Into the Wild: What Can We Learn About Aquarium Fish from Stream Fish?

My name is Angie and I am a stream ecologist. My passion for fish and biology started when my family moved to Wisconsin in the mid 1990's. Lake Fishing became a favorite pastime with my father and sister. It impacted me so much in my early years that I chose to study freshwater ecosystems in my higher education, starting out as a field technician and research assistant with my professor, fisheries biologist Dr. Robert Anderson, at Wisconsin Lutheran College. After earning a Bachelor's of Science degree in Biology, I obtained my Masters of Science degree in Biological Sciences at the University of Wisconsin-Milwaukee, focusing on stream ecology and urban stream restoration. Additionally, I spent time studying marine ecology in Jamaica and the rainforests and streams of Costa Rica. I've taught collegiate level courses in Zoology, Biology, Anatomy and Physiology, and Life Sciences in hopes of spreading my passion and getting more students fascinated with biology.

Today, I work with Ozaukee County in the Planning & Parks Division's Fish Passage Program. The team and I investigated the fish habitat and impediments (any feature that prevents fish migration, such as a dam or log jam) on the direct Lake Michigan tributaries in the county. We also studied fish communities and fish habitat on the streams that empty into the Milwaukee River. Our goal is to improve and increase native fish spawning and populations.

Did you know that fish are the most diverse group of vertebrate animals? There are 33,700 species of known fish according to FishBase, a globally recognized non-profit and non-governmental organization biodiversity information system on ray-finned fishes. Due to the large amount of species of fish inhabiting the same region, they have developed special adaptations, or traits, that allow them to coexist. Many of these traits lead to very slight changes in DNA (it's survival of the fittest out there!), which are then passed down from generation to generation. An example of a heritable trait involves the anatomy, or structure, of a fish's mouth and what it likes to eat.

The Three Types of Fish Mouths

Fish found in lakes and rivers display a wide variety of feeding behaviors. When comparing species, it is easy to determine where wild fish like to find their favorite food items and what those foods may be, based on the design of their mouths.

There are 3 basic mouth shapes in bony fish:

1. Terminal

2. Inferior (turned downward)

3. Superior (turned upward)

Terminal Mouths

In most fishes, the mouth is terminal (at the end of the head, like a dog or cat). Species with terminal mouths can be omnivorous, eating both plant and animal matter, or exclusively carnivores, feeding exclusively on animals.

Wild fish: The Iowa darter (Etheostoma exile) is a colorful fish living in shallow, cool, clear water streams in the Midwest. This small fish loves to eat aquatic insect larvae, crustaceans (such as the small shrimp-like critters that move with currents), and zooplankton. While zooplankton are found throughout the water column, the larvae and crustaceans are found on the stream bed. Primarily a carnivore, this fish will feed at the top, middle, and bottom of the stream.

view of a small Iowa darter fish inside a went hand

Iowa darter

Aquarium fish: "A common aquarium fish with a terminal mouth is the Oscar, Astronotus ocellatus. Native to the Amazon region of South America, Oscars feed primarily on smaller fish, but are also known to eat crustaceans, insects, and aquatic and terrestrial plant matter. While they prefer meaty foods, they will eat fruit and vegetation when food is scarce."

orange and black oscar fish


Inferior Mouths

Fishes with inferior, or downward-turned mouths, are omnivorous or herbivorous, feeding exclusively on the bottom of the river or lake.

Wild fish: One of the most common fish in the Midwest is the white sucker (Catostomus commersonii). This fish has a very muscular body and can grow to nearly 2 feet in length. With the mouth positioned just underneath the head, this fish will eat algae growing on rocks, mussels, aquatic insect larvae, and crustaceans. As long as it fits in their mouth, the white sucker will eat it!

person in boat on the water, their hands are visible holding a white sucker fish

White sucker

Aquarium fish: An inferior mouth fish is often a good idea to have in an aquarium as they help keep it clean and free of algae. A commonly sold fish is the flying fox fish (Epalzeorhynchos kalopterus). This powerhouse loves to snack on all kinds of algae and detritus, which is decaying organic matter (such as the food that falls to the bottom of the tank when your pet fish is being a picky eater). While this little algae eater is a ravenous consumer, they typically do not damage live aquatic plants.
close-up of a striped flying fox fish

Flying fox fish

Superior Mouths

The superior mouth is indicative of an ambush predator. These fish are sneaky and generally hide and wait for the prey or food pellets to come close before they strike.

