Actinopterygii: The Ray-Finned Fish

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  • Actinopterygii: The Ray-Finned Fish
    • The largest group of vertebrates
      • Skeleton of paired fins is formed from many small bones (fin rays) in a fan-like arrangement
        • Supported at the bases of fins by parallel rows of bones called radials
      • Actinopterygian Skeleton Components
        • Ural region consists of highly modified vertebral elements and arches supporting the caudal fin rays
          • Epicentrals or dorsal ribs are membrane bones without homologs in tetrapods
      • Skeletal support among Osteichthyans
        • Last common ancestor of bony fishes had polybasal condition where proximal skeletal elements of the fin articulated with the shoulder girdle
          • Pectoral fin consisted of anterior propterygium, the middle mesoterygium and the posterior metapterygium
        • Lineage leading to tetrapods, the propterygium and mesoterygium were lost along with dermal fin rays; new long bones that were articulated end on end were added along the main axis (PD axis) of the metapterygium, ultimately forming the limb
        • Teleosts, which are the largest group of ray-finned fish, and in some holosteans, such as the gar
          • Metapterygium was lost while the propterygium and mesopterygium were retained
            • No teleost shows end-on-end articulation of long bones in the pectoral fin
      • Primitively: Heterocercal tails (upper and lower fin lobes differ in length)
        • Derived: increasing trend of tail symmetry- homocercal tail
          • Fewer bones often fused up into larger elements as in triggerfish
            • Net effect: skeletal elements associated with caudal fin tend to become increasingly specialised
              • Become physically consolidated at the end of the tail, with clear distinction between elements associated with the tail's upper and lower lobes
        • Prim: numerous bones in tail, as in salmons
      • Dorsal fin
        • Prim: single fin un-subdivided
        • Derived: two separate portions; usually anterior portions carries sturdy rays modified as rigid spines; posterior portion carries flexible and soft rays
        • Some have an additional adipose fin posterior to the dorsal fin (small, soft and fleshy)
      • Fin spines
        • Prim: 0 (rays soft, flexible as in herrings)
        • Der: development of hollow, robust spines, which are modified fin rays
        • Various fins are often associated with poison glands
      • Skull ornamentation
        • Prim: Smooth-surfaced skull bones
        • Der: crests, ridges, tubercles and spikes
      • Jaw mechanics
        • Prim: simple jaws (gar); upper jaw bones connected to skull
        • Der: connection between cheekbone and posterior bone of maxilla lost
        • Articulations and levers- complex movements
      • Body shape
        • Prim: slender, elongate, large adult sizes, fusiform heads, bodies and tails, long guts, high vertebral count (50-75), no fin spines, fins other than pectorals placed posteriorly
        • Der: body highly variable, small adult body sizes, short and deep heads and bodies, short guts, low vet count (24), fins other than pectorals placed far forward, fin spines present
        • Rover predators (salmon, tuna, bass): fusiform bodies, pointed heads, terminal mouths, narrow tail fins, forked tail fins, actively chasing prey, continuous swimming
        • Ambush predators (pike, barracuda and needlefish): streamlined bodies, flat heads. large toothed mouths, narrow tail base, large tail fins, dorsal and anal fins situated far back, surprising prey, fast swimming
        • Surface-orientated dwellers (halfbreak, killifish, and flyingfish): small bodies, flat heads, dorsally directed mouths, large often dorsally facing eyes
        • Bottom dwellers (flatfish, sucker, anglerfish): flat/compressed bodies, very small subterminal mouths or very large gaping mouths, small eyes
        • Reef dwellers (angelfish, surgeonfish, mandarinfish): short, compressed bodies, pectoral fins high on body, pelvic fins below pectoral fins, long dorsal and anal fins, large eyes, small mouths, short snouts
        • Agile hunters (loach, eel, gunnel): very long bodies, stout heads, tapering or rounded tails, very long dorsal and anal fins running the length of the body, paired fins small or absent
    • Neopterygii
      • Most extant species
      • Lepisosteriformes
        • Gars
          • Most surface periodically to take a gulp of air, doing so more frequently in stagnant or warm water (O2 conc low)
        • Freshwater; brackish water; occasionally marine
        • Heavily vascularized swim bladders that can effectively function as lungs
        • Ganoid scales
        • Elongate and very slender jaws armed with small needle-like teeth (act as simple opposable devices)
        • Heterocercal tails; dorsal fin close to tail
      • Amiiformes
        • Bowfin
          • Long, undulating dorsal fin consisting of 145 to 250 rays, running from middle of the back to base of tail
        • Ambush, predator, freshwater, near bottom of vegetated sloughs, lowland rivers and lakes, swamps and backwater area
        • Swim bladder that serves to maintain buoyancy
          • Allows them to breathe air by means of a small pneumatic duct that connects foregut to bladder; can also gulp air
      • Teleostei
        • Largest radiation of verts.
          • Pinnacle of ray-finned fish diversity
            • Diverse habitats (poles to equator) various dentition inside oropharyngeal cavity
            • Body sizes, shapes and proportions vary greatly
        • Movable premaxilla in upper jaw; variable degrees of jaw mobility
        • Scales highly differentiated
        • Reproductive strategies and biology among most complex in animal kingdom
        • Perch-like teleosts: ctenoid (toothed) scales
    • Basal Actinopterygians
      • Very few extant species
      • Polypteriformes
        • Bichirs & reedfish
        • Series of dorsal small fins (7-18)
          • Have bifid (double edged) tips); only fins with spines; rest are soft rays
        • Elongate fish
        • Thick, ganoid scales
        • Jaws: simple and operate as scissor-like devices
        • Fleshy pectoral fins
        • Slit-like spiracles on the top of their heads that are used to breathe air
        • Paired ventral lungs (left lung shorter than the right)
        • 4 pairs of gill arches
      • Chondrostei
        • Sturgeons and paddlefish
          • Sturgeons: 5 rows of longitudinal bony scutes; bodies spindle-shaped; flattened snouts
          • Paddlefish: the sword-shaped snout and head up to the level of the operculum are extensively covered in sensory ampullae
        • Little degree of ossification
        • Retain a considerable amount of cartilage in their skeletons
        • Retain a spiracle
        • Heterocercal tail
        • Scale-less

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