Nature is home to an immense biodiversity, and within the animal kingdom, fish constitute one of the oldest and most fascinating groups of aquatic vertebrates. Among the great variety of species that exist, one peculiar group stands out: the cartilaginous fish, scientifically known as chondrichthyans (Chondrichthyes). This article explores in depth all of their characteristics, from their anatomy and physiology to their classification, behavior, habitats, diet, reproduction, ecology, and evolutionary relevance, integrating and comprehensively explaining all available relevant information.
What are cartilaginous fish?
The cartilaginous fish o chondrichthyans They are a group of aquatic vertebrates whose main characteristics are that its skeleton is composed entirely of cartilage, a tissue that is lighter and more flexible than bone, which provides advantages for swimming and maneuverability in the water. This group is considered one of the most old and conservatives of vertebrates, and although their diversity is lower compared to bony fish (osteictians), maintain a dominant presence in many marine ecosystems thanks to their morphological, sensory and reproductive adaptations.
The term 'cartilaginous fish' groups three main types of animals:
- Sharks (Selachimorphs)
- Rays and manta rays (Batoids)
- Chimeras (Holocephali)
These fish have a fundamental ecological importance and play vital roles as top predators, controlling the population of other species and maintaining the balance of aquatic ecosystems.
Origin and evolution of cartilaginous fish
The chondrichthyan group first appeared during the Upper Devonian period, at a time when the seas and oceans were undergoing significant evolutionary changes. The oldest known fossils belong to the genus CladoselacheThis group has gone through two major periods of evolutionary expansion, which allowed them to diversify widely in shapes, sizes, and lifestyles.
Over the course of evolution, cartilaginous fish adapted their cartilaginous skeleton, dispensing with the calcified bone present in their agnathan (jawless) ancestors in favor of greater flexibility and buoyancy. Their sensory and physiological development has led them to become dominant predators in many marine environments, and although they are not as numerous as bony fish, they have thrived in ecosystems from coastal waters to the abyssal depths.
The cartilaginous structure should not be seen as a primitive feature, but as a secondary adaptation which has given them advantages for survival and specialization.
Anatomical and morphological characteristics of cartilaginous fish
Cartilaginous fish have a fascinating anatomy and several Unique features that differentiate them from other groups de peces:
- Cartilaginous skeletonComposed entirely of cartilage, which reduces body weight, increases flexibility, and improves swimming and buoyancy. Shark cartilage, for example, is especially strong and different in composition from that of other vertebrates.
- Body shapeIn most sharks, it's fusiform (elongated and cylindrical), while in rays and manta rays, it's dorsoventrally flattened (from top to bottom). This facilitates different lifestyles: active predators or bottom-dwelling inhabitants.
- fins: They usually have large, rigid, laminar fins supported by cartilage, distributed in pairs (pectoral and pelvic) and unpaired fins (dorsal, caudal, and anal), although the anal fin may be absent in some species. In sharks, the caudal fin is asymmetrical (heterocercal), with a larger upper lobe, which favors rapid impulses and buoyancy.
- Placoid scales or dermal denticles: Its skin is covered by tiny tooth-like scales (dermal denticles), which are homologous to teeth and are composed of enamel, dentin, and pulp. This gives them a rough yet hydrodynamic surface, reducing friction with water and improving their resistance and defense against predators and parasites.
- notochord: When they are young, they have a notochord (a flexible embryonic structure), which later evolves into the definitive cartilaginous column.
- Advanced sensory organs: They have a lateral line that runs along the sides, capturing vibrations and water pressure, and, in sharks and rays, the Ampullae of Lorenzini, specialized in detecting electrical fields and muscle movements in prey and other animals. Furthermore, their sense of smell is extremely well-developed; they can detect minute concentrations of blood and track prey at great distances. Their vision, however, tends to be less acute than that of other fish, but they can adapt to low-light conditions.
- gill respiration: They breathe through gills, 5 to 7 pairs in number, open to the outside (without an operculum like bony fish), except in chimeras, which have a single covered slit.
- Absence of swim bladderThey lack the buoyancy characteristic of bony fish, which forces them to keep moving to avoid sinking. They compensate for this lack with a very large liver rich in oils (squalene), which aids buoyancy, especially in pelagic sharks.