Wild fish: At first glance the blackstripe topminnow (Fundulus notatus) may look like the flying fox fish. However, the topminnow has a superior, or upward-turned, mouth. Found near the surface in the slow moving waters of streams, rivers, and edges of lakes, the blackstripe topminnow loves to eat insects that fall or are blown onto the surface of the water. The algae growing along the surface of the water is another favorite food.

close-up image of a blackstripe topminnow fish

Blackstripe topminnow (Tony Terceira/TFH Magazine Archives)

Aquarium fish: Many common aquarium fish have an upward turned mouth and feed from the top of the water such as: betta fish, archer fish, and hatchet fish. In their natural environment, betta fish (Betta splendens) eat insects; at home, bettas gobble up small pellets, flakes, and treats high in protein that float on the surface of the water. The archer fish is famous for its ability to shoot water and knock down insects from overhanging leaves and branches. It has a large mouth compared to the betta and is able to eat large insects. Having an aquarium set up with both underwater and above water plants will allow pet archer fish to demonstrate this natural behavior when provided with live insect prey.

Close-up of a betta fish

Betta fish

Do Fish Have Tongues?

Despite the various mouth shapes, most fish have a rough, flat tongue that is attached to the bottom of the mouth. Unlike mammals that can freely move the tongue, a fish's tongue is made of tough cartilage and can only move up and down which assists in swallowing food.

Do Fish Have Teeth?

Most fish have teeth, with the number and shapes of teeth dependent upon the fish's primary diet. Carnivorous fish have teeth that can pierce, hold, and cut prey; the most common tooth found in both fish and mammalian carnivores is the canine. Herbivorous fish possess teeth that are able to shred plant material, such as algae and aquatic plants. Bottom-feeders that prefer to feed on animals with tough shells like snails or clams have molars to assist in crushing and grinding. Fish teeth can be located on both lower and upper jaws as well as inside the throat (these are called pharyngeal teeth).

Fish teeth are made of a bony substance called dentin and have an enamel coating, nerves, and blood vessels similar to a human tooth. Most fish are able to replace a tooth if it becomes injured or damaged, a feat that would be much appreciated by many adult humans!

Tetras, which are often brightly colored, active, and relatively easy to maintain, are members of the Family Characidae. Like their cousin the piranha, tetras have a row of sharp teeth located on both the upper and lower jaws that they use in biting and tearing apart food.
close-up of a dark orange tetra fish


The familiar suckermouth catfish (also known as plecostomus or pleco) have small cone-shaped teeth and specialized cells with brush-like projections that allow them to scrape and graze on the algal growth.

Even the beloved Betta has teeth! Although very tiny in size, these teeth help chew up food. The male bettas, which are well-known for their aggressiveness, also use their teeth when fighting other males. Their teeth are the perfect shape and size to shred the fins of competitors.

Final Thoughts

Next time you watch or feed aquarium fishes, take a moment to observe how each species consumes its food and identify the size and placement of their mouths. An inexpensive magnifying glass will allow you to see teeth located along the jaws, which can be quite impressive. It is interesting and beneficial to research the natural prey items and feeding methods of your pet fishes and compare it to the ingredients of their food and treats. It is important that your fish have a well-balanced diet as coloration and behavior are greatly dependent upon nutrition. The investigation and discovery of all these small, unique characteristics make our finned-friends fantastically fascinating creatures!

Best fishes!
Stay in schools :)

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Note: None of the fish pictured in this article were injured or harmed in any way. All wild fish were collected during stream fish community surveys using approved Wisconsin DNR and EPA protocols. All fish were promptly returned to the water following identification.

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