Fish are aquatic, cold blooded vertebrates that breathe with gills. There are three classes of fishes: jawless, cartilaginous, and bony.

Agnathans are jawless fishes. They have a skeleton made from cartilage. They represent the first jawless fish evolved from invertebrates. Agnathans lack a true backbone but possess a notocord for support. Even though they do not have jaws, they are voracious feeders. They live as parasites by attaching to, and feeding on the blood and tissues of a host fish. Examples include the sea lamprey and hagfish.

Day 131.1

Osteichthyes are the bony fish and make up majority of all fish. Bony fishes have a skeleton of bone, mucus-covered scales over their skin, a swim bladder, and gills covered by an operculum. Mucus protects the fish from parasites and bacteria. Bony fish typically utilize external fertilization. The females will produce thousands of eggs and the males will release sperm near the eggs in hopes of fertilization. The anatomy of bony fish includes the following structures:

  • Operculum – protective gill cover
  • Swim bladder – gas filled sac that inflates or deflates to maintain neutral buoyancy which allows the fish to “hang” in the water.

Day 131.2

The color of fish is determined by chromatophores and iridocytes. The chromatophores help give the fish countershading which allows them to appear darker on the top and lighter on the underside. The iridocytes give fish that characteristic reflective or iridescent sheen. Fish breathe by taking water in through their mouths and pushing the water over their gills and out the operculum.

By the way, Is it Fish or Fishes?

Groups of fish that are the same species are called fish.

Groups of fish that include different species are called fishes.