- Reproductive organs: Males have claspers or pterygopodia, copulatory structures located between the pelvic fins for internal fertilization.
- Renewable teethBoth sharks and rays continually replace their teeth; they are not fixed to the jaw, but embedded in the gums, and are replaced throughout their lives.
- Size and weightIn general, they tend toward gigantism compared to other aquatic vertebrates. Some species of sharks and manta rays can reach several meters in length.
These characteristics have been fundamental to their evolutionary success over millions of years and have facilitated their adaptation to diverse aquatic ecosystems worldwide.
Differences between cartilaginous fish and bony fish
- SkeletonCartilaginous fish have a skeleton made of cartilage; bony fish have a skeleton made of calcified bone tissue.
- finsIn cartilaginous fish, the fins are more rigid and laminar; in bony fish, they are usually soft and flexible.
- GillsCartilaginous animals have open gill slits, while in bony animals they are protected by an operculum.
- ScalesCartilaginous cells have dermal denticles; bony cells have true scales (cycloid, ctenoid, or ganoid).
- Swim bladder: Absent in chondrichthyans; present in most de peces bony.
- ReproductionCartilaginous mammals tend to have internal fertilization with low fertility; bony mammals have external fertilization and a higher number of offspring.
- Ampullae of Lorenzini: Exclusive to cartilaginous animals, for electro-sensory detection.
- TeethIn cartilaginous bones, they are constantly replaced; in bony bones, they are fixed in the jaw.
Physiological and sensory adaptations of cartilaginous fish
Chondrichthyans are experts in stimulus detection and show a repertoire of advanced physiological adaptations that have allowed them to colonize from surface waters to great depths:
- Sideline: Sensory system that runs along both sides of the body, allowing the detection of movements and vibrations in the water, essential for locating prey and avoiding predators.
- Ampullae of Lorenzini: Specialized in capturing electric fields produced by other living organisms and natural variations in the environment, which is of utmost importance for hunting, even in dark or murky waters.
- Chemoreception and olfaction: Very sensitive nostrils (nasal openings), capable of detecting minute amounts of chemical substances, such as blood, at great distances.
- Adapted viewAlthough generally less acute than in other fish, some chondrichthyans can detect contrasts and movements in very low light conditions, such as in deep water.
- ThermoregulationSome species have physiological mechanisms that allow them to maintain parts of their body, such as the brain and swimming muscles, at temperatures slightly higher than that of the surrounding water (white shark, mako shark), which optimizes their hunting performance.
Habitat and distribution of cartilaginous fish
Cartilaginous fish have a wide distribution and occupy a wide variety of aquatic environments, from polar regions to tropical zones, and from shallow coastal waters, rivers, estuaries, reefs and bays to abyssal ocean depths.
Some species, such as sharks, can travel great distances and undertake seasonal migrations in search of food or breeding grounds. Rays and manta rays typically inhabit sandy and silty seabeds, where they find shelter and food. Chimaeras prefer deep areas and seabeds, where they develop their life cycle more discreetly.
Regarding adaptation to the habitat:
- Most are marine, but there are species that can tolerate brackish and even fresh water.
- Some species use specific habitats to raise their young, including estuarine areas, mangroves, and protected areas.
- Chimeras are usually found on deep seabeds, away from direct sunlight.
Feeding of cartilaginous fish
Cartilaginous fish have developed various feeding strategies which have allowed them to occupy different niches within the aquatic food chain:
- Top predators: Most sharks and many rays are fully developed predators. They feed de peces, mollusks, crustaceans, cephalopods, and even marine mammals. Their teeth adapt to the type of prey and their diet, featuring sharp, serrated, or flat teeth for cutting, tearing, or crushing.
- FiltersSharks like the whale shark and manta rays are filter feeders. plankton, small fish and crustaceans through their gills.
- omnivoresSome species may include plant matter, algae and organic detritus in their diet.
- Specialization in the fund: Rays and many chimaeras feed on small invertebrates, crustaceans and mollusks that they find buried in the seabed.
- sensory systems: Its advanced senses (smell, electroreception and lateral line) are essential for locating food and for hunting at night or in low visibility conditions.
