By Gary Hendry

Fish are cold-blooded vertebrates that breathe using gills and live in aquatic habitats. Fish make up the most diverse class of vertebrates with more than 3,000 species. They come in a wide variety of sizes, shapes, and colors. Marine environments alone are home to more than 18,000 species of fish, while more than 12,000 species of fish live in freshwater habitats. The most abundant fish species in the world is the grass carp with a commercial catch of more than five million tons annually.  

Fish Habitat 

Fish are adapted to living in various aquatic habits though some fish species such as mudskippers can live outside water. The common fish habitats include rivers, oceans, streams, coastal wetlands, inland lakes, streams, coral reefs, rocky shores, kelp forests, deep seas, and many more.  

2 Dolphin during Daytime

Classification of Fish

Fish are classified into three classes: bony fish, jawless fish, and cartilaginous fish. Bony fish have a skeleton composed of bone tissue. Examples of bony fish include ray-finned fishes, eel, tuna, marlin, oarfish, catfish, and cichlid. Jawless fish are simply fish that lack jaws. Examples of jawless fish include lampreys and hagfishes. Cartilaginous fish have a skeleton made of cartilage rather than bone, including skates, rays, sharks, sawfish, and chimeras.

Characteristics of Fish

Fish are cold-blooded

Unlike mammals that maintain a constant body temperature via internal process, fish do not maintain a constant internal body temperature. Their body temperature is directly linked to the external environment.  A fish’s body temperature fluctuates depending on the water’s temperature. 

Fish have gills for breathing

Fish have gills located on the side just behind the head for breathing underwater. Water is pumped over the gills whereby they absorb oxygen from the water and release carbon dioxide and other waste. However, fish that live in areas with very low amounts of oxygen have developed lungs for breathing. The lungfish is an example of fish that uses lungs to breathe. 

Most fish have scales as skin covering

Most fish have scales on the outside of the skin which protect fish from injury and predators. The scales also reduce friction for more efficient movement through the water. They secrete a mucous substance over the scales to protect the body from infection.  The hagfish is an example of fish that lacks scales. 

Fish can detect movement in the surrounding water

Fish have a highly developed sensory organ called a lateral line along the length of the body. The lateral line detects movements and vibrations in the nearby water allowing the fish to avoid predators.  Even in total darkness, fish can detect food and predators with the help of this specialized sensory organ. 

Fish have a swim bladder for buoyancy and sleep

All fish have a swim bladder which is located in the abdomen.  This unique internal organ helps fish to maintain stable buoyancy without sinking or floating upward. Fish adjust the amount of air in the swim bladder accordingly to allow the fish to sleep without sinking or floating too much.

Fish also have fins to provide balance and help with motion and maneuverability.  

Three Brown Sailing Boats Surrounded by Body of Water

Fish are an important economic resource for humans. They are a source of food across the world and can be kept as pets.