspan citationid=\"CR4\" class=\"CitationRef\"\u003e2014\u003c/span\u003e).\u003c/p\u003e\u003cp\u003eIn conclusion, our observations substantially expand the depth range of three species of marine elapid snakes.\u003c/p\u003e\u003cp\u003eIn particular, sea kraits regularly move across large areas (\u0026gt;\u0026thinsp;20 km from the home island) of the reef to forage on the bottom and return to land (Brischoux et al. \u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2007\u003c/span\u003e). Our observations of sea kraits occurring in the mesophotic zone suggests their role in translocating nutrients between mesophotic and shallow reef systems, as well as with the terrestrial habitat that they utilize (Slattery et al. \u003cspan citationid=\"CR34\" class=\"CitationRef\"\u003e2011\u003c/span\u003e; Estupi\u0026ntilde;\u0026aacute;n-Monta\u0026ntilde;o et al. \u003cspan citationid=\"CR7\" class=\"CitationRef\"\u003e2023\u003c/span\u003e).\u003c/p\u003e\u003cp\u003eIn Ashmore Reef in north-western Australia, where the once abundant sea snakes have largely disappeared from shallow waters, sea snake populations were found to persist in the mesophotic zone (Speed et al. \u003cspan citationid=\"CR37\" class=\"CitationRef\"\u003e2022\u003c/span\u003e). The increase in numbers of top predators, namely sharks, has been a major plausible explanation for the drastic decline of sea snakes in Ashmore Reef (Somaweera et al. \u003cspan citationid=\"CR35\" class=\"CitationRef\"\u003e2021\u003c/span\u003e). The mesophotic zone may have acted as a refuge for sea snakes from predatory sharks, which are significantly more abundant in shallow waters (Speed et al. \u003cspan citationid=\"CR37\" class=\"CitationRef\"\u003e2022\u003c/span\u003e). Similarly in the Ryukyu Islands, sharks, particularly tiger sharks \u003cem\u003eGaleocerdo cuvier\u003c/em\u003e, are known to be a major predator of sea kraits and sea snakes (Masunaga et al. \u003cspan citationid=\"CR23\" class=\"CitationRef\"\u003e2008\u003c/span\u003e), and deep diving by snakes may serve to avoid predation by sharks.\u003c/p\u003e\u003cp\u003eCoral reefs have experienced widespread habitat degradation, notably through heat-induced bleaching events caused by elevated ocean temperatures (Hoegh-Guldberg et al. \u003cspan citationid=\"CR14\" class=\"CitationRef\"\u003e2007\u003c/span\u003e; Hughes et al. \u003cspan citationid=\"CR16\" class=\"CitationRef\"\u003e2017\u003c/span\u003e). While the direct impacts of reef degradation on marine elapids remain poorly quantified, declines in the abundance of reef-associated fishes following habitat loss are relatively well documented, though the magnitude varies across taxa (Prachett et al. 2011; Huang et al. \u003cspan citationid=\"CR15\" class=\"CitationRef\"\u003e2023\u003c/span\u003e). Marine elapids that primarily feed on coral reef fishes (e.g., sea kraits) may be indirectly affected by such declines (Ota \u003cspan citationid=\"CR24\" class=\"CitationRef\"\u003e2014\u003c/span\u003e). Mesophotic reefs represent a major habitat type that may be less impacted by heat-induced bleaching events that severely affect shallow reefs (Bongaerts and Smith \u003cspan citationid=\"CR1\" class=\"CitationRef\"\u003e2019\u003c/span\u003e; P\u0026eacute;rez-Rosales et al. \u003cspan citationid=\"CR27\" class=\"CitationRef\"\u003e2021\u003c/span\u003e). These deep areas may serve as important refugial foraging habitats for reef-associated snakes. Future research integrating standardized visual survey methods using ROVs and/or BRUVS and implanted data loggers are needed to understand the population trends, diving behavior, and ecological roles of marine elapids in mesophotic systems. Such information will be crucial in informing conservation policies such as marine protected areas (MPAs) regarding mesophotic habitats (Gress et al. \u003cspan citationid=\"CR10\" class=\"CitationRef\"\u003e2018\u003c/span\u003e; Pulido Mantas et al. \u003cspan citationid=\"CR29\" class=\"CitationRef\"\u003e2024\u003c/span\u003e).