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Fin

A fin is a thin, membranous appendage extending from the body of many aquatic animals, particularly fish, consisting of skin supported by bony or cartilaginous rays or spines and controlled by underlying muscles; it serves essential roles in locomotion, balance, steering, and sensory functions.^[1] Fins are classified into unpaired and paired types, with unpaired fins including the dorsal fin (for stability and protection), caudal fin (for propulsion), and anal fin (for steering and balance), while paired fins comprise the pectoral and pelvic fins (analogous to limbs in tetrapods, aiding in maneuvering and braking).^[2]^[3] In fish anatomy, fins exhibit diverse structures adapted to specific environments and behaviors; for instance, the caudal fin's shape varies from forked in fast-swimming species to rounded in more maneuverable ones, directly influencing hydrodynamic efficiency.^[4] Some species, like certain salmonids, possess an adipose fin—a small, fleshy dorsal structure without rays—whose exact function remains under study but may aid in sensory perception or stability.^[2] Fin development in teleost fish, such as the pectoral fin, mirrors evolutionary patterns seen in vertebrate limbs, originating from fin folds in embryonic stages and involving complex vascular and skeletal formation.^[5] Beyond locomotion, fins play critical roles in reproduction, display, and defense; males of many species use elongated fins for courtship rituals, while spines in fins of predatory fish like sharks deter attackers.^[1] Evolutionary adaptations have led to fin loss or modification in some aquatic vertebrates, such as eels, where reduced fins prioritize undulatory swimming over fin-based propulsion.^[3]

Anatomy and Types

Structure of Fins

Fins in aquatic animals, particularly fish, are primarily composed of a thin membrane or web of skin that stretches between supportive skeletal elements, enabling flexibility and interaction with water. In ray-finned fish (Actinopterygii), these supports are fin rays known as lepidotrichia, which are dermal structures formed from two opposing hemirays that articulate at overlapping joints, creating segmented, flexible rays capable of independent movement.^[6]^[1] In contrast, chondrichthyans such as sharks and rays feature fins supported by cartilaginous radials—elongated, segmented elements that radiate from the fin base—along with fibrous ceratotrichia that provide additional rigidity without bony ossification.^[7]^[8] The key structural components of fins include the leading edge, which faces the direction of motion and may incorporate stiff spines for reinforcement or protection in certain species; the trailing edge, where the membrane tapers to minimize drag; and the base, which attaches to the body via muscles, ligaments, and skeletal girdles such as the pectoral or pelvic structures, without direct articulation to the axial skeleton in most cases.^[3]^[1] Variations in these components, such as anterior spines on the leading edge, enhance durability while maintaining flexibility through the membranous webbing.^[7] At the microscopic level, fin surfaces are richly vascularized with blood vessels that supply nutrients and oxygen to the tissues, intertwined with nerves that innervate sensory receptors and control musculature for precise movements.^[9] In bony fish, the skin covering includes overlapping scales—typically cycloid or ctenoid—that reduce friction and protect underlying structures, while shark fins bear dermal denticles, tooth-like placoid scales embedded in the dermis that provide abrasion resistance and hydrodynamic benefits.^[9]^[10] For example, the dorsal fin in sharks consists of a rigid, triangular membrane supported by cartilaginous radials and often an anterior spine, with the entire surface clad in dermal denticles for enhanced toughness.^[7]^[10] In comparison, the dorsal fin of ray-finned fish forms flexible, fan-like lobes through arrays of lepidotrichia that allow undulating motion, covered by smooth scales that facilitate seamless water flow.^[6]^[3]

Classification of Fins

Fins in vertebrates are broadly classified into median and paired types based on their anatomical position and developmental origins. Median fins are unpaired structures located along the body's midline, including the dorsal fin on the back, the anal fin on the ventral surface anterior to the tail, and the caudal fin at the posterior end. These fins typically consist of soft rays supported by fin membranes and contribute to overall body orientation.^[3] Paired fins, in contrast, occur symmetrically on either side of the body and include the pectoral fins positioned behind the gills and the pelvic fins located ventrally further posterior. These structures are evolutionarily homologous to the limbs of tetrapods, sharing developmental pathways involving similar genetic controls such as Hox genes.^[11]^[3] From an evolutionary perspective, fins are further categorized by their structural composition and phylogenetic distribution within bony fishes (Osteichthyes). Ray-finned fishes (Actinopterygii), comprising over 30,000 species, feature fins supported by slender, unbranched or branched lepidotrichia (fin rays), enabling flexible and lightweight appendages. In contrast, lobe-finned fishes (Sarcopterygii), a smaller group including coelacanths and lungfishes, possess fleshy fins with robust internal bones resembling limb precursors, often lacking extensive ray support. Some species exhibit vestigial fins, such as in certain eels (e.g., swamp eels in Synbranchidae), where pectoral and pelvic fins are absent and dorsal/anal fins are reduced to rudimentary ridges.^[12]^[13]^[14] Fin morphology varies significantly across vertebrate taxa, reflecting adaptations to aquatic lifestyles. In teleost fishes, caudal fins often display homocercal symmetry, with equal upper and lower lobes for balanced propulsion in advanced species like tuna. Primitive fishes, such as sharks and early bony fishes, typically have heterocercal caudal fins, where the upper lobe is enlarged and the vertebral column extends into it, aiding lift in ancestral forms. Cetaceans, fully aquatic mammals, retain a dorsal fin for stability, as seen in dolphins and whales, while their paired appendages are modified into flippers without true fin rays. Ichthyosaurs, extinct marine reptiles, evolved paddle-like paired fins from forelimbs and possible dorsal fins for streamlined swimming. In amphibians and reptiles, fins are generally absent, replaced by limbs derived from paired fin homologues during the fin-to-limb transition, though some aquatic forms like sea turtles exhibit flipper modifications.^[15]^[16]^[17]

