Did you know that not all fish have bones? The structure of some of them is made-up by cartilage. Today we will know which are the bony and cartilaginous fish as well as their main differences.
Table Of Content
Bony fish
Bony fish are those vertebrate and gnathostomes fish (vertebrates characterized by having articulated jaws). They are endowed with an internal bone skeleton, hence their name. They are also known as Osteichthyes
This internal bone skeleton is the main condition that differentiates them from the other large group of fish: cartilaginous fish.
As a curiosity, it must be said that there is a third group of fish, which is made up of jawless fish. The latter are very scarce and there is hardly a very small number of species among which are, for example, lampreys.
Bony fish are, mainly, the fish that we are used to seeing, that is, the most common species such as typical salmon, trout , aquarium fish, etc. Instead, cartilaginous fish, broadly speaking, are sharks, rays and blankets. We invite you to read our article goldfish to learn about these striking aquarium’s bony fish
Main Characteristics of Bony fish
Their skeleton is divided into several sections: the axial skeleton, which is the one that occupies the central part; the cephalic skeleton occupies the head; the zonal skeleton, (the one that is close to the pelvic and thoracic fins); and the appendicular skeleton, which is what makes-up the fins.
Perhaps, many of the differences between bony fish and cartilaginous fish are internally. Bony fish don’t have a spiral valve, but have pyloric blinds and lack that rectal gland.
The respiratory system of bony fish has gills located within the gill chamber, and covered by a sort of operculum that only exposes a small brachial opening on each side of the creature.
Rare is the time, although it’s is also possible, that a preperculum appears, but we insist that it’s something very unusual. It should be noted that these gills are not separated by septa.
In some species of bony fish, the swim bladder has evolved into a lung that helps them stay afloat, move vertically.
The mouth of these animals is called the terminal mouth, which is capable of very accurate movements, thanks mainly to the articulated dermal bones by which it’s made-up.
The teeth are usually small extensions of such dermal bones, and it’s important to point out that a fracture or loss of them becomes irreparable damage.
Another feature is that in addition to that internal skeleton; bony fish also have bones in some areas of their skin as scales.
When identifying whether a bony fish belongs to one species or another, such scales (mainly those found in their lateral and transverse lines) will be quite helpful. We invite you to read our article the anatomy of fish to learn deeply about such scales
Regarding the fins issue, bony fishes have a pair of pelvic fins, a pair of thoracic or pectoral fins (they are symmetrical in shape and body layout) and one or several dorsal or anal fins.
Depending on where the pelvic and thoracic fins are located, four types of bony fish arise: abdominal (if the pelvic fins are always behind the thoracic fins), thoracic (if the pelvic fins are at the same height or slightly delayed to the thoracic), yogular (if the pelvic fins are ahead of the thoracic fins) and, finally, the anodes (are those bony fish that lack pelvic fins).
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How Do Bony Fish Reproduce?
The reproduction of bony fish is sexual, presenting individuals of different sexes. It must be said that in many of the species of bony fish it’ s very, very difficult to distinguish between males and females, since they don’ t have a clear sexual dimorphism.
On the other hand, there are some species in which the sexes, over time, are invested. This variety of bony fish is known as sequential hermaphrodite fish. 
Males lack a copulatory organ since fertilization is external, except in some exceptions in which males have anal fins that make an internal fertilization process possible.
The vast majority are oviparous fish, although it’s normal for cases of ovoviviparous and viviparous bone fish to appear.
Some variety of bony fish take care of their eggs until the hatchlings hatch, but it’s unusual. The preferred time for reproduction depends a lot on the species.
Classification of Bony fish
Within the own family of bony fish, we can establish a new differentiation or classification, whose protagonists are ray-finned fish (Actinopterygii )and and lobe-finned
fish (Sarcopterygii)
(Actinopterygii are bony fish that have radiated fins made-up by
bony tissues. Their skull is mainly made of cartilaginous tissue.
Moreover, they have two gill openings protected by an operculum, and the scales are overlapping and rudimentary.
Finally, it’s good to point out that Actinopterygii lack internal internal nose or sewer.
