There are 2 main types of fish!
There are just two types of fish, but here's the catch there's some big words coming that I can't really avoid, even one of the two simpler english words is a little long, but if we explain them it'll make sense. You see scientists like to use long words especially in dead languages like Latin to describe things!
-Types of Fish are there?
We can see there are some differences between the two types of fish but what does it all MEAN? I hear you ask! Well let me explain by asking some of the questions you're probably thinking and answering them for you....
Sharks do not have bones -
Whoa! how come they're not like a lump of jelly ? - Instead of bones, sharks have cartilage instead.
Well what is cartilage ? - Pinch your nose and wiggle it! Good, now pinch the top of your ears and wiggle it with your hands! Good - That's cartilage! Now, tap your finger against your forehead - That's Bone
Sharks don't have a single bone in their body, but they do have skeletