What's in that fish on your dish? While we may be used to the simple tuna fish sandwich or of simply wondering what the heck our goldfish is thinking about, there's a tremendous amount we can learn by investigating the anatomy of a fish. If you've always wanted the smell of fish permanently lining the insides of nostrils, then fish dissection may be the right thing for you. This works great whether you're out fishing or in a classroom. Take a moment to see how a fish works.

What: Fish Dissection!
OMG: EWWWWW!!!!!
Concepts: anatomy, physiology, organ systems, evolution, biology
Materials:
- 1 Fish (or 1 fish per group of two students)
- 2 gloves
Tools:
- Tray
- Cutting device (X-acto, razor blade, sharp craft scissors)
- Tweezers
- Wood skewer for prodding (optional)
- Fish Anatomy Guide

A Note on Dissection

Dissecting animal organisms is a tricky subject which ultimately some will object to and that's okay. For us, we believe that with careful scientific exploration of animals who were treated in humane ways, we can learn and teach our students to be greater stewards of the environment, and especially toward the species they just learned about. There will always be some students who wish not to do a dissection, and we always allow whatever level of participation they choose in these experiences. Many prefer to just watch. Many of those eventually join in. But whatever it is, feel it out with your classroom or audience.

Okay, let's go fisihing!

Step 1: Which Fish Do You Wish?

For dissections with a class, often you're looking for a medium sized fish that isn't too expensive. I often choose to dissect perch because they have easy to see organs, don't have too much meat, and are cheap. However, this time at the grocery store, I was caught by the iridescence of the milkfish.

Step 2: Finding Fins

Get ready for a loud and resounding, "EWWWWWWWWW" if you're teaching in a classroom. I heard a great thing from another teacher, who said that while most might say, "EWWWW!," scientists say, "OooOooOoo!" So often I have the class do both together, because it's okay to feel both things!

There's a lot to learn from your fish before you cut in. After you put on gloves, and grab your tray, start by expanding your fish fins. Your fish is likely to have a pronounced and spiny dorsal fin (on top), pectoral fins (imagining chest level), a caudal fin (the tail), and an anal fin or two (in the back). Each one folds slightly differently.

How many do they have? What are the largest ones? How can they move and help the fish move?

Step 3: Color, Scales, and the Lateral Line

Fish win the rainbow contest for animals, and vary greatly in their appearance, even within a species. A common trait that you can see is to have a dark top and a light bottom. This is so that they can camouflage from predators looking down at them (blend in with the sea floor), or looking up (blend in with the sky).

Fish vary greatly on the colors of their sides, with many fish having iridescent scales like the milk fish shown here. Iridescence is an amazing phenomena, and is caused by tiny guanine crystals in the scales of the fish that give them that metallic sheen. There is a great paper on this here!

You can peel off a scale or two and look at it up close.

Additionally, the lateral line is an amazing system of sensory organs that you can see clearly in the third photo of this step. Fish and other aquatic vertebrates use these to detect movement and vibration in the surrounding water. These are very important in schooling behavior, and in tracking prey. Fish can use these to track prey even in complete darkness!

Step 4: The Head

Fish heads! Oft neglected in the culinary realm, there's a lot to see in the scientific one!

The natural start is to look at the eye, which as students can see they don't have eyelids like us (no why would this be?) In the case of the milkfish, they have an adipose eyelid which is a transparent membrane over the eye. If you look directly into the center of the pupil, you can even see the lens, which is much more spherical than ours.

Moving on, if you look at the flaps on the side (called the operculum), you can see the gills inside. How many gills are there? Why would they have these flaps? If you open the mouth, you can tell a bit about the way in which your species of fish eats. In the case of the milkfish, it is mostly by opening their mouth rapidly, creating negative pressure, and forcing prey in. They have a tongue for assistance, too. If you poke through the mouth opening with your tweezers, you can sometimes see your tweezers come out the side flaps. Fish literally have holes in the side of their heads!