Primary Functions in Locomotion

Thrust Generation

Fins generate forward propulsion in aquatic animals primarily through oscillatory motions that exploit hydrodynamic interactions with water, producing net thrust forces directed rearward on the fluid and forward on the body. The caudal fin is the dominant structure for this in most fishes, oscillating laterally to shed vortices into the wake, forming linked vortex rings or loops that create a reactive jet propelling the animal forward.^[18] This vortex shedding mechanism is evident in the wakes of freely swimming fishes, where each tail beat expels coherent vortical structures, enhancing momentum transfer.^[19] In undulatory swimmers, pectoral fins contribute to thrust via coordinated flapping or rowing motions that interact with body waves, recapturing wake vortices to boost overall efficiency.^[20] Hydrodynamic thrust arises from two key principles: lift-based generation and reactive forces from fluid displacement. In lift-based propulsion, fins function as dynamic airfoils, with oscillatory angles of attack creating pressure differences across the fin surface; faster flow over the curved upper surface lowers pressure per Bernoulli's principle, yielding upward lift that, when inclined, provides rearward thrust.^[21] Reactive forces, meanwhile, stem from the inertia of water accelerated by the fin's motion, producing a counterforce via momentum change as fluid is displaced laterally or posteriorly during beats.^[22] These mechanisms combine in most species, with vortex dynamics—such as leading-edge vortices stabilizing flow—amplifying lift and thrust during the oscillatory cycle.^[23] Efficiency in thrust generation depends on fin morphology and kinematics, notably the aspect ratio (span squared divided by area, or length-to-width) and stroke phasing. High-aspect-ratio caudal fins, as in fast cruisers, minimize induced drag while maximizing lift for sustained high speeds, achieving efficiencies up to 60-70% in cruising.^[24] Low-aspect-ratio fins favor maneuverability but at the cost of cruising efficiency. In flapping propulsion, the power stroke (typically the downstroke or high-angle phase) generates most thrust through strong vortex shedding, while the recovery stroke feathers the fin to reduce drag; at higher speeds, both strokes become active, with upstrokes contributing via reversed angles of attack.^[25] Swimming modes illustrate these principles' variations: thunniform locomotion in tuna relies on high-frequency, low-amplitude caudal oscillations for thrust-dominated propulsion, enabling speeds over 10 body lengths per second with body stability.^[26] Conversely, anguilliform mode in eels uses full-body undulations propagating as waves, with the tail amplifying thrust through vortex rings but distributing effort for flexibility in low-speed environments.^[26] The magnitude of thrust can be quantified using the equation

T = \frac{1}{2} \rho v^2 A C_T

where T is thrust force, \rho is water density, v is the fin's relative velocity through water, A is effective fin area, and C_T is the dimensionless thrust coefficient encapsulating shape, kinematics, and flow effects (typically 0.2-1.0 for efficient swimmers).^[27] This formulation highlights how thrust scales with dynamic pressure and geometry, guiding evolutionary adaptations for speed versus efficiency.^[28]

Motion Control

Fins facilitate steering in aquatic locomotion primarily through the asymmetric deployment of paired structures, allowing fish to generate differential hydrodynamic forces for directional changes. Pectoral and pelvic fins, positioned laterally, enable control over yaw (lateral turning), pitch (nose-up or nose-down adjustments), and roll (tilting along the longitudinal axis) by varying their angle of attack or oscillation amplitude on one side relative to the other.^[1] In bluegill sunfish (Lepomis macrochirus), for instance, pectoral fins actively modulate these rotational degrees of freedom to maintain precise orientation during steady swimming or evasion maneuvers.^[29] Dorsal and anal fins, located medially, contribute to yaw stability by acting as passive keels that resist unwanted lateral deviations, particularly during high-speed travel.^[1] Stability during swimming is achieved through strategic fin placement that dampens oscillatory motions and balances hydrodynamic forces. Fins positioned posterior to the center of gravity (COG) shift the center of pressure (COP)—the point where net hydrodynamic force acts—aft of the COG, promoting inherent static stability and preventing uncontrolled tumbling or rolling.^[30] This configuration generates restoring moments that counteract perturbations, such as currents or sudden accelerations, while drag from fin surfaces further damps dynamic oscillations in pitch and yaw.^[30] In sharks, for example, the pectoral fins' forward positioning relative to the caudal fin helps trim the body by adjusting the COP to offset the upward lift from the heterocercal tail, ensuring level progression.^[1] Fins enhance maneuverability by enabling rapid, high-agility turns distinct from linear propulsion. In many coral-reef fishes, pectoral fins facilitate tight turns with radii as small as 0.2–0.5 body lengths through independent flapping or feathering, allowing precise navigation amid complex habitats.^[31] Larger cetaceans, such as humpback whales (Megaptera novaeangliae), employ elongate pectoral fins to generate lift during acrobatic maneuvers like breaching, where asymmetric fin extension provides the necessary torque for body rotation and reorientation out of water.^[32] Sensory feedback from fins integrates with motor control for real-time adjustments during motion. Fin rays contain arrays of mechanosensory neurons that detect bending, pressure, and flow, functioning as proprioceptors to monitor fin position and external loads relative to the body.^[33] This proprioceptive input allows fish to reflexively modulate fin kinematics, such as altering stroke amplitude to correct for drag imbalances during turns, thereby maintaining equilibrium without relying solely on visual or lateral line cues.^[33] The effectiveness of fin-based turning is quantitatively linked to the moment arm of the applied force, influencing the minimum achievable turning radius. Torque (\tau) generated by a fin is given by \tau = r \times F, where r is the perpendicular distance (lever arm) from the rotation axis (typically near the COG) to the line of force F produced by the fin.^[34] Longer moment arms, as in elongated pectoral fins, amplify torque for tighter radii, enabling agile species like reef fish to execute turns with radii under 0.3 body lengths, while shorter arms in streamlined swimmers prioritize speed over sharp maneuvers.^[35]

