Diversity in Actinopterygii

Actinopterygii or the ray-finned fishes are the most successful of all vertebrates in terms of diversity and sheer numbers. They constitute around 26,900 species out of a total of around 31,500 species of fish, thus representing more than 95 of the living fish species, and adding up to more than half of all the extant vertebrate species. They include many familiar varieties such as bass, perch, sturgeons, birchers, tuna, goldfish, cod, seahorse, eels, carp, herrings, anchovies, catfishes, goldfishes, gars, pikes, piranhas, oarfish, cichlids, mackerel, pickerel, salmon, trout sardines, and the flying fish.

Actinopterygii and Sarcopterygii are the two classes of fishes with bony endoskeletons, i.e., a cranium, vertebral column with ribs, and bones supporting their moveable fins, constituting the superclass Osteichthyes.  Actinopterygii (aktinos  ray) have the distinguishing characteristic of fins supported by thin and flexible bony rays, while the other group of fleshy-finned fishes (sarcos  flesh) bear fins supported by muscles. Ray-finned fishes have always been more diverse and thriving than their fleshy-finned counterparts and preponderate the seas, rivers and lakes in vastly greater numbers. There are in fact only a handful of living species in Sarcopterygii.

Osteichthyes first appeared in the Silurian period, 445  415 million years ago, and diverged toward the end of that period. The first Actinopterygii evolved in freshwater but gradually moved into marine habitats. Actinopterygians have a two-boned structure of the upper jaw which has proven remarkably adaptable. With the upper jaw being protrusible as a result of this structure, Actinopterygii have been able to develop a wide range of feeding habits.

The most predominant group of actinopterygii are called teleosts they have first appeared during the Late Triassic (190 mya) and evolved during the Mesozoic era (155  70 mya). The group contains the most number of living fish species and makes up 95 of Actinopterygii, nearly 24,000 species in all.

The class Actinopterygii has two subclasses, Chondrostei and Neopterygii, with the former consisting of the two orders Acipenseriformes (sturgeons and paddlefishes) and Polypteriformes (reedfishes and bichirs), and the latter consisting of the infraclass Holostei (gars and bowfins) and the infraclass Teleostei (salmon, trout, carp, goldfishes, electric eels and so on). Teleostei consists of several superorders such as Osteoglossomorpha, Elopomorpha, Cluepeomorpha, Ostariophysi, Protacanthopterygii. In all, there are 38 orders, 426 families and 4064 genera in Teleostei (Ostrander, 2000). Of teleost diversity, the encyclopedic reference Smiths Sea Fishes says the following
The tremendous diversity of size, body shape, fin development, feeding adaptations, eye structure, scale morphology, skeletal modifications, sound production, bioluminescence, production and detection of electrical currents, behavior and reproduction is unparalleled among the vertebrates. (Smith, Heemstra, 2003)  

Actinopterygii indeed sport a fantastic diversity in terms of appearance, morphology, habitat, feeding habits, defense mechanisms, behavior, lifestyle and so on. Fish usually only swim, but some of the actinopterygian species can crawl and walk (walking catfish, mudskipper), fly, or be immobile. They come in a bewildering variety of colors, shapes, skin textures and patterns within a vast range of sizes. The smallest vertebrate is a teleost called goby, a coral reef dwelling fish, that on adulthood measures 8mm. Minnows are also very tiny.  Toward the other end of the spectrum, another teleost fish called the arapaima, found in the Amazon River system, may reach a length of 2.5 meters (Wootton, 1998). Giant blue marlin can be 5 meters long with a mass of 820 kg, while sickle fish (Pachychalybinae) reach up to 9 meters. 

Actinopterygii vary widely in terms of their sensory apparatuses which extend their purview beyond vision and hearing into chemoreception, electroreception, lateral line sensation, and so on. Electric eels and some other members of Actinopterygii display an interesting defense  predatory mechanism, in that they can produce electric organ discharges which can incapacitate their prey or attackers. As a group Actinopterygii are omnivorous, feeding on all sorts of organic matter, but pose little danger to humans. Piranhas are perhaps the most notorious of Actinopterygii, for their assumed dangerous behavior toward humans. Discussing the feeding habits of Piranhas, Helfman et al (1997) say this in their book The Diversity of Fish
This subfamily , the Serrasalminae, contain about 60 species, some of which are predatory, other that are scale-eating opportunists and specialists, and some that are largely herbivorous, such as the pacus and silver dollars. Despite their reputation and potential for doing damage, many purported attacks on human by piranhas actually result from postmortem scavenging on drowning victims.

Our water planet is rich in habitats and ecosystems, and Actinopterygii thrive in almost all of the aquatic environments, including extreme types such as subterranean caves, polar seas, desert springs, turbulent rivers, ephemeral pools, and mountain creeks. The diversity of the habitats is helped by the fact that some of the ray-finned fishes can spend considerable time out of water. The Siamese fighting fish can breathe oxygen directly from the air, besides possessing the capability of extracting oxygen from the water like normal fish.

