Sharks and rays are among the most distinctive vertebrates in the ocean, combining flexible skeletons, specialized senses, and evolutionary adaptations that have enabled their ancestors to thrive for hundreds of millions of years. Although sharks are often associated with powerful jaws and streamlined bodies, and rays with broad fins and flattened shapes, both belong to the same ancient group of fishes. Their biology reveals how evolution can produce remarkably different body forms from a shared anatomical foundation.
Sharks and rays are cartilaginous fishes, meaning their skeletons are made primarily of cartilage rather than bone. They belong to the class Chondrichthyes, which also includes chimaeras, a less familiar group of deep-water fishes. Within this lineage, sharks and rays share many fundamental features: replaceable teeth in most species, paired fins, specialized sensory organs, and a body plan adapted to life in water. Their differences reflect the diverse demands of finding food, avoiding predators, reproducing, and moving through environments ranging from shallow coastal habitats to the deep sea.
Understanding these animals requires looking beyond their external appearance. Their success depends on the interaction of anatomy, physiology, sensory biology, behavior, and evolutionary history.
The biology that distinguishes sharks and rays
The most obvious difference between sharks and rays is body shape. Many sharks have elongated, streamlined bodies, with the head, trunk, and tail arranged along a relatively narrow axis. This form helps them move efficiently through the water. Rays typically have flattened bodies and enlarged pectoral fins that extend outward from the head region, creating a broad surface for generating lift and propulsion.
These forms are not universal. Some sharks, such as angel sharks, have flattened bodies, while some rays have long, narrow tails and compact bodies. The manta rays and their relatives are especially specialized for swimming in open water, using broad pectoral fins to generate powerful, winglike strokes. Body shape therefore reflects ecological function rather than a simple division between two rigid designs.
Sharks and rays belong to the subclass Elasmobranchii, a major branch of cartilaginous fishes. Rays are closely related to sharks, and many familiar anatomical differences between them evolved as their lineages adapted to different ways of life.
Cartilage instead of bone
The skeleton of a shark or ray consists mainly of cartilage, the flexible, resilient tissue also found in the joints, nose, and ears of humans. Cartilage provides structural support without the density of a fully mineralized bony skeleton.
However, the skeletons of cartilaginous fishes are not simply soft. Much of the cartilage is strengthened by mineralized tissues, including small, hard structures that reinforce its surface. This combination provides both flexibility and rigidity, helping the animal withstand the forces generated during swimming and feeding.
A cartilaginous skeleton can reduce structural mass compared with a heavily ossified skeleton, but it does not by itself explain the buoyancy or swimming efficiency of sharks and rays. These properties also depend on body composition, fin shape, swimming behavior, and the surrounding water.
Most sharks lack the gas-filled swim bladder used by many bony fishes to regulate buoyancy. Instead, many species rely partly on a large liver containing oils, including squalene in some species, that are less dense than seawater. Dynamic lift generated by the fins and body can provide additional support while swimming. Some sharks must keep moving to maintain their position in the water column, while others can rest on the seabed or use other behaviors to manage their position.
Rays generally spend more time near the bottom, although many species swim actively in open water. Their flattened bodies help them maneuver close to the seafloor, where they can search for prey hidden in sediment.
Skin, scales, and teeth
The skin of sharks and rays is covered with tiny structures called dermal denticles, or placoid scales. Each denticle has a hard, toothlike surface, often with a shape that helps manage water flow over the body. These structures differ in size and form among species and body regions.
Dermal denticles are made from tissues similar to those found in vertebrate teeth. They protect the skin and influence its interaction with moving water. Depending on their shape and arrangement, they can help reduce certain forms of drag or limit turbulence in ways that benefit swimming.
The relationship between scales and teeth is especially revealing. Shark teeth and dermal denticles share important developmental and structural features, illustrating how evolution can modify related biological structures for different functions.
Many sharks continually replace their teeth. New teeth develop in rows within the jaw and move forward as older teeth are lost or worn down. This replacement system is useful for animals that capture slippery prey, bite through tough tissues, or feed on hard-shelled animals.
Ray teeth vary with diet. Species that crush mollusks and crustaceans often have broad, flattened tooth surfaces, while others have smaller teeth suited to different prey. Filter-feeding rays, including manta rays, do not rely on large, grasping teeth to obtain their primary food.
