Chondrichthyes

Chondrichthyes constitute a major class of jawed vertebrates whose skeletons are composed primarily of cartilage rather than true bone. These cartilaginous fishes include the sharks, skates, rays and chimaeras, representing some of the oldest extant vertebrate lineages. They form a distinctive group within the infraphylum Gnathostomata and are easily distinguished from the bony fishes (Osteichthyes) by their skeletal structure, dermal denticles and unique anatomical and physiological traits. Their evolutionary success is reflected in their wide distribution, from shallow coastal zones to the deep ocean, and their remarkable adaptations to various ecological niches.

General Biology and Distinctive Characteristics

Members of Chondrichthyes are fully aquatic vertebrates with paired fins, paired nostrils and a skin covered with placoid scales (dermal denticles), giving them a tough, sandpaper-like texture. They lack a bony operculum and do not possess a swim bladder, features typical of many bony fishes. Respiration occurs via five to seven pairs of gill slits, and buoyancy is maintained by an oil-rich, enlarged liver rather than a gas bladder.
Cartilaginous fishes range dramatically in size, from the minute finless sleeper ray measuring only a few centimetres to the whale shark, the largest living fish. Their sensory systems, feeding mechanisms and locomotory specialisations exhibit a high degree of evolutionary refinement.
Chondrichthyes are divided into two major subclasses:

• Elasmobranchii – sharks, skates, rays and sawfishes.
• Holocephali – chimaeras or ghost sharks.

These subdivisions reflect deep evolutionary splits and are marked by differences in skeletal elements, reproductive morphology and cranial structure.

Skeleton and Internal Anatomy

The skeletal system of Chondrichthyes is made of cartilage, a strong yet flexible tissue. Although lighter than bone, the cartilage is fortified with calcium salts to enhance durability. In most species, the notochord becomes replaced by a vertebral column during development, but in the Holocephali it persists throughout life.
Chondrichthyans lack bone marrow, and their red blood cells are instead produced in the spleen, the epigonal organ surrounding the gonads and, in many species, the Leydigs organ, a specialised haematopoietic structure located near the oesophagus. Members of the Holocephali lack both the Leydigs and epigonal organs, reflecting their more specialised physiology.
The heart contains a conus arteriosus, a muscular chamber supporting blood flow, a feature shared with certain early vertebrates. Their digestive system is relatively short and includes a spiral valve intestine, which increases absorptive surface area.

Body Covering and External Features

The skin of most cartilaginous fishes is characterised by dermal denticles, small toothlike scales that streamline the body, reduce drag and provide protection from predators and parasites. In many species, denticles are aligned in one direction, producing a smooth surface when stroked anteriorly and a rough one posteriorly.
Electric rays represent a major exception: they possess soft, loose skin, and their body is modified to house electric organs, used for defence or predation. Chimaeras also differ, as adults lose their dermal denticles except for those retained on the male clasping organ.
The evolution of dermal denticles and oral teeth has long been debated. It is commonly proposed that oral teeth originated through a modification of dermal denticles, although fossil evidence remains inconclusive. Early vertebrates such as placoderms had bony plates rather than true teeth, and the origin of oral dentition remains an important subject in vertebrate evolutionary biology.

Fins, Locomotion and Body Form

Chondrichthyans possess paired pectoral and pelvic fins, together with one or two dorsal fins and a caudal fin. A hallmark of sharks is the heterocercal tail, where the upper lobe of the caudal fin is larger than the lower. This asymmetry generates lift, compensating for the absence of a swim bladder.
Early cartilaginous fishes had pectoral and pelvic girdles that were not connected. Later lineages evolved ventral fusion points through the formation of the scapulocoracoid and puboischiadic bars. In rays and skates (Batomorphi), the pectoral fins have extended and fused with the head region, forming broad “wings” used for undulating or flapping locomotion.

Respiratory System

Most cartilaginous fishes respire through five to seven pairs of gills. Pelagic, fast-swimming sharks often rely on ram ventilation, swimming continuously to force water over their gills. Demersal species commonly use the spiracle, a small opening behind the eyes, to draw water into the gill chamber when resting on the seabed.
The spiracle varies in form: small and round in some benthic sharks, elongated in wobbegongs and entirely absent in highly active pelagic sharks such as thresher and mackerel sharks. Oxygen uptake efficiency is enhanced by the elaborate lamellae of the gill filaments, supporting high metabolic demands.

Sensory and Nervous Systems

Chondrichthyans possess some of the most sophisticated sensory systems among vertebrates. Their nervous system consists of a brain, eight to ten pairs of cranial nerves and a spinal cord with numerous spinal nerves.
Key sensory features include:

• Ampullae of Lorenzini, electroreceptive organs that detect electric fields and temperature gradients.
• A lateral line system, detecting vibrations and pressure changes in the surrounding water.
• Highly developed olfactory organs, enabling detection of minute chemical traces.
• Well-developed eyes, sensitive to low light and capable of colour perception in some species.
• Inner ears with three semicircular canals, aiding balance and spatial orientation.

Some species, particularly rays, possess electric organs, used offensively or defensively.
The myelin structure in their nervous system is nearly identical to that of tetrapods, providing important clues to the evolution of vertebrate neural architecture.

Immune System and Evolutionary Significance

Chondrichthyans possess a fully developed adaptive immune system similar to that of other jawed vertebrates. Their evolutionary history stretches back to the Devonian period, and fossil evidence—primarily teeth—indicates wide diversity in extinct forms. Although the fossil record is fragmentary, numerous ancient orders have been identified from distinctive dental remains, highlighting a long evolutionary lineage that preceded the diversification of many modern vertebrates.

Reproduction and Development

Reproduction in Chondrichthyes is characterised by internal fertilisation, facilitated by the male’s claspers, specialised extensions of the pelvic fins. Development varies across species and includes:

• Oviparity – egg-laying species enclosing embryos in tough, keratinous egg cases.
• Ovoviviparity – embryos develop inside eggs retained within the mother until hatching.
• Viviparity – true live birth, involving maternal-embryo nutrient transfer in certain advanced species.

Many sharks and rays show no parental care after birth, although egg cases of oviparous species may be strategically anchored to substrates. A notable phenomenon is capture-induced parturition, where pregnant females abort embryos under stress, a factor that complicates fisheries management for vulnerable species.

Taxonomic Structure and Major Groups

The taxonomic structure of Chondrichthyes comprises a wide array of extant and extinct lineages. Modern classifications recognise two principal subdivisions:
Subclass Elasmobranchii – sharks, rays and skates

• Division Shark
  • Order Carcharhiniformes
  • Order Lamniformes
  • Order Orectolobiformes
  • Order Heterodontiformes
  • Order Squaliformes
  • Order Echinorhinus
  • Order Squatiniformes
  • Order Pristiophoriformes
  • Order Hexanchiformes
• Division Batomorphi
  • Order Myliobatiformes
  • Order Rajiformes
  • Order Rhinopristiformes
  • Order Torpediniformes

Subclass Holocephali – chimaeras

• Superorder Holocephalimorpha
  • Order Chimaeriformes

Numerous extinct groups remain incertae sedis due to incomplete fossil preservation. These include ancient genera and families such as Bandringa, Delphyodontos, Listracanthidae, Nanocetorhinus, Plesioselachus, Psammodontiformes and Xiphodolamia.
Other fossil orders reveal the immense diversity of ancient cartilaginous fishes across geological eras, illustrating the long evolutionary history of the group.