When watching a fish swim, you might be charmed by the grace many species display. While they have entire systems designed to propel them, it’s easy to recognize that their skeleton plays a major role in their movement. Fish seem so fluid and flexible that it begs the question do fish have backbones?!
Vertebrates vs Invertebrates
When diving into the question of fish backbones, we need to first talk about vertebrates and invertebrates. Of the two, all fish are vertebrates. Many fish species don’t have true bones, such as jawless fish, but the majority of aquarium fish do.
Backbones grow from an internal skeleton structure. This flexible bone support structure surrounds the spinal cord, a bundle of critical nerves that leads to the brain. Ribs, limb bones, and other skeletal components protect vital organs and give muscles a place to anchor.
What Are Invertebrates?
Invertebrates include animals like jellyfish, insects, spiders, squid, and starfish. They are called invertebrates because they lack a backbone.
Some invertebrates instead use an exterior skeleton (exoskeleton) that acts like a suit of armor. The benefit is that their soft flesh isn’t so exposed to the elements and predators.
But the downside is that an exoskeleton can’t grow along with the soft internal organs. That’s why the majority of invertebrates with hard parts have to shed their skins periodically.
Unfortunately for them, this is dangerous since predators love nothing more than a freshly molted crab or spider that’s soft and defenseless.
Do All Vertebrate Fish Have a Backbone?
Yes, all fish have a backbone of some sort. While it isn’t always made of bones, As vertebrates (chordates), a backbone is the defining characteristic of this group of animals! Their body plan includes a backbone, cartilaginous rod, naked nerve cord, or other main skeletal structure. From this connective tissue, internal organs and other body parts branch out from it.
Chordates (phylum Chordata) is one of the most familiar classifications for animals. It includes not only fish but amphibians, reptiles, birds, and mammals like us! All chordates have a familiar bilateral symmetry that’s very obvious when looking at the skeleton and all their vital organs.
In their eggs, embryonic, or larval stages, all chordates have a primitive notochord that is the precursor to true backbone development. This notochord is similar to that found in tunicates and other primitive chordate ancestors.
What is the Backbone for in a Vertebrate Fish?
Fish have backbones in order to provide structure and support for their muscles, organs, and the other bones in their bodies. The backbone is used the same way in the bodies of other vertebrates as well, including animals like mammals and birds.
What is the Difference Between Cartilaginous Fish and Bony Fish?
What Are Cartilaginous Fish
The first thing that might stand out to you is the word “cartilaginous,” from the word cartilage. This group includes sharks, rays, skates, and sawfish. Rather than bone, they have softer cartilage to provide structure for their bodies.
Other fish are classified as bony fishes: fish that use bones in their skeleton. This includes the majority of aquarium and game fish.
Many cartilaginous fish also grow their backbones using a very different process from bony fish. Using specialized cells called somites, they form their cartilaginous backbone.
What’s interesting about this is that land-dwelling vertebrate species, including birds, mammals, and humans, use somites to form their backbones.
Bony fish, on the other hand, are unlike all other vertebrate animals. They use a mixture of somites and cells called chondroblasts to form their backbone bone structure.
This suggests that sharks, rays, and other cartilaginous fish are much more than just “primitive fish.” They are very distinct in a number of ways and closer in some respects to land-dwelling vertebrates!
What Are Bony Fish?
Bony fish have backbones like other vertebrates. There are two clades (groups) within this category: the ray-finned fish, which have thin membranes held up with rays.
And the lobe-finned fish, which have fleshy paired fins that eventually evolved into limbs for ancestral amphibians. Lobe-finned fish still alive today include the many kinds of lungfish as well as the prehistoric-looking Coelocanth.
The only exception is a bony fish that blurs the boundaries between bony and cartilaginous fish: the hagfish. Affectionally known as the “snot eel,” they are the only animal known with a proper skull yet no spinal column!
They do have rudimentary vertebrae that have almost entirely disappeared. But instead of a spinal column, they have a cartilaginous cord running along their spinal nerve.
Hagfish are also some of the only jawless fish left in the world, alongside the lamprey. The rest were some of the first vertebrates but evolved into different forms or went extinct millions of years earlier.
So why would a fish give up its vertebral column when other vertebrates don’t? The answer is in how hagfish feed and defend themselves!
The snot eel has the ability to literally tie itself in knots! They do so in order to gain leverage when ripping chunks of rotting flesh from dead fish (lampreys prefer live fish).
And if that wasn’t gross enough, they also expel copious amounts of blue slime when they feel threatened.
The slime from a single hagfish can clog a predator’s gill openings and make an entire bucket of water sticky! By losing their backbone, hagfish gained the ability to be impossibly flexible.
Since fish are vertebrates, the spinal column forms the body’s main structure from which other bones, muscles, and organs branch off from. Therefore, all fish have backbones and rely on them to support their body parts.
Cartilaginous fish and one species of bony fish (hagfish) bend this rule, though, since they rely on cartilage as a support structure rather than a vertebral column like most fish. Other fish have small amounts of cartilage in their skeletons but not nearly as much as cartilaginous ones.
This is the opposite of invertebrates, animals which don’t have a backbone or any other bones. Or they use a tough outer exoskeleton for a defense that needs to be shed in order for the animal to grow.
I hope you’ve appreciated this dive into what makes the vertebral column so essential not only to fish propulsion and evolution but how they compare to other animals and invertebrate species! Let me know your thoughts if any other questions arise!