Chondrichthyes
Overview
 
Chondrichthyes or cartilaginous fishes are jawed fish
Fish
Fish are a paraphyletic group of organisms that consist of all gill-bearing aquatic vertebrate animals that lack limbs with digits. Included in this definition are the living hagfish, lampreys, and cartilaginous and bony fish, as well as various extinct related groups...

 with paired fins, paired nares, scales, two-chambered hearts, and skeletons made of cartilage
Cartilage
Cartilage is a flexible connective tissue found in many areas in the bodies of humans and other animals, including the joints between bones, the rib cage, the ear, the nose, the elbow, the knee, the ankle, the bronchial tubes and the intervertebral discs...

 rather than bone
Bone
Bones are rigid organs that constitute part of the endoskeleton of vertebrates. They support, and protect the various organs of the body, produce red and white blood cells and store minerals. Bone tissue is a type of dense connective tissue...

. The class is divided into two subclasses: Elasmobranchii
Elasmobranchii
Elasmobranchii is a subclass of Chondrichthyes or cartilaginous fish, that includes the sharks and the rays and skates .-Evolution:...

 (shark
Shark
Sharks are a type of fish with a full cartilaginous skeleton and a highly streamlined body. The earliest known sharks date from more than 420 million years ago....

s, ray
Batoidea
Batoidea is a superorder of cartilaginous fish commonly known as rays and skates, containing more than 500 described species in thirteen families...

s and skate
Skate
Skates are cartilaginous fish belonging to the family Rajidae in the superorder Batoidea of rays. There are more than 200 described species in 27 genera. There are two subfamilies, Rajinae and Arhynchobatinae ....

s) and Holocephali
Holocephali
The subclass Holocephali is a taxon of cartilaginous fish, of which the order Chimaeriformes is the only surviving group.Holocephali has an extensive fossil record that starts during the Devonian period. However, most fossils are teeth, and the body forms of numerous species are not known, or, at...

 (chimaera
Chimaera
Chimaeras are cartilaginous fish in the order Chimaeriformes, known informally as ghost sharks, ratfish , spookfish , or rabbitfishes...

s, sometimes called ghost sharks, which are sometimes separated into their own class).

Within the infraphylum Gnathostomata
Gnathostomata
Gnathostomata is the group of vertebrates with jaws. The term derives from Greek γνάθος "jaw" + στόμα "mouth". Gnathostome diversity comprises roughly 60,000 species, which accounts for 99% of all living vertebrates...

, cartilaginous fishes are distinct from all other jawed vertebrates, the extant members of which all fall into Teleostomi
Teleostomi
Teleostomi is a clade of jawed vertebrates that includes the tetrapods, bony fish, and the wholly extinct acanthodian fish. Key characters of this group include an operculum and a single pair of respiratory openings, features which were lost or modified in some later representatives...

.
The skeleton is cartilaginous.
Encyclopedia
Chondrichthyes or cartilaginous fishes are jawed fish
Fish
Fish are a paraphyletic group of organisms that consist of all gill-bearing aquatic vertebrate animals that lack limbs with digits. Included in this definition are the living hagfish, lampreys, and cartilaginous and bony fish, as well as various extinct related groups...

 with paired fins, paired nares, scales, two-chambered hearts, and skeletons made of cartilage
Cartilage
Cartilage is a flexible connective tissue found in many areas in the bodies of humans and other animals, including the joints between bones, the rib cage, the ear, the nose, the elbow, the knee, the ankle, the bronchial tubes and the intervertebral discs...

 rather than bone
Bone
Bones are rigid organs that constitute part of the endoskeleton of vertebrates. They support, and protect the various organs of the body, produce red and white blood cells and store minerals. Bone tissue is a type of dense connective tissue...

. The class is divided into two subclasses: Elasmobranchii
Elasmobranchii
Elasmobranchii is a subclass of Chondrichthyes or cartilaginous fish, that includes the sharks and the rays and skates .-Evolution:...

