CLASS OSTEICHTHYES - Jawed with bony skeleton – 96% of all fishes


The bony fish comprise the largest section of the vertebrates, with over 20,000 species worldwide. They are called bony fishes because their skeletons are mineralized bone tissue, much harder and heavier than the cartilage skeleton of the chondrichthyes.  Bony fish have evolved into every imaginable shape and size, and exploit most marine and freshwater habitats on earth. Many of them have complex physiologies, organs, and behaviors allowing them to deal with their environment in highly sophisticated and unique manners.


Osteichthyes are divided into two sub-classes, the Sarcopterygii (coelacanths and the lungfishes – closest relatives of tetrapods) and the Actinopterygii (various "primitive fishes" plus the enormous group of fishes known as the teleosts). 



Actinopterygians = Ray-finned Fishes



Teleosts – vast majority of ray-finned fishes – complete bony skeleton

Scales are of light bone and are small or absent in many fast swimmers

Drag is reduced by the production of surface mucous.

Most of the bony fishes have a sac like "swim bladder" to provide buoyancy

Gills covered by operculum


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Bony fishes secrete a layer of mucus that covers the entire body.  Mucus reduces friction and helps protect a fish from infection.  In some bony fishes, mucus may serve additional functions.  Some species of parrotfishes (family Scaridae) envelop their bodies in mucus bubbles at night while they rest. Nocturnal predators that rely on their sense of smell to locate prey may not be able to detect the parrotfish. Clownfish use specialized mucus that prevents some anemones from stinging. 


Swim bladder

Bony fish have swim bladders to help them maintain buoyancy in the water. Sharks are lighter than bony fishes because cartilage is lighter than bone tissue.  In addition, sharks increase buoyancy with their fatty livers.  Swim bladder is the bony fish solution.  Mechanisms for filling and emptying the bladder vary. Some fishes gulp air into their mouths and push it into the bladder while others add and remove gases via the circulatory system. In many fishes, the bladder is also used for sound amplification (croaking).




Skeletal system

The skeleton of bony fishes is made of bone and cartilage

Provides structure, protection, & leverage for muscles, and is a site of blood cell production

Muscular system

Tail and trunk muscles consist of a series of muscle blocks called Myotomes or myomeres. The myotomes are separated by

connective tissue called myosepta, which gives the fish flesh you eat its white lines. Fin

connections to muscle and bone are highly variable.





Bony fish show various degrees of fin fusion and reduction.  Fins help stabilize and propel the fish in the water.  Except in the lungfishes and the coelacanth, fins lack bones. Some bony fishes have soft, flexible fin rays.  Others have spiny, rigid fin rays at the leading edges of the dorsal, anal, and pelvic fins.  Both soft and spiny fin rays are modified scales. Some bony fishes use their pectoral fins to help them stay at the bottom and on reef areas.  Others support themselves on land with their pectoral fins. The dorsal fin of remoras is modified into a sucking disc. Fishes with blunted or rounded caudal fins are usually strong, but slow, swimmers.  Many continuously swimming fishes have forked caudal fins.  Fishes with crescent caudal fins tend to be the fastest fishes and maintain a rapid speed for long durations.


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Reproduction - Over 97% of all known fish are oviparous. However, a few oviparous fish practice internal fertilization, with the male using some sort of penis-like organ to deliver sperm into the genital opening of the female


1) Some bony fishes lay eggs that drift through the water column (spawning). Some bony fish eggs have oil droplets that help them float.

(2) Some bottom-dwelling bony fishes produce eggs that sink and remain on the ocean bottom.

(3) Some eggs may be sticky or may have tendrils that allow them to attach to plants and other living or nonliving materials in the environment.

(4) Some species brood eggs in their mouth or on their skin, fins, or gill areas.

5. Ovoviviparous – eggs retained in abdomen but no nutrition – i.e. pregnant female guppy

6. Viviparous – surf perch – nutrition for developing embryos - Nutrients and gases are exchanged through he female's ovarian walls and the embryo's vascular fin membranes and hind gut regions. The young are 54-61 mm (2.1-2.4 in) long and fully developed miniature replicas of the adults when they emerge from the female.


Parental care: In fishes, parental care is very rare as most fish are broadcast spawners, but there are a few instances of parental care. Some guard eggs in nests until they are born.  Chiclids, clownfish, damselfish


Sequential Hermaphrodites: Some damselfishes (family Pomacentridae) begin life as males and change into females. In some, females can revert back to males.  Some seabasses (family Serranidae) change from female to male, and are capable of reverting back to female.  Most wrasses (family Labridae) are born female, grow into sexually mature females, and have the potential to transform into functional males later in life. In many of the wrasses, sex change correlates with social hierarchy and social behavior: social structure includes a large dominant male and many smaller, subordinate females. Removing the male from the group triggers the largest female to begin transforming into a male.