\u003c/p\u003e"},{"header":"Declarations","content":"\u003cp\u003e\u003ch2\u003eCompliance with Ethical Standards\u003c/h2\u003e\u003cp\u003eNo ethics approval was required for the present study as all sampling was conducted using non-invasive underwater video and photography techniques.\u003c/p\u003e\u003c/p\u003e\u003cp\u003e\u003ch2\u003eCompeting interests\u003c/h2\u003e\u003cp\u003eThe authors declare that they have no competing interests.\u003c/p\u003e\u003c/p\u003e\u003ch2\u003eAuthor contributions\u003c/h2\u003e\u003cp\u003eAll authors contributed to the study conception and design. Kanta Fujishima carried out video analysis, species identification, and writing of the first manuscript draft. Takuji Higashiji provided resources for ROV surveys at the Churashima Foundation, and associated data. Frederic Sinniger conceived scuba diving field work and provided the records and associated data. Saki Harii supported the field work. All authors contributed and reviewed the final version of the manuscript.\u003c/p\u003e\u003ch2\u003eAcknowledgements\u003c/h2\u003e\u003cp\u003eWe thank members of Motobu Fisheries Cooperative Association for their cooperation and technical divers Thomas Jonsson, Matthew Broughton, Raeni Liu, Samuel Mason, and Sonia Rowley for their help in the deep-diving surveys. The second author is also indebted to the staff in charge of the Deep-Sea Animal Section of the Okinawa Churaumi Aquarium for their encouragement. A grant from the National Geographic Society (NGS-73985R-20) to FS allowed the exploration of the deeper parts of the reef with the help of mixed gas rebreather divers. We are grateful to Christine and Hinrich Kaiser for their valuable comments on the manuscript.\u003c/p\u003e\u003ch2\u003eData availability\u003c/h2\u003e\u003cp\u003eAll raw data supporting the conclusions of this study are available from the corresponding author upon reasonable request.\u003c/p\u003e"},{"header":"References","content":"\u003col\u003e\n\u003cli\u003eBongaerts P, Smith TB (2019) Beyond the \u0026ldquo;deep reef refuge\u0026rdquo; hypothesis: a conceptual framework to characterize persistence at depth. In: Mesophotic Coral Ecosystems. Springer, Cham, pp 881\u0026ndash;895\u003c/li\u003e\n\u003cli\u003eBrischoux F, Bonnet X, Shine R (2007) Foraging ecology of sea kraits \u003cem\u003eLaticauda\u003c/em\u003e spp. in the Neo-Caledonian Lagoon. Mar Ecol Prog Ser 350:145\u0026ndash;151. https://doi.org/10.3354/meps07133\u003c/li\u003e\n\u003cli\u003eBrischoux F, Bonnet X, Legagneux P (2009) Are sea snakes pertinent bio-indicators for coral reefs ? a comparison between species and sites. Mar Biol 156:1985\u0026ndash;1992. https://doi.org/10.1007/s00227-009-1229-7\u003c/li\u003e\n\u003cli\u003eCook TR, Brischoux F (2014) Why does the only \u0026ldquo;planktonic tetrapod\u0026rdquo; dive? Determinants of diving behaviour in a marine ectotherm. Anim Behav 98:113\u0026ndash;123. https://doi.org/10.1016/j.anbehav.2014.09.018\u003c/li\u003e\n\u003cli\u003eCook TR, Bonnet X, Fauvel T, Shine R, Brischoux F (2016) Foraging behaviour and energy budgets of sea snakes: Insights from implanted data loggers. 