 (shark
Shark
Sharks are a type of fish with a full cartilaginous skeleton and a highly streamlined body. The earliest known sharks date from more than 420 million years ago....

s, ray
Batoidea
Batoidea is a superorder of cartilaginous fish commonly known as rays and skates, containing more than 500 described species in thirteen families...

s and skate
Skate
Skates are cartilaginous fish belonging to the family Rajidae in the superorder Batoidea of rays. There are more than 200 described species in 27 genera. There are two subfamilies, Rajinae and Arhynchobatinae ....

s) and Holocephali
Holocephali
The subclass Holocephali is a taxon of cartilaginous fish, of which the order Chimaeriformes is the only surviving group.Holocephali has an extensive fossil record that starts during the Devonian period. However, most fossils are teeth, and the body forms of numerous species are not known, or, at...

 (chimaera
Chimaera
Chimaeras are cartilaginous fish in the order Chimaeriformes, known informally as ghost sharks, ratfish , spookfish , or rabbitfishes...

s, sometimes called ghost sharks, which are sometimes separated into their own class).

Within the infraphylum Gnathostomata
Gnathostomata
Gnathostomata is the group of vertebrates with jaws. The term derives from Greek γνάθος "jaw" + στόμα "mouth". Gnathostome diversity comprises roughly 60,000 species, which accounts for 99% of all living vertebrates...

, cartilaginous fishes are distinct from all other jawed vertebrates, the extant members of which all fall into Teleostomi
Teleostomi
Teleostomi is a clade of jawed vertebrates that includes the tetrapods, bony fish, and the wholly extinct acanthodian fish. Key characters of this group include an operculum and a single pair of respiratory openings, features which were lost or modified in some later representatives...

.

Skeleton

The skeleton is cartilaginous. The notochord
Notochord
The notochord is a flexible, rod-shaped body found in embryos of all chordates. It is composed of cells derived from the mesoderm and defines the primitive axis of the embryo. In some chordates, it persists throughout life as the main axial support of the body, while in most vertebrates it becomes...

, which is present in the young, is gradually replaced by cartilage. Chondrichthyes also lack ribs, so if they leave water, the larger species' own body weight would crush their internal organs long before they suffocate.

As they do not have bone marrow
Bone marrow
Bone marrow is the flexible tissue found in the interior of bones. In humans, bone marrow in large bones produces new blood cells. On average, bone marrow constitutes 4% of the total body mass of humans; in adults weighing 65 kg , bone marrow accounts for approximately 2.6 kg...

, red blood cell
Red blood cell
Red blood cells are the most common type of blood cell and the vertebrate organism's principal means of delivering oxygen to the body tissues via the blood flow through the circulatory system...

s are produced in the spleen
Spleen
The spleen is an organ found in virtually all vertebrate animals with important roles in regard to red blood cells and the immune system. In humans, it is located in the left upper quadrant of the abdomen. It removes old red blood cells and holds a reserve of blood in case of hemorrhagic shock...

 and the epigonal organ (special tissue around the gonad
Gonad
The gonad is the organ that makes gametes. The gonads in males are the testes and the gonads in females are the ovaries. The product, gametes, are haploid germ cells. For example, spermatozoon and egg cells are gametes...

s, which is also thought to play a role in the immune system). They are also produced in the Leydig's organ
Leydig's organ
Leydig's Organ is a unique structure that is only found in elasmobranchs , although some elasmobranchs lack this organ. Along with the spleen and special tissue around the gonads, this structure produces red blood cells and it is nestled along the top and bottom of the esophagus....

 which is only found in cartilaginous fishes, although some do not possess it. The subclass Holocephali
Holocephali
The subclass Holocephali is a taxon of cartilaginous fish, of which the order Chimaeriformes is the only surviving group.Holocephali has an extensive fossil record that starts during the Devonian period. However, most fossils are teeth, and the body forms of numerous species are not known, or, at...

