Fish - Anatomy
Fish are vertebrates, meaning they have a backbone. All fish have fins and most have scales (with a few exceptions, like catfish which do not). Fish are cold blooded animals that lay eggs and are well suited for living in water. Learn about the different fish adaptations below that allow a fish to survive in water.
External Anatomy
Eyes: Used for sight, fish can detect colors and see short distance with their eyes. They use their vision to escape predators and find food.
Nares: Similar to nostrils, except nares are used for smelling only (nostrils are used for both smelling and breathing).
Mouth: The mouth is used to consume food.
Operculum: The operculum is the bony flap that protects the gills from harm. It opens and closes to allow water to pass over the gills.
Pectoral Fin: The pectoral fin allows for abrupt changes in side-to-side direction and speed. It also acts as a brake to decrease speed while swimming.
Pelvic Fin: The pelvic fin stabilizes the fish while swimming and allows for up-and-down movement in the water.
Vent: The vent removes waste and extra water. It is also the outlet for eggs or milt (sperm) during spawning.
Anal Fin: The anal fin stabilizes the fish while swimming.
Caudal Fin: The caudal fin moves, propels or pushes the fish through the water.
Adipose Fin: The adipose fin is not pictured; it is not present on a lot of fish species. Its purpose is unknown. Trout, salmon and catfish have an adipose fin. It is the small, thick, fleshy fin located between the dorsal and caudal fins.
Dorsal Fin: The dorsal fin helps maintain balance while swimming.
Scales: Scales protect the fish from injury.
Barbels: Barbels are not pictured. They are the "whiskers" found on the head area of fish such as catfish or bullheads. On the catfish and bullheads, barbels are thought to be a sensory organ to help track down prey or food. Sturgeon also have barbels.
Internal Anatomy
Gills: Gills are the feathery tissue structure that allows fish to breathe in water. Water flows in through their mouth and over their gills where oxygen is extracted and passed into the bloodstream.
Swim Bladder: The swim bladder is a long, skinny organ that can inflate/deflate with air allowing fish to float at different levels in the water column.
Fish Senses
Eyesight: Fish can see in two directions (one eye focusing on an object independent of the other whereas human’s eyes can only focus on one object at a time).
Hearing: Fish have ears but not external ear openings like humans do. Their ears lack a middle and outer ear because sound travels faster in water than in air. Fish have internal ears with pairs of inner ear bones called otoliths. The otoliths allow fish to sense sounds in the water. Fisheries biologists can also use these bones (otoliths) to age fish and determine the health of fish populations.
Smell: Fish use their sense of smell to locate food and to aid in migrating.
Taste: Some fish have taste buds, however, these taste buds are located on the outside of the fish’s head and fins in small pores. Some fish like catfish have a very developed sense of taste.
Lateral Line: The lateral line, found alongside a fish’s body from the operculum to the tail (caudal fin), senses vibrations or movements in the water. It allows fish to locate predators and find prey. This system is made up of a series of fluid-filled canals just below the skin of the fish’s head and alongside the body. The canals are filled with tiny hair-like structures that detect changes in the water pressure via tiny pores connected to the system.