As for Sarcopterygii, they are bony fish that have even fins of a fleshy or lobular tissue. These fins are very similar to the fins of some amphibians, which is a very clear signal of the evolutionary process. Within them, we have another subdivision: the coelacanths and the lungfish or Dipneos.
Lampreys
Lampreys are primitive fish that lack jaw and that resemble eel, although they are different inside, having a jelly-like and quite slippery body, which has no scales and cylindrical shape.
Lampreys have horny and suction-shaped teeth, which allow them to grab their prey, suck their blood and feed on them. It’s common to see them fixed to salmons, cod or even sharks.
This is an anadromous species, which lives in the sea, but reproduces in rivers. Especially those that have a large flow, of not very fast waters and that are placed in the low and medium sections.
Lampreys are usually along the North Atlantic, on both sides of the ocean. The increase that has been produced by artificial barriers and marine pollution have resulted in the area of distribution of this creature has dropped significantly.
We hope we were able to let you know this group of animals that are present in our day to day and in our daily lives, but of which we might not know certain things that make them very special fish.
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Cartilaginous fish
Cartilaginous fish, also known as chondrichthyes, are a class of fish characterized by having their skeleton made-up by cartilage and not by bone as occurs in bony fish. This group includes well-known marine animals such as: sharks, sawfish and rays.
An interesting aspect about these fish relies on their lack of ribs, so if they left the water, their own weight would cause their internal organs to be crushed long before drowning.
Unlike bony fish that are usually laterally flattened, cartilaginous fish are flattened on the dorso-ventral axis.
In addition, they generally have bodies of great proportions with respect to Osteichthyes fish; For example, some sharks or rays can be several meters long.
Cartilaginous fish have their body covered with scales with a shape similar to teeth known as dermal denticles, which have a double function: on the one hand to protect the animal and on the other to provide greater hydrodynamics.
All dermal denticles are oriented in the same direction, so if you caress the body of a cartilaginous fish in one direction, it can be completely smooth, while in the other it may seem extremely rough. In addition, some cartilaginous fish may have mucous glands that secrete a substance that improves their ability to swim efficiently.
Breathing in these fish is carried out through between five and seven pairs of gills, depending on the species.
Many of them need to swim continuously so that water can pass through the gills, although some species have the ability to actively pump water through their respiratory organs.
Cartilaginous fish may also breathe through spiracles, rather than gills, which are found on top of the heads of all rays and skates, and some sharks.
These openings allow the fish to rest on the ocean bottom and draw oxygenated water in through the top of their head, allowing them to breathe without breathing in sand.
Unlike the bony fishes, most cartilaginous fish lack any specialized organ to protect gills and these are exposed by gill slits of variable number depending on the species.
Moreover, it’s very important to point out that Cartilaginous fish also lack, (from the point of view of their anatomy), swim bladder so they can’t stay afloat without actively swimming.
This characteristic is also related to the need that most of them have to move to be able to breathe effectively.
However, some species can remain stationary and in this case they are at the bottom of the sea.
The most important sense for cartilaginous fish is smell, in fact they have the ability to detect small amounts of substances and follow their trail easily.
They also have specialized organs for capturing vibrations such as the lateral line. However, the sight is not specially developed in these animals.
How Do Cartilaginous fish Reproduce?
The reproduction of these fish is sexual and dioecious, since there are male and female individuals. Unlike bony fishes, cartilaginous ones usually do internal fertilization. Males have a copulatory organ through which they transfer their sperm to females.
However, not too many details are known about mating rituals or the coupling itself. This copulatory organ is called claspers.
The development of the young is usually oviparous or ovoviviparous, although some species are viviparous.
Normally for each reproductive cycle the number of offspring is much smaller than that found in bony fish.
The eggs are much larger than those of other fish and contain a lot of yolk. Some species take care of their eggs, but once they are born, the behaviors related to the care of the young are not widely known.
Let’s learn about sexual reproduction
Classification of cartilaginous fish.
Within the class of Cartilaginous there are two clearly differentiated subclasses: the Elasmiobranchs and the, hyostyly. The first of these are the typical cartilaginous fish that we can imagine, for example a shark or a ray, while the hyostyly are less abundant and less popular. Within the subclass of the Elasmiobranchs there are two superorders: the Selacimorphs and the Batoids.
