Big Picture

Fish have many adaptations for aquatic life, such as gills and swim bladders. Fish reproduce sexually and often participate in spawning, where a group of fish releases gametes at the same time to increase the chance of fertilization. There are five classes of fish, and each class has adapted differently for survival in their respective habitats.

Key Terms

Fish: Vertebrate that lives in water and has gills for absorbing oxygen

Swim Bladder: Gas-filled internal organ that allows fish to move up and down in the water.

Spawning: A group of fish all release their gametes at the same time.

Endoskeleton: Internal skeleton that provides support and protection.

Cartilage: Tough tissue containing a protein called collagen.

Bone: Hard tissue containing a collagen matrix filled with minerals like calcium.

Structure & Function

Fish have several adaptations for water:

  • Gills to breathe in water - the gills absorb oxygen from water that passes over the gills
  • Streamlined body(typically long and narrow to reduce water resistance),fins, muscle systems, and swim bladder for swimming
  • Fish can move up and down by changing the amount of gas in the swim bladder
  • Organ systems:
  • Circulatory system with 2-chambered heart
  • Digestive system
  • Nervous system with brain, lots of sensory organs that allow them to see, hear, feel, smell, and taste

Reproduction & Development

  • Most fish reproduce sexually
  • Fertilization usually takes place outside the body
  • Males and females release gametes into the water
  • Many fish participate in spawning. This increases the chance of fertilization and increases the probability that some offspring will survive predators
  • Eggs hatch into larvae
  • Larvae are attached to yolk sacs that provide nutrients/food
  • Larvae go through metamorphosis to become adults

Classification, Evolution, & Ecology


Fish are divided into 5 classes:

  • Hagfish: Very primitive fish that keeps the notochord and has no backbone, scales, or fins; secrete lots of mucus to make themselves slippery.
  • Lamprey: Fish with no scales but have fins and a partial backbone with a large sucker to suck blood of other fish.
  • Cartilaginous Fish: Fish with an endoskeleton made of cartilage, complete backbone, large brain, no swim bladder.
  • Ray-finned Fish: Fish with an endoskeleton made of bone and a swim bladder.
  • Lobe-finned Fish: Fish with fins containing bone and muscle; some species can breathe air.


  • Earliest vertebrates were ancestors of hagfish.
  • Lobe-finned fish are ancestors of amphibians.
  • Ray-finned fish were the first fish to evolve in fresh-water.


  • Hagfish live at the bottom of the ocean. They go inside their prey and eat them from the inside out.
  • Lampreys live in shallow ocean water/freshwater. They drink the blood of other fish.
  • Cartilaginous fish live in the middle/bottom of the ocean. They prey on smaller fish/mammals and plankton.
  • Bony fish live in any kind of water. They eat algae, detritus, smaller fish, etc.