Ray-Finned Fishes:

The most diverse group of fish

The ray-finned fishes (actinopterygians) are the largest group of living fishes and compose half of the living vertebrates. There are around 42 orders, 431 families and nearly 24,000 species. Almost all fish that you see belong to this class of fish.  The most notable exceptions include lampreys, hagfish, lungfish, and coelocanths.

Ray-finned fish have several physical characteristics worth mentioning.  The following is a general description of the characters of the class Actinopterygii, Ray finned fishes.

The following is a detailed description of the family summarized in part from information on the Animal Diversity Web: 

Ray-finned Fish Scales

There is no one type of fish scale that all actinopterygians share. Ancestral actinopterigians generally had heavy and complex scales known as ganoid scales. Today, surviving ray finned fishes generally have more flexible scales with reduced weight known as leptoid scales.  It is believed that the lighter scales, while providing less protection allowed the fish to avoid predators and find food easier, thus increasing fitness.  

Ganoid Scales: These scales are extremely heavy and inflexible. The inner layer of the scale is made of lamellar bone.  They were common in ancestral actinopterygians.  They can however be seen today in primitive fish like gars, bichirs, and reedfishes have ganoid scales.

Leptoid Scales: As leptoid scales grow they add concentric rings to the scale.  They are arranged so that they overlap head to tail, much like the roofing shingles.  This allows the fish to swim with less water resistance.  There are two basic types of leptoid scales.  Cycloid scales have a smooth outher edge (found in salmon and carp).  Ctenooid scales have a toothed or rough outer edge (found in bass, crappie, etc). 

Branchiostegal Rays

In Ray-finned fishes, the branchiostegal rays evolved from the bones at the base of the branchial cavity.  These rays increased the efficiency that water was pumped across the gills.  The interopercular bones, which were formed by the modification of branchiostegal rays at the bottom of each gill cover, further improved the pumping efficiency by enlarging the opercular cavity, thus increasing the volume of water that could flow though the gills.  

Ray-finned Fish Swim Bladders

The swim bladder in ray-finned fish most likely served as a type of primitive lung in early actinopterygians. However, in more derived fishes in the group, it serves almost entirely as a buoyancy device. Fish with swim bladders can control the amount of gas in the bladder and thus control their buoyancy. This ability made it so that the fins were no longer needed to function as hydroplanes and could evolve into devices for greater maneuverability.

In some fish, like the beta fish (also known as the Siamese fighting fish) and gouramis, the air bladder actually works somewhat like the lungs in mammals. These fish can gulp air into the swim bladders and pull oxygen from it. For more info see (Helfman, Collete, and Facey 1997) (Liem 1998) (Moyle and Cech 2004)

Ray-finned Fish Jaws

Jaws developed in conjunction with the branchiostegal rays and show a trend towards more flexibility. (See an illustration of jaw anatomy).Two major bones of the upper jaw, the maxilla, and the premaxilla, were previously firmly attached to the skull and had teeth. However, in recently derived actinopterygians, there are fewer attachments andteeth are rarely present. This has allowed for the upper jaw to extend, making it protrusible (dramatically illustrated by some wrasses), and permitted a variety of feeding specializations to develop, such as plankton straining (usually zooplankton). One result of increased flexibility of the upper jaw has been that processing could not easily occur at the rim of the mouth any longer. Therefore, many fish with protrusible jaws have a second set of jaws in the throat, termed pharyngeal jaws , that process food and free the outer jaw to continue feeding. (Helfman, Collete, and Facey, 1997; Moyle and Cech, 2004)

Ray-finned Fish Tails

Changes in the lobes of the tail, from heterocercal (upper lobe longer than lower lobe) to homocercal (upper and lower lobes same size) in recently derived actinopterygians, are also related to the achievement of neutral buoyancy. Heterocercal tails (still found in sharks) provide lift, which is unnecessary for neutrally buoyant fish. Homocercal tails provide uniform thrust and allow for precise movements (each ray can be controlled individually), which are quite important for fast-swimming, pelagic fish and small, maneuverable fish,respectively. (See an illustration of locomotion in fish). (Helfman, Collete, and Facey, 1997; Liem, 1998; Moyle and Cech, 2004)

Ray-finned Fish Fins

With the loss of heavy, armoring scales, actinopterygians developed spines, which are used as anti-predator devices when individuals are unable to use speed to escape (see Predation). The positioning of the pelvic and pectoral fins also changed along with the achievement of neutral buoyancy. Instead ofhaving pelvic fins located well behind the pectoral fins, in more derived actinopterygians the pelvic fins are located just below or even slightly in front of the pectoral fins; here they aid in increased maneuverability instead of being used simply as stabilizers, as in earlier actinopterygians. (See an illustration of fin function). (Helfman, Collete, and Facey, 1997; Moyle and Cech, 2004)

Classification

Ray finned fishes can be divided up into two basic groups, Chondrosteins and Neopterygiians. Chondrostians are generally considered to be primitive fish, such as sturgeons, paddlefish, bichirs and reedfishes. Neopterygians are divided between Holosteins (gars and bowfins) and Teleost fish (the rest). The following is a list of ray-finned fish orders. Classification follows that of FishBase.

• Polypteriformes (Bichirs, Reedfishes)
• Acipenseriformes (Sturgeons, Paddlefish)
• Lepisosteiformes (Gars)
• Amiiformes (Bowfins)
• Osteoglossiformes (bony-tongued fishes)
• Hiodontiformes (mooneye, goldeneye ...)
• Elopiformes (ladyfishes, tarpon ... )
• Albuliformes (bonefishes)
• Notacanthiformes (halosaurs, spiny eels)
• Anguiliiformes (true eels, gulpers)
• Saccopharygiformes (gulper eels)
• Clupeiformes (herrings, anchovies)
• Gonorynchiformes (milkfishes)
• Cypriniformes (barbs, carp, danios ...)
• Characiformes (characins, pencilfishes ...)
• Gymnotiformes (electric eels, knifefishes)
• Siluriformes (catfishes)
• Argentiniformes (barreleyes, slickheads)
• Salmoniformes (salmon, trout)
• Esociformes (Pike)
• Osmeriformes (smelts, galaxiids)
• Ateleopodiformes (jellynose fish)
• Stomiiformes (bristlemouths, marine hatchetfishes)
• Aulopiformes (lancetfishes, Bombay duck)
• Myctophiformes (lanternfishes)
• Lampriformes (oarfish, opay, ribbonfishes)
• Polymixiiformes (beardfishes)
• Percopsiformes (cavefishes, trout-perches)
• Batrachoidiformes (toadfishes)
• Lophiiformes (anglerfishes)
• Gadiformes (cods)
• Ophidiiformes (pearlfishes)
• Mugiliformes (mullets)
• Atheriniformes (silversides, rainbowfishes)
• Beloniformes (flyingfishes)
• Cetomimiformes (whale-fishes)
• Cyprinodontiformes (livebearers, killifishes)
• Stephanoberyciformes (ridgeheads)
• Beryciformes (fangtooths, pineconefishes)
• Zeiformes (dories)
• Gobiesociformes (clingfishes)
• Gasterosteiformes (sticklebacks)
• Syngnathiformes (seahorses and pipefishes)
• Synbranchiformes (swamp eels)
• Tetraodontiformes (filefishes and pufferfish)
• Pleuronectiformes (flatfishes)
• Scorpaeniformes (scorpionfishes, sculpins)
• Perciformes (all the other fish ... )