Secondary Biological Roles

Temperature Regulation

Fins facilitate thermoregulation in aquatic animals primarily through their high surface-to-volume ratio, which enhances heat exchange with the surrounding water, acting as efficient radiators for both heat gain and loss.^[36] In species with specialized vascular arrangements, countercurrent heat exchange in fin blood vessels allows arterial blood to warm cooler venous blood returning from the periphery, thereby conserving metabolic heat and minimizing passive loss to colder environments.^[37] This mechanism is particularly vital in maintaining thermal gradients, where the fin's vascular structure—briefly referenced in anatomical studies—enables precise control over heat transfer.^[37] For ectothermic aquatic animals, such as most fish, fins play a key role in absorbing environmental heat to elevate body temperature above ambient levels. During basking behaviors, species like common carp (Cyprinus carpio) position their fins near the water surface to capture solar radiation, resulting in body temperatures 0.7–2.2°C warmer than surrounding water and supporting faster growth rates.^[38] Similarly, basking sharks (Cetorhinus maximus), traditionally viewed as ectotherms, exhibit surface-oriented behaviors that expose fins to warmer waters, aiding initial heat uptake despite their emerging regional endothermic traits.^[39] In endothermic species like tunas (Thunnus spp.), adaptations such as intricate vascular patterns enable controlled cooling to counteract overheating from elevated metabolic rates. These fish can rapidly adjust whole-body thermal conductivity by up to two orders of magnitude, to balance internal heat production with environmental conditions.^[40] Vascular countercurrent systems further prevent excessive heat loss during dives into cooler waters while allowing dissipation when needed.^[41] Physiological mechanisms in fin tissues, including vasodilation to increase blood flow for enhanced cooling and vasoconstriction to reduce flow for heat retention, help maintain core temperature gradients across varying activity levels.^[36] Environmental water temperature significantly influences these processes, with colder conditions triggering reduced fin circulation rates to conserve heat and warmer waters promoting increased flow to avert hyperthermia. In tunas, such adjustments ensure stable muscle temperatures despite ambient shifts of 10–15°C.^[41]

Ornamentation and Sensory Uses

In many fish species, fins serve ornamental roles through vibrant coloration and exaggerated shapes that enhance mating displays. Male guppies (Poecilia reticulata) exhibit polymorphic coloration on their caudal fins, with orange, black, and iridescent patches that are actively displayed during sigmoid courtship behaviors to attract females.^[42] These ornaments signal genetic quality and are preferred by females, influencing mate choice.^[43] Similarly, in threadfin rainbowfish (Iriatherina werneri), males possess elongated, filamentous dorsal and anal fins that are flared during both courtship and intrasexual competitions, amplifying visual appeal.^[44] Fins also facilitate threat postures and aggressive signaling. Triggerfish (family Balistidae), such as the gray triggerfish (Balistes capriscus), can erect and lock the spines of their first dorsal fin into a rigid position when threatened, deterring predators by anchoring themselves in crevices or displaying an imposing silhouette.^[45] In poeciliid fishes like sailfin mollies (Poecilia latipinna), males extend their enlarged dorsal fins during confrontations to intimidate rivals, a behavior linked to establishing dominance.^[46] Beyond static ornamentation, fins play key roles in dynamic communication. Fin flicking and flaring serve as visual signals for aggression and courtship across species; for instance, male guppies flare their fins in circular swims to court females, while in Siamese fighting fish (Betta splendens), opercular and fin flaring escalates during male-male aggression to assess opponent intent. In characin fishes like the glowlight tetra (Hemigrammus erythrozonus), rapid fin flicking acts as an alarm signal to conspecifics, indicating predator detection and prompting evasive responses.^[47] Deep-sea species further employ bioluminescence in fins for signaling; viperfish (Chauliodus sloani) possess a glowing lure on their dorsal fin ray, which may facilitate mate attraction or species recognition in low-light environments.^[48] Fins contribute to sensory functions through specialized receptors that detect environmental cues. Paired fins in damselfishes (family Pomacentridae) bear extraoral taste buds, enabling short-range chemoreception to identify prey or suitable habitats via dissolved chemicals in water.^[49] These gustatory structures, integrated with the fish's lateral line system, allow precise localization of food sources. In sharks and other elasmobranchs, the ampullae of Lorenzini—electroreceptive pores concentrated around the head—detect weak bioelectric fields from prey muscle contractions, aiding hunting even in murky waters.^[50] Camouflage via fins involves adaptive patterns generated by chromatophores, pigment cells that enable rapid color shifts. Flatfishes like the peacock flounder (Bothus lunatus) use dermal chromatophores to match fin patterns to sandy or rocky substrates, reducing visibility to predators through disruptive coloration.^[51] In reef fishes such as wrasses, fin chromatophores produce mottled or banded patterns that blend with coral environments, with neural control allowing instantaneous adjustments to background changes.^[52] Peacock-like displays, seen in male peacock gudgeons (Tateurndina ocellicauda), contrast this by temporarily overriding camouflage for courtship, flaring iridescent fins to reveal bold spots and edges.

