A Basic Guide to Fish Anatomy

There are many fish species, and they are all unique, but they share the same basic anatomy. This includes the head, body, fins, gills, air bladder and lateral lines.

Feb 26, 2024By Tanya Taylor
basic guide fish anatomy

Fish are cold-blooded creatures, meaning they depend on the sun to stay warm, unlike mammals, which create their body heat. They look incredibly different to you or me, but surprisingly, we share much of the same internal anatomy with fish. The main difference is that fish don't have lungs, arms, or legs. If you want to know more about what makes aquatic creatures tick, take a look at my basic guide to fish anatomy.

The Fish Head

clown fish
Photo Credit: Dustin Humes on Unsplash

Fish are unique aquatic creatures, and the best place to start when looking at basic fish anatomy is at the head. A fish has the same features as most creatures on their head, including eyes, ears, nostrils and a mouth. They also have an extra feature, gills, which we’ll explore later in the article. Fish use their mouth for ingesting water and eating, and their position reflects their feeding habits. Species with low mouths generally feed off the floor, while a fish with a high mouth usually feeds off the surface.

Fish also have nostrils, which they use for smelling and ears for hearing and to help with balance. They also have two eyes, similar to human ones, so like us, they have a lens, retina and optic nerve. Each fish has a different type of vision - some fish see in color, and others have UV vision. Like in mammals, fish with eyes at the front of their head are generally predators - and the ones with eyes on the side are usually prey.

The Fish Body

wild fish
Photo Credit: Sara Kurfeß on Unsplash

A fish’s body includes a substantial backbone, running from the head to the tail, which supports two large lateral muscles. The large muscles are a fish's primary swimming aid. Fish have many of the same internal organs as mammals, except the lungs. They have a digestive and circulatory system with arteries, veins, blood vessels, and a heart.

All fish have skin on their body and are often covered in scales - but not all fish have scales. Fish scales protect the skin and internal organs - and come in many shapes, sizes, and layouts. Some species have protective spines, which are modified scales.

A fish’s body color usually depends on its environment. Fish that live in the open ocean, such as barracudas, are generally gray or blue-colored, while brightly colored fish are usually from coral habitats. Color is significant in the fish world and can influence fish behavior - they use it for camouflage, attracting a mate, and warning others about danger.

Fish Fins

koi fish
Photo Credit: Max Ducourneau on Unsplash

Fins are a crucial part of a fish's anatomy and are used for balance, to propel themselves through the water, and to change direction. Most fish species have two sets of paired fins and three unpaired fins which connect to the skeleton.

The pectoral and pelvic fins are paired fins, and they are in the corresponding area on a fish’s body to our arms and legs. Pectoral fins help fish maneuver, and pelvic fins help regulate speed and push a fish up or down.

The dorsal, anal, and tail fins are unpaired and are also known as median fins because they are in the middle of a fish’s body. Medial fins primarily help fish with stability. The dorsal fin, on the top of a fish’s body, helps with rolling and sharp turns. The anal fin is on the underside of a fish and supports the dorsal fin, and the tail fin provides the main propulsion for swimming.

Fish Gills

yellow fish
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Fish don't have lungs, but they still need oxygen to function. Instead of breathing it in from the air, they absorb it from the water through their gills. They drink water through the mouth, and the buccal pump pushes water to the gills. Fish gills filter the oxygen from the water and absorb it into the body. Some fish, such as tuna, don't have a buccal pump and must constantly swim for the water to pass over their gills.

Fish usually have 4 gills on each side of their neck, and they are complex anatomical structures. The gills contain the gill arch, a rigid structure that supports the gill filaments. The filaments contain many blood vessels, are soft tissue structures, and absorb oxygen into the bloodstream. The gill rakers separate food and keep particles inside the mouth, and it’s all protected by a bony gill cover called the Operculum. Some species, such as sharks, have naked, unprotected gills.

The Air Bladder

aquarium fish
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Fish don’t have lungs, but they have an air-filled sack inside the body called the air, gas or swim bladder. The swim bladder is unique to fish and sits just below the kidneys. Unlike the lungs, it doesn’t have anything to do with breathing - it controls a fish’s gravity and buoyancy.

The gas bladder is like a balloon and expands or contracts depending on the water density, which helps fish stay stable in the water. The gas bladder is usually closed but is sometimes in two segments. In some species, it connects to the alimentary tract with a duct or tube. The air bladder works slowly, and fish can be bloated or injured if they move too quickly to the surface. Not all fish have an air bladder, especially bottom dwellers who generally don't need them.

The Lateral Lines

orange fish
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The lateral lines are the final section in our guide to basic fish anatomy. Most fish have them, and they run along the side of a fish, from the head down to the tail. Lateral lines are covered in tiny pits which contain sensory hairs. They are crucial for survival because they help fish sense vibrations in the water. The ocean is full of subtle vibrations, and each one represents something different. Fish can tell if the vibrations are from prey, predators, obstacles, or schools of fish through the lateral lines.

Tanya Taylor
By Tanya Taylor

Tanya is a trusted animal care professional and has devoted her life to animals. In her 25-year career, she’s worked with all kinds of creatures in many environments, including three years caring for small animals as a veterinary nursing assistant and five years birthing down racehorses.

She is an expert farm and dog sitter - and has spent many hours volunteering at her local pony sanctuary. Tanya is originally from Liverpool in the UK, but now she lives in Ibiza, Spain, with her cheeky red terrier Leo and three Leopard tortoise hatchlings, Ninja, Tiny, and Orwell.