The depth of actinopterygian water habitats can range from 0 to 7,000 meters, and temperatures from -1.8C to 40C. In terms of salinity and pH, 0  90 ppm and 4  10 are the numbers respectively. These fishes can live in environments that have almost no oxygen to fully oxygen-saturated ones. Some species of Actinopterygii migrate between fresh and salt water (called diadromous) for example, salmon migrate from saltwater to freshwater in order to spawn while freshwater eels do the reverse. As for geographical concentrations, freshwater ray-finned species teem in places like Amazon River Basin and wetlands of Southeast Asia, while marine species have particularly proliferated around coral reef systems such as the Great Barrier Reef of Australia.

Most of these fish lay eggs but some species which include the familiar guppies and swordtails withhold the eggs inside their bodies until they are ready for hatching. In seahorse and a few other rare species it is the male that receives the eggs. Such species are called ovoviviparous the males keep the eggs inside a special pouch inside the body.   Usually these fish do not provide any care for their offspring, but certain species protect them in various peculiar ways. For example, the females of some African cichlids keep their eggs in their mouth and even after they hatch the young are retained in the mouth for a period.

The diversity with the actinopterygians can be better appreciated by taking a look at some of the unique features of some of the major families of this class

Despite comprising a meager 23 species, sturgeon are relatively quite abundant both in marine and freshwater environments. The females produce an enormous number of eggs, which is a factor that has contributed to their success. They are among the largest fish occurring in freshwater environments. The Russian beluga sturgeon can reach up to 8.5 meters, weighing 1,300 kg, and the North American white sturgeon can reach a length of 4 meters, weighing 590 kg.

While sturgeon feed on whatever they can suck up off the bottom, Gars catch their prey by employing ambush tactics. Gars have a heavy protective armor, and in order to keep their weight buoyant they have a large air bladder which also functions as a lung. Gars reach one to two meters in length, only the alligator gar reaches up to 3 meters. 

Herrings are exquisitely adapted to avoid the predators. They have large silvery scales that reflect light to dazzle predators during the daytime. Furthermore, these scales are deciduous, so even when even caught a herring can escape shorn of its scales and grow them again. They have a streamlined shape which reflects minimal light, making them less visible to predators attacking from below.

There are over six hundred species of eels this family shows a surprising variety of forms. Both European and American eels migrate thousands of kilometers in ocean to Sargasso Sea (Atlantic Ocean), where they spawn in deep water. From leaf-like larva they transform into elvers at which stage they migrate long distances upstream to some freshwater source and mature into adulthood (Moyle, Cech, 2004). The African electric fishes, belonging to the same family as eels and herrings has over one hundred species and live in murky swamps of tropical Africa. They find prey and communicate with one another using electrical signals. An enigmatic aspect of these fish is that in some species the brain-to-body ratio is very close to that of humans. These fish are also found in other continents, including North America, but in much lesser numbers.

Salmon, trout, whitefish, and grayling and about one hundred and fifty other species make up the family Salmonidae. They are a highly migratory species, superbly adapted to cold water. They are the dominant fishes in the streams and lakes of north temperate regions. Besides being highly valuable as commercial fish, they are very popular with anglers as well as fish biologists.

By contrast, bristlemouths are fishes known to few people, and yet they are perhaps the most numerous fishes in the world. Tiny in size, reaching only 5 cm or less in length, they teem in deep seas. At times sonar beams from submarines are reflected back from them creating a false notion of the bottom.

The teleost superorder Ostraiophysi comprises fishes that dominate most of the world, including minnows, suckers, loaches, South American electric fishes, catfishes, popular aquarium fishes such as tetras, and the notorious piranhas. One quarter of all known fishes belong to this group. Of the many reasons for their success particularly noteworthy are their acute sense of hearing, their pharyngeal teeth, and their fear scent. They have auditory apparatus remarkably like that of mammals the pharyngeal teeth make the mouths of these fish specialized for many types of foods just like mammalian teeth the fear scent is given off by an injured fish so that its school members can dive for cover. The family Cyprinidae of this superorder, consisting of minnows, carps, goldfish and over sixteen hundred species is the largest single family of fishes. A prominent reason for their success is the presence of stout spines on the dorsal and anal fins which makes them harder for predators to capture. Another family suckers, with about sixty species, is a very successful group in North America. They are specialized for sucking and scraping algae, small invertebrates and organic debris from the bottom. Catfish is another successful family, with forty species. There are catfish that can reach over 5 meters and in length as well as those that are so tiny they can slip into the gill cavities of larger fish. One of the reasons for their success is their secretive, night-oriented behavior. While carps, minnows and the Cyprinidae rule the day, catfishes rule the night.

Flying fish are among the most curious members of the actinopterygian group, for obvious reasons. They leap out of water and glide as much as 100 meters with the help of their wing-like pectoral fins. The flying fish use their wings only for gliding and not to power their flight.

By contrast, there is a freshwater family of hatchet fishes which have hatchet-shaped breasts that contain large muscles needed to move their pectoral fins rapidly up and down, thus achieving true flight.

The famous evolutionary biologist, Richard Dawkins, pays this tribute to the teleost variety in his book, The Ancestors Tale A Pilgrimage to the Dawn of Evolution (2004)

The teleost fish are the great success story among modern vertebrates  there are some 23, 500 species of them. They are prominent at many levels of underwater food chains, in both salt and freshwater. They have managed to invade hot springs at one extreme, and the icy waters of the Arctic seas and high mountain lakes at the other. They thrive in acid streams, stinking marshes, and saline lakes... The teleost pilgrims arrive in a jostling crowd, brilliant in their variety.


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