Evolutionary Development

Origins of Fins

Fins originated in early chordates during the Cambrian explosion, with median fins evolving as dorsal and ventral structures from a continuous median fin fold around 535 million years ago, providing stability and propulsion in primitive swimming.^[53] Fossils from the Middle Cambrian Burgess Shale, such as Pikaia gracilens, reveal these early chordates possessed a notochord-supported body with tail structures resembling simple fins, enabling undulatory locomotion through myomere contractions.^[54] This transition from notochord-based support to fin-augmented structures marked a key step in vertebrate swimming efficiency, predating more specialized appendages.^[55] Paired fins emerged later in the Silurian period, approximately 420 million years ago, among jawless vertebrates known as ostracoderms, particularly in osteostracans, where pectoral fin-like extensions arose from the pectoral girdle for enhanced maneuverability.^[56] These structures were homologous to the limb girdles in later vertebrates, consisting of dermal and endoskeletal elements that anchored to the body wall.^[57] In placoderms, the first jawed vertebrates during the Devonian, fins underwent elaboration with the addition of paired pelvic fins supported by internal girdles, increasing diversity in locomotion and body control.^[58] However, paired fins were subsequently lost in certain jawless lineages, such as modern lampreys and hagfish, where regulatory genes like Tbx5 fail to extend expression into the lateral plate mesoderm necessary for appendage initiation.^[59] Developmentally, fin origins involve conserved genetic mechanisms, with Hox genes regulating the positioning and patterning of fin buds during embryogenesis in fish.^[60] In zebrafish, posterior Hoxa and Hoxd cluster genes display tri-phasic expression in pectoral fin mesenchyme, initiating bud formation and proximal-distal outgrowth akin to early limb development.^[60] The apical ectodermal ridge (AER), a thickened ectodermal fold at the fin bud's distal margin, secretes signaling molecules like Fgf to sustain mesenchymal proliferation and prevent apoptosis, ensuring proper fin elongation.^[61] Comparative embryology highlights the homology between fish fins and tetrapod limbs, particularly in sarcopterygians like lungfish and coelacanths, where shared Hox expression domains and AER-like structures underpin the fin-to-limb transition around 390 million years ago.^[62] In these lobe-finned fish, fin endoskeletons exhibit proximodistal patterning similar to limb buds, with genetic modules enabling the evolutionary innovation of digits from fin radials.^[63] This developmental framework illustrates how fins provided the scaffold for terrestrial appendage evolution without altering core patterning genes.^[62]

Adaptations and Diversity

Fins exhibit remarkable morphological and functional diversity shaped by environmental pressures, enabling species to exploit varied aquatic niches. In fast-swimming pelagic fish such as tunas and mackerels, fins are streamlined and fusiform to minimize hydrodynamic drag, facilitating sustained high-speed locomotion through open water.^[64] Conversely, bottom-dwelling species like sea robins (Prionotus spp.) possess enlarged pectoral fins that aid in substrate manipulation, camouflage, and slow maneuvering over benthic environments.^[65] This contrast highlights how fin shape correlates with habitat demands, with pelagic forms prioritizing efficiency and demersal ones emphasizing stability and utility.^[66] A striking example of specialized environmental adaptation is the ribbon-like anal fin in gymnotiform knifefishes (e.g., Eigenmannia spp.), which integrates myogenic electric organs derived from modified muscle tissue to generate electric fields for navigation and communication in murky freshwater habitats.^[67] These electric fin rays, comprising up to 150 undulating segments, produce weak discharges (around 1-10 V) that support electrolocation without relying on traditional vision.^[68]^[69] Such innovations underscore the evolutionary plasticity of fins beyond mere propulsion, adapting to sensory challenges in low-visibility ecosystems.^[70] Beyond fish, fin-like structures have convergently evolved in other vertebrates, demonstrating broad adaptive radiation. In cetaceans like dolphins (Delphinidae), flukes represent modified tail structures analogous to caudal fins, providing thrust via oscillatory movements while the body maintains streamlining for efficient cruising.^[71] Similarly, penguins (Spheniscidae) have transformed forelimbs into rigid, flattened flippers derived from avian wings, optimized for rapid underwater propulsion through reduced drag and enhanced lift in polar marine environments.^[72] These non-homologous appendages illustrate how selective pressures for aquatic locomotion can repurpose diverse anatomical precursors across taxa.^[73] Sexual dimorphism further diversifies fin morphology, often driven by reproductive competition. In many species, males develop elaborate, elongated fins—such as the extended dorsal and anal fins in male guppies (Poecilia reticulata)—to attract females or intimidate rivals during courtship displays.^[74] Coral reef fishes exhibit polymorphic fins, where color and shape variations (e.g., in labrids like Thalassoma bifasciatum) signal alternative mating strategies, with terminal-phase males sporting brighter, larger fins for harem defense.^[75] These traits enhance mating success but impose energetic costs, balancing display with survival.^[76] Pathological alterations reveal fins' vulnerability to external stressors and regenerative potential. Teleost fishes, including zebrafish (Danio rerio), demonstrate robust fin regeneration, restoring amputated caudal fins to near-original size within 2-3 weeks via blastema formation and signaling pathways like Wnt/β-catenin.^[77] This capacity, conserved across actinopterygians, involves coordinated proliferation of epidermal and mesenchymal cells.^[78] However, pollution induces deformities; exposure to contaminants like selenium in California's Sacramento-San Joaquin Delta causes spinal and fin malformations in native fish such as delta smelt (Hypomesus transpacificus), reducing mobility and survival.^[79] Heavy metals similarly lead to eroded or absent fins in polluted waters, as observed in Swedish coastal species.^[80] Evolutionary trends in fin morphology reflect long-term adaptations to extreme conditions. Cave-dwelling populations of the Mexican tetra (Astyanax mexicanus) show pelvic fin reduction or loss, an energy-saving regression in nutrient-poor, dark environments where vision and active swimming are deprioritized.^[81] In contrast, flying fish (Exocoetidae) exhibit hyper-specialized pectoral fins, hypertrophied into wing-like structures spanning up to 40% of body length, enabling glides of 200-400 meters to evade predators.^[82] These enlarged, asymmetrical fins generate lift at speeds of 15-20 m/s, with pelvic fins providing additional stability during aerial phases.^[83] Such polarizations—from simplification to elaboration—exemplify fins' role in niche specialization across evolutionary timescales.^[84]

