There are many interesting aspects to the anatomy of fish that are worth exploring. Why? Because the evolution of fish actually sets the stage for the evolution of every other vertebrate on earth!
Nearly all of the features that amphibians, reptiles, birds, and mammals have are adaptations and alterations from anatomical features fish first developed. Even lungs have their roots in formerly aquatic features that fish still use today!
Fish are a very old, highly successful animal lineage. They have roots going back into deep history; all the way to the Cambrian Explosion, over 530 million years ago! This was a period where the fossil record shows a sudden explosion in animal diversity. From soft worms, mollusks, and other more primitive life we suddenly see more advanced animal forms arising.
The first fish are a very strange lot. Many had no jaws, for example, and simple spinal columns formed mostly or entirely of cartilage. Most also had thick, bony plates of armor at first; the small scales we associate with modern bony fish were a later development.
Sharks vs Bony Fish
Cartilaginous fish (sharks, rays, sawfish, etc) split from other fish around 420 million years ago. They are still fish – but they are about as distantly related as you can possibly be while still being a fish. They are superficially similar because they share common ancestors and live in the same environment.
But when looking at a shark, you’ll notice that they are quite different from other fish in several ways. Cartilaginous fish don’t have scales, for example, nor do they have bone in their skeletons. They don’t have swim bladders and many don’t have symmetrical tails like bony fish do, with the top and bottom lobe being the same size.
Today’s article will focus on Betta fish, which are bony fish. Since 99% of all aquarium fish fall under this category, most of what you read here will also apply to all tropical freshwater fish. All except for freshwater stingrays – and all saltwater fish except sharks and rays!
Betta Fish Body Parts
As you read this article on the anatomy of Betta fish, take some time to compare what you read to the fish anatomy diagram included in this article. It will give you a much clearer picture of what it is to be a fish!
We are going to focus on the parts of Betta fish that make them unique from other animals. Since fish have a stomach, eyes, and mouth that are mostly similar to those of other animals we will talk less about those. On the other hand, their gills, scales, fins, and other parts are very different from other animals and even unique to fish!
Betta Fish Gills and their Function
If you had to choose one fish body part that sets it apart from air breathing animals it has to be their gills. Betta fish use their gills to breathe and they actually work using the same principle that our own lungs do!
Fish gills bring oxygen-poor blood in close proximity to the water, where oxygen can diffuse across the cells into the bloodstream. Carbon dioxide takes the opposite path, leaving the blood and being expelled into the water. Our lungs work in exactly the same way.
Fish don’t have a diaphragm to pull in oxygen like we do, though. Instead they have to pump their mouths open and shut, pulling in water with vacuum pressure and forcing it over their gills to breathe. Interestingly, open water sharks can’t do this. They have to swim continually, which forces water over their gills.
This is called ramjet breathing. It’s extremely effective, especially for fast, powerful swimmers like Great White Sharks. The downside is that if the shark can’t move, it will slowly suffocate. But in the open ocean that’s a very rare occurrence.
Bettas and Other Air Breathing Fish – the Labyrinth Organ
Gourami and Betta anatomy is very special in this regard. They have a highly evolved gill structure called the labyrinth organ. The labyrinth organ allows them to take in oxygen from the air directly – without giving up their ability to breathe water!
Now, you might be thinking this is where air breathing animals took off. But not exactly; we are actually descended from lungfish relatives that left the water nearly 400 million years ago. Rather than a labyrinth gill arch they use a modified swim bladder full of blood vessels. Over time, as these fish spent more time out of water the swim bladder became more and more efficient until they served as true lungs.
Interestingly, amphibians of all kinds retain their gills when young, growing lungs as they get older. And some species of amphibians, especially salamanders, can keep their gills even into adulthood through a process called neoteny. But the majority of amphibians are air (and skin) breathers.
Many fish use their gills for tasks other than breathing! Some have special gill arches that allow them to filter feed. Paddlefish and Kissing Gourami do this; they can filter microscopic plankton from the water to eat. Kissing Gourami also have the same labyrinth organ as Bettas. In short, the different parts of their gills perform triple duty as feeding, water breathing, and air breathing organs!
Understanding the Swim Bladder
Swim bladders are an organ found in most bony fish including Bettas and are used to control their buoyancy – how well they float in the water. Many fish aim for neutral buoyancy, meaning they neither float or sink. This allows them to move wherever they wish with minimal effort.
Some fish have body parts that allow them to swallow or burp out air to precisely control their buoyancy. While others can’t do this and need to slowly add or remove gases through the bloodstream. This is true of many deep sea fish, which never come anywhere near the surface.
As I mentioned before, the swim bladder is also the organ that our lungs evolved from. Lungfish, bichirs, and other fish that use their swim bladder to supplement their gills were the ancestors of all land dwelling animals!
Betta Fish Scales
If you’ve ever touched a fish before then you’ve likely marveled at how hard and rigid their scales are. Fish scapes are a marvelous adaptation. Most aren’t bone, as you might have first thought, though some fish do have plate or knob-like scutes, which are usually made of bone.
True scales are extremely diverse in form and function, with descriptive names like cycloid, ganiod, and elasmoid to describe these traits. One function of fish scales is defense; not just from large predators but also bacteria, viruses, and other opportunists.
The slime coat that most fish have furthers this defense while also lowering their water resistance, allowing them to swim quickly. This is why you should never touch a fish with dry skin; your skin easily strips away this slime coating. Plus the bacteria and fungal spores on your hands are potentially infectious to fish.