, which is a very specialized group, lacks both the Leydig's and epigonal organ.

Appendages

Their tough skin is covered with dermal teeth (again with Holocephali as an exception as the teeth are lost in adults, only kept on the clasping organ seen on the front of the male's head), also called placoid scales or dermal denticles, making it feel like sandpaper. In most species, all dermal denticles are oriented in one direction, making the skin feel very smooth if rubbed in one direction and very rough if rubbed in the other. Another exception are the electric rays, which have a thick and flabby body, with soft, loose skin devoid of dermal denticles and thorns.

Originally the pectoral and pelvic girdles, which do not contain any dermal elements, did not connect. In later forms, each pair of fins became ventrally connected in the middle when scapulocoracoid and pubioischiadic bars evolved. In rays
Batoidea
Batoidea is a superorder of cartilaginous fish commonly known as rays and skates, containing more than 500 described species in thirteen families...

, the pectoral fins have connected to the head and are very flexible.

One of the primary characteristics present in most sharks is the heterocercal tail, which aids in locomotion.

Body covering

Chondrichthyes have toothlike scales called denticles or placoid scales. Denticles provide two functions, protection, and in most cases streamlining. Mucous glands exist in some species as well.

It is assumed that their oral teeth evolved from dermal denticles which migrated into the mouth, but it could be the other way around as the teleost
Teleostei
Teleostei is one of three infraclasses in class Actinopterygii, the ray-finned fishes. This diverse group, which arose in the Triassic period, includes 20,000 extant species in about 40 orders; most living fishes are members of this group...

 bony fish Denticeps clupeoides has most of its head covered by dermal teeth (as does, probably, Atherion elymus, another bony fish). This is most likely a secondary evolved characteristic which means there is not necessarily a connection between the teeth and the original dermal scales.

The old placoderms did not have teeth at all, but had sharp bony plates in their mouth. Thus, it is unknown which of the dermal or oral teeth evolved first. Neither is it sure how many times it has happened if it turns out to be the case. It has even been suggested that the original bony plates of all the vertebrates are gone and that the present scales are just modified teeth, even if both teeth and the body armor have a common origin a long time ago. But for the moment there is no evidence of this.

Respiratory system

All Chondrichthyes breathe through 5-7 gills, depending on the species. In general, pelagic species must keep swimming to keep oxygenated water moving through their gills whilst demersal species can actively pump water in through their spiracles and out through their gills. However, this is only a general rule and many species differ.

A spiracle
Spiracle
Spiracles are openings on the surface of some animals that usually lead to respiratory systems.-Vertebrates:The spiracle is a small hole behind each eye that opens to the mouth in some fishes. In the primitive jawless fish the first gill opening immediately behind the mouth is essentially similar...

 is a small hole found behind each eye. These can be tiny and circular, such as found on the Nurse Shark (Ginglymostoma cirratum), to extended and slit-like, such as found on the Wobbegongs (Orectolobidae). Many larger, pelagic species such as the Mackerel Sharks (Lamnidae) and the Thresher Sharks (Alopiidae) no longer possess them.

Biology

Fertilization is internal. Development is usually live birth (ovoviviparous species) but can be through eggs (oviparous). Some rare species are viviparous. There is no parental care after birth; however, some Chondrichthyes do guard their eggs.

Phylogeny

Subphylum Vertebrata
└─Infraphylum Gnathostomata
├─Class Placodermi
Placodermi
Placodermi is a class of armoured prehistoric fish, known from fossils, which lived from the late Silurian to the end of the Devonian Period. Their head and thorax were covered by articulated armoured plates and the rest of the body was scaled or naked, depending on the species. Placoderms were...

 — extinct (armored gnathostomes)
└Microphylum Eugnathostomata
Eugnathostomata
-Taxonomy and phylogeny: Subphylum Vertebrata └─Infraphylum Gnathostomata ├─Class Placodermi — extinct └Microphylum Eugnathostomata...