Contemporary Applications

Biomimicry in Robotics

Biomimicry in robotics draws inspiration from fish fins to develop propulsion systems for underwater vehicles, emphasizing flexible structures that replicate the undulating and oscillating motions observed in aquatic locomotion. These designs often incorporate soft robotics principles, utilizing compliant materials such as silicone or elastomers to mimic the ray-supported architecture of pectoral or caudal fins, enabling distributed deformation along the fin surface for enhanced hydrodynamic interaction.^[85] Oscillating fin propulsors, in particular, emulate the flapping or waving patterns of biological fins, where actuators drive periodic motions to generate thrust through vortex shedding and lift forces.^[86] This approach contrasts with rigid propellers by allowing adaptive shaping in response to flow conditions, as demonstrated in soft-rigid hybrid robots like those inspired by pangasius fish, which use servo-driven fin rays for precise control.^[87] Such fin-inspired mechanisms find primary application in autonomous underwater vehicles (AUVs) designed for ocean exploration, where they facilitate navigation in complex marine environments such as coral reefs or deep-sea trenches. Early examples include the RoboTuna, developed at MIT in the 1990s, which used a flexible tail fin to mimic thunniform swimming for efficient cruising speeds up to 1.25 m/s, serving as a foundational model for biomimetic propulsion testing.^[88]^[89] Modern undulating fin drones, such as those employing long-based fin arrays, extend this to multi-degree-of-freedom maneuvering, enabling AUVs to perform tasks like seabed mapping or environmental sampling with reduced acoustic signatures compared to traditional rotors.^[90] Fin-based systems offer distinct advantages over conventional propellers, including superior maneuverability in turbulent currents due to their ability to generate vectored thrust through asymmetric flapping, allowing for agile turns and hovering without additional control surfaces.^[91] Biomimetic flapping also enhances energy efficiency, with studies showing up to 30-40% lower power consumption for steady-state swimming in soft robotic fish prototypes, attributed to optimized wake structures that minimize drag.^[92] These benefits are particularly evident in undulating propulsors, which provide better stability in variable flows, outperforming propellers in scenarios requiring low-speed precision.^[93] Despite these gains, challenges persist in material durability, as flexible polymers degrade under prolonged saltwater exposure, leading to reduced elasticity and biofouling accumulation that impairs fin oscillation.^[94] Developing robust, corrosion-resistant composites remains critical, alongside advancing control algorithms to synchronize multi-actuator fin motions, where real-time feedback from sensors is needed to counteract hydrodynamic instabilities.^[95] Recent developments as of 2025 have integrated artificial intelligence to enhance underwater robot designs, such as MIT's use of AI to optimize shapes for autonomous gliders, improving efficiency in ocean exploration tasks like current tracking and environmental monitoring.^[96]

Other Technological and Cultural Uses

In naval engineering, bilge keels serve as fin-like appendages attached along the hulls of ships near the bilge to counteract rolling motions by generating hydrodynamic lift and drag forces that dampen oscillations.^[97] These structures, typically steel plates extending from one-third to one-half the ship's length, enhance stability in rough seas without significantly impeding forward motion.^[98] Hydrofoils, analogous to underwater wings, are engineered lifting surfaces that elevate vessel hulls above the water surface at high speeds, reducing drag and enabling efficient travel for ferries and military craft.^[99] In aviation, bioinspired designs drawing from fish fin flexibility have informed morphing wing technologies, such as 3D-printed prototypes that adapt shapes for improved aerodynamic control in aircraft and drones.^[100] Fin concepts permeate cultural narratives, notably in mermaid mythology where hybrid human-fish figures feature tail fins symbolizing the enigmatic boundary between land and sea, often embodying themes of allure, danger, and transformation across global folklore from European sirens to Asian sea spirits.^[101] In modern sports, swim fins—footwear extending the surface area of human feet for propulsion—were pioneered by inventor Owen Churchill, who developed and patented a practical rubber design in 1943 based on observations of Pacific Islanders, revolutionizing underwater activities like diving and training.^[102] These devices, first commercialized in the late 1930s, amplify swimmer efficiency by mimicking fish fin hydrodynamics.^[103] Artistic expressions of fins appear in body art and adornments, with fish or shark fin motifs in tattoos representing freedom, strength, or marine heritage, often stylized in tribal or realistic forms to evoke oceanic power.^[104] Jewelry incorporating fin shapes, such as pendants or rings mimicking dorsal or pectoral structures, draws from nautical themes to symbolize resilience and fluidity in wearable art.^[105] In architecture, fin-like elements manifest as decorative protrusions or structural fins on buildings, inspired by aquatic forms to enhance aesthetics and provide shading, as seen in modern coastal designs blending form and function. Aquariums and marine parks utilize fin displays in exhibits to educate visitors on aquatic locomotion, highlighting how these appendages enable species survival while fostering public appreciation for marine biodiversity.^[106] Conservation efforts have targeted shark fins through international regulations, with finning—the practice of removing fins and discarding carcasses—addressed by FAO's 1999 International Plan of Action for Sharks, which urged sustainable practices, followed by binding prohibitions such as the ICCAT's 2004/2005 finning ban and measures enforced by over two dozen nations since the mid-2000s to curb overexploitation by requiring full shark retention or fin-to-body ratios.^[107]^[108]^[109] Emerging applications include fin-shaped heat sinks on photovoltaic panels, where extended surfaces like L-shaped or pin fins facilitate water or air cooling to mitigate efficiency losses from overheating, with studies showing temperature reductions of up to 3-5% under natural convection.^[110] In wearable technology, fin-inspired devices for swimmers, such as webbed gloves or smart attachments, enhance propulsion while integrating sensors for performance tracking, blending biomimicry with electronics for training optimization.^[111]