Gills and respiration
Like other fishes, sharks and rays extract dissolved oxygen from water using gills. Water passes over delicate gill surfaces where oxygen diffuses into the blood and carbon dioxide diffuses out.
Sharks typically have several separate gill slits on each side of the head. Rays usually have five pairs of gill openings on the underside of the body. Their position is closely related to the animals’ body shapes and feeding habits.
For a ray resting on the seabed, drawing water through the mouth can be difficult if sediment blocks the opening. Many bottom-dwelling rays therefore take in water through openings called spiracles, located behind the eyes. Water enters through the spiracles and passes over the gills before leaving through the gill slits.
Spiracles are also present in many sharks, especially species that spend time near the bottom. Their use varies among species and circumstances.
Some sharks ventilate their gills by actively pumping water across them with movements of the mouth and throat. Others depend substantially on forward movement to force water into the mouth and over the gills, a process often called ram ventilation. These strategies are not mutually exclusive, and ventilation patterns differ among species. The widespread idea that all sharks must swim continuously to breathe is therefore incorrect.
How sharks and rays move through water
Swimming depends on generating thrust while limiting the energy required to overcome water resistance. Sharks and rays have evolved different ways to achieve this, reflecting their contrasting body forms and ecological roles.
Most sharks produce thrust through side-to-side movements of the body and tail. The tail pushes water backward, driving the animal forward. The body and fins also contribute to stability, steering, and lift. The relative importance of each part varies with swimming speed and species.
Many fast-swimming sharks have crescent-shaped tails, with a large upper lobe and a smaller lower lobe. This design can support efficient propulsion at sustained speeds. Other sharks have different tail shapes suited to acceleration, maneuverability, or life near the seabed.
Rays typically generate propulsion through movements of their enlarged pectoral fins. Bottom-dwelling species often undulate the fins in waves that travel along their margins, allowing them to maneuver over sand, mud, or reef surfaces. More active swimmers, including eagle rays and manta rays, flap their fins through the water in broad strokes.
These movements are not merely different ways of producing thrust. They reflect differences in how the animals interact with their surroundings. A shark pursuing mobile prey in open water faces different physical challenges from a ray searching for buried invertebrates along the seafloor.
Buoyancy and stability are also important. Sharks use their fins and body posture to control lift and direction. Rays use the broad surfaces of their pectoral fins to control both propulsion and movement through the water column. Their flattened shape is advantageous for many bottom-associated activities, while the more winglike bodies of pelagic rays support sustained swimming in open water.
The sensory systems that guide hunting and navigation
Sharks and rays possess a suite of sensory systems that detect different kinds of information. Vision, smell, hearing, touch, water-motion detection, and electroreception each provide distinct signals. Together, these senses help the animals locate prey, orient themselves, recognize environmental features, and respond to threats.
No single sense explains their hunting ability. The importance of each varies with species, habitat, prey type, light conditions, and behavior.
Vision in changing light
The eyes of sharks and rays are broadly comparable to those of other vertebrates, with a lens that focuses light onto a light-sensitive retina. Their visual capabilities differ among species according to habitat and lifestyle.
Species that hunt in dim environments may have adaptations that improve sensitivity to low light. Some sharks possess a reflective layer behind the retina called the tapetum lucidum. Light that passes through the retina can be reflected back through it, giving the photoreceptors another opportunity to detect it. This adaptation can improve vision in low-light conditions, although it may affect image sharpness under some circumstances.
The retina contains photoreceptor cells that respond to light. Rods are particularly sensitive in dim conditions, while cones support color vision and finer detail in suitable light. The balance of these cells differs among species.
Sharks were once widely assumed to have little or no color vision. Research on their visual pigments indicates that many examined species possess only one type of cone photoreceptor, suggesting that their color discrimination may be limited compared with that of humans. This does not mean every shark sees the world in exactly the same way, and the visual capabilities of many species remain incompletely understood.
Vision can be especially useful during the final stages of an attack, when an animal needs to judge the position, shape, or movement of nearby prey. It can also support social interactions and navigation through complex habitats.
