Anatomy of a Fish: Internal & External

Instructor: Julie Zundel

Julie has taught high school Zoology, Biology, Physical Science and Chem Tech. She has a Bachelor of Science in Biology and a Master of Education.

While fish seem pretty simple, there's a bunch of vocabulary that goes along with them. Sigh. Don't worry! This lesson will help you learn all of the terms you need to understand the internal and external anatomy of a fish.

What Is A Fish?

Okay, you probably know what a fish is, right? I mean they are scaly little buggers that swim in the water, and some taste pretty good. What else is there to know? Well, for starters, there are nearly 28,000 species of fish and there is a lot of variation amongst them. For example, some are enormous, like the 40-foot whale shark. Others don't look like fish at all, like the hagfish.

The hagfish does not look much like a fish
hag fish image

Some like rivers, others prefer lakes, and some inhabit the oceans. While there are exceptions, most fish share the following characteristics and/or structures:

  • Are vertebrates (have a backbone)
  • Use gills instead of lungs
  • Have a brain, eyes, and a mouth
  • Have fins
  • Are ectotherms (cannot regulate their own body temperature)
  • Have a protective scale covering

Of course there are always a few exceptions to the rule (hagfish do not have vertebrae, lungfish have lungs, etc.). But you get the general idea.

External Anatomy

Now that you know what it takes to be a fish, let's take a tour of a fish's external, or outer, anatomy. Let's begin our fish tour at the front of the fish and work our way around.

External anatomy
fish image

Take a look at the fish in the image, which is a bony fish.

  • Let's start with number 1, which is a bony structure that covers the gills and aids in respiration (breathing) called the operculum.
  • The next structure is the lateral line (2), which like the name implies, is a line that follows the length of the fish. This sensory structure helps the fish detect movement or vibrations in the water. This assists in hunting, escaping, and moving in unison.

The lateral line helps fish swim in unison in a school
school of fish

  • The next stop on the tour is 3, or the dorsal fin, which is a fin on the backside (dorsal side) of the fish, which stabilizes and aids in turning. If you've ever seen the movie Jaws, the dorsal fin is the fin that pops out of the water as the shark approaches and you hear ominous music.
  • Number 4 is the adipose fin, which is seen in various families of fish (including salmon). It is located between the dorsal fin and the tail. Scientists once believed it was to store fat (adipose), hence the name. Now many scientists believe it helps with swim efficiency.
  • The next fin on our tour is the tail (5), or caudal fin (cauda is Latin for tail), and the fish uses it to propel itself.
  • Number 6 is the anal fin (which is located near the anus, hence the name). This fin helps keep the fish stable.
  • We are almost done with our external tour! Two more stops. The pelvic fins (7) are located closer to the front of the fish than the anal fin. These are paired fins that help keep the fish stable.
  • Finally, the paired pectoral fins (8) are towards the front, near the gills, and help stabilize and steer the fish.

Internal Anatomy

Now let's go over some of the internal anatomy of a fish.

Internal anatomy
image of fish insides

If you look at the image, you might become overwhelmed. Don't. We won't go over every single thing in painstaking detail.

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