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Lift generation can be explained by the Bernoulli principle: relatively higher °uid speeds and lower pressure on the upper surface of an airfoil compared to ...
[23]
Biomechanics of fast-start swimming in fish - ScienceDirect.com
Reactive forces are produced from the displacement of each section of the fish, and for fast-starting E. lucius the caudal body segments have the greatest ...
[24]
Vortex dynamics and fin-fin interactions resulting in performance ...
The leading-edge vortex (LEV) formation on the caudal fin (CF) has been identified as playing a key role in efficient lift-based thrust production of fish-like ...
[25]
Numerical Study on the Hydrodynamic Performance of a Flexible ...
Jul 21, 2024 · They indicated that large-aspect-ratio caudal fins have a higher efficiency during cruising, while low-aspect-ratio caudal fins are more ...
[26]
Experimental Hydrodynamics of Fish Locomotion - Oxford Academic
At low speed each fin downstroke produces a single vortex ring (Fig. 5A), whereas at high speed both the downstroke and upstroke are hydrodynamically active, ...
[27]
Convergence of undulatory swimming kinematics across a diversity ...
The length of the propulsive body wave was shorter for species classified as anguilliform and longer for those classified as thunniform, although substantial ...
[28]
Study of the thrust–drag balance with a swimming robotic fish
Sep 11, 2018 · The thrust produced by the flexible robot is proportional to A2f2, where A and f are the respective tail-beat amplitude and oscillation ...
[29]
Performance evaluation and optimization for two-dimensional fish ...
Aug 1, 2021 · The paper aims to present a systematic hydrodynamic study to further understand propulsion performance of fish-like swimming.
[30]
[PDF] Morphology and Experimental Hydrodynamics of Fish Fin Control ...
The leading edge of the pectoral fin plays a critical role during locomotion, as demonstrated by kinematic analyses of fin movement which show that the first ...
[31]
Control surfaces of aquatic vertebrates: active and passive design ...
Dec 1, 2017 · Aquatic vertebrates display a variety of control surfaces that are used for propulsion, stabilization, trim and maneuvering.
[32]
Maneuverability of four species of coral-reef fish that differ in body ...
Pectoral-fin morphology also determined the use of maneuvering behaviours within gaits that are used by fish to fractionate their locomotor-performance range.Missing: tight | Show results with:tight
[33]
Performance evaluation of humpback whale-inspired shortboard ...
Apr 21, 2020 · It is well-known that humpback whales (Megaptera novangelaie) use their pectoral fins in two primary ways: to generate lift and maneuver [18–20] ...
[34]
The function of fin rays as proprioceptive sensors in fish - Nature
Apr 16, 2013 · Here we test mechanosensory abilities of afferent nerves in the pectoral fin rays, limb structures used by many fish species in propulsion and manoeuvreing.
[35]
Jellyfish and Fish Solve the Challenges of Turning Dynamics ... - MDPI
Torque (τ) generation relies upon a force (F) applied at a distance (r) from the axis of rotation (r is also termed the lever arm) according to the ...
[36]
Mean fin surface area at the time of maximum excursion. Fin area at...
Fishes with longer moment arms can produce greater torque, requiring ... turning radius, improving maneuverability. However, a deep body compromises ...<|control11|><|separator|>
[37]
The relationship between heat flow and vasculature in the dorsal fin ...
Nov 15, 2002 · This study suggests that the dorsal fin is a spatially heterogeneous thermal surface and that patterns of heat flux are strongly influenced by underlying ...
[38]
[PDF] physiological and behavioral thermoregulation in - nc docks
In the dorsal fin, vascular countercurrent heat exchangers permit the transfer of heat from arterial blood at core body temperature to the cooler venous blood, ...
[39]
Sun-basking fish benefit from body temperatures that are higher ...
May 30, 2018 · The higher heat capacity and thermal conductivity of water compared with air have been universally assumed to render heat gain from sun basking ...
[40]
Regionally endothermic traits in planktivorous basking sharks ...
Jul 20, 2023 · Basking sharks have red muscle at the trunk, a compact myocardium, and white muscle temperatures 1.0 to 1.5°C above ambient, suggesting ...
[41]
Physiological and behavioural thermoregulation in bigeye tuna ...
Jul 30, 1992 · Here we show using telemetered data that free-ranging bigeye tuna (Thunnus obesus) can rapidly alter whole-body thermal conductivity by two orders of magnitude.Missing: fins cooling
[42]
(PDF) Thermoregulation in Yellowfin Tuna, Thunnus albacares
Aug 5, 2025 · ... dorsal fin (fig. 1). The. final thermistor position, later verified ... Because tunas possess countercurrent vascular pathways serving the trunk ...
[43]
High-resolution characterization of male ornamentation and re ... - NIH
Guppies are sexually dimorphic, with males displaying sexually selected coloration resulting from female preference. Previous work has suggested that much of ...
[44]
The effect of turbidity on female mate choice in the guppy, Poecilia ...
Males have conspicuous colour patterns on their body, with orange, black and iridescent patches which the males show off to females during 'sigmoid displays' ...
[45]
Extravagant ornaments of male threadfin rainbowfish (Iriatherina ...
Apr 27, 2013 · Male threadfin rainbowfish frequently flare their extravagant fins during both courtship and intrasexual displays, and the extensions of the ...
[46]
Gray Triggerfish – Discover Fishes - Florida Museum of Natural History
Feb 5, 2025 · The first dorsal fin has three spines that can be locked into an erect position for use as predator-defense and as an anchoring device. The ...Missing: posture | Show results with:posture
[47]
In love and war: The morphometric and phylogenetic basis of ...
Here, we apply a phylogenetic approach to study the evolution of ornamental dorsal fins in male poeciliid fish of the subgenera Mollienesia and Limia, which ...
[48]
https://oceanexplorer.noaa.gov/wp-content/uploads/2025/04/bioluminescent-creatures.pdf
[49]
(PDF) Fin-flicking behaviour: A visual antipredator alarm signal in a ...
These results support our hypothesis that fin flicking in tetras serves as both a visual alarm signal to conspecifics and a deterring signal to predators, and ...
[50]
[PDF] bioluminescent-creatures.pdf - NOAA Ocean Exploration
This fish also has a glowing light on its adapted dorsal fin ray that hangs in front of its mouth. And, like many fishes of the deep sea, the viperfish has ...
[51]
Extraoral Taste Buds on the Paired Fins of Damselfishes
Aug 11, 2022 · The paired fins of fishes function in a diversity of behaviors. For those with taste buds, fins can function as short-range chemical detectors ...Introduction · Methods · Results · Discussion
