Superclass Osteichthyes
Patrick Doran
Taxonomy:
Subclass Sarcopterygii: (Fleshy Finned Fishes)
These fish are primarily cartilaginous with limited ossification. Most of these fish are extinct and this class represents an important evolutionary step that led to the animals that are most numerous today. The species that are still extant are considered living fossils. These fish evolved into the very bony ray-finned fish that make up 96% of all fish species and are what most people consider stereotypical fish. These fish also evolved into the tetrapods when they populated land.
Superorder Dipnoi- Lungfish
Lungfish live in shallow swamps and marshes and have primitive lungs that developed from airbladders. Due to their lack of an airbladder, these fish primarily inhabit the bottom of their habitats, only briefly swimming to the surface for air. While some of these fish rarely use their lungs and rely primarily on their gills for respiration except for in times when dissolved oxygen levels are especially depleted, other species are obligatory air breathers who can not survive solely on respiration from their limited gills and must go to the surface to inhale air on a regular basis. Some of these fish burrow into the ground when their habitat dries up and go into a state of animal dormancy called aestivation during which they only use a small fraction of their original metabolic rate and only breathe air.
Superorder crossopterygii- (Lobe finned fishes)
There are only two extant fishes in this superorder, both coelacanths. These fish live at the bottom of their habitats and many times walk along the bottom of these habitats. These fish eventually developed lungs and walked onto land, evolving into the tetrapods. They have very few bones. Most of what is known about these fish is known from the fossil record and they were studied extensively from fossils long before the first live specimen was discovered in the 1930’s.
Subclass Actinopterygii- (Ray Finned Fish)
This subclass contains almost every extant species of fish and most likely evolved after the fish in subclass sarcopterygii and is therefore more evolutionarily advanced. Most of the fish in this subclass are bonier than the fish in subclass sarcopterygii. Most of these fish also have more developed airbladders that make them more buoyant allowing them to inhabit all levels of their habitat, not only the bottom. Many of these fish are fast swimmers with the fastest reaching speeds up to
80 kilometers per hour.
Infraclass Chondrostei

These are the most cartilaginous and therefore the most primitive of the actinopterygii. Many of their characteristics resemble sharks but they still have an operculum and a limited amount of bone. These fish are an evolutionary step that led to the rest of the actinopterygii and there are only two extant orders in this infraclass, polypteriformes and acipenseriformes.
Polypteriformes are bichirs. Bichirs are fish that live in Africa and can survive in water with very low levels of dissolved oxygen. Their fins are intermediates between the lobed fins of the sarcopterygii and the ray fins of the actinopterygii. The order acipenseriformes contains the sturgeons and the paddlefish. These fish have eggs that are valuable for their use as caviar.
Because the eggs of many fish in this order are harvested for profit, it contains many endangered species.
Infraclass Holostei
This infraclass is evolutionary step between the more primitive infraclass chondrostei and the more advanced infraclass teleostei. They have a welldeveloped bony skeleton but not nearly as many bones as the fish in infraclass teleostei. There are two orders in this infraclass, one contains the gars and one contains the bowfins. The gars are carnivores that swim slowly through their habitat until they detect pray, which they can speed up to capture. They have a very bony snout with many teeth (Whitmore). There is only one extant species of bowfin. They are predators that live in the shallow areas of rivers and they have major areas of their skeleton that are primarily cartilaginous.
Infraclass teleostei
This infraclass contains most extant species of fish and is mostly composed of archetypical ray-finned fish. It has many orders due to the large amount of species in this infraclass. Most of these fish are very bony and have an advanced airbladder system that allows them to swim at all levels of their habitat.
Habitat: These fish inhabit every aquatic habitat on the planet including a wide variety of freshwater and saltwater habitat. Most evolutionary biologists believe that these fish originally evolved in freshwater and then went through a series of evolutionary adaptations that allowed them to inhabit saltwater. Some fish, including salmon undergo these adaptations during their lifecycle when they migrate from freshwater to saltwater. The salmon makes many changes between when it is in a freshwater environment and when it’s in a saltwater environment to deal with the obvious osmotic problems that must be dealt with when crossing between environments. Many of these same changes may have happened between species when osteichthyes first inhabited saltwater. When the salmon is in a saltwater environment, it drinks immense amounts of water to combat the movement of water out of its body due to the hypertonic environment it inhabits. In contrast, when the salmon is in a freshwater environment, it almost never drinks and gets most of its hydration from the movement of water into its body due to the

hypotonic environment it is in. The kidneys of the salmon also adapt to the environment. In a freshwater environment, the salmon produces a large amount of dilute urine because it is taking in a lot of water and not many ions. In contrast, the salmon produces very little highly concentrated urine in saltwater environments to counter the high ion concentration of the water it’s taking in. This reduces the ion level in the body of the salmon making it more suitable to life. Salmon also have a transmembrane protein in its gill cells that uses ATP to actively transport sodium and chloride ions against their concentration gradients. When the salmon is in freshwater, this protein actively transports salt into the body of the salmon to utilize the limited amount of salt that exists in freshwater. In saltwater environments this protein pumps salt out of the fish so that the levels of salt in the fish don’t get high enough to kill the fish due to the immense amounts of salt water that the fish drinks.
These adaptations most likely mimic the adaptations that happened over many generations when the ancestors of the extant osteichthyes first evolved to inhabit saltwater environments.
Feeding mechanisms: Many species of osteichthyes are predators who eat other fish, mollusks, crustaceans and worms. There are also species of bony fish that eat plants and omnivorous bony fish that are opportunistic feeders and will eat whatever is most readily available. Some bony fish are also filter feeders that ingest plankton from the water that surrounds them. The digestive system of most bony fish is very similar to the mammalian digestive system. Many bony fish have accessory teeth in the roof of their oral cavity and on the tongue. Many have accessory teeth or gill rakers in the pharynx both of which assist in the digestion of food.
Reproduction: Reproduction in bony fish is typically external. Typically a female will lay many eggs in the sand or mud in the body of water that the fish typically live in. These unfertilized fish eggs are called roe and can be sold as caviar if they belong to certain species. The male will proceed to release milt, a milky fluid that contains sperm, onto the eggs fertilizing most of them. While there is little parental protection of young after the eggs hatch, parents often protect fertilized eggs. A quintessential example of this is the male seahorse that protects fertilized eggs in a pouch. Many consider this to be male pregnancy. Some species of bony fish are hermaphrodites. There are also a few species of bony fish that undergo internal reproduction but these are rare exceptions.
Unique Characteristics: Most bony fish have swim bladders. These are cavities in the fish that the fish either emits or releases air into to control buoyancy. This is how fish control the level they are at in the body of water they inhabit. The fish generates gas to fill the bladder with a gas gland that emits carbon dioxide. The swim bladder is evolutionarily homologous to the lungs in that terrestrial and aquatic animals both have structures in the same area of their body that or filled with air. It is clear that the swim bladder evolved into the lungs and Charles Darwin observed this in his book, On the Origin of Species. This is most obvious in the lungfish, which has primitive lungs in the same place that other fish have swim

bladders. The lungfish is in the same infraclass that eventually evolved to form all terrestrial animals.