Fish scales also contain pigments that reflect, absorb, and refract light. This can help them blend into their environment, find mates, signal to rivals, and more. Cartilaginous fish like sharks stand apart from bony fish because they don’t have true scales at all. Their skin feels like sandpaper thanks to the millions of denticles that cover their flanks, like tiny teeth!
What Makes Fish Bones Unique?
Betta fish bones are very similar to our bones though they aren’t always in the same places. They serve the same function: structural support, immune cell production via the bone marrow, and protection. But one look at a fish, especially a cooked one, and you’ll see that there is a lot of complexity there!
Fish have many structural bones along their backs and tails to support their fins and the muscles that power their swimming motion. They also provide core support to the body cavity without inhibiting swimming power.
Nearly all aquarium fish are “bony fish,” which lets you know that all fish in your tank likely have bones. Unlike Betta fish, sharks, stingrays, and other cartilaginous fish anatomy doesn’t include bones. Instead their entire skeleton is made from the flexible but rigid material cartilage; the same protein that is found in between your own bones at joints.
Do Betta Fish Have Ears?
Many fish do have ears! Their ears work a little different from ours but still allow them to hear in the water and process sound. The main barrier to hearing if you are an aquatic organism is that your body is likely around the same density as water.
If it wasn’t then you’d either float if you were less dense – or sink if you were more dense. However, sounds travel directly through objects of the same density, such as water and the water-filled tissues of fish. So how to hear things in the water?
Betta fish have a bony ear structure called the otolith, something that most vertebrates have. In humans we use this “ear bone” to sense acceleration. But in fish the otolith captures sounds directly, vibrating in response to incoming sound waves and stimulating cilia in the surrounding cavity. They also help the fish sense acceleration and balance and come in three pairs for most bony fish.
Fish otoliths can also be used to estimate the age of fish in many species! When sliced in half they often show growth rings just like a tree. These rings document growth speed, the age of maturity, life span, and other important factors for scientists trying to understand both population dynamics and the parts of a fish.
Fish and their Many Fins
Betta fish have several fins that are used most directly for swimming purposes. They catch water, directing it through both muscular and passive action along their bodies. Fish have several groups of fins that are shared across many species.
The pectoral fins of a fish are the first pair along the sides of the body. They serve as lift generators in many species such as sharks – and extreme lift generators in flying fish! Other fish use theirs to fine tune their movements in complex swimming environments or even swim backwards.
Look at your own arms and hands right now. These forelimbs evolved from the pectoral fins of your distant lungfish ancestors! You can see the proof of this in how fish that “walk” on land like mudskippers move. They push and pull themselves along using muscled pectoral fins!
The second set of fins along the belly are the pelvic fins. These led to the hindlimbs of land dwelling animals – or the legs of us upright walking mammals.
The first fin you encounter moving from the head to the back is the dorsal fin. This fin prevents rolling while swimming and also helps the fish make sudden turns and stops.
Along the belly, past the pectoral fins is the anal fin. This fin is another stabilizer, similar in function to the dorsal fin. Last comes the caudal fin, or tail fin! The tail fin is the main propulsion engine of the fish, though the other fins all help too.
Besides swimming, Betta fish also use their fins for display purposes. If you keep social fish like cichlids or tetras together, you can watch them signal to rivals, impress mates, and “talk” to one another by stretching, closing, and waving their fins in precise ways. Over time you may even be able to decipher this fish talk, figuring out who is who, who is the most dominant, who the males and females are, and more!
Fish fins also give you clues to fish health. Clamped fins are often a sign of stress, disease, and anxiety in Betta fish. Torn fins can reveal territorial disputes or diseases like fin rot. There is a lot of information hidden in the secret language of fish fins!
Fish Mouths and Teeth
This tour of fish anatomy would not be complete without talking about fish mouths and teeth. Fish mouths and teeth are extremely diverse in form and function. In fact, you can thank fish for the jaws that you use to chew your food and shape the words coming out of your mouth. Jaws are an innovation going back to the Silurian Period, around 440 million years ago.
Fish jaws were originally modified sets of the frontmost gill arches. Remember how I mentioned that some fish use their gill arches to strain plankton from the water? Well these ancient fish used theirs to help snare prey, growing protrusions that eventually specialized into true teeth!
What are Fish Teeth Like?
Fish teeth are also diverse; much more so than ours. Some fish, such as Pacus, have teeth that are frighteningly similar to ours because they eat the same things that our ape ancestors do: fruit, leaves, and other crunchy vegetables.
Predatory fish like Piranhas tend to have sharp, pointed teeth for either grabbing into or shearing flesh from their prey. Many bottom dwelling algae eaters have thick lips and rasping teeth for scraping algae and biofilm from hard surfaces like rocks and driftwood.
The strangest fish anatomy of all might be pharyngeal teeth! Rather than sitting inside the jaw these teeth are found in the upper part of the throat. These fish have to first grab their food with their mouth and then partially swallow it to begin chewing. Fish with pharyngeal teeth include goldfish, barbs, and moray eels (who also use these teeth to snare prey and pull it into the throat).
The anatomy of Betta fish and their relatives is a fascinating subject because so much of their anatomy is both very similar and very different from our own. While we share identical structures like eyes, fish also have scales, swim bladders, and other organs with radically different functions. I hope you’ve enjoyed this brief dive into the world of Betta fish anatomy!