(true jawed vertebrates)
├─Class Chondrichthyes (cartilaginous fish)
└─(unranked) Teleostomi
Teleostomi
Teleostomi is a clade of jawed vertebrates that includes the tetrapods, bony fish, and the wholly extinct acanthodian fish. Key characters of this group include an operculum and a single pair of respiratory openings, features which were lost or modified in some later representatives...

 (Acanthodii & Osteichthyes)
├─Class Acanthodii
Acanthodii
Acanthodii is a class of extinct fishes, sharing features with both bony fish and cartilaginous fish. In form they resembled sharks, but their epidermis was covered with tiny rhomboid platelets like the scales of holosteans...

 — extinct ("spiny sharks")
└Superclass Osteichthyes
Osteichthyes
Osteichthyes , also called bony fish, are a taxonomic group of fish that have bony, as opposed to cartilaginous, skeletons. The vast majority of fish are osteichthyes, which is an extremely diverse and abundant group consisting of over 29,000 species...

 (bony fish)
├─Class Actinopterygii
Actinopterygii
The Actinopterygii or ray-finned fishes constitute a class or sub-class of the bony fishes.The ray-finned fishes are so called because they possess lepidotrichia or "fin rays", their fins being webs of skin supported by bony or horny spines , as opposed to the fleshy, lobed fins that characterize...

 (ray-finned fish)
└─Class Sarcopterygii
Sarcopterygii
The Sarcopterygii or lobe-finned fishes – sometimes considered synonymous with Crossopterygii constitute a clade of the bony fishes, though a strict classification would include the terrestrial vertebrates...

 (lobe-finned fish)

 
Note: lines show evolutionary relationships.

Taxonomy

The extant members of the Chondrichthyes are the shark
Shark
Sharks are a type of fish with a full cartilaginous skeleton and a highly streamlined body. The earliest known sharks date from more than 420 million years ago....

s and rays
Batoidea
Batoidea is a superorder of cartilaginous fish commonly known as rays and skates, containing more than 500 described species in thirteen families...

, belonging to the subclass Elasmobranchii
Elasmobranchii
Elasmobranchii is a subclass of Chondrichthyes or cartilaginous fish, that includes the sharks and the rays and skates .-Evolution:...

, and the chimaera
Chimaera
Chimaeras are cartilaginous fish in the order Chimaeriformes, known informally as ghost sharks, ratfish , spookfish , or rabbitfishes...

s, belonging to the subclass Holocephali
Holocephali
The subclass Holocephali is a taxon of cartilaginous fish, of which the order Chimaeriformes is the only surviving group.Holocephali has an extensive fossil record that starts during the Devonian period. However, most fossils are teeth, and the body forms of numerous species are not known, or, at...

. Nelson's 2006 Fishes of the World arranges the class as follows:
  • Subclass Elasmobranchii
    Elasmobranchii
    Elasmobranchii is a subclass of Chondrichthyes or cartilaginous fish, that includes the sharks and the rays and skates .-Evolution:...

    • Plesioselachus
    • †Order Squatinactiformes
    • †Order Protacrodontiformes
    • †Infraclass Cladoselachimorpha
      • †Order Cladoselachiformes
        Cladoselachiformes
        Cladoselachidae is an extinct family of cartilaginous fishes and among the earliest predecessors of modern sharks. They are the only members of the order Cladoselachiformes and were characterized by having an elongated body with a spine in each of the two dorsal fins.-References:*, Dictionary of...

    • †Infraclass Xenacanthimorpha
      • †Order Xenacanthiformes
    • Infraclass Euselachii (sharks and rays)
      • †Order Ctenacanthiformes
      • †Division Hybodonta
        • †Order Hybodontiformes
          Hybodontiformes
          Hybodontiformes was an order of sharks, they were characterised by possessing 1-2 pairs of hooked protrusions on their heads.-External links:* http://www.palaeos.com/Vertebrates/Units/070Chondrichthyes/070.600.html#Hybodontiformes...