Selacimorphs
In this group we find all sharks, also known as escualiforms. They stand out for their fusiform morphology more similar to that of bony fishes and for having the gill openings on the sides, 
presence of spiracles and the mouth is mostly ominous, that is, in ventral position.
Most of them are large predators, being at the top of the trophic chain of their habitat.
Batoids
In this group of cartilaginous fish, the most representative aquatic animals are the rays and the blankets (stingrays), although it also includes others such as sawfish (pristiform).
Blankets and Rays
Rays and stingrays are characterized by having the body dorsoventrally flattened and with highly developed pectoral fins that have merged with the head and implanted horizontally. The eyes and head are located, in most species, in the dorsal part. The opening of their gills is located on the ventral surface.
Stingrays are batoids in which many species acquire enormous proportions; in fact some stingrays can reach up to nine meters in length (they are commonly known as manta rays).
Their pelagic habits must really be highlighted as well as their skill to jump out of the water. in addition their mouth is oriented in a terminal or slightly inclined position, since they are filtering animals that need to swim with their mouths open to ingest the water and retain the food.
They have two cephalic appendages and their eyes stand out for being oriented laterally at the base of their fins.
On the other hand, the rays outstand because of their benthic customs and small size. They feed on invertebrates that hunt on the prowl, that is, they wait for their prey half-buried to go unnoticed.
Some species have a stinger or toothed spine in the tail, which produces toxins and is very useful as a means of defense against their main predators.
Special mention deserves the group of rhinobatids, known as guitar fish, which are rajiform with a curious morphology that resembles a guitar or a violin.
Electric Rays
The torpediniformes have ray form morphology highlighting their rounded disc, bare skin, mouth with presence of teeth, (although attached to their pectoral fins), small
Eyes seated on the dorsal part, thick and short tail, in which the dorsal fins and the caudal are very developed.
They have special organs on each side of the head that emit electric shocks, an adaptation to stun prey and capture them easily.
Saws fish
The pisiforms or saws fish have a morphology that differs quite a lot from the previous groups; their body is elongated, highlighting their snout (also elongated) with rostral teeth in lateral position.
In many occasions they are confused with the saw sharks included in the group of the escualiformes, although they differ from the sawfish because they have their gill openings in lateral position instead of ventral.
It’s the main reason why zoologists integrate pisiforms into the group of Baitoids.
Subclass Holocéfalos (Holocephali)
The majority of animals that comprise this group of cartilaginous fish are already extinct and currently only the order of the Chimeriforms is maintained, where the chimeras stand out.
They have a very peculiar aspect since their head protrudes from their body much more than in the other fish. Their mouth is protuberant, located in the ventral part, and their elongated tail reminds of some species of mammals.
In their morphology there are certain characteristics similar to that of bony fish, such as the presence of operculum.
However, they only have a hole with access to the outside on each side of the head, which communicates with the gill chamber, where the gill arches.
An important fact in chimeras is the fact that they lack a spiracle. They usually live at the bottom of the ocean and feed on small mollusks and other small invertebrates.
Chimeras
This curious and rare animal is nothing more and nothing less than a cartilaginous fish, close relative of sharks, although it’s estimated that both species separated approximately 400 million years ago.
These «ghost sharks», as they are also known (erroneously since they are not sharks) are usually found in deep ocean trenches, at depths of up to approximately 4,000 meters, although a few species can live in shallow waters.
This characteristic has made these animals truly enigmatic to science, since the arrival of man to such depths is relatively recent.
Previously, they only appeared in a very isolated way caught by mistake in fishing nets, constituting an event for fishermen, nothing accustomed to an animal with such a strange morphology.
Chimeras have a large protruding head in which a strange mouth is remembered that recalls that of a rabbit, as well as a long, sharp tail that can remind of that of a rat. This has motivated precisely that they have baptized it with the name of Chimera (order Chimaeriformes).
These fish can reach sizes up to a meter and a half long, although it’s not usual to find individuals of such size.