Smell and chemical detection
The sense of smell is highly developed in many sharks and rays. Water carrying dissolved chemicals enters the nasal openings and passes over sensory tissue containing receptor cells. These cells detect chemical compounds and transmit signals to the brain.
Smell can help an animal recognize prey-associated chemicals, investigate potential food sources, and detect other biologically relevant signals. Its effectiveness depends on the chemical properties of the substance, water movement, concentration, and the animal’s sensory capabilities.
A common misconception is that sharks can detect a single drop of blood from an enormous distance under any conditions. Chemical sensitivity does not work that way. A substance must reach the sensory organs at a detectable concentration, and currents, dilution, turbulence, and background chemicals affect how it travels.
Sharks and rays also possess paired nasal openings that are separate from the mouth and respiratory passages. In some species, water flows through these openings in ways that allow comparisons between chemical signals reaching the two sides. Such comparisons may help animals orient toward a chemical source, although the details vary and are not fully understood across the group.
Smell works alongside other senses. A shark may detect chemical cues over a distance, use water-motion information to investigate nearby activity, and rely increasingly on vision or other short-range senses as it approaches an object.
The lateral line and water movement
Along the sides of sharks and rays is a sensory system called the lateral line. It consists of fluid-filled canals and specialized sensory organs that detect water movement and pressure changes around the body. Related sensory structures are also found on the head.
The receptor organs, called neuromasts, contain hair cells that respond when tiny hairlike projections are displaced by water movement. The resulting signals provide information about nearby disturbances, including the movement of prey, obstacles, and the flow of water around the animal.
This system is particularly useful in dark or murky environments, where vision may provide little information. A shark or ray can detect the water movements produced by a struggling animal without necessarily seeing it.
The lateral line is not a long-range sonar system. It detects local water motion and pressure gradients rather than creating sound pulses and interpreting their echoes. Its useful range depends on the strength and frequency of the disturbance, environmental conditions, and the animal’s sensory sensitivity.
Together with vision and other senses, the lateral line helps sharks and rays make sense of their immediate surroundings. It also contributes to swimming by providing information about the flow of water over the body.
Electroreception: detecting the electrical signals of life
One of the most remarkable sensory abilities of sharks and rays is electroreception: the detection of weak electrical fields in the environment.
Living animals generate electrical signals through the activity of nerves, muscles, and other tissues. When these signals extend into the surrounding water, they can create electrical fields. Sharks and rays possess specialized receptors that detect these fields, allowing them to locate nearby animals even when visual or chemical cues are limited.
These receptors are called the ampullae of Lorenzini. They appear as small pores on the skin, especially around the head, connected to sensory cells through jelly-filled canals. The canals conduct electrical signals from the surrounding water to the receptors.
Electroreception is particularly valuable during close-range prey detection. A ray searching for an animal buried beneath sediment, for example, may be unable to see it and may receive little useful information from smell alone. The prey’s electrical activity can nevertheless provide a detectable signal.
The receptors can respond to extremely weak electrical fields, but their usefulness depends on the field’s characteristics and the surrounding environment. Electrical signals become less distinct with distance, and the sensory system does not give sharks and rays a detailed electrical picture of everything around them.
Electroreception may also contribute to orientation and navigation. Some sharks and rays can detect electric fields associated with Earth’s magnetic field when they move through seawater, providing a possible mechanism for sensing aspects of the geomagnetic environment. However, the role of electroreception in magnetic navigation is not fully resolved for all species, and other sensory mechanisms may also contribute.
Electroreception is one reason sharks and rays can remain effective predators under conditions that make visual hunting difficult. It is not a substitute for their other senses but an additional channel of information that is especially valuable at close range.
Hearing and other sensory cues
Sharks and rays detect sound through structures in the inner ear. Sound travels through water as pressure fluctuations and particle motion, and fishes can detect aspects of these signals through the sensory systems of the ear and, in some cases, associated body structures.
Sharks lack external ear openings like those of mammals, but they are not deaf. They can respond to sounds, particularly some low-frequency signals, although hearing ranges and sensitivity vary among species. Their inner ears also help maintain balance and provide information about body orientation and movement.
Touch contributes to interactions with the environment, including contact with the seabed, prey, or other animals. Taste receptors in and around the mouth help assess food once it is close enough to be sampled.