      • Division Neoselachii
        • Subdivision Selachii (modern sharks)
          • Superorder Galeomorphi
            • Order Heterodontiformes (bullhead sharks)
            • Order Orectolobiformes (carpet sharks)
            • Order Lamniformes
              Lamniformes
              Lamniformes is an order of sharks commonly known as mackerel sharks . It includes some of the most familiar species of sharks, such as the great white shark, as well as more unusual representatives, such as the goblin shark and the megamouth shark.Members of the order are distinguished by...

               (mackerel sharks)
            • Order Carcharhiniformes
              Carcharhiniformes
              The ground sharks, order Carcharhiniformes, are the largest order of sharks. With over 270 species, carcharhiniforms include a number of common types, such as the blue shark, catsharks, swellsharks, and sandbar shark....

               (ground sharks)
          • Superorder Squalomorphi
            • Order Hexanchiformes
              Hexanchiformes
              Hexanchiformes is the order consisting of the most primitive types of sharks, and numbering just five extant species. Fossil sharks that were apparently very similar to modern sevengill species are known from Jurassic specimens....

               (frilled and cow sharks)
            • Order Echinorhiniformes (bramble sharks)
            • Order Squaliformes
              Squaliformes
              Squaliformes is an order of sharks that includes about 97 species in seven families.Members of the order have two dorsal fins, which usually possess spines, no anal fin or nictitating membrane, and five gill slits. In most other respects, however, they are quite variable in form and size...

               (dogfish sharks)
            • †Order Protospinaciformes
            • Order Squatiniformes (angel sharks)
            • Order Pristiophoriformes (sawsharks)
        • Subdivision Batoidea
          Batoidea
          Batoidea is a superorder of cartilaginous fish commonly known as rays and skates, containing more than 500 described species in thirteen families...

           (rays)
          • Order Torpediniformes (electric rays)
          • Order Pristiformes (sawfishes)
          • Order Rajiformes
            Rajiformes
            Rajiformes is one of the four orders of batoids, flattened cartilaginous fishes related to sharks.Rajiforms are distinguished by the presence of greatly enlarged pectoral fins, which reach as far forward as the sides of the head, with a generally flattened body. The undulatory pectoral fin motion...

             (skates and guitarfishes)
          • Order Myliobatiformes
            Myliobatiformes
            Myliobatiformes is one of the four orders of batoids, cartilaginous fishes related to sharks. They were formerly included in the order Rajiformes, but more recent phylogenetic studies have shown that the myliobatiforms are a monophyletic group, and that its more derived members evolved their...

             (stingrays and relatives)
  • Subclass Holocephali
    Holocephali
    The subclass Holocephali is a taxon of cartilaginous fish, of which the order Chimaeriformes is the only surviving group.Holocephali has an extensive fossil record that starts during the Devonian period. However, most fossils are teeth, and the body forms of numerous species are not known, or, at...

    • †Superorder Paraselachimorpha
      • †Order Orodontiformes
      • †Order Petalodontiformes
        Petalodontiformes
        Order Petalodontiformes is a group of extinct marine cartilaginous fish related to modern day chimaera found in what is now the United States of America and Europe. With a very few exceptions, they are known entirely from teeth...

      • †Order Helodontiformes
      • †Order Iniopterygiformes
        Iniopterygiformes
        Iniopterygiformes is an extinct order of chimaera-like cartilaginous fish that lived from the Devonian to Carboniferous periods . Fossils of them have been found in Montana, Indiana, Illinois, and Nebraska...

      • †Order Debeeriiformes
      • †Order Eugeneodontiformes*
    • Superorder Holocephalimorpha
      • †Order Psammodontiformes*
      • †Order Copodontiformes
      • †Order Squalorajiformes
      • †Order Chondrenchelyiformes
      • †Order Menaspiformes
      • †Order Coliodontiformes
      • Order Chimaeriformes (chimaeras)


* position uncertain
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