Do All Fish Have a Backbone? (Solved!)

Do All Fish Have a Backbone? (Solved!)

Most figure that all fish have backbones…or at least have bones to support and shape their bodies but the answer is not as obvious as you might think. Fish are vertebrates, which in simple terms mean that they have a spine or a backbone. So this means that all fish have a backbone, right? If you agree that they do, then you are wrong. The one true fish that does not have a backbone is the Hagfish, which has a cartilaginous rod through its entire body and it is not classified as a backbone.

Vertebrate Fish

There are two types of fish that have backbones. One is a bone that is found in bony fish and the other is a skeleton structure that is made up of cartilage. All fish do have some type of backbone.

Purpose of a Backbone

The main purpose is to help protect the fish’s vital muscles, organs, and other body parts. It also helps to support the structure of the fish. Without a backbone, the fish would look like a deflated balloon. It is part of the main structure of a fish and without a backbone, the fish could not live.

How Fish Backbones Grow

Even if all fish have a backbone, they do not grow them all the same way. Ray-finned fish, which are the ones that have bony spines supporting fins of skin, grow their spines from using two different cells. These cells are called chordoblasts and somites. Land dwelling vertebrates grew their spines from cells called somites. Research has shown that the backbone development in sharks is more similar to land-dwelling vertebrates than to bony fish like salmon.


At this time, this is the only fish that does not have a backbone, which makes them an invertebrate. Invertebrates, in general, make up the largest species in the ocean. The hagfish are scavengers and feed off rotting flesh and the remains of dead fish at the bottom of the sea. They have attacked live fish but that is rare for that to happen. The hagfish is also referred to as a snot eel. The reason is that this fish has the ability to release a blue slime from their skin. This will occur as a form of defense when they feel threatened or under attack. It is not a true eel either. The hagfish’s flexible rod in their body enables them to tie themselves in a knot.

Evolution of a Backbone

Although the first fish to start developing a backbone looked more like a worm than a fish was called a Pikaia. The nerve cord that ran down the length of its body was not protected by a backbone so although it was not technically a vertebrate, it did lay the foundation for future vertebrate fish. During the Ordovician Period of evolution, 443 million years ago, in many fish, the vertebrate, or backbone, had formed and was present. 

Two Non-real Fish who do not Have a Backbone

The two “non-real” fish that do not have a backbone are the jellyfish and starfish. Why they have fish in their name is unknown because both are invertebrates, which means no backbones. They are considered plankton and do not even have a brain, heart, or any bones. 


A jellyfish has a dome-shaped body that pulsates through the water with tentacles that dangle behind. One of the reasons that they are not considered a fish is that the anatomy of a fish is centered around its backbone, which a jellyfish does not have. The jellyfish are more closely related to sea anemones and corals. The reason is that all of them have a harpoon-like stinging cell that they use to catch prey. The jellyfish is a cnidarians 

The jellyfish have limited control over their movement. They use a muscle to be able to propel themselves a short distance. It is done by expanding and contracting its bell. Mostly they will drift in currents. There are about 20 species of jellyfish and they can be divided into two types. One is free-swimming and one is the type that is attached to seaweed or other things by a stalk. The free-swimming jellyfish are found in all oceans and usually found drifting along the shoreline. They have a short life span, usually just a few weeks. They range in size from one to 16 inches.


The starfish are considered echinoderms, which also include sand dollars and sea urchins. There are approximately 2,000 species of starfish in the oceans globally, from frigid waters to the tropics. They are also invertebrates like jellyfish and typically have five arms and a central disc. Some species have more than five arms. Some scientists are taking on the task of replacing their common name to sea star because they are not fish. The scientists want to make sure there is no more confusion as to whether it is a fish or not.

Even though a starfish is an invertebrate, they do have a kind of a skeleton but not a backbone like all fish, except for the hagfish, do. Their skeleton is their body, which is composed of calcium carbonate plates. This is their endoskeleton and can take on a variety of forms like granules or spines. They do not have a brain but do have a primitive nervous system.

Fun Facts

  • Fish were the first vertebrates on the planet
  • They provided the basic body and shape for the millions of species that followed and for millions of years of evolution
  • The first evidence of a backbone was discovered about 500 million years ago


After much research, it has been determined that all fish, with the exceptions listed above, are vertebrates and have a backbone. When you catch and fillet a fish, look at the structure of their bones and the way their backbones are either straight or arched. A backbone is part of a fish’s skeleton and helps give the fish its shape, whether it is long and slim, or oval or round and thick.

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