Synchronous hermaphrodites have both sperm- and egg-producing organs at the same time. In a few species, self-fertilization is possible.





Digestive system

The esophagus in bony fishes is short and expandable so that large objects can be swallowed. The esophagus walls are layered with muscle.  The length of the intestine in bony fishes varies greatly. Herbivorous bony fishes generally have long, coiled intestines, and carnivorous bony fishes have short intestines.



Cardiovascular system

Bony fishes have a heart with two chambers: the atrium and the ventricle. Some tunas maintain a body temperature several degrees higher than that of the surrounding water using modifications to their circulatory system.  Higher body temperature is an adaptive advantage because it provides extra power for high-speed swimming.  A similar modified circulatory system warms the brain and eye of some species. 

Gas Exchange system

Water enters the gill chamber through the mouth. A bony fish can remain motionless and breathe by pumping water over its gills by opening and closing its mouth.  Blood in the gill filaments absorbs oxygen from the incoming water.  Water exits through the gill openings located under the operculum.

Lungfish must return to the surface to breathe air. A lungfish swallows air to fill up an air sac. This "lung" is surrounded by vessels that bring blood to be oxygenated. Its gills alone can't keep a lungfish supplied with enough oxygen to live. Other species such as tarpon can gulp air at the surface to supplement their oxygen demand.  Some species of bony fishes can absorb considerable amounts of oxygen through their skin.


Water is continually leaving the body of a marine fish through its skin and gills.  To keep from becoming dehydrated, a marine fish drinks large amounts of water and produces a small amount of concentrated urine. In addition, its gills are adapted to secrete salt.  The tissues of a freshwater fish are saltier than its surrounding environment.  Water is continually entering the body of a freshwater fish through its skin and gills.  Freshwater fishes do not drink water, and they produce large amounts of dilute urine.





Development may take place within the egg supported by yolk or during free-living stages. 

Egg, Yolk Fry, Fry, juvenile, adult.

Fish typically show larval stages.  Life cycles include marine, freshwater, and anadromous forms. Dramatic metamorphosis occurs in many species including sex change in tropical reef fish, movement of organs during development i.e. flounder and changes during migration in anadromous species i.e. salmon. Fish never stop growing entirely, though growth rate in mature fish is reduced.




Environmental Response

Nervous system

A bony fish's brain is divided into three sections: the forebrain, the midbrain, and the hindbrain.  The forebrain is responsible for the bony fish's ability to smell.  The midbrain is responsible for vision, hearing, learning, and motor responses. The hindbrain contains the medulla oblongala and cerebellum, which coordinates movement and balance. The spinal cord and a matrix of nerves serve the body. In addition to the usual special senses, some bottom-dwelling fishes have taste buds and touch receptors on their pectoral fins used to locate food.




Fish have excellent hearing with no external ear canals. 


Small, mineral structures found in the head of all bony fishes.  Otoliths provide a sense of balance to fish in much the same way that the inner ear provides balance in humans. Fish otoliths also aid in hearing.

To the fisheries biologist, the otolith is one of the most important tools for understanding the life of fish and fish populations. Growth rings (annuli) not unlike those of a tree record the age and growth of a fish from the date of hatch to the time of death.




The spiny fin rays of some species are associated with venom glands. Protective spines are common in slow-swimming bony fishes and bony fishes that hunt without fast swimming.




Many species of small bony fishes swim together in a coordinated fashion, called schooling.  Schooling is an adaptation for avoiding predators: An individual fish has a lesser chance of being eaten by a predator when in a school than when alone. A school of small fish may give the impression of a large animal, discouraging predators. Schooling poses a hydrodynamic advantage and increases reproductive success. It also may facilitate locating food sources.      

Territorial Behavior

Various species of bony fishes have sharply contrasting territorial behavior. Although damselfishes are relatively small, they are fearless in defending a territory. However, most large groupers will retreat from their territory if approached by another animal.

Sound Production          

Many bony fishes produce sound, sometimes in association with reproductive, social, territorial, or aggressive behavior.  Depending on the species, a bony fish can produce sound by rubbing its teeth together, vibrating its swim bladder, or by flexing and contracting muscles.  Most sounds produced by bony fishes are so low they are not audible to humans.

Symbiotic Relationships          

Several species of small bony fishes, such as the cleaner wrasse, eat debris and parasites from the skin and scales of larger fishes that report for the procedure.  Remoras commonly attach themselves to sharks or other large fishes, whales, and sea turtles using a modified dorsal fin. They eat scraps left over from the meals of their hosts and may eat parasites as well.  Some bony fishes have symbiotic relationships with non-fish species. Clownfish live unharmed among the venomous tentacles of sea anemones, which protect the clownfish from potential predators.


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Fish Learning Behavior

Fish can be trained to complete a number of complex tasks.  Forget what you heard about goldfish not being able to remember!