The combination of sensory systems allows sharks and rays to move between different stages of perception. Long-range cues may indicate where a potential food source is located, while local water movement, vision, and electroreception help resolve its position as the animal approaches. The exact sequence depends on the species and the situation; hunting does not follow a single universal sensory script.
Feeding strategies and ecological roles
Sharks and rays occupy a wide range of positions in marine food webs. Some are active predators of fishes, marine mammals, or other large animals. Others specialize in crustaceans, mollusks, worms, or small fishes living near the seafloor. Still others feed primarily on plankton.
Their feeding structures and sensory abilities reflect these different diets.
Many predatory sharks have jaws that can extend or move independently of the skull to help capture prey. Their teeth are often arranged in multiple replacement rows, and tooth shape is associated with feeding function. Sharp, narrow teeth may help grasp slippery prey, while broader or more robust teeth can be useful for cutting or crushing tougher food.
Bottom-feeding rays commonly have jaws and teeth adapted for handling hard-shelled invertebrates. Their flattened bodies and spiracles allow many of them to remain close to the seabed while breathing and searching for food. Some can stir sediment with movements of their bodies or fins, exposing hidden prey.
Manta rays and other large filter-feeding rays use specialized structures in the mouth and gill region to capture small organisms suspended in water. They feed on plankton, including small crustaceans and other drifting organisms, rather than relying on large prey. Their broad mouths and swimming behavior allow them to process substantial volumes of water as they feed.
These differences matter ecologically. Predatory sharks can influence the abundance and behavior of prey species, while rays can affect communities of bottom-dwelling invertebrates through their feeding. Filter-feeding rays connect planktonic food webs with larger marine animals.
The effects of sharks and rays are rarely as simple as a single predator controlling an entire ecosystem. Food webs contain many interacting species, and the consequences of changes in shark or ray populations depend on local conditions, prey availability, habitat, and the presence of other predators. Nevertheless, these fishes can play important roles in maintaining the structure and functioning of marine ecosystems.
Reproduction and life history
Sharks and rays generally reproduce through internal fertilization. Males possess paired reproductive organs called claspers, which are modified portions of the pelvic fins used to transfer sperm to the female. Fertilization occurs inside the female’s reproductive tract.
Their reproductive strategies vary widely. Some species lay eggs, while others retain developing embryos inside the body. In species that give birth to live young, the embryos may receive nourishment primarily from yolk or, in some cases, through more direct maternal support.
Egg-laying and live birth
In egg-laying species, the embryo develops inside a protective egg case. These cases may have tough, flexible coverings and structures that help secure them to seaweed, rocks, or other surfaces. The familiar objects sometimes called mermaid’s purses are egg cases produced by certain sharks, skates, and related fishes.
In many live-bearing species, embryos develop inside the female until they are ready to be born. Some rely on yolk stored in the egg, while others have more complex forms of maternal nourishment. A few sharks have reproductive systems in which the developing embryos receive substantial nutrients from the mother through structures that function somewhat like a placenta.
The diversity of these strategies reflects different evolutionary solutions to the challenge of developing young in aquatic environments. Egg-laying can protect embryos within a contained structure, while live birth can allow the mother to retain them during development. Neither strategy is universally superior; its advantages depend on habitat, predation risk, energy costs, and other ecological factors.
Slow growth and vulnerability
Many sharks and rays mature relatively late, grow slowly, and produce comparatively few young per reproductive cycle. These traits are not universal, but they occur often enough to make many species vulnerable to sustained fishing pressure.
A population with slow reproduction may recover only gradually after numbers decline. Removing large adults can be especially damaging when those individuals contribute disproportionately to reproduction. Fishing can also affect young animals before they reach maturity, reducing the number that eventually breed.
The life histories of sharks and rays therefore have important implications for conservation. Protecting habitat, reducing excessive fishing mortality, and limiting accidental capture can help populations persist, but effective measures must account for differences among species. A fast-growing species may respond to pressure differently from a long-lived species with a low reproductive rate.
The evolutionary history of sharks and rays
Sharks and rays belong to an ancient lineage of vertebrates whose evolutionary history extends back hundreds of millions of years. Their ancestors lived long before modern marine ecosystems took shape, and their descendants diversified through repeated changes in body form, feeding structures, sensory systems, and habitat use.
