Parts of a Fish
Many external and internal features of fishes are equivalent to our own although several notable exceptions exist, such as the gas bladder (same as swim bladder) and gills. Major anatomical differences address significantly different challenges between living in water vs. air. Great similarities reflect common ancestry among all vertebrates.
External and internal features may be familiar because it is common to see fishes up close, e.g. in an aquarium, at a fish market, or one you've just caught.
Explore the diagram below to learn the names of fish parts and find out what each one does, or use it as a reference as needed.
We couldn't include all the features of every kind of fish, so we started by showing three different types - a shark, spiny-rayed fish and soft-rayed fish.
General anatomy of fishes
Interactive diagram -
Roll over a fish silhouette and click to choose an external, internal of MRI (magnetic resonance image) view.
Note there is no rollover text on the MRI view.
MRI (3-D) views
Comparison of scientific diagrams with MRI images of internal organs
Interactive diagram -
Click on an image to see the interactive diagram. Drag the slider slowly to see different views.
Online fish dissection resources
Examples of anatomy
Coming in the future - Evolutionary significance of cartilage, bone, teeth and types of scales.
Bone, teeth, scales
The secrets of irridescence lie in the advanced crystal structure of this fish scale. Natural materials like this fish scale offer a wealth of ideas for materials of the future.
Photographer - Chana Nudelman-Faust,
image courtesy Avital Levy-Lior .
Photographer - Hank Brooks, VIMS Juvenile Fish Trawl Survey.
Radiograph courtesy California Academy of Sciences Dept. of Ichthyology.
Fish otoliths or "earstones" are stones that grow just behind the brain of many fishes, excluding sharks, rays and skates. Three pairs of otoliths help fishes hear and maintain balance. The largest of the otolith pairs are called sagittae and these are what most people refer to as otoliths.
Otoliths are made of calcium carbonate (CaCO3) in the mineral form called aragonite.
They grow in concentric layers as calcium carbonate is deposited over time. The number of rings can be used to determine a fish's age when the relationship between number of rings and time has been confirmed.
To see the rings, the otoliths are cut open and thinly sliced to reveal the inner layers.
Because the otoliths are more durable than the soft flesh of the fish, otoliths are commonly found in recent sediments and can also be preserved as fossils in older sedimentary strata.
When the otoliths found in different layers of sediment, their comparison can offer clues about changes in the population of fishes and environmental conditions over time.
Bluefin Tuna at Funchal Fish Market in Madeira, Portugal.
Photographer and © Bill Anderson.
Fish muscle can be white or light-pink to red and dark red. (Salmon muscles are orange depending on what they eat.) On a fish steak, you can often see darker red muscle along the lateral line of a fish. Fast swimmers like the Mako Shark and Bluefin Tuna show this clearly.
"Apart from their relative positions, the red fibres are smaller and more uniform in size and contain greater quantities of mitochondria, myoglobin, fats and glycogen. They are also provided with a more abundant vascular supply" (Greer-Walker and Pull, 1975) . Like hemoglobin, myoglobin is a protein that carries oxygen, except it exists in muscle rather than blood.
Swim (gas) bladder
Salmon. The elongate white object is the swim bladder.
Image courtesy of Salmonids in the Classroom, Fisheries and Oceans Canada.
Gas-filled bladders help fish to position themselves vertically in the water column and maintain buoyancy. Without it, it is difficult to move up-and-down.
On the other hand, gas bladders limit the up-and-down range of motion due to physiological problems related to dissolved gases in the body, like those experienced by human divers. Thus deep-living fishes do not tend to have gas-filled bladders and maintain their buoyancy in other ways. Many sharks have particularly large livers containing lower density oils.
Swim bladders also help fish to receive and generate sound.
Buoyancy of shark liver.
Demonstration by marine biologist,
Don't click and look if you don't want to see a dissection.
Birch Aquarium at Scripps.
• More video of
Before information about the external world can be analyzed on a computer, it must be detected by a sensor, collected and transmitted to a computer (or human note taker). In the same way, the sensory organs of fishes and humans must first detect motion, light, chemicals and other aspects of their environment before being transmitted through the nervous system to the brain for processing, and then acted upon.
Like instrumentation aboard an observational satellite, fish are loaded with various receptors that collect information such as sights and sounds. Major sensors are located on the head close to the brain. That information is used to initiate action — to capture prey, locate suitable mates, evade predators, and more.
- Photoreceptors that detect light
- Electroreceptors that detect electric fields
- Neuromast sensors that detect motion and sound
- Olfactory receptors that detect chemical substances
Lateral line on the side of a Barred Surfperch marked by disruption in color pattern.
Barred Surfperch usually live in the surfzone of sandy beaches where people wade and swim. One of the few fishes that give birth to fully formed live young.
Photographer and © Kip Evans.
Unusual transparent eye configuration of the Barreleye or Spookfish.
Image © The American Society of Ichthyologists and Herpetologists (ASIH). Permission granted by ASIH.
Image from "Macropinna microstoma and the Paradox of Its Tubular Eyes By Bruce H. Robison and Kim R. Reisenbichler, Copeia 2008, No. 4, 780-784.
Eye position and size are related to light levels and depth where a fish lives.
Electromagnetic field detction
Great White Shark
Electromagnetic field sensors lie at the ends of pores at one end of the Ampullae of Lorenzini on the snout of a shark.
Photographer - © George T. Probst.
Photographer - Monika Betley.
Creative Commons 3.0
The barbels or "whiskers" of fishes such as catfishes are used to detect odors and locate food, even in murky water.
Fishes that return from the sea to the streams where they hatched navigate at least partially by recollection of smell.