Fish: Gnathostomes (Jawed Fish) & Osteichthyans (Boney Fish)

Gnathostomes

Infraphylum Gnathostomes

• Jawed fish
• Gnath = "jaw", stoma = "mouth"
• Evolution of 2 pairs of fins
• Bigger brain
• Enables more complex movements
• More duplications of Hox genes
• 2 x 2 = 4 clusters of Hox genes
• Added genetic diversity
• Jaws: enabled diverse feeding behavior
• Larger Brain
• Improved vision and smell
• Sharks lost hard skeleton
• Reverted to cartilage
• Evolution of jaws
• Modified 2 pairs of skeletal rods previously used to support pharyngeal slits
• More ways to get food
• Posterior slits became specialized for gas exchange
• Became gill slits

Class Chondrichthyes

• Cartilaginous fish
• Chondro = "cartilage", ichthys = "fish"
• Endoskeleton made out of cartilage
• Do have bony teeth (calcified)
• Cartilaginous skeleton
• Not a primitive character
• Character is derived
• Evolved from ancestors with bony skeletons
• Development of most vertebrates
• Skeleton (cartilaginous) becomes ossified (bony)
• Cartilage is replaced by calcium phosphate
• Cartilage ossifies as embryo develops
• Chondrichthys skips this process

Sharks

Swimming

• propulsion comes from the tail
• Lacks gas-filled swim bladder
• Buoyancy from oil in liver
• Must use muscles to gulp water if not in motion

Skin

• Rough due to dermal placoid scales
• Dermal denticles
• Hydrodynamic
• Scales are homologous to vertebrate teeth
• Unclear which came first

Teeth

• Continuously replaced (conveyor belt of teeth)
• Common in fossil records

Sharks (Continued)

Reproduction

• Copulation
• Claspers: male pelvic fins modified for copulation
• Internal fertilization
• Progeny production
• Oviparous: Lays eggs
• Ovoviviparous: Eggs hatch inside female
• Viviparous: Live birth
• Eggs protected in leathery egg case

Nostrils

• Smelling (not gas exchange)
• Detect changes in electrical field generated by other animal muscle movements
• Uses pores around the head
• Detects vibrations in water
• Uses Lateral Line
• Lateral Line: pores that detect disturbances in the water
• Exists in bony fish as well

Osteichthyans

Overview of Osteichthyans

• Bony Fish
• Oste = "bony", Ichthyans = "fish"
• Hard ossified endoskeleton
• Group contains all tetrapods
• Operculum: A bony cover that protects fish gills
• Swim Bladder: air sac used to control buoyancy
• Evolved from lungs
• Mostly external fertilization

Fish Diversity

Ray-finned fishes

• Actinopterygii
• Over 27,000 species
• Major protein source for humans
• Fisheries have seriously depleted natural fish populations

Lobe-finned fishes

• Sarcopterygii
• Rod-shaped bones with thick layer of muscle in fins
• Likely used these robust fins to occasionally move on land
• Large predators in the Devonian era
• Only three lineages survived today
• Coelacanths: thought to be extinct 75 MYA, but found in 1938!
• Dipnoi: Lungfish
• Found in ponds and swamps
• Gulp air to supplement gills
• Can estivate when ponds are dry
• Tetrapods: Vertebrates with limbs and feet
• Far more diverse
• Includes humans!