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Nature 546:82\u0026ndash;90. doi: 10.1038/nature22901\u003c/li\u003e\n\u003cli\u003eHuang M, Wei S, Li Q, Gao K, Peng Z, Chen Y, Zhou W, Wei F (2023) Degradation of coral reefs altered the community trophic structure and reduced the shoaling size of fish. Frontiers in Conservation Science. doi: 10.3389/fcosc.2023.1229513\u003c/li\u003e\n\u003cli\u003eKidera N, Uyeno D, Naruse T (2015) Notes on the occurrence and habitat of \u003cem\u003eHydrophis ornatus\u003c/em\u003e (Gray, 1842) (Reptilia: Squamata: Elapidae) in the Ryukyu Islands, Japan. Fauna Ryukyuana 20:7\u0026ndash;13.\u003c/li\u003e\n\u003cli\u003eLukoschek V, Beger M, Ceccarelli D, Richards Z, Pratchett M (2013) Enigmatic declines of Australia\u0026rsquo;s sea snakes from a biodiversity hotspot. Biol Conserv 166:191\u0026ndash;202. https://doi.org/10.1016/j.biocon.2013.07.004\u003c/li\u003e\n\u003cli\u003eMasunaga G, Ota H, nagai Y, Tanase H (2005) A record of the black-headed sea snake, \u003cem\u003eHydrophis melanocephalus\u003c/em\u003e (Reptilia: Elapidae), from Wakayama Prefecture, Japan. Curr Herpetol 24:37\u0026ndash;41. https://doi.org/10.5358/hsj.24.37\u003c/li\u003e\n\u003cli\u003eMasunaga G, Kosuge T, Asai N, Ota H (2008) Shark predation of sea snakes (Reptilia: Elapidae) in the shallow waters around the Yaeyama Islands of the southern Ryukyus, Japan. 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PLoS One 12:e0179871. https://doi.org/10.1371/journal.pone.0179871\u003c/li\u003e\n\u003cli\u003eP\u0026eacute;rez-Rosales G, Rouz\u0026eacute; H, Torda G, Bongaerts P, Pichon M, Parravicini V, H\u0026eacute;douin L (2021) Mesophotic coral communities escape thermal coral bleaching in French Polynesia. R Soc Open Sci. 8: 210139. https://doi.org/10.1098/rsos.210139\u003c/li\u003e\n\u003cli\u003ePratchett MS, Hoey AS, Wilson SK, Messmer V, Graham NAJ (2011) Changes in biodiversity and functioning of reef fish assemblages following coral bleaching and coral loss. Diversity 3:424\u0026ndash;452. https://doi.org/10.3390/d3030424\u003c/li\u003e\n\u003cli\u003ePulido Mantas T, Roveta C, Calcinai B, Campanini C, Coppari M, Falco P, Di Camillo CG, Garrabou J, Lee MC, Memmola F, Cerrano C (2024) Mesophotic zone as buffer for biodiversity protection: A promising opportunity to enhance MPA effectiveness. Mar Environ Res 201:106676 https://doi.org/10.1016/j.marenvres.2024.106676\u003c/li\u003e\n\u003cli\u003eRasmussen AR, Hay-Schmidt A, Boneka F, Allentoft ME, Sanders KL, Elmberg J (2020) Viviparous sea snakes can be used as bioindicators for diverse marine environments. Philipp J Syst Biol 19:1\u0026ndash;16. https://doi.org/10.26757/pjsb2020b14013\u003c/li\u003e\n\u003cli\u003eSeymour RS (1978) Gas Tensions and blood distribution in sea snakes at surface pressure and at simulated depth. Physiol Zool 51:388\u0026ndash;407.\u003c/li\u003e\n\u003cli\u003eShine R, Shine TG, Brown GP, Goiran C (2020) Life history traits of the sea snake \u003cem\u003eEmydocephalus annulatus\u003c/em\u003e, based on a 17-yr study. Coral Reefs 39:1407\u0026ndash;1414. https://doi.org/10.1007/s00338-020-01974-y\u003c/li\u003e\n\u003cli\u003eSinniger F, Harii S, Humblet M, Nakamura Y, Ohba H, Prasetia R (2019) Ryukyu Islands, Japan. In: Mesophotic Coral Ecosystems. Springer, Cham, pp 231\u0026ndash;247\u003c/li\u003e\n\u003cli\u003eSlattery M, Lesser MP, Brazeau D, Stokes MD, Leichter JJ (2011) Connectivity and stability of mesophotic coral reefs. J Exp Mar Biol Ecol 408:32\u0026ndash;41. https://doi.org/10.1016/j.jembe.2011.07.024\u003c/li\u003e\n\u003cli\u003eSomaweera R, Udyawer V, Guinea