Chondrichthyes is the scientific class that includes all sharks, rays, skates, and chimaeras, a group of roughly 1,200 living species united by one defining trait: their skeletons are made of cartilage rather than bone. That distinction sounds simple, but it underpins a radically different body plan from the bony fishes most people picture when they think of ocean life. Chondrichthyans have been around for at least 440 million years, predating dinosaurs by a wide margin, and their biology is full of surprises that challenge the popular image of sharks as mindless eating machines.
A Skeleton Made of Cartilage, But Not Just Cartilage
The cartilaginous skeleton is what gives the class its name (chondros is Greek for cartilage), but calling it simply “cartilage” undersells the material. The outer surface of a chondrichthyan skeleton is covered in a mosaic of mineralized tiles called tesserae, small blocks of hardened tissue that form a shell around the softer cartilage core. Research on stingrays has shown that each tessera has a layered structure: an upper “cap” built on type I collagen (the same collagen found in bone) merging into a lower “body” zone made of type II collagen (the kind typical of cartilage). The cells inside tesserae remain alive and intact in their own little chambers, which is unusual for mineralized tissue. The whole setup defies the neat division biologists usually draw between bone and cartilage, and it has attracted attention as a model for studying how mineralization is controlled in vertebrates more broadly.1PubMed. Calcified cartilage or bone? Collagens in the tessellated endoskeletons of cartilaginous fish (sharks and rays)
This tiled arrangement is not merely structural curiosity. In ancient sharks that reached lengths over seven meters, heavily mineralized tesserae may have served a dual role: storing phosphate and adding skeletal density to support a massive body. In modern species, the reinforced tiling is concentrated in areas under the most stress, particularly the jaws, where it stiffens cartilage that would otherwise flex too much during biting.2PubMed. Architectural and ultrastructural features of tessellated calcified cartilage in modern and extinct chondrichthyan fishes The lighter overall skeleton gives chondrichthyans a weight advantage over bony fish of similar size, which matters when you lack a swim bladder and need every edge you can get to stay neutrally buoyant.
How They Stay Afloat Without a Swim Bladder
Bony fish inflate and deflate a gas-filled swim bladder to control their depth. Chondrichthyans never evolved one. Instead, they rely on an enormous, oil-rich liver that can account for a quarter or more of their body weight. The key ingredient in many species is squalene, a hydrocarbon oil with a density of about 860 kilograms per cubic meter, well below that of seawater. Squalene appears to be metabolically inert in these livers, meaning the body does not burn it for energy. Its only job is to provide lift.3Integrative and Comparative Biology. Buoyancy in Marine Fishes: Direct and Indirect Role of Lipids
The system works even at extreme depths, which is not obvious. Gases compress under pressure, which is why a swim bladder becomes less useful the deeper a fish goes. Oil, however, compresses at nearly the same rate as the surrounding seawater, so the buoyancy contribution of a squalene-loaded liver stays relatively stable whether the animal is at 100 meters or 2,000 meters. Squalene also has a lower viscosity than other liver oils, which may help the liver respond to small changes in body orientation.4Journal of Experimental Biology. Near-equal compressibility of liver oil and seawater minimises buoyancy changes in deep-sea sharks and chimaeras Deep-sea sharks and chimaeras often carry especially large, oil-packed livers, reflecting how central this organ is to their ability to inhabit the deep ocean.
An Unusual Blood Chemistry
Walk into a seafood market and sniff a piece of fresh shark meat. The faint ammonia smell is a clue to one of the strangest physiological tricks in chondrichthyan biology. Unlike bony fish, which keep their blood much less salty than the ocean around them and constantly work to replace lost water, most marine chondrichthyans take the opposite approach: they load their blood with urea until their body fluids are roughly the same concentration as seawater. This lets them avoid the energetic cost of constantly pumping water and ions.
Urea in high concentrations is toxic to proteins, though, so cells also accumulate a counteracting molecule called trimethylamine N-oxide (TMAO). In shallow-water species, the ratio of urea to TMAO sits near two to one, the mix that best stabilizes proteins.5PubMed. Decreasing urea∶trimethylamine N-oxide ratios with depth in chondrichthyes: a physiological depth limit? In deeper-dwelling species, TMAO levels climb even higher, possibly to counteract the destabilizing effects of hydrostatic pressure on proteins. That same TMAO is responsible for the “fishy” smell that intensifies as shark meat ages and the compound breaks down into trimethylamine.