The earliest history of cartilaginous fishes is difficult to reconstruct in detail because cartilage fossilizes less readily than bone. Many fossils preserve teeth, fin spines, or mineralized fragments rather than complete skeletons. As a result, paleontologists must often infer relationships from incomplete evidence.
Sharklike fishes appeared early in vertebrate evolution, but the history of modern sharks and rays is not a simple, uninterrupted sequence of animals that all resembled today’s familiar species. The group includes extinct lineages with distinctive anatomy, and many features associated with modern sharks evolved at different times.
How sharks and rays diverged
Sharks and rays share a common ancestry within cartilaginous fishes. Their evolutionary history includes a major split between the lineage that gave rise to modern sharks and the lineage that includes rays, with the latter known as Batoidea.
The exact timing of key evolutionary transitions remains an area of scientific investigation because the fossil record is incomplete and interpretations can change as new specimens and analyses become available. However, it is clear that the flattened body plan characteristic of many rays evolved within a lineage closely related to sharks, rather than representing an entirely separate origin among fishes.
In many rays, the pectoral fins expanded and became integrated with the sides of the head and trunk. The gill openings shifted to the underside of the body, and the overall shape became suited to moving along the seabed or swimming through open water with broad fin strokes. These changes were accompanied by modifications in feeding structures and sensory arrangements.
Not every ray follows the same ecological pattern. Skates, stingrays, electric rays, eagle rays, and manta rays differ substantially in their anatomy and behavior. Their diversity demonstrates how a shared evolutionary foundation can produce a range of specialized forms.
Evolution through adaptation and constraint
Evolution does not plan ahead or produce perfect organisms. Instead, inherited variations arise through genetic changes and recombination, and natural selection can favor variants that improve survival or reproduction in particular environments. Other changes can spread through genetic drift or arise as consequences of developmental processes and evolutionary history.
Sharks and rays illustrate this process in several ways. Dermal denticles and teeth share developmental similarities, yet their shapes and functions differ. The same basic paired fins can support very different swimming styles. Sensory systems can become especially important in habitats where light is limited or prey is difficult to see.
Evolution also operates within constraints. An adaptation that improves one function may impose costs elsewhere, and structures inherited from ancestors cannot always be redesigned freely. The diversity of shark and ray anatomy reflects both adaptation to current environments and the historical pathways through which their lineages evolved.
Their long evolutionary history does not mean sharks and rays have remained unchanged. Both groups have diversified extensively, and their modern species represent the outcomes of ongoing evolutionary processes rather than unchanged survivors from a distant past.
Sharks and rays in a changing ocean
Despite their evolutionary history, sharks and rays face significant pressures in modern marine environments. Fishing is a major source of mortality, both through targeted capture and incidental catch in fisheries aimed at other species. Habitat loss, pollution, and changes in ocean conditions can add further stress.
Their vulnerability varies by species. A widely distributed, fast-growing species may be able to withstand some pressures that would severely affect a slow-growing species confined to a limited habitat. The same conservation strategy may therefore produce different results across different populations.
Sharks and rays are also important to human communities. They support fisheries, contribute to marine tourism, and hold scientific value because their biology offers insights into sensory evolution, vertebrate anatomy, and the functioning of marine ecosystems. Some species can pose risks to people, but the danger varies greatly across the group and should not obscure their broader ecological importance.
Conservation requires reliable information about population trends, reproductive biology, movement, and habitat use. It also requires attention to how fishing practices, regulations, and local livelihoods interact. Protecting these animals is not simply a matter of preserving ancient forms of life; it is a practical effort to maintain marine biodiversity and the ecological relationships on which ocean communities depend.
Sharks and rays demonstrate how a common evolutionary heritage can support an extraordinary range of biological solutions. Their cartilage-based skeletons, specialized fins, replaceable teeth, and diverse sensory systems reflect millions of years of adaptation to aquatic life. From the shark that detects a moving animal through subtle water disturbances to the ray that finds prey hidden beneath sediment, their biology reveals the power of combining multiple senses with anatomy suited to a particular ecological role. Understanding that diversity offers a clearer view of both the history of vertebrate life and the complex systems that sustain the world’s oceans.
