Chondrichthyes

• 1. Introduction

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  Chondrichthyes is the class of jawed fish whose skeletons are built not from bone but from cartilage, the same flexible tissue that shapes a human ear or nose. That single anatomical fact sets them apart from every other jawed vertebrate on Earth. The group includes sharks, rays, skates, sawfish, and the strange deep-sea chimaeras, sometimes called ghost sharks. Living members range in size from the finless sleeper ray, barely 10 centimetres long, to the whale shark, which can exceed 10 metres. What does a skeleton made of cartilage actually mean for how these animals live, breathe, sense the world, and reproduce? And how far back does their story reach? The answers stretch across hundreds of millions of years, into some of the oldest chapters of vertebrate life.

• 2. A Body Built Differently

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  Cartilage in place of bone is not simply a material substitution; it reshapes almost every system in the body. Because cartilaginous fish lack bone marrow, they cannot produce red blood cells the way most vertebrates do. Instead, blood cells form in the spleen, the epigonal organ, a specialised tissue wrapped around the gonads, and, in certain species only, the Leydig's organ. The subclass Holocephali, which is among the most specialised of all cartilaginous fish, lacks both the Leydig's and epigonal organs entirely.

  The skin of most chondrichthyans carries its own distinctive armour: tiny tooth-like structures called placoid scales, or dermal denticles. Run a hand along the skin in one direction and it feels like sandpaper; reverse the stroke and it smooths out. In most species all these denticles point the same way, creating that directional texture. Electric rays are the main exception, with thick, flabby bodies and loose, soft skin.

  The question of how oral teeth evolved in this lineage remains genuinely open. Researchers have long assumed that the mouth teeth migrated inward from the outer dermal denticles. But two bony fish, Denticeps clupeoides and probably Atherion elymus, have most of their heads covered by dermal teeth, suggesting the relationship between surface scales and oral teeth is more tangled than a simple inside-out migration. It has even been proposed that the original bony plates of all vertebrates eventually gave way to what are now just modified teeth, though no current evidence confirms that.

• 3. Senses and the Nervous System

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  Ampullae of Lorenzini are one of the most remarkable sensory tools in the animal kingdom. These are networks of small jelly-filled pores that act as electroreceptors, letting cartilaginous fish detect electric fields in the water around them. They use this sense to find prey, navigate, and even gauge temperature.

  The lateral line system complements this capability. Modified epithelial cells positioned externally pick up motion, vibration, and pressure changes in the water. The eyes of most species are large and well-developed, and the olfactory organs are described as very powerful. The inner ear carries three large semicircular canals that manage balance and orientation. The sound-detecting apparatus tends to work better at lower frequencies.

  Some species add yet another tool: electric organs capable of both defence and predation. Despite this sensory richness, the brain itself is relatively simple, with the forebrain not greatly enlarged. What has drawn the attention of evolutionary biologists is the structure of myelin, the insulating sheath around nerve fibres. In chondrichthyans, myelin formation is nearly identical to that found in tetrapods, the four-limbed land vertebrates. That similarity has led researchers to place cartilaginous fish as a cornerstone group in the evolutionary history of myelin.

• 4. Breathing and Moving

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  All chondrichthyans breathe through five to seven pairs of gills, and the number varies by species. Open-water, or pelagic, species generally must keep moving to push oxygenated water through their gills continuously. Bottom-dwelling, or demersal, species take a different approach: they can actively pump water in through spiracles, small holes located just behind each eye, and out through the gills.

  Spiracles range from tiny and circular, as seen on the nurse shark, known scientifically as Ginglymostoma cirratum, to extended and slit-like, as found on the wobbegongs of the family Orectolobidae. Many larger pelagic species, including mackerel sharks in the family Lamnidae and thresher sharks in the family Alopiidae, no longer possess spiracles at all.

  Locomotion in most sharks relies on the heterocercal tail, an asymmetrical structure where the upper lobe is longer, which aids in propulsion. In rays, the pectoral fins fuse to the head and become extremely flexible, enabling the sweeping glide that makes rays so distinctive. The tail in Holocephali is long and thin, and these animals move by sweeping their large pectoral fins rather than by using a conventional tail-driven stroke.

• 5. Two Subclasses, Two Paths

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  Elasmobranchii, the first subclass, contains the sharks, rays, skates, and sawfish. These fish lack swim bladders and have five to seven pairs of gill clefts that open individually to the exterior. The upper jaw is not fused to the skull, giving it considerable mobility. Males carry a grooved clasper on the inner margin of each pelvic fin, used to transmit sperm during internal fertilisation.

  Holocephali, whose name translates roughly as "complete-heads," take a different form. The only surviving order is Chimaeriformes, which includes rat-fishes such as Chimaera, rabbit-fishes such as Hydrolagus, and elephant-fishes in the genus Callorhynchus. Their mouths are small apertures rimmed by lips that give the head a parrot-like look. They have no stomach in the conventional sense; the gut is simplified, and what would be the stomach merges with the intestine. The dorsal fin carries an erectile spine that is sometimes venomous. Holocephali live near the bottom and feed on molluscs and other invertebrates. In certain species, males possess an additional structure called a spine-brush complex instead of a first dorsal fin, a feature linked to heavy sexual dimorphism in the extinct order Symmoriiformes.

• 6. Reproduction and a Hidden Crisis

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  Fertilisation in all chondrichthyans is internal. Most species give birth to live young, a mode called ovoviviparity, while others lay eggs. A small number of species are truly viviparous. No species provides parental care after birth, though some do guard their eggs while the young develop.

  A largely overlooked consequence of fishing has been documented in sharks and rays that carry live young. When these animals are caught, capture can trigger premature birth or abortion, a phenomenon researchers call capture-induced parturition. Recreational fishers often mistake these events for natural births and report them as such. Commercial fisheries management has rarely factored in this effect, even though studies show it occurs in at least 12 percent of live-bearing sharks and rays, spanning at least 88 species.

• 7. Deep Roots, Ancient Cousins

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  Cartilaginous fish are considered to have evolved from acanthodians, an assemblage of ancient spiny fish. The fossil genus Doliodus is regarded as among the closest relatives to Chondrichthyes; it carried a mosaic of chondrichthyan and acanthodian traits, making it a transitional figure in two lineages at once.

  The oldest unequivocal fossils of acanthodian-grade cartilaginous fish are Qianodus and Fanjingshania, found in the early Silurian, specifically the Aeronian stage, of Guizhou, China. They date to around 439 million years ago and hold the additional distinction of being the oldest unambiguous remains of any jawed vertebrates at all. A separate species, Shenacanthus vermiformis, which lived 436 million years ago, bore thoracic armour plates that resembled those of the much better-known placoderms.

  By the opening of the Early Devonian, some 419 million years ago, jawed fishes had already branched into three distinct groups: the now-extinct placoderms, the bony fishes, and the clade containing spiny sharks and early cartilaginous fish. The first abundant shark genus, Cladoselache, appeared during the Devonian. Among the more unusual extinct lineages, Iniopterygia were early chondrichthyans that resembled flying fish, while Xenacanthiformes were eel-like cartilaginous fish that typically lived in freshwater. The fossil record of Holocephali extends back to the Devonian, though most of what survives are teeth, and the body plans of many species remain poorly understood or entirely unknown.