Sensory Systems That Put Human Perception to Shame
Chondrichthyans are loaded with sensory hardware that has no equivalent in land animals. The most famous example is electroreception: small gel-filled pores on the head and snout called ampullae of Lorenzini detect the faint bioelectric fields produced by the muscles and nerves of other animals. The gel inside the ampullae behaves like a semiconductor, converting tiny voltage differences into nerve signals that let a shark sense prey buried in sand or hidden in murky water.6PubMed Central. Semiconductor gel in shark sense organs? The sensitivity is extraordinary, on the order of nanovolts per centimeter.
Smell, too, works differently than most people assume. Experiments with smooth dogfish sharks showed that when an odor reaches one nostril slightly before the other, the shark turns toward whichever side was stimulated first, even if the opposite side received a stronger pulse. In other words, timing beats concentration. This steering algorithm means that every time a shark encounters an odor patch, it turns into it, which keeps the animal locked onto an intermittent scent trail. The researchers suggested that having widely spaced nostrils would allow better angular resolution at higher swimming speeds, a feature that may help explain the bizarre head shape of hammerhead sharks.7PubMed. The function of bilateral odor arrival time differences in olfactory orientation of sharks
Smell does not work alone. Sharks also rely on their lateral line, a series of fluid-filled canals that detect water movement and pressure changes. When researchers blocked the lateral line in nurse sharks, the animals could still detect and follow an odor plume using their nose and eyes, but they lost the ability to precisely pinpoint where the scent was coming from. The lateral line lets sharks read the tiny swirling eddies in the water that carry odor molecules, essentially mapping the texture of a scent trail in three dimensions.8Journal of Experimental Biology. Sharks need the lateral line to locate odor sources: rheotaxis and eddy chemotaxis Studies of leopard sharks in the wild reinforced the importance of olfaction for large-scale navigation: sharks whose sense of smell was temporarily blocked advanced only about a third as close to shore as control animals and showed no directed movements beyond very short distances.9PLoS ONE. Olfaction Contributes to Pelagic Navigation in a Coastal Shark
Warm-Blooded Exceptions
Most chondrichthyans are ectothermic, meaning their body temperature matches the surrounding water. A handful of species break this rule. Mako sharks, great whites, and a few of their relatives have evolved specialized networks of blood vessels called retia mirabilia, essentially counter-current heat exchangers. Warm blood leaving the muscles passes alongside cool blood arriving from the gills, transferring heat inward so that it stays in the body rather than being lost at the gill surface. In makos, this system keeps the red swimming muscle and viscera significantly warmer than the ocean.10Journal of Experimental Biology. Water-tunnel studies of heat balance in swimming mako sharks The payoff is faster muscle contraction and more efficient digestion, advantages that help explain why makos are among the fastest fish in the sea.
Reproduction and the Surprise of Virgin Birth
Chondrichthyans are remarkably diverse in how they reproduce. Some lay eggs in leathery cases (the “mermaid’s purses” that wash up on beaches). Others carry developing embryos internally, nourishing them through a yolk sac, uterine secretions, or even a placenta-like structure. This range of strategies is wider than what you find in most other vertebrate groups.11PubMed Central. The adaptability of facultative parthenogenesis and ‘multiple embryos per eggcase’ as alternative reproductive strategies in Chondrichthyes
One of the more startling discoveries in recent years is that some sharks and rays can reproduce without mating at all. Parthenogenesis, or “virgin birth,” has now been documented in multiple species. In a population of smooth-hound sharks in captivity, researchers confirmed through genetic analysis that females produced offspring year after year without any male involvement, alternating between two females. The pups were homozygous at every genetic marker tested, consistent with a process called terminal fusion automixis. This was not a one-off fluke: the pattern repeated annually, ruling out long-term sperm storage as an explanation.12Scientific Reports. First report of recurrent parthenogenesis as an adaptive reproductive strategy in the endangered common smooth-hound shark Mustelus mustelus The finding raises interesting questions about whether parthenogenesis might serve as a backup strategy when mates are scarce, which could matter for small or fragmented populations.
Smarter Than Their Reputation
The popular image of sharks as instinct-driven predators does not survive contact with the experimental evidence. A review of chondrichthyan cognition research concluded that sharks and rays tested so far perform on par with most other vertebrates, including mammals and birds, in areas like spatial learning, social behavior, and discrimination tasks.13PubMed Central. Smart sharks: a review of chondrichthyan cognition Grey bamboo sharks, for instance, learned to navigate a T-maze to find food and retained that spatial knowledge for up to six weeks without any reinforcement, the first demonstration of long-term spatial memory in a shark species.14PubMed. Spatial learning and memory retention in the grey bamboo shark (Chiloscyllium griseum) The small number of species tested so far is itself a limitation. Most cognition research has focused on a handful of species that adapt well to captivity, and large pelagic sharks remain almost completely unstudied in this regard.
