BIOL 112 Lecture 34

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Chordata: Osteichthyes

bony fish

Ray-Finned Fish

fins supported by dermal rays

Characteristics

• swim-bladder (buoyancy control)
  • c.f. cartilaginous fish with lighter endoskeleton and body oils; no swim bladder
• Flattened bony scales covered with mucus
• Gas exchange: gills covered by operculum
  • Easy for aquatic organisms to keep gas exchange surface moist
  • Water oxygen concentration ~0.4% (c.f. air ~21%), so exchange system must be efficient.

Gas Exchange Abstraction

• gas exchange surface: large area of thin tissue between organism and external environment
  • must stay moist to dissolve gases (easy for aquatic organisms)
• some kind of circulation from gas exchange area and rest of organism

Counter-Current Exchange System

Figure 42.22

Capillary beds in gills and in tissues to maximize gas exchange surface area

Two-chambered heart that pumps blood through lungs and straight to body:

• heart → gills → body → heart (single circulation)
• (c.f. double circulation in other organisms: heart → gas exchange system → heart → body → heart)

blood flow moves in opposite direction of water flow across surface

• blood always introduced to a higher concentration

Lobe-Finned Fish

fins supported by bone and muscle

Extant tetrapods probably evolved from these types of fish.

• Lungfish (use swim-bladder as lungs)
• Coelocanths

Fins are sturdy and can support body weight

Evolved during Devonian in O₂-poor environments

Fishapods

Figure 34.20

• halfway between lobe-finned fish and tetrapods
• many species
• driving forces
  • climatic change: increase in shallow, oxygen-poor water habitats
  • progressive drying of continents