A Fishy Biology Lesson From Paul

Some fish, such as the Banded Pipefish (Doryrhamphus dactylophorus), must eat constantly to stay healthy

Some fish, such as the Banded Pipefish (Doryrhamphus dactylophorus), must eat constantly to stay healthy

Though fish are our distant cousins, there are more differences than similarities between us. In today’s post, I’d like to explore the various characteristics that make a fish a fish—or not.

Skeletal distinctions

Of course, not all fish are the same and the oldest fish are not truly fish. True fish have bones somewhat like us, but not exactly. All of our bones are connected together by cartilage, but in fish, the fins are not connected to anything but muscle. The tail is connected to the spine, but none of the other fins are. Sharks and rays have no bones, only cartilage.

Built-in buoyancy compensators

The swim bladder in most fish (but not all) allows them to maintain neutral buoyancy, or the ability to stay in one place in the water without sinking. In sharks and rays, a large liver filled with oil helps maintain buoyancy, but they do sink if they stop swimming.

Some bony fish that lack a swim bladder are tuna and mackerel. They swim so fast that they don’t need one, as their fins are shaped to guide them where they want to go and there is no need for them to float, as they never stop swimming.

The vast majority of fish have no direct control over their swim bladder. Rather, it is controlled by air dissolved in the fish’s bloodstream, which is exuded into the bladder.

Supercharged immune systems

I talk a lot about the immune systems of fish. It is my favorite topic, and I feel it is vastly overlooked in our hobby. A fish’s immune system is more complicated than ours. Fish make macrophages (which attack bacteria, viruses, and parasites) in a few different places. They use their spleen (as we do), but they also use their kidneys, gills, skin, guts, and gonads.

Remember, a fish is bathed in disease-laden water. We normally live in air, and bacteria, parasites, and viruses can swim better than they can fly. We can also go to a doctor when we get sick, but fish have to depend on their immune system to prevent them from getting sick in the first place, and they rarely do unless they are kept in less-than-perfect conditions—like many of our tanks.

No sweat and the skinny on fish skin

Fish do not have sweat glands, but they do have mucous glands. Mucus is secreted by all fish; it is part of their immune system. Our skin is mostly dead cells that we shed all the time, but a fish’s skin is alive all the way through. It is protected in most fish by scales, or body armor. Fishes’ skin is clear but contains chromatophores, which allow the animal to change color.

Picking up good vibrations

A major difference between us and fish is their ability to detect minute traces of electricity in the water. Some fish, such as stingrays and freshwater elephantnose fish, use this capability to detect prey, but all fish have a lateral line, which is a set of nerves that runs along both sides of the fish. If you look closely at almost any fish, you can see these nerves. They start near the gills and arch back to the tail. These nerves allow the fish to “feel” or detect what is around it.

The lateral line allows fish in a school to all swim together and turn at the same time without hitting each other. It also allows them to escape a net being pushed behind them. Did you ever see a fish crash into the glass of a tank? That’s because they never do, even though they can’t see the glass from inside the tank. Fish can also dive into a sharp coral head without suffering a scratch. Try that the next time you scuba dive!

Sharks and other ancient fish have even sharper senses and can detect the electric current from a small prey fish from quite a distance away. When we go swimming in the ocean, sharks and many other fish know exactly where we are. We are just lucky that most of us don’t taste particularly good to them. If we did, we could never venture into the sea.

Cold-blooded—or nearly so

Fish are cold-blooded creatures, and most fish are the same temperature as the water, but there are a couple of fish that keep the temperature of their blood a little higher than the surrounding water. These include some large sharks and tuna, which have developed a mechanism of warming their blood through the action of their muscles. Our blood is warmed by our metabolism and requires food and oxygen, but this is costly for us and requires a lot of energy. Those slightly warm-blooded fish do it more efficiently than we do by using waste heat from their muscles. They don’t require any more food or effort to keep warm other than swimming, which they are doing anyway.

A shark can go for months without eating or watching TV. We would have a problem with that. Sharks are so efficient that they can swim many hundreds of miles with no food at all. When they do find a nice, juicy seal or walrus, they can eat the entire thing and store the fat as oil in their liver. I myself would have a hard time eating a fatty meal that is almost as large as myself.

Different food-processing capabilities

Some of the fish we normally keep in our tanks can store food for a very long time, and some, such as seahorses, pipefish, and mandarinfish, must eat constantly, as they don’t have a real stomach and can’t store much fat in their liver. Those fish were designed to eat tiny food on a continual basis. That is why it’s not a good Idea to train a mandarin to eat pellets. It will eat them, but it can’t store the calories it needs for the rest of the day.

An old girlfriend of mine used to remind me of a certain fish—a flounder. And she also wanted to eat continually. The only difference between her and a mandarin in that regard was that she only ate lobster.

Photo credit: Klaus Stiefel


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About Paul Baldassano

Paul Baldassano has been in the hobby since the 50s and holds two aquarium-related patents. His current reef aquarium was set up in 1971. He is also an avid SCUBA diver and Vietnam veteran.

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