A teleost is a member of Teleostei, the largest and most diverse group of ray-finned fishes. The name “teleost” comes from Greek words meaning “complete bone,” reflecting their fully ossified skeletons. This infraclass encompasses nearly all living fish species, representing 96% of all extant fish and about half of all vertebrate species. Teleosts are the dominant aquatic vertebrates today, distinguished by advanced anatomical features.
Defining Physical Characteristics
A distinguishing feature of teleosts is their highly mobile and protrusible jaw. This allows them to extend their mouth outwards, creating a suction force that pulls prey into their mouth. The premaxilla, a bone in the upper jaw, is unattached to the neurocranium and plays a role in this protrusion.
The lower jaw and maxilla then retract, enabling the fish to firmly grasp its prey. This feeding mechanism provides an advantage, allowing teleosts to capture various fast-moving or elusive food sources.
Another characteristic is the homocercal tail, where the upper and lower lobes of the caudal fin are roughly equal in size. Unlike some other fish groups where the spine extends into the upper lobe of the tail, in teleosts, the spine terminates at the caudal peduncle. This symmetrical design provides efficient forward propulsion and enhanced swimming agility.
Teleosts also possess a gas-filled swim bladder, located in the dorsal portion of their body. This organ plays a primary role in buoyancy control, allowing fish to maintain their position in the water column without expending excessive energy. The thin membrane of the swim bladder contracts or expands in response to ambient pressure.
Unprecedented Diversity and Habitats
Teleosts exhibit vast diversity, encompassing over 30,000 known species globally, making them the most speciose group of vertebrates. They inhabit nearly every aquatic environment, showcasing an extraordinary range of adaptations.
Their habitats span from the freezing waters of the Arctic and Antarctic oceans to desert hot springs exceeding 38 degrees Celsius. Some species, like deep-sea anglerfish, thrive in the lightless depths of ocean trenches, while others, such as freshwater trout, inhabit turbulent mountain streams.
The desert pupfish, for instance, can survive in hot, saline desert water bodies. This group also includes the minute male anglerfish, Photocorynus spiniceps, and the giant oarfish.
Varied Reproductive Strategies
Teleosts display varied reproductive strategies. Most species engage in external fertilization, where females release numerous eggs that are fertilized by males in the water column. Many offshore marine teleosts produce planktonic eggs, while most freshwater species lay demersal eggs.
Some teleosts exhibit parental care behaviors, such as the male stickleback building nests and fanning the eggs. Certain African cichlids and marine cardinalfishes are oral brooders, with either the male or female incubating the eggs inside their mouth.
Sequential hermaphroditism is another strategy, where individuals change sex at some point in their lives. Clownfish, for example, begin as males and transition to females, while bluehead wrasse may change from female to male. This adaptability allows them to maximize reproductive success based on social or environmental conditions.
Long-distance migrations are also common, exemplified by salmon, which are anadromous, returning to freshwater rivers to spawn. Conversely, catadromous species like the European eel migrate from freshwater to the Sargasso Sea to breed.
Evolutionary Success and Origins
Teleosts emerged during the Mesozoic Era, with the oldest known fossils dating back to the late Triassic period. They evolved from earlier ray-finned fish, such as those in the Holostei clade. Their diversification accelerated during the Late Cretaceous and Paleogene periods.
The development of the protrusible jaw allowed teleosts to exploit more varied food sources. This innovation provided a competitive advantage over fish with less flexible jaw structures.
The evolution of the homocercal tail further enhanced their swimming capabilities, offering greater speed and maneuverability. This efficient propulsion system contributed to their ability to inhabit diverse aquatic environments and escape predators. These anatomical innovations, along with a lighter skeletal structure, contributed to their ability to outcompete other fish groups.
While a whole-genome duplication event occurred early in their lineage, current research suggests that their diversification was not solely due to this event. Instead, the success of teleosts is attributed to a combination of specific adaptive features and their ability to evolve new body plans and strategies over millions of years.