ML, Ceccarelli DM, Clarke RH, Glover M, Hourston M, Keesing J, Rasmussen AR, Sanders K, Shine R, Thomson DP, Webber BL (2021) Pinpointing drivers of extirpation in sea snakes: a synthesis of evidence from Ashmore Reef. Front Mar Sci 8:658756. https://doi.org/10.3389/fmars.2021.658756\u003c/li\u003e\n\u003cli\u003eSomaweera R, Udyawer V, Fresnes J De, Amarasinghe AAT, Molchanova G (2023) Coordinated and communal hunting behaviours by Erabu sea krait \u003cem\u003eLaticauda semifactiata\u003c/em\u003e. Sci Rep 1\u0026ndash;7. https://doi.org/10.1038/s41598-023-48684-3\u003c/li\u003e\n\u003cli\u003eSpeed CW, Wilson NG, Somaweera R, Udyawer V, Meekan MG, Whisson C, Miller K (2022) Video surveys of sea snakes in the mesophotic zone shed light on trends in populations. Front Mar Sci 9:1\u0026ndash;10. https://doi.org/10.3389/fmars.2022.921542\u003c/li\u003e\n\u003cli\u003eSu Y, Fong SC, Tu MC (2005) Food habits of the sea snake, \u003cem\u003eLaticauda semifasciata\u003c/em\u003e. Zool Stud 44:403\u0026ndash;408.\u003c/li\u003e\n\u003cli\u003eTabata R, Tashiro F, Nishizawa H, Takagi J, Kidera N, Mitamura H (2017) Stomach contents of three sea kraits (Hydrophiinae: Laticauda spp.) in the Ryukyu Islands, Japan. Curr Herpetol 36:127\u0026ndash;134. https://doi.org/10.5358/hsj.36.127\u003c/li\u003e\n\u003cli\u003eUdyawer V, Cappo M, Simpfendorfer CA, Heupel MR, Lukoschek V (2014) Distribution of sea snakes in the Great Barrier Reef Marine Park: observations from 10 yrs of baited remote underwater video station (BRUVS) sampling. Coral Reefs 33:777\u0026ndash;791. https://doi.org/10.1007/s00338-014-1152-3\u003c/li\u003e\n\u003cli\u003eUdyawer V, Simpfendorfer CA, Heupel MR (2015) Diel patterns in three‑dimensional use of space by sea snakes. Animal Biotelemetry 3:1\u0026ndash;9. https://doi.org/10.1186/s40317-015-0063-6\u003c/li\u003e\n\u003cli\u003eVoris HK, Voris HH (1983) Feeding strategies in marine snakes: an analysis of evolutionary, morphological, behavioral and ecological relationships. Am Zool 425:411\u0026ndash;425.\u003c/li\u003e\n\u003c/ol\u003e"}],"fulltextSource":"","fullText":"","funders":[],"hasAdminPriorityOnWorkflow":false,"hasManuscriptDocX":true,"hasOptedInToPreprint":true,"hasPassedJournalQc":"","hasAnyPriority":false,"hideJournal":false,"highlight":"","institution":"","isAcceptedByJournal":true,"isAuthorSuppliedPdf":false,"isDeskRejected":"","isHiddenFromSearch":false,"isInQc":false,"isInWorkflow":false,"isPdf":false,"isPdfUpToDate":true,"isWithdrawnOrRetracted":false,"journal":{"display":true,"email":"[email protected]","identity":"marine-biology","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":false,"externalIdentity":"mabi","sideBox":"Learn more about [Marine Biology](https://www.springer.com/journal/227)","snPcode":"227","submissionUrl":"https://submission.nature.com/new-submission/227/3","title":"Marine Biology","twitterHandle":"","acdcEnabled":true,"dfaEnabled":true,"editorialSystem":"em","reportingPortfolio":"Springer Hybrid","inReviewEnabled":true,"inReviewRevisionsEnabled":false},"keywords":"Diving behavior, Foraging, Remotely operated vehicles, Sea snakes, Laticauda, Hydrophis","lastPublishedDoi":"10.21203/rs.3.rs-7818285/v1","lastPublishedDoiUrl":"https://doi.org/10.21203/rs.3.rs-7818285/v1","license":{"name":"CC BY 4.0","url":"https://creativecommons.org/licenses/by/4.0/"},"manuscriptAbstract":"\u003cp\u003eMarine snakes in the Family Elapidae are key mesopredators in tropical and subtropical Indo-Pacific ecosystems. While some species have been recorded in the mesophotic zone (30\u0026ndash;150 m) and in even deeper water, such