Ever wondered whether fish are reptiles? They both live in water, so they must be right? The similarities don’t stop there either, so surely, they are? Well, not quite.
To put it bluntly: no, fish are not reptiles. But we’ll get to the bottom of things in a lot more detail for you below…
What is a fish?
It feels a little funny asking this question. After all, everyone knows what a fish is. What we mean by asking this is: what class do fish belong to? Humans belong to the class Mammalia, while reptiles belong to the class Reptilia.
95% of fish, on the other hand, belong to the class Actinopterygii, which can be split into three main subcategories: jawless fish, bony fish and cartilaginous fish (which means their skeletons are mainly cartilage). Fish are also considered animals, and they are vertebrates, which means they have a backbone inside their body. Still with us?
In all of the world’s water, there are over 33,000 known species of fish! In comparison, there are only approximately 6,500 known species of mammals.
Key characteristics of a fish
So, we now know what a fish is from a scientific basis, but does that mean in practice? What are the practical ways that we can spot a fish in the wild or in our aquarium?
An underwater home
So, what makes a fish a fish? Probably the most obvious feature is that, of course, they live entirely underwater. To do this, they breathe through gills – though certain kinds of fish (like lungfish) can actually breathe air! These gills absorb oxygen from water and release carbon dioxide, which is what allows them to breathe.
Fins and scales
They also have fins rather than feet, which lets them move through water in a smooth, streamlined manner. The pelvic and pectoral fins let the fish remain stable, while the dorsal and ventral fins stop them from rolling over, and help them to turn. The tail fin, meanwhile, is what keeps the fish moving forward when swimming.
Many of them are covered in scales, and they aren’t just for decoration either – some fish have what’s called a lateral line, a special row of scales with their own in-built sensors to detect movement. Handy, right?
Cold-blooded
Fish are cold-blooded too (or ectothermic to give it its fancy name). Cold-blooded species can control their body temperature by using the outside temperature; so the body temperature of a fish changes as the temperature of the environment around it changes, too.
Swim bladders
This strange-sounding thing is an essential part of a fish’s anatomy. A special organ filled with air, the swim bladder keeps fish buoyant in the water, stopping them from sinking to the bottom or floating too far to the top, which comes in especially handy when it’s time for fish to sleep.
Although there’s some overlap between the features of fish and reptiles, as you’ll soon see, there’s a far more that sets them apart.
Key differences between fish and reptiles
As we said earlier, reptiles belong to the class Reptilia, which includes species of snakes, lizards, turtles, alligators and crocodiles. Like fish, they are cold-blooded vertebrates, but we’ll soon see that there are plenty of things that set them apart, which explains why fish are not reptiles. Let’s take a look.
While fish only live in water, reptiles can live in both land and water. These kinds of reptiles, known as marine or water reptiles, may spend all their time in water, or they may move between land and water.
All reptiles are tetrapods, which is another way of saying that they have four limbs (like turtles and crocodiles) or are descended from four-limbed animals (like snakes).
All reptiles breathe through lungs. What’s interesting is that different kinds of reptiles/ breathe in completely different ways. Lizards breathe using the same muscles they use to run, so when they’re running, they actually have to hold their breath!
Like fish, reptiles also have scales. The scales of reptiles help provide them with a layer of protection since they’re made of a tough protein called keratin. Their scales also prevent water loss, so they can stay hydrated too.
Key differences between fish and mammals
Fish and mammals could not be more different. There are three key characteristics that define a mammal and differentiate it from a fish.
- Firstly, mammals have hair or fur; even a sphynx cat has tiny hairs on its almost hairless body.
- Secondly, they have mammary glands that produce milk, which they use to feed their young.
- And lastly, they have three middle ear bones. These three bones improve hearing, particularly high frequencies.
Fish do not possess any of these characteristics. In fact, they do not even have a middle ear to have bones in!
5 important facts about fish you must know
Ready to learn some fish facts to impress your friends? We’ve got 5 facts that are sure to impress!
Fish can be venomous
It’s not just snakes and spiders that can be venomous; some species of fish are also venomous. The stone fish are a subfamily of fish with needle-like spines on their back, which inject venom when depressed.
Inflatable fish
Pufferfish are incredibly famous for inflating when they feel threatened. If a potential predator gets near, they swallow air and water to make themselves look as big as possible. Almost all species of pufferfish are also poisonous if eaten, so if their size doesn’t put you off, their deadly toxins should!
Fish of all shapes and sizes
From whale sharks (Rhincodon typus) weighing up to 18,000 kg, to the tiny Paedocypris progenetica, fish are some of the most diverse animals on the planet in terms of their size.
Glow in the dark
Deepsea fish, like the angler fish (Lophiiformes) possess the incredible ability to glow in the dark using bioluminescence. They their glowing orb to attract prey, which they quickly gobble up. Fish like neon tetras don’t glow in the dark, but make use of highly reflective scales to increase ambient light so that they can group together.
Some fish don’t need eyes
In areas with little natural light, like caves or deep underwater, some animals have lost their eyes over millions of years of evolution. Animals like the Mexican tetra (Astyanax mexicanus) make use of a lateral line organ to navigate rather than using their eyesight.
Learn more about fish at Hastings Aquarium
Hastings Aquarium is a fantastic place to learn about an enormous number of fish and other marine life. Our exhibits expertly mimic the natural environments that these animals would be in in the wild.
During our talks and feeds, you will have the opportunity to learn and ask any questions you might have to the team who look after the animals.
We hope everything you’ve just read has helped you understand the differences between fish and reptiles. For more know-how about the deep and who lives there, head to the blog or come and say hi and see these amazing creatures up close and in person by booking your tickets now!