Do Fish Have Bones? Why Do They Need Them?

Do fish have bones? Yes, fish have bones, of course, they do! After all, haven’t we all found one occasionally in our fish and chip supper? You bet we have and this has to be proof that fish most definitely have bones, right?

Do fish have bones? Well, actually no, what we think we know, that fish have bones is not strictly the truth. There are fish that do, a fish that doesn’t, and also some in-between. Confused? Well, don’t be as everything is about to be revealed.

The Fish With No Bones

Hagfish, sometimes known as the slime eel, is the only fish (to our knowledge) that can be classed as not having bones per se. This is due to the fact that unlike other fish which are vertebrates, meaning they have a backbone, the Hagfish does not. Instead, this fish has a notochord – a flexible rod made of cartilage – which it uses to do the same job.

Hagfish are also jawless, so do not have a jawbone where other fish’s teeth would be. Rather, the Hagfish has two rows of tooth-like structures made of keratin that they use to burrow into their food face first. Because of this, they are often thought of as disgusting scavengers of the sea. They are, however, much more than this, they are fascinating and unique.

The In-Betweeners

The in-betweeners are the cartilaginous fish which are members of the Chondrichthyes family. Species in this group include Batoids, Rays, Chimaera, Sawfish, and of course Sharks. These fish’s skeletons are made up of cartilage and not bone. However, it is harder cartilage than in the human body as it contains extra minerals.

There are also other notable differences between cartilaginous fish and ones with bones. These are:

  • Cartilaginous fish have gills that open through slits rather than a bony covering.
  • Cartilaginous fish may breathe through spiracles rather than their gills. These are located on the top of the head and enable a fish to rest on the bottom of the ocean and still draw in oxygenated water.
  • Cartilaginous fish are covered in placoid or dermal denticles rather than the flat scales (ganoid, ctenoid, cycloid) of bony fish.
  • Cartilaginous fish are only found in the saltwater of oceans and seas.
cartilaginous fish-sharks
Sharks are cartilaginous fish

The Fish With Bones

Fish with bones belong to the Osteichthyes family and are vertebrates, meaning they have a skeleton that includes a spine and skull. They are found all across the world in both saltwater and freshwater. Most of the fish we keep in our aquariums belong to this family.

There is no typical or average for the number of bones a fish will have. This is due to the multitude of species, sizes and anatomical structures of all the fish around. The main features of a fish’s skeletal system are the vertebral column, jaw, ribs, cranium, and intramuscular bones.

Interestingly, it is believed that a fish skull contains more bones than that of a human (22), alligator (53), or mammal (43 before fusing) at approximately 130. This is thought to be because fish rely on a skull that is highly mobile. Fish skulls also tend to be lighter and more flexible than those of creatures that live on land. Probably due to the fact that they don’t have to deal with gravity constantly dragging at their skulls.

Do fish have a backbone? Find out by reading our in-depth article on this topic.

volitan lionfish

Why Do Fish Need Bones?

The main reason that fish need bones is to help support and protect the softer parts of their body, such as organs and muscles. For example, the cranium protects the brain from any external stresses, whilst the pin bones (commonly found in our fish supper) anchor the fish’s muscles. Pin bones are also used by the fish to easily access calcium for the muscles. This is possible as they run directly through the muscles themselves, and important as muscles need calcium to work.

In addition, part of a fish’s skeleton grows within its skin. These growths then become the hard spines of the fins and also tiny hard plates within the fish’s scales. Both of these are used for protection, mainly against predator attacks, although the spines are also used vitally by the fins to help steer the fish through the water.

Fish Bone Facts

  1. Some fish can rattle their bones in order to convey a message to other fish.
  2. Triggerfish have three dorsal spines (bones) that lock together and allow them to securely lodge themselves in crevices to avoid predators.
  3. A fish’s jawbone is not always attached to its skull. This allows many to shoot their mouths forward like a spring to catch prey.
  4. The heaviest bony fish ever caught was a Mola Alexandrini Ocean Sunfish in 1996. It weighed in at an incredible 5070 lbs or 2300 Kilos.
  5. The smallest bony fish is thought to be the endangered Dwarf Pygmy Goby which reaches lengths of no more than 15 millimeters.
  6. The Pufferfish has spines all over its body which remain flat until the fish needs to puff up.

Conclusion: Do Fish Have Bones?

As you can see the answer is not straight forward. But the majority of fish do have bones and only a few species don’t.

There are thousands of species of fish and most of them do have bones. The exceptions make up a small percentage of the fish in seas and rivers.