[52]
Molecular basis of ancestral vertebrate electroreception - PMC
Elasmobranch fishes, including sharks, rays, and skates, use specialized electrosensory organs called Ampullae of Lorenzini to detect extremely small changes in ...
[53]
[PDF] Electroreception in sharks
All chondrichthyan fishes can detect electric fields with their electroreceptors, the ampullae of Lorenzini. This electrosense enables them to locate ...
[54]
The Incredible Diversity of Fish: How Form Equals Function
Some flatfish species can further conceal themselves using specialized cells called chromatophores to rapidly change color and blend with their surroundings.
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More than Skin Deep: Color and Pattern in Marine Fishes, Part One
Nov 3, 2024 · One of the most common reasons for color and pattern adaptations among reef fish is for camouflage. ... chromatophores, in their skin to ...
[56]
https://www.researchgate.net/publication/258494420_Paired_fins_of_jawless_vertebrates_and_their_homologies_across_the_agnathan-gnathostome_transition
[57]
Fin modules: an evolutionary perspective on appendage disparity in ...
Apr 27, 2017 · This study addresses fin disparity in a phylogenetic perspective, while focusing on the presence/absence and number of each of the median and paired fins.Missing: amphibians | Show results with:amphibians
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The Middle Cambrian fossil Pikaia and the evolution of chordate ...
Jun 13, 2012 · Pikaia gracilens, a well-known Cambrian fossil and supposed basal chordate, and propose on this basis some new ideas about Pikaia's anatomy and evolutionary ...
[59]
A new interpretation of Pikaia reveals the origins of the chordate ...
Jul 8, 2024 · Problematic Cambrian fossils that have been considered as candidate chordates include vetulicolians, Yunnanozoon, and the iconic Pikaia.
[60]
Paired fins of jawless vertebrates and their homologies across the ...
Paired fin-like extensions of the body wall [3, 4] originate in stem gnathostomes (vertebrates capable of skeletal mineralization) and in derived jawless groups ...
[61]
PART I: PAIRED FINS AND GIRDLES IN OSTRACODERMS ...
The problem of the origin and evolution of the paired appendages and girdles has been considered as if it could be settled by itself, but the cumulative ...
[62]
Unusual anal fin in a Devonian jawless vertebrate reveals complex ...
Key among these is paired appendages (fins, legs and wings), which first evolved at some point on the gnathostome stem. The anatomy of fossil stem gnathostomes ...
[63]
Regulatory evolution of Tbx5 and the origin of paired appendages
Aug 8, 2016 · Jawless vertebrates, such as lamprey and hagfish, do not possess paired fins, whereas jawed vertebrates have two pairs of appendages. Although ...
[64]
Tri-phasic expression of posterior Hox genes during development of ...
Oct 1, 2008 · We found that, similar to tetrapod Hox genes, expression of hoxa/d genes in zebrafish pectoral fins occurs in three distinct phases.
[65]
Mechanism of pectoral fin outgrowth in zebrafish development
Aug 15, 2012 · In early paired-fin development, the apical ectodermal ridge (AER) rims the distal edge of the fin bud along the dorsoventral boundary and ...
[66]
Sarcopterygian fin ontogeny elucidates the origin of hands with digits
Aug 19, 2020 · Our findings provide a genetic framework for the transition of fins into limbs that supports the significance of classical models proposing a ...
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Fins into limbs: Recent insights from sarcopterygian fish - Amaral
Aug 19, 2017 · Limbs with digits evolved as sarcopterygian fish transitioned to a terrestrial life, giving rise to modern tetrapods.
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[PDF] MinnAqua Fishing: Get in the Habitat! Lesson 2:6 - Adapted for ... - files
Some prey fish and many predator fish are torpedo-shaped. This streamlined shape enables the fish to swim quickly, with less drag in the water. Examples of ...
[69]
[PDF] Form and Function Background
Sea robins are a bottom dwelling fish with very large pectoral fins. The large pectoral fins make the fish look larger to predators swimming through the water.
[70]
Fish - Swimming, Fins, Adaptations | Britannica
Oct 28, 2025 · Many fishes have a streamlined body and swim freely in open water. Fish locomotion is closely correlated with habitat and ecological niche.
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The diversity and evolution of electric organs in Neotropical knifefishes
Apr 1, 2022 · All Gymnotiform species possess a myogenic electric organ (mEO) derived from the muscle tissue, and members of the Apteronotidae family uniquely ...
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Kinematics of the ribbon fin in hovering and swimming of the electric ...
Like other knifefish, it propels itself by sending traveling waves along an extended anal fin attached to the ventral midline of the fish (Fig. 1A,B).
[73]
Development of the electric organ in embryos and larvae of the ...
South American Gymnotiform knifefish possess electric organs that generate electric fields for electro-location and electro-communication.
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Anatomical adaptations of aquatic mammals - Reidenberg
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[75]
Adaptations | manoa.hawaii.edu/ExploringOurFluidEarth
This topic will work towards building an understanding of evolutionary adaptations that have enabled mammals to thrive and diversify throughout the world ocean.
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Locomotion in the ocean: how sea animals move
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https://www.pnas.org/doi/10.1073/pnas.1900475116
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Colour dimorphism in labrid fishes as an adaptation to life on coral ...
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[79]
Intrasexual competition facilitates the evolution of alternative mating ...
Dec 23, 2010 · Here, we explore how overt male aggression can influence male mating success and facilitate the persistence of colour polymorphism in the ...
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Deep evolutionary origin of limb and fin regeneration - PNAS
Jul 3, 2019 · Among actinopterygians, teleosts, such as zebrafish, have been broadly used for fin regeneration studies, yet their regenerative abilities are ...
[81]
Zebrafish fin regeneration involves generic and regeneration ... - eLife
Jun 24, 2022 · In contrast, teleost fish and urodele amphibians are able to fully regenerate limbs/fins. Thus, the zebrafish caudal fin has become a ...
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Lifetime Chronicles of Selenium Exposure Linked to Deformities in ...