observations remain rare and limit our understanding of habitat use and the ecological role of these snakes. We present 15 observations of marine elapids at depths\u0026thinsp;\u0026gt;\u0026thinsp;50 m in the Ryukyu Islands of southwestern Japan, obtained through rebreather diving and remotely operated vehicles. Black-banded Sea Kraits (\u003cem\u003eLaticauda semifasciata\u003c/em\u003e) were observed at depths of 50\u0026ndash;112 m across both hard and soft substrates (\u003cem\u003en\u003c/em\u003e\u0026thinsp;=\u0026thinsp;13), and one Blue-lipped Sea Krait (\u003cem\u003eL. laticaudata\u003c/em\u003e) was observed at 122 m depth. Four of the observed \u003cem\u003eL. semifasciata\u003c/em\u003e exhibited foraging behavior. Additionally, one Ornate Reef Sea Snake (\u003cem\u003eHydrophis ornatus\u003c/em\u003e) was recorded at 95 m. These represent the deepest known records for all three species. As benthic foragers, these marine elapids may play a role in nutrient transfer between deep and shallow habitats. Furthermore, deep-water environments may serve as foraging refugia, offering protection from predators such as sharks, as well as the impacts of marine heatwaves and coastal development.\u003c/p\u003e","manuscriptTitle":"Observations of marine elapid snakes in the mesophotic zone in the Ryukyu Islands, Japan, with the deepest records for three species","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2025-11-10 09:27:01","doi":"10.21203/rs.3.rs-7818285/v1","editorialEvents":[{"type":"communityComments","content":0},{"type":"decision","content":"Acceptable after minor revision","date":"2025-12-15T04:37:49+00:00","index":"","fulltext":""},{"type":"reviewerAgreed","content":"","date":"2025-11-02T07:23:00+00:00","index":0,"fulltext":""},{"type":"reviewersInvited","content":"","date":"2025-10-30T11:33:13+00:00","index":"","fulltext":""},{"type":"editorAssigned","content":"","date":"2025-10-10T08:37:57+00:00","index":"","fulltext":""},{"type":"submitted","content":"Marine Biology","date":"2025-10-09T10:16:12+00:00","index":"","fulltext":""}],"status":"published","journal":{"display":true,"email":"[email protected]","identity":"marine-biology","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":false,"externalIdentity":"mabi","sideBox":"Learn more about [Marine Biology](https://www.springer.com/journal/227)","snPcode":"227","submissionUrl":"https://submission.nature.com/new-submission/227/3","title":"Marine Biology","twitterHandle":"","acdcEnabled":true,"dfaEnabled":true,"editorialSystem":"em","reportingPortfolio":"Springer Hybrid","inReviewEnabled":true,"inReviewRevisionsEnabled":false}}],"origin":"","ownerIdentity":"3c188c84-525a-463b-90d9-b61f157313b3","owner":[],"postedDate":"November 10th, 2025","published":true,"recentEditorialEvents":[],"rejectedJournal":[],"revision":"","amendment":"","status":"published-in-journal","subjectAreas":[],"tags":[],"updatedAt":"2026-02-16T16:05:19+00:00","versionOfRecord":{"articleIdentity":"rs-7818285","link":"https://doi.org/10.1007/s00227-026-04796-8","journal":{"identity":"marine-biology","isVorOnly":false,"title":"Marine Biology"},"publishedOn":"2026-02-13 15:57:04","publishedOnDateReadable":"February 13th, 2026"},"versionCreatedAt":"2025-11-10 09:27:01","video":"","vorDoi":"10.1007/s00227-026-04796-8","vorDoiUrl":"https://doi.org/10.1007/s00227-026-04796-8","workflowStages":[]},"version":"v1","identity":"rs-7818285","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-7818285","identity":"rs-7818285","version":["v1"]},"buildId":"8U1c8b4HqxoKbykW_rLl7","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}