Sharks are fish but not fish. Confusing? Absolutely. You can instinctively recognize a distinction between sharks and fish; you know they are different animals, but you cannot quite pin down what the difference is. Sharks fall under the label of “fish,” but they are not the same, and you know it. Here are the 5 things that make sharks different from fish.
1. Where Are The Bones?
Think of the word “fish” and you probably picture all kinds of things: fishermen reeling in a giant tuna or the colorful variety of creatures around a coral reef, a nice salmon dinner, some fresh-cut fillets at the grocery store.
You probably do not immediately imagine a 16 foot 3,000 pound great white shark, but this too is a fish. Those first fish listed – and the animals we typically refer to as “fish” – are “bony fish,” simply meaning they have skeletons made of bone.
Sharks are cartilaginous fish, a type of fish that includes sharks, rays, and skates. Instead of a rib cage, they have a skeleton made of cartilage. The only bones in a shark’s body are the jawbone, teeth, and the spine.
The only bones in a shark’s body are the jawbone, teeth, and the spine.
Bony fish have come to colloquially equate with fish as a whole, meaning sharks in this sense are released from the bondage of the term. As far as common-use definitions, they are not strictly fish; here are 4 more ways they are different:
This makes them lighter and allows them to move more efficiently.
2. Where Are The Scales?
Why don’t sharks have scales? Well, they do, they are just so small they’re hard to see.
The scales of the fish we most often encounter, such as salmon and carp (cycloid scales), or perch and sunfish (ctenoid scales), are basically collagen covered in a layer of calcium.
The shark’s placoid scales, also called dermal denticles, are much smaller than those of a bony fish. They are basically very tiny versions of the same teeth in their mouths, only not serrated: a pulp cased in dentine and coated with enamel.
That’s right: a shark is covered in teeth.
Touching a shark will not shred you, the effect is more like a coarse sandpaper: a bump or collision with a shark will not slice you to pieces, but it can leave abrasions.
3. Sharks Do Not Have Swim Bladders
You know that if something like a leaf falls into the water it will just float on top, whereas a rock will sink. The leaf is buoyant, the rock is negatively buoyant.
Allow me to blow your mind: sharks, and all fish, have more in common with the rock than the leaf; they are negatively buoyant.
If you put a shark or a fish in the water, it will sink to the bottom.
Have you ever wondered why fish neither float to the top nor sink to the bottom, but seem to effortlessly float at whichever depth they like? The typical bony fish can do this because of their swim bladders, a sack of air somewhat comparable to a lung; but this lung is not for breathing. These fish can change the pressure and amount of air in these swim bladders to float at whichever depth they prefer.
Most fish have the ability to create gas inside their bodies; if they are deep below the surface and want to move to a shallower depth, they produce more gas and fill their swim bladder, making them rise to a higher level.
The swim bladder in fish also works in a similar manner to the ballast tanks of large ships. A ballast tank, filled with water, weighs down a ship into the water to add stability.
The swim bladder acts on the same principle, but in reverse. Instead of water, the swim bladder fills with gas, and instead of stabilizing the ship by pulling it down, the swim bladder stabilizes the fish by pulling it upwards. Most fish have their swim bladder near the top and center of their bodies, helping with this stabilization effect.
Some fish, though they have a swim bladder, cannot produce their own gas. These fish need to rise to the surface and take a gulp of air from above the water.
Sharks have no swim bladder. The closest thing they have is their massive liver filled with oil, composing up to 30% of their body weight. They also have an advantage from their cartilage-based skeleton, less dense and more buoyant than bone. Though these traits give them some lift in the water, even combined they are not as effective as a swim bladder and sharks need to actively swim to maintain their depth; if not, they will sink.
Sharks need to actively swim to maintain their depth; if not, they will sink.
Without a swim bladder, they also lose the stability this organ adds. They make up for this loss via their 8 (in most cases) large fins which help keep them balanced in the water. The large fins also help to provide lift while the sharks move forward.
The giant liver, however, makes the great white (in particular) a target of orcas, who will hunt the shark down for its liver, leaving the rest of the shark behind.
As often, there are exceptions. The sand tiger shark takes gulps of air from the surface and keeps the air in its stomach, and bottom dwelling sharks have no real need for the extra buoyancy.
4. Sharks Do Not Lay Eggs
Most sharks give live birth. Unlike mammals, however, the majority begin with an egg that hatches inside of their body, where the newborns are further carried before the mother gives birth. They do not live off of a placenta, as in mammals, but rather from the nutrients left from the eggs. The idea is to maximize the size of the shark pups when they enter the world and increase their chances of survival. These sharks are “ovoviviparous.”
A good many well known sharks give birth with no eggs involved whatsoever. Some of these even have a placenta, like mammals; in these, the offspring have an umbilical cord and survive off of nutrients directly from the mother. These sharks include the tiger, bull, and hammerhead, as well as many others. These sharks are “viviparous.” Interesting how the sharks most dangerous to us are, in this way, the most similar to us.
A few species do lay eggs, such as the horn shark. These sharks are “Oviparous.”
The vast majority – over 90% – of bony fish lay eggs. In most species the eggs are internally fertilized, but in some major species such as salmon and tuna, the eggs are laid unfertilized and must be fertilized in the water.
Unlike sharks, most of which are born fully-formed, bony fish usually hatch very small and poorly-formed.
The difference between most sharks and bony fish can be summed up in the age-old quality versus quantity debate. Bony fish lay as many eggs as possible hundreds or thousands. They overwhelm predators by the sheer number of larvae, meaning that on the law of averages at least some should survive, and they do. This mass quantity method of survival reproduction is called r-selection.
Sharks, on the other hand, give birth to just a few, very strong offspring. Again, many will be eaten, but from birth they have a spot higher on the food chain than many bony fish will reach even in adulthood. Some will die, but from their strength, rather than their numbers, enough will survive to carry on the next generation. This high quality method of survival reproduction is called k-selection.
5. Sharks Have Large Brains
Brain size in relation to body has been used traditionally as an indicator of intelligence. This method is debatable and far from exact. Nevertheless, it gives us a useful tool for comparison.
Sharks have some of the highest brain-to-body mass ratios of all fish. Shark brains are comparable in proportion to some mammals, such as the hippopotamus, whereas the average fish brain-to-body mass ratio is about 1/15th that of a bird or mammal of comparable size.
Sharks have been shown capable of learning and problem solving, as well as exhibiting a level of curiosity. Sharks, nevertheless, still have not quite learned not to bite hooks or that license plates and tires are not food, indicating their intelligence has a limit. They are smarter than most fish, but not quite the savants some people imagine them to be.
Conclusion
So now it makes sense: sharks are fish but not fish. Even though you knew sharks were fish, you knew they were also something more, something different. Now you know exactly how, and you have 5 concrete ways in which sharks are not like fish: they have no skeleton and no swim bladder; they have scales made of teeth, they give live birth and they have big brains. Sharks are in some ways more like mammals, in other ways like freaky mutant fish, and in still others they are a creature entirely unto themselves.