Step 5: Making the Cuts

Start by finding the anus (the only opening on the underside), and cutting a line not too deep all the way up to the gills. Open it up a bit, and then cut a lateral line up the side of the fish so you can open it up to see the way the guts are inside while they're in place.

Already you'll notice the ribs of the fish, and the thickness of the meat. In milkfish, they have a bulky muscular wall, and so there is a large amount of meat. If at the end of your dissection, you're still in the mood for fish, you can clean and fry it up!

Step 6: Digestive System

Track the digestive system either starting from the mouth or from the anus. You'll see a tiny fist-like organ which is mushy but tough, and that's the stomach! Start by removing it, and cutting it open to see what's inside. This is how biologists do gut exams and diet analysis. Our food material looks pretty well digested, but if they ate smaller fish shortly before they were caught, you can often see them in here.

Afterwards, remove the intestines from the fish, which are often quite long.

Step 7: Heart and Liver

Two pronounced organs in the fish near the pectoral fins are the heart and liver. The heart is often very dark and often has coagulated black blood surrounding it, and can be found just below the gills. You can remove it, and in larger fish even dissect it to see the two chambers inside. Human hearts, by contrast, have four chambers. The fish circulatory system is a single circuit, with blood flowing from the heart to the gills and then the rest of the body.

The liver is the largest item by volume in our milkfish. It is red, and often large in other fish, too, but varies by sex, species, and stage of life. Besides working to clean blood, the liver also often acts as a food storage of fats, blood sugar, and vitamins for the fish and so is essential in times of low food availability.

Step 8: Swim Bladder

If you're careful with your cuts, you can see a neat membrane that comprises a sac that is the swim bladder of your fish. All aquatic animals have different strategies for changing buoyancy, and how they float or sink in the water. They are also called "gas bladders" or "air bladders" because they are air-filled organs that allow fish to maintain a level of buoyancy in the water without wasting energy swimming. This is big news for being able to sleep comfortably.

We humans have borrowed this adaptive biology to do the same while scuba diving in Buoyancy Control Devices, which we inflate or deflate with tanks to control our position in the water.

Step 9: Grills of Gills!

Go to the head and remove the gills of the fish. How many does your fish have? You can see that they are often fan-like, and kind of look like a radiator. This is an adapted characteristic, so that they can have maximum surface area in contact with the water around them. Fish need all the help they can get to get oxygen, as they need it like we do. While air is approximately 21% oxygen or 210,000 parts per million, water often only has about 4-8 parts per million dissolved within. That means those gills have to do a lot of work!

Read about the amazing way that gills absorb oxygen and disperse carbon dioxide in this article!

Step 10: Fish Eyes

Okay, they're weird. As stated, fish don't have eyelids like we do (except sharks, kind of), but instead often have a translucent covering to protect their eyes that, of course, never dries out. If you cut away this protective layer, you can see how the optics of a fish eye work. Using your tweezers on the pupil, you can find the lens, which once you pull the gunk away, is almost a perfect sphere! If you really want to get into it, if you hold it up to light, you can see the way light passes through and flips images upside-down!

Step 11: Clean and Keep Exploring

Hope you enjoyed your fish dissection, whether it was virtual or physical. Hopefully with a greater knowledge about fish anatomy, we can come to understand how to protect them, manage populations sustainably, and understand more about human life and life of all things around us.

Different fish will have different pronounced characteristics, and there are a few that weren't pronounced here that you can find in others. Fish gametes are one, where often if they have two large sacs toward their rear, they are male, and one are female. Our milkfish was a juvenile and had not developed them yet. Fish brains are another, which are quite small and difficult to find, but are often somewhere between the eyes. There is always more, too, including spleens, olfactory bulbs, kidneys, and more.

If you are interested in other dissections, you can find our squid dissection here, too!

As always, keep exploring, and share what you find!