Marine Fish External Anatomy & Physiology Guide

By: Michael Phife


Ever wonder what the true names to parts of a fish were called… instead of calling them names such as “little fins, the top fin, the second pair of little fins”? You get the idea. In this guide, I will not only be identifying the exterior anatomy of a fish, but also what each part and its function does for the fish. Please keep in mind that the anatomy and physiology to saltwater fish are slightly different than freshwater, so some of the portrayals will differ between the two. Also, the anatomy is different between bony and cartilaginous fish, which I will explain later in this article.


External Anatomy (Bony Fish):


In my crude depiction of my Moorish Idol (before he passed from a horrible disease), you can see the major anatomy of a fish labeled above. There are several anatomical parts I have left out in this picture, which I don’t find to be very relevant or useful to the common reader. I will list each of these parts below going counter-clockwise, starting with the dorsal fin, and what their functions are for the fish.

  • Dorsal fin: Think of this like a mast on a ship. Helps the fish stay in position that way it doesn’t topple over in the water, along with turning. Some fish may have a second pair of dorsal fins. Dorsal is also a fancy way of saying “top.”
  • Caudal fin: Another name for this is the tail fin. This helps propel the fish forward like a motor for a boat.
  • Anal fin: Easy to remember as they are located on the back of a fish. The anal fins help a fish steer left and right. Works in conjunction with the caudal fin.
  • Vent: Opening to the urinary and reproductive tract. Almost always found in front of the anal fins.
  • Pelvic fin: Helps with balance and works in conjunction with the pectoral fins.
  • Pectoral fin: Found on the bottom-frontal part of a fish. Helps a fish steer and “push” its way through water, along with keeping balance.
  • Mouth: This is where the yummies go. Also where water is taken in and then expelled through the gills.
  • Nare: The nose of a fish. Fish have a very keen detection of smells around them.
  • Eye: For seeing… just like everything else that has eyes. Fish eyes are much rounder than anything on land because of the refraction in water. The lens moves in and out to help with the refractive index. And yes, fish can see many colors.
  • Scales: Some fish will have smooth scales (cycloid), like the Moorish Idol above. Others will have rough or jagged scales (ctenoid) like that of a salmon. Nearly all fish have a slime coat that helps ward off infection. Also important why you shouldn’t handle fish for an extended period of time. Doing so rubs the slime coat off and oils in your hand seep into their scales.
  • Gill cover: The area that covers and protects the gills. Water is expelled through the gills whenever they breathe in through their mouths to regulate osmosis.



External Anatomy (Cartilaginous fish):


This is a Bamboo Cat Shark that I had the pleasure of viewing at the Texas State Aquarium. It will be my reference for cartilaginous fish anatomy, yet most will be a repeat from the bony fish examples. I will label this one going clockwise this time, starting from the pectoral fin.

  • Pectoral fin: Same function as above. Though, cartilaginous fish use more energy to help move themselves through water since they travel at a faster rate.
  • Gills: Same function as above. It is a myth that all sharks will drown if they do not constantly pump water through their gills. Some definitely have to keep swimming, but others like the bamboo cat shark shown above can rest at the sea floor and pump water with their pharynx which helps them maintain osmo-regulation.
  • Eye: Same function as above.
  • Mouth: Same function as above.
  • Nare: Same function as above. Though, their smell is even greater than that of a bony fish. Most sharks can smell 1/4 of a mile away, and even smell a single drop of blood in that entire radius.
  • Scales: Same function as above.
  • Dorsal fin 1: The frontal dorsal fin in fish that have multiple.
  • Lateral line: This is actually present on both bony and cartilaginous fish… just harder to pinpoint on some fish. It is a bit complex, but is essentially a fluid-filled sac on their sides that help them detect currents, pressure, and movement in the water.
  • Dorsal fin 2: This is found on some fish but not others, but easy to know if a fish has it (second huge fin coming off its back). Same purpose as the first dorsal fin, which is to help maintain balance and for turning.
  • Anal fin: Same function as above.
  • Caudal fin: Same function as above. However, the caudal fin in some sharks are so powerful that they have been reported to break bones or even kill humans when the shark is taken out of the water. There was a report perhaps a decade ago of a person who caught a Thresher Shark, and the shark swiped its tail, hitting the human in the upper back which dislocated his spine from his brain.


These are the most important external anatomical parts for both bony and cartilaginous fish. I will be writing an internal anatomy guide in the near future, too. It will be very informative to those of you who wish to know how a fish regulates salinity, keeps equilibrium, traverses through oceanic pressure, and overall how they stay alive in the ocean.


Bonus picture of me petting the Bamboo Cat Shark!