They have a soft skin and covered with placoid scales that give them colorations ranging from brownish gray to black.
Chimeras lack sharp or replaceable teeth like many sharks, but have very strong dental plates that are used to crush the hard structures of their prey, such as the strong shells of mollusks or exoskeletons of crustaceans.
To locate and capture them, they use very sensitive sensory structures that they have mainly in their head and that detect the weak electric fields of their victims. As a defense mechanism, most species have a poisonous spine located in front of their dorsal fin.
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Main Differences between Bony fish and Cartilaginous fish
The main difference between the bony fish and the cartilaginous fish is obviously the structure of the skeleton. As mentioned earlier, bony fish have a bony skeleton while cartilaginous fish have a skeleton made of cartilage.However, there are many other differences between these two kinds of fish. These differences are listed below.
Habitat
The vast majority of cartilaginous fish live in marine or salt water habitats.
These fish can be found in all the seas and oceans of the world. Bone fish, on the other hand, are found in both saltwater and freshwater habitats.
Gills
Fish gills are tissues located on both sides of the throat. These tissues contain ions and water in the fish system, where the water’s oxygen and the fish’s carbon dioxide are exchanged. In other words, the gills of the fish act like lungs.
In the bony fish, the gills are covered by an external skin flap, known as the operculum.
In cartilaginous fish, the gills are exposed and not protected by any external skin. Most fish, whether bony or cartilaginous, have five pairs of gills.
Heart and Blood
In both kinds of fish, the heart is divided into chambers. In the hearts of cartilaginous fish, one of these chambers is known as the arterial cone, a special heart muscle that contracts. Instead of this 
chamber, bony fish have a bulbous artery, a muscle that doesn’t contract.
Another difference between bony and cartilaginous fish lies in how each class produces red blood cells. In bony fish, red blood cells are produced in the bone marrow, (the central part of the bone). This process is known as hemopoiesis.
Cartilaginous fish lack bone marrow for hemopoiesis. Instead, these fish produce red blood cells in the spleen and thymus organs.
Mandibular structure
The jaw is part of the mouth, allowing it to open and close to grab and digest food. Its structure is different in cartilaginous and bony fish.
For example, bony fish have two sets of jaws: the oral jaw and the pharyngeal jaw. The oral jaw allows bony fish to trap food, bite and chew it.
The teeth usually only grow along one side of the jaw. The pharyngeal jaw, (located in the throat), digests the food further by processing it before it moves from mouth to stomach.
In contrast, cartilaginous fish lack the pharyngeal jaw. The oral jaw of these fish is made-up of cartilage and is divided into an upper and lower section.
Each section can contain several teeth, which grow in multiple sets. Cartilaginous fish can even grow their teeth again as they wear out over time.
Digestive system
The digestive system between bony and cartilaginous fish is also different. The intestine of cartilaginous fish is typically shorter than that of bony fish.
However, it spirals internally to create a larger surface area that optimizes nutrient absorption. In the bony fish, the intestine is longer and has no spiral shape.
J-shaped stomachs can be found in cartilaginous fish, while bony fish have a wide variety of stomach shapes and in some cases, they have no stomach at all.
The sewer, the opening through which urine and feces are excreted is also different. It can only be found in cartilaginous fish and with lobular fins. In other bony fish, the urinary tract, genitals and anus each have a separate opening.
Neutral buoyancy
Fish must have an internal flotation system to prevent them from floating on top of the water or sinking at the bottom, which is known as neutral floating. Bony fish are able to maintain neutral buoyancy with the help of the swim bladder.
The swim bladder is typically a two-bag organ that controls the volume of internal gases to help fish maintain a certain position in the water.
This allows them to conserve energy that they could otherwise use swimming to maintain neutral buoyancy.
Some bony fish have lost the swim bladder through evolution; most of these are species that inhabit the bottom.
Cartilaginous fish can achieve neutral buoyancy due to the lighter weight of their cartilaginous skeleton and its more hydrodynamic exteriors.
Some cartilaginous fish, such as sharks, even swim to the surface of the water to absorb the air that helps them maintain their position in the water.
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