Life in the Deep Sea
Hundreds of chondrichthyan species live in the mesopelagic and bathypelagic zones, the vast dark layers of the ocean between about 200 and 4,000 meters. Some of these deep-sea sharks are bioluminescent, producing their own light through specialized cells in their skin. Lanternsharks in the family Etmopteridae are the best-studied group. Their eyes have adapted to detect the blue-green wavelengths that dominate in the deep, with rod visual pigments tuned to peak sensitivities between 484 and 491 nanometers. Researchers also identified a translucent area in the upper eye orbit of etmopterids that had never been described before, which may help the sharks calibrate their own glow for counterillumination (matching the faint downwelling light to erase their silhouette from below) or even break the camouflage of other bioluminescent animals.15PLoS ONE. Photon Hunting in the Twilight Zone: Visual Features of Mesopelagic Bioluminescent Sharks
The deep ocean has also played a role in the group’s long-term survival. Fossil evidence shows that certain ancient chondrichthyan lineages survived the devastating end-Permian extinction, which wiped out the majority of marine species, by retreating into deep-water habitats that were more buffered from the environmental chaos affecting shallow seas.16PubMed. Cretaceous stem chondrichthyans survived the end-Permian mass extinction The pattern echoes what happened to coelacanths, another ancient lineage that seemingly vanished from the fossil record only to turn up alive in deep water. During the later Cretaceous-Paleogene extinction, the one that killed the non-avian dinosaurs, chondrichthyans were hit unevenly: rays and species that specialized in crushing hard-shelled prey suffered extinction rates above 72 percent, while sharks and species with broader diets fared better.17PubMed. Global impact and selectivity of the Cretaceous-Paleogene mass extinction among sharks, skates, and rays Species with large geographic ranges also tended to survive at higher rates, a reminder that flexibility, whether in diet, habitat, or range, has repeatedly been the ticket to getting through catastrophic upheaval.
Chimaeras and Rays
Sharks dominate public attention, but the class Chondrichthyes includes two other major groups that are easy to overlook. Chimaeras (subclass Holocephali) are strange, big-eyed fish that split from sharks and rays early in chondrichthyan history. They have smooth skin without the sandpaper-like denticles of sharks, fused upper jaws, and ever-growing tooth plates instead of replaceable teeth. Metabolically, chimaeras resemble their elasmobranch relatives: their muscles show a reduced capacity for burning fat, their blood carries lower levels of free fatty acids, and they rely heavily on ketone bodies as fuel.18PubMed. Metabolic organization of the spotted ratfish, Hydrolagus colliei (Holocephali: Chimaeriformes): insight into the evolution of energy metabolism in the chondrichthyan fishes Most chimaeras live in deep water and are rarely encountered outside of deep-sea trawl surveys, which means we know far less about their ecology and behavior than we do about most sharks.
Rays and skates (the batoids) are the most species-rich group within Chondrichthyes. Their flattened bodies and enlarged pectoral fins have been reshaped for life on or near the seafloor, though some species, like manta rays, are fully pelagic. Batoids show a range of swimming styles from undulation (passing a wave down the pectoral fin, as stingrays do) to oscillation (flapping the fins up and down, as mantas do). Even within smaller species like the little skate, swimming kinematics shift with speed: the fins beat faster without changing much in amplitude, suggesting that frequency modulation is the main way these animals generate more thrust.19Journal of Experimental Biology. Batoid locomotion: effects of speed on pectoral fin deformation in the little skate, Leucoraja erinacea
Why Losing Sharks Reshapes Entire Ecosystems
The ecological importance of chondrichthyans, especially large sharks, extends far beyond what they eat. When great shark populations in the northwest Atlantic declined over a 35-year period, 12 of 14 prey species (smaller sharks, rays, and skates) increased in abundance. One of those prey species, the cownose ray, grew so numerous that its predation on bay scallops was enough to collapse a commercial scallop fishery that had operated for over a century.20PubMed. Cascading effects of the loss of apex predatory sharks from a coastal ocean This kind of trophic cascade, where the removal of a top predator sets off a chain reaction down the food web, has been documented in coastal systems and is increasingly suspected in open-ocean communities as well.21PubMed. Patterns and ecosystem consequences of shark declines in the ocean
Sharks also change the behavior of their prey even when they are not eating them. The mere presence of predators in an area can alter where prey species feed, how long they stay in a given patch, and how they move between habitats. Losing that “landscape of fear” can be just as disruptive to an ecosystem as the direct removal of predation pressure.22PubMed. Cascading top-down effects of changing oceanic predator abundances
A Conservation Crisis Driven by Overfishing