Some species, especially sharks, show intrauterine cannibalism (oophagy) during embryonic development, where more developed embryos feed on eggs or less developed embryos in the maternal uterus.
Reproduction of cartilaginous fishes
La reproduction of cartilaginous fishes is another distinctive feature and shows a great diversity of strategies:
- internal fertilizationMales insert one of their classic spermatozoa into the female's cloaca, ensuring sperm entry. This technique allows for greater survival of the offspring under adverse environmental conditions.
- DioeciousThere are well-differentiated male and female individuals.
- Types of embryonic development:
- Oviparous: Females deposit large eggs, protected by resistant capsules. Examples: dogfish, some sharks, and rays.
- ovoviviparousThe eggs develop and hatch inside the mother, giving birth to fully formed young. Example: bull shark.
- viviparous: The offspring develop in the mother's uterus and are fed by the placenta or through fluid exchange with the mother.
- Low fertility and K strategy: They usually produce few offspring per reproductive cycle, but with high viability and early development. The offspring are born practically independent.
- Absence of parental care: Generally, after birth or hatching, the young are left to their own devices, although there are exceptions where a certain degree of temporary protection is observed.
- Intrauterine cannibalism: Especially in some sharks, oophagy occurs, where the most developed embryo consumes its siblings or unfertilized eggs.
These adaptations make populations de peces cartilaginous are especially vulnerable to overexploitation, since their low reproduction rate makes it difficult for their populations to recover in the face of indiscriminate fishing or the destruction of key habitats.
Taxonomic classification of cartilaginous fish
The chondrichthyans they are divided into two main subclasses, with a wide variety of orders and families:
- Elasmobranchs (Elasmobranchii)
- selachimorpha (Sharks):
- Carcharhiniformes (dogfish, hammerhead sharks)
- lamniformes (white shark, mako, basking shark)
- orectolobiformes (carpet shark, whale shark)
- Heterodontiformes (horn shark)
- squaliformes (cigar sharks, spiny sharks)
- Squatiniformes (angel shark)
- Pristiophoriformes (sawsharks)
- Hexanchiformes (six-gill shark)
- Bathoid (Rays and blankets):
- Rajiformes (true stripes)
- Myliobatiformes (blankets and whip rays)
- Pristiformes (sawfish)
- Torpediniformes (electric torpedo rays)
- selachimorpha (Sharks):
- Holocephali: Chimeras
- Chimaeriformes
- Callorhynchidae
- Rhinochimaeridae
- Chimaeridae (chimeras)
- Chimaeriformes
Meanwhile, the chimeras They are solitary fish of unusual appearance, with large heads, oversized eyes, and, in some cases, a poisonous spine on their tail, which they use as a defense. They lack dermal denticles, have a single gill slit covered by a soft gill operculum, and have an upper jaw fused with the skull. They live in deep waters and their diet is based primarily on mollusks and benthic invertebrates.
Examples of species de peces cartilaginous
- White shark (Carcharodon carcharias)
- Whale shark (Rhincodon typus)
- Hammerhead shark (Sphyrna spp.)
- Redpepper (Scyliorhinus canicula)
- Common ray (Raja clavata)
- giant blanket (birostris blanket)
- Sawfish (pristis pristis)
- Torpedo fish (Torpedo marmorata)
- Quimera (Chimaera monstrosa)
These examples illustrate the diversity of morphologies, habits and adaptations of chondrichthyans.
Ecological importance and threats
Cartilaginous fish are keys to ecological balance of the seas and oceans. As top predators, they regulate the abundance and health of populations de peces, mollusks and other marine animals, avoiding population explosions and promoting healthy and diverse ecosystems.
However, human activities have put many species at serious risk due to overfishing, habitat destruction, pollution and bycatchAdded to this is their low fertility rate and slow growth, factors that make the recovery of overexploited populations very slow or even impossible in some cases.
The conservation and study of these animals is essential for the preservation of marine biodiversity and to ensure the sustainability of aquatic ecosystems and global fisheries.
Cartilaginous fishes are a fascinating and essential group of the animal kingdom, with unique adaptations and an evolutionary history rich in successes and challenges. Understanding their biology and ecology is essential to valuing and protecting the richness of the oceans and the balance of the planet.