Introduction: When one thinks of animals that live in the water, chances are the first thing that comes to mind are the fish. Fish have been on Earth for millions of years, with some species (e.g. the coelacanth) remaining virtually unchanged for all that time. Defining characteristics of fish include living in the water (fresh or salt) and using gills for respiration (although there are other animals that use gills as well).

As mentioned, fish have been swimming the oceans for millions of years. The earliest fish appeared about 510 million years ago. While they did live in the water, they had no jaws or teeth. These animals were the ancestors of the hagfishes and lampreys alive today. These species do not have vertebrae in their adult forms and are very snake-like in shape (see image below). They are filter feeders and produce incredible amounts of slime.

In addition to the jawless fishes, another large group of fish evolved, this time with jaws and teeth. These species developed a skeleton made of cartilage, which is strong, yet flexible. Over time, they also evolved fins, which allowed the body to be steered in different directions and provide enormous amounts of powerful thrust. This speed was (and still is) used to chase and capture prey. Animals in this group, called the Chondrichthyes, include the sharks and rays. There are hundreds of different species, include the great white shark and the manta ray (see image below).

The final, and largest group of fish are those that have a skeleton made of bone. Called the Osteichthyes, this group includes fish that have fins that are ray-like and those that are lobe-like. Most fish belong to the ray-finned fishes. Salmon, swordfish, catfish, and even your pet goldfish all belong to this group. The "ray" refers to the bones connected to a layer of skin to form the fins.

The lobe-finned fishes evolved fins that are more globular and have much larger bones for support. Members of this group include the lungfish and the coelacanth (see image below). The coelacanth was thought to be extinct for a millions of years, but recently fishermen off the African coast pulled some up in their nets. Additional study has shown that there is a thriving population of these fish in this area. It is believed that this group of fish are the ancestors of the four-limbed vertebrates that moved onto land.

Directions for this Lesson: Answer the practice questions and then watch the video to learn more about fish.