Yuuki Y. Watanabe

Buoyancy is an omnipresent force that governs how aquatic animals move through three-dimensional space, leading to physiological and behavioural adaptions across species. Swimming effort during vertical movements is increasingly being used to infer buoyancy in animals that exhibit oscillatory diving behaviours and for which collecting direct measurements of buoyancy is infeasible. Here, we estimated the relative buoyancy of 29 Greenland sharks (Somniosus microcephalus) tagged with biologgers by comparing swimming effort [overall dynamic body acceleration (ODBA)/speed] during ascending and descending dives, and accounting for dive geometry. We found that buoyancy in Greenland sharks varied on both sides of the spectrum, with effort in the counter-buoyancy direction reaching as high as 225% of the effort exerted in the buoyancy-aided direction in negatively buoyant individuals, and up to 177% for positively buoyant individuals. Despite this, there was no overall bias towards positive or negative buoyancy for this species, supporting the expectation that slow-moving deep-sea species face selective pressure towards neutral buoyancy. Given that gliding behaviour was rare and that small deviations from neutral buoyancy could drive large differences in effort for a minimally active species, more research is needed to determine the exact magnitude of the buoyancy variation observed here. Even so, its impact on swimming efficiency probably regulates movement and foraging strategies, as sharks are forced to either cope with, or exploit, their tendency to sink or float. These findings also highlight the need for studies directly linking biologger-inferred buoyancy to true buoyancy, with implications for the study of body condition in sharks.

Cruising speed is a key factor affecting prey-search efficiency and migration range in continuously swimming animals. Tunas and lamnid sharks (e.g., white sharks) have convergently evolved traits for high-speed cruising, including the ability to maintain slow-twitch, aerobic red muscle (RM) warmer than ambient water, known as RM endothermy. Despite their well-known high cruising speeds, kinematic features underlying their elevated speeds remain unclear. Swim speed is the product of tailbeat frequency (TBF; Hz) and stride length (SL, the absolute distance traveled per tailbeat; m). RM endothermy is expected to elevate TBF by enhancing muscle contraction performance. Furthermore, within RM-endothermic fishes, tunas and lamnid sharks may exhibit distinct kinematic features because of differences in caudal fin morphology and tailbeat amplitude. Here, we compiled kinematic parameters from 20 fish species, including five RM-endothermic species, measured in the wild using animal-borne sensors. Comparative analyses showed that, for a given body mass and water temperature, RM-endothermic fishes exhibited 1.9 times higher cruising speed and TBF than ectothermic fishes, while SL remained similar. Within RM-endothermic fishes, tunas exhibited 2.3 times higher TBF than similar-sized lamnid sharks, whereas lamnid sharks showed 1.7 times longer SL than similar-sized tunas. These results indicate that RM endothermy is generally associated with higher TBF, while significant kinematic differences remain between tunas and lamnid sharks. This divergence may be partly explained by the greater caudal fin area and tailbeat amplitude in lamnid sharks. It may also reflect contrasting skeletal types of teleosts and elasmobranchs, which potentially influence body stiffness and swimming kinematics.

Incidental capture of sharks by fisheries is common, making it critical to assess species’ physiological and behavioural stress responses and post-release survival. Previous studies have examined blood chemistry, post-release behaviour or long-term survival in various bycatch species; however, few have combined these approaches within a single species. Here, we integrated hook timers, blood chemistry, accelerometry and acoustic and satellite telemetry to assess the impacts of longline capture on Greenland sharks Somniosus microcephalus, a deep-sea species encountered in Arctic fisheries. Due to extremely slow growth and late maturity, this species is considered vulnerable to anthropogenic impacts. Lactate and glucose concentrations were within normal ranges for related species and lower than most species. Following release, individuals exhibited reduced vertical movement with elevated swim speed and tailbeat frequency, returning to baseline behaviour after ~ 10 h. Recovery duration was positively correlated with lactate concentration, but not with time hooked. Acoustic and satellite tracking confirmed survival for > 30 days in 9 of 10 tagged individuals, with evidence of multiyear survival up to 6.7 years. The remaining individual moved offshore over 4 days, but long-term survival could not be confirmed. In addition, one shark that was tail-wrapped during capture, for which tracking data were unavailable, may have experienced immediate post-release mortality. Overall, our findings indicate that Greenland sharks are resilient to longline capture stress when properly handled. This resilience likely reflects their slow swim speed and low metabolic rate associated with cold environments, their limited fight response when hooked, and the narrow vertical temperature range of Arctic waters, which minimizes thermal stress during capture. We emphasize the importance of appropriate handling practices and recommend further monitoring of Greenland shark post-release survival from (i) longline fisheries by capture mode (i.e. mouth-hooked, body-tangled or tail-wrapped) and (ii) other fishing gears such as trawl and gillnet.