Feb 24, 2020 · Several factors besides Se toxicity can result in spinal deformities such as kyphosis, lordosis, and/or scoliosis in fish. These include ...
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Skeletal anomalies in fish from polluted surface waters - ScienceDirect
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Dark ages: Genomic analysis shows how cavefish lost their eyes
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Aerodynamic characteristics of flying fish in gliding flight - PubMed
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[86]
Take-off performance of flying fish Cypselurus heterurus doederleini ...
hyper trophied pectoral fins at an air speed of 15 to. 20 m s−1. The propulsion is generated by the entire caudal fin and posterior body when under water, and.
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Fossils Show How Flying Fish Started to Glide
Jan 7, 2015 · The flying fish's wings are actually enlarged front fins, and they help the fish escape predators. Some species even have larger pelvic fins, ...
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Underwater Soft Robotics: A Review of Bioinspiration in Design ...
Jan 10, 2022 · Fish are able to generate lift and thrust in order to swim. They can achieve swimming using their fins or swimming propulsors (Figure 1b).
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[PDF] Design of a Robotic Fish Propelled by Oscillating Flexible Pectoral ...
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Design and control of soft biomimetic pangasius fish robot using fin ...
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MIT's Robotic Fish Takes First Swim
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Underwater Undulating Propulsion Biomimetic Robots: A Review
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(PDF) Advantages of Natural Propulsive Systems - ResearchGate
Aug 9, 2025 · While traditional propeller-based UUVs operate well in deep water environments, bio-inspired fins offer greater agility and maneuverability ...
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Review of biomimetic flexible flapping foil propulsion systems on ...
This superior performance of biomimetic propulsors results in efficient cruising, maneuverability, and low noise motion of the underwater vehicles.
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Underwater Undulating Propulsion Biomimetic Robots: A Review
By contrast, underwater biomimetic robots show better stability and maneuverability in harsh marine environments. This is particularly true of undulating ...<|separator|>
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Recent Advances on Underwater Soft Robots - Qu - 2024
Oct 5, 2023 · This review presents the recent advances of underwater soft robots in the aspects of intelligent soft materials, fabrication, actuation, ...Missing: saltwater | Show results with:saltwater
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[PDF] Advancements and Challenges in Underwater Soft Robotics
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https://www.marineinsight.com/naval-architecture/importance-of-ships-keel-and-types-of-keel/
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Innovation Timeline | DARPA
DARPA announced a multi-year investment of more than $2 billion on artificial intelligence research and development in a portfolio of some 50 new and existing ...
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BionicFinWave''s Undulating Fin Propulsion May Give Underwater ...
Jan 14, 2025 · The wave-shaped movement of the fins allows the fish to push the water behind them, thereby creating a forward thrust. Conversely the creatures ...Missing: ocean exploration
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On the Structural Design of Bilge Keels - ASME Digital Collection
Bilge Keels are positioned symmetrically on each side of the vessel, normal to the bottom or/and bilge plating and fitted amidships. Dimensions vary from 1/3 to ...
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Importance Of Ship's Keel and Types Of Keel - Marine Insight
May 30, 2021 · The keel is the primary structural member and backbone of a ship. Learn about the importance of keel and types of keels used for a ship.
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2.972 How a Hydrofoil Works - MIT
A steep angle of attack is not needed in the design of the hydrofoil. On the contrary, small angles of attack are used on hydrofoils to optimize the lift to ...
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A 3D-Printed 'Fish Fin' Offers New Ideas for Morphing Aircraft
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Mermaids: Myth, Kith and Kin - JSTOR Daily
Dec 24, 2022 · Mermaids have been a fixture of different cultures and regions throughout history, representing, among other things, queer identity.Missing: fins significance
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https://www.jaefr.com/articles/unveiling-the-mysteries-of-fish-fins-a-comprehensive-exploration-106179.html
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Owen Churchill Fin - Northwest Diving History Association
Sep 8, 2015 · The Churchill fin was the first commercially manufactured diving fin. Conceived and developed around 1937, the design was applied for U.S. ...
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Life and Death: Central Motifs in Barim's Tattoo Art
May 7, 2025 · Light and shadow, life and death, skulls and balloons. In the poetic and symbolic world of Barim, contrast becomes creation.
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Cece Jewellery: Turning Tattoo Art into ... - Prestige Hong Kong
Mar 22, 2022 · Cece Jewellery takes fairytales and folklores and transforms them into delicate signet rings and pendants in hand-painted enamel and gold.Missing: fin | Show results with:fin<|separator|>
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Unveiling the mysteries of fish fins: A comprehensive exploration
Furthermore, fins play a crucial role in thermoregulation, enabling fish to regulate their body temperature in response to environmental fluctuations. By ...
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International Shark Protection Measures | Animal Welfare Institute
The agreement initially prohibited the practice of shark finning and the landing of unattached fins and applied to seven shark species—basking, great white, ...
[112]
A new cooling method for photovoltaic module using U-shape ...
This paper provides a solution for the existing problem by proposing a new design which is an integration of fins and tubes in a single cooling unit.
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Become Aquaman with these wearable swimming fins
Dec 23, 2018 · These silicone webbed swim gloves will let you channel your inner Aquaman or mermaid. They are like driving gloves for swimmers.