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The marine life of planet Earth is vast and ancient. Fish reside in every ocean, sea, and almost any body of water. Different species of fish carry many different characteristics.

Many of us may take these ancient creatures for granted; they’ve been on this earth far longer than humans.

So, it begs the timeless question: Do all fish have backbones?

By all technical understanding, yes, they do. But it’s not as cut-and-dry as you may think. Many fish species do have backbones, but there are a few fish species considered to be true invertebrates.

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The Evolution of Fish

The first vertebrates on Earth were fish. They provide the shape and body base for millions of other living life forms over the course of epochs of evolution.

If it weren’t for the evolution of fish, humankind would not be here today.

Fish are avid survivors. Not only have they continued through tumultuous climate differentiation and environmental hardships, but they have also survived several periods of extinction on a massive scale.

Where dinosaurs failed in their ability to survive, fish have lived on.

The Cambrian Period

Many scientists claim that the Cambrian period is when fish began to propagate the earth’s oceans. This occurred in particular during the Paleozoic era, over 540 million years ago.

The theory comes from the fact that most of the oldest fish fossils we have are from that ancient period in time.

What we know to be fish now looks nothing like the fish from these ancient days. They hadn’t yet developed vertebrae or a backbone and appeared more like worms with armor plates for protection.

These “Agnatha” were fish without jaws, which meant they didn’t have a backbone. Even though they still could eat with a mouth, they didn’t have a jaw.

Jawless fish are extinct for the most part, but a few species survive.

Fish from the Cambrian period, like conodonts, had stiff cartilage that gave their bodies support. It was over 100 million years of evolution that developed the backbone along with gills for breathing.

Three Types of Fish

After millions of years of evolutionary survival, three main types of fish are still around today.

Jawless Fish

Lampreys are one fish living today that can give us a good idea of how jawless fish used to look.

They have cartilage shaped in an arc around the spinal column and made of thin cartilage. These tiny pieces of cartilage have the name “vertebral arches.”

Other types of jawless fish still in existence are the Snot Eel and Hagfish. Hagfish are close cousins to Lampreys.

They look like big, slimy worms who eat dead fish by crawling into them and using their sandpaper-like tongue to wipe the inside out.

Neither the Snot Eel nor Hagfish has any vertebrae. They have some arches in their tail, but no backbone and are fish in classification.

Cartilaginous Fish

Cartilaginous fish have skeletons made of cartilage and are similar to jawless fish. They do have jaws and fins with bodies made of scales that appear like little, pointed teeth.

Skates, rays, and sharks all fall under this type of fish and are carnivorous.

They have vertebrae composed of two tubes, an upper and a lower. Both of these serve to give structure and strength to these types of fish. The lower often has many calcification layers to serve these purposes.

Cartilaginous fish do have backbones, but not in the same way as bony fish.

Skates and sharks actually have spinal development like that of humans. This puts into question how primitive these two species are.

Bony Fish

Bony fish are the ones with which we are most familiar from lakes, oceans, and other water bodies and comprise 95% of all fish.

Clownfish, salmon, tuna, catfish, zebrafish, and goldfish are common examples. Although they have fragile fins, the fins have an intricate network of bones stemming from a spine.

What We Know; Identify As A Fish Today

The appearance of what we identify as fish today has a spine with a series of strung-together vertebrae.

But the basics of their bodies, from tail to head, resemble that of their ancestors. They don’t have a sacrum, thorax, or neck, though.

What defines a fish is its ability to breathe underwater by using gills. But some underwater creatures are said to be fish, but this is a mistake.

Further Reading: Most Popular Freshwater Aquarium Fish

Some Fish Aren’t Actually Fish.

Despite popular thought, jellyfish and starfish are not actually fish.

They are plankton because they don’t have a heart, bones, or brain. The only two known invertebrate fish are Hagfish and Lampreys, that’s it.

Wales are not fish either. They are actually mammals because mothers feed their young with milk produced from mammary glands in the body.

Skeletons, Backbones; Vertebrae of Fish

Two types of skeletons are available for fish. There are the exoskeleton and the endoskeleton.

Whether made of cartilage or bone, the exoskeleton refers to a solid outer structure, while the endoskeleton is the inner support structure where the spine attaches to the ribs.

Although no fish species actually has an entire exoskeleton, bony fish will have something of one around their fins. All fish do have some endoskeleton.

Why Fish Have Backbones

Regardless of the type of skeleton a fish has, its purpose is to protect organs, aid in movement, and give the body strength and support.

This is why most fish are vertebrates, the same as humans. They have a stiff, long rod running through their body’s central length, also known as the spinal cord or notochord.

Whether you’re a fisherman, aquarist, marine biologist, or one who experiences icthyophilia (a love for fish), understanding the different types of fish in the world will enrich your experience with them.

It should deepen your appreciation for their ancestry and evolution. After all, they’ve existed far longer on the earth than humans have.

Video: Vertebrate Animals for kids: Mammals, fish, birds, amphibians, and reptiles

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