About a third of all chondrichthyan species are now threatened with extinction. When data-deficient species are included using statistical estimation, that figure rises to roughly 37.5 percent. Three species have been classified as Critically Endangered and possibly extinct, representing what may be the first global marine fish extinctions caused by overfishing. Overfishing is the threat affecting every single one of the 391 threatened species. For about two-thirds of them, it is the only threat; for the remaining third, it interacts with habitat loss, climate change, or pollution.23PubMed. Overfishing drives over one-third of all sharks and rays toward a global extinction crisis
Marine protected areas are one of the main tools available to help. Satellite tracking of grey reef sharks at Palmyra Atoll, part of the U.S. Pacific Remote Islands Marine National Monument, found that two-thirds of the tagged animals stayed exclusively within the protected area for up to over a year, suggesting that large reserves can provide substantial protection for resident species.24Biological Conservation. Assessing the effectiveness of a large marine protected area for reef shark conservation The picture is less encouraging for wide-ranging species. Satellite tracking of bull, great hammerhead, and tiger sharks in the western Atlantic found that the proportion of core habitat areas that fell inside fully protected zones ranged from zero percent for bull sharks to about 35 percent for tiger sharks. Expanding protections to include territorial waters would theoretically cover all the core habitat areas identified in that study, but political and economic realities make that a complicated proposition.25Diversity and Distributions. Use of marine protected areas and exclusive economic zones in the subtropical western North Atlantic Ocean by large highly mobile sharks
Shark Skin as an Engineering Template
The skin of most sharks is covered in tiny tooth-like structures called dermal denticles, which give it a sandpaper texture when rubbed the wrong direction. Each denticle has a crown with fine ridges (riblets) that interact with the flow of water passing over the body. These riblets reduce drag by disrupting the tiny vortices that form in the turbulent boundary layer close to the skin. Engineers have been trying to replicate this effect for decades, with potential applications on aircraft, ship hulls, and wind turbine blades. State-of-the-art manufacturing can now produce sub-millimeter-scale artificial denticles and nanotextured surfaces that have achieved reported drag reductions of up to 31 percent in laboratory settings.26Oxford Academic (Integrative and Comparative Biology). Experimental Studies of Bioinspired Shark Denticles for Drag Reduction Scaling those results to real-world conditions at higher flow speeds remains one of the central challenges in the field, since most tested surfaces work best at speeds under one meter per second.
Immune Systems and Tumor Research
The idea that “sharks don’t get cancer” is a myth that has been used to sell dubious shark cartilage supplements. Sharks do get cancer. But their immune systems are genuinely interesting, and the myth is rooted in a kernel of legitimate science. Chondrichthyans lack bone marrow and lymph nodes, two key immune organs in mammals. In their place, they have evolved unique tissues called the epigonal organ (associated with the gonads) and the Leydig organ (associated with the esophagus) that produce immune cells. When researchers cultured cells from the epigonal organ of bonnethead sharks and collected the substances those cells released, the resulting medium inhibited the growth of a range of mammalian tumor cell lines in the lab, including fibrosarcoma, melanoma, lymphoma, leukemia, pancreatic cancer, ovarian cancer, and breast carcinoma cells.27Integrative and Comparative Biology. Elasmobranch immune cells as a source of novel tumor cell inhibitors: Implications for public health Inhibiting cells in a dish is a far cry from curing cancer in a living person, and no therapy has emerged from this line of research so far. But the finding does point toward genuinely unusual immune mechanisms worth investigating, even as the supplement industry has run far ahead of the science.
The Economics of Living Sharks
One of the more persuasive arguments for chondrichthyan conservation is an economic one. An analysis of shark-related dive tourism in the Bahamas found that operators facilitated over 72,500 shark encounters in a single year, generating an estimated $78 million in economic input when all local services used by dive clients were factored in.28Elsevier / Biological Conservation. Effects of tourism-related provisioning on the trophic signatures and movement patterns of an apex predator, the Caribbean reef shark That same study investigated a common concern about shark feeding tourism: that regular provisioning would alter the animals’ behavior, making them dependent on handouts or changing their movement patterns. A small number of dominant sharks monopolized most of the bait, and those individuals showed elevated nitrogen levels in their tissues (a chemical signature of a shifted diet), but they did not change their overall residency patterns or daily movement distances compared to unfed sharks. Whether long-term feeding alters behavior in subtler ways remains an open question, but the initial evidence suggests sharks do not become couch potatoes when offered free meals.