Body size and temperature set metabolic rates and the pace of life, yet our understanding of the energetics of large fishes is uncertain, especially of warm-bodied mesotherms, which can heavily influence marine food webs. We developed an approach to estimate metabolic heat production in fishes, revealing how routine energy expenditure scales with size and temperature from 1-milligram larvae up to 3-tonne megaplanktivorous sharks. We found that mesotherms use approximately four times more energy than ectotherms use and identified a scaling mismatch in which rates of heat production increase faster than heat loss as body size increases, with larger fish becoming increasingly warm bodied. This scaling imbalance creates an overheating predicament for large mesotherms, helping to explain their cooler biogeographies. Contemporary mesotherms face high fuel demands and overheating risks, which is a concern given their disproportionate demise during prior climate shifts.

Background: Understanding how predatory animals efficiently locate prey with limited knowledge of its location is challenging. Optimal foraging theory suggests that animals improve their food intake through experience-based adjustments of search patterns. For example, animals feeding on clustered prey may repeatedly search successful areas and move farther away when unsuccessful (the ‘win-stay, lose-shift’ strategy). A related concept, area-restricted search, predicts that animals initially search broadly and then switch to a more localized, tortuous search upon finding clustered prey. However, few studies have empirically supported these predictions for large aquatic predators due to difficulties in recording their foraging success on known prey species. Methods: We used biologging techniques to record the fine-scale foraging behaviour of Baikal seals in Lake Baikal, which hunt tiny, clustered, planktonic amphipods at high rates. We reconstructed their three-dimensional movement paths during dives and estimated the timing of prey capture events based on video-validated body acceleration data. Results: Seals moved shorter horizontal distances and exhibited greater directional changes after more successful dives, supporting the ‘win-stay, lose-shift’ strategy. Consistent with area-restricted search, successful foraging dives led to decreased speed and increased tortuosity in the horizontal plane. Conclusions: These findings suggest that experience-based behavioural adjustments at a dive-to-dive scale are crucial for Baikal seals—and possibly other large aquatic predators—to maintain high foraging rates. Furthermore, they illustrate how an exceptionally high predator-prey body mass ratio (> 500,000) for a single-prey-feeding (non-filter-feeding) predator is maintained in the unique Lake Baikal ecosystem.

Some large, wide‐ranging teleosts and elasmobranchs are converged to have regional endothermy, retaining metabolic heat via vascular countercurrent heat exchangers. Yet, their adaptive significance remains debated. While previous studies proposed potential benefits of elevated body temperature, enhanced controllability of body temperature enabled by heat exchangers may also be important. Some endothermic teleosts (e.g. bigeye tuna) alter rates of body temperature change depending on dive phases to maximize foraging time in deep, cold waters while minimizing recovery time in shallow, warm waters. However, whether endothermic elasmobranchs possess similar abilities remains unclear. Using animal‐borne tags, we recorded diving behaviours and muscle temperatures of shortfin mako sharks, a possible elasmobranch equivalent to bigeye tuna. Warming and cooling rates were estimated with a heat exchange model. Further, we conducted literature‐based, phylogenetically informed comparative analyses of heat exchange rates across 25 fish species (mass range, 0.01–1600 kg). All four mako sharks repeatedly dived below the thermocline with ambient temperature changes of up to 7–14°C. On average, muscle temperatures were 1.5–3.9°C warmer than the ambient water. Two individuals dived deep (up to 286–327 m) and showed a 14–47 times higher warming rate than cooling rate, whereas the other two individuals that dived shallowly exhibited one to two times differences. One individual warmed its muscle above sea surface temperature before a deep dive, possibly preparing for the coming deep excursion using internal heat sources. Comparative analyses showed that the ratio of warming to cooling rate and its range across individuals was larger in endothermic bigeye tuna, swordfish and mako sharks than in most other fishes. Our results demonstrate that enhanced temperature controllability has convergently evolved among some endothermic teleosts and elasmobranchs that inhabit low‐to‐middle latitude waters with strong thermal gradients. By contrast, some other endothermic species (e.g. salmon sharks and Atlantic bluefin tuna) that migrate to subpolar waters are specialized for body temperature elevation. We propose that the controllability and elevation of body temperature have different adaptive significance, reflecting species' habitats and foraging ecology. Our findings help explain the diversity and success of endothermic fishes as apex predators across the world's pelagic oceans.

The recent Kunming-Montreal Global Biodiversity Framework (GBF) sets ambitious goals but no clear pathway for how zero loss of important biodiversity areas and halting human-induced extinction of threatened species will be achieved. We assembled a multi-taxa tracking dataset (11 million geopositions from 15,845 tracked individuals across 121 species) to provide a global assessment of space use of highly mobile marine megafauna, showing that 63% of the area that they cover is used 80% of the time as important migratory corridors or residence areas. The GBF 30% threshold (Target 3) will be insufficient for marine megafauna’s effective conservation, leaving important areas exposed to major anthropogenic threats. Coupling area protection with mitigation strategies (e.g., fishing regulation, wildlife-traffic separation) will be essential to reach international goals and conserve biodiversity.

Global climate‐driven ocean warming has decreased dissolved oxygen (DO) levels (ocean deoxygenation) leading to expansions of hypoxic zones, which will affect the movements, behaviour, physiology and distributions of marine animals. However, the precise responses of animals to low DO remains poorly understood because movements and activity levels are seldom recorded alongside instantaneous DO in situ. We describe a new animal‐attached (dissolved oxygen measuring, DOME) archival tag with an optical oxygen sensor for recording DO, in addition to sensors for temperature and depth, a triaxial accelerometer for fine‐scale movements and activity, and a GPS for tag recovery. All sensors were integrated on a single electronic board. Calibration tests demonstrated small mean difference between DOME tag and factory‐calibrated DO sensors (mean relative error of 5%). No temporal drift occurred over a test period three times longer than the maximum deployment time. Deployments on four blue sharks (Prionace glauca) in the central North Atlantic Ocean showed regular vertical oscillations from the surface to a maximum of 404 m. Profiles from diving sharks recorded DO concentrations ranging from 217 to 272 μmol L⁻¹, temperatures between 13°C and 23°C, and identified an oxygen maximum at ~45 m depth, all of which were consistent with ship‐based measurements. Interestingly, the percentage of time sharks spent burst swimming was greater in the top 85 m compared to deeper depths, potentially because of higher prey availability in the surface layer. The DOME tag described blue shark fine‐scale movements and activity levels in relation to accurately measured in situ DO and temperature, with the potential to offer new insights of animal performance in low oxygen environments. Development of a tag with physico‐chemical and movement sensors on a single electronic board is a first step towards satellite relay of these data over broader spatiotemporal scales (months over thousands of kilometres) to determine direct and indirect responses of marine animals to heatwave and deoxygenation events.

Megamouth sharks Megachasma pelagios Taylor, Compagno, and Struhsaker, 1983, are a large-bodied, planktivorous, deep-sea species with peculiar morphology. Since their initial description in the late 20th century, many individuals of different sizes have been reported, but few studies examined ontogenetic changes in body shapes. Here, we assess the growth changes in their heads and fins based on length measurements from nine different-sized individuals (177–544 cm in total length). Bivariate analyses showed that the head becomes larger relative to body length with increasing body size (i.e., positive allometry), whereas the relative size of the caudal fin remains constant (i.e., isometric growth). This trend differs from basking sharks and apparently resembles whale sharks and some baleen whales, although they are all large-bodied filter feeders. Given that relative mouth size is linked to feeding modes, our results suggest that megamouth sharks have different feeding modes from ram-feeding basking sharks and may have some similarity with suction-feeding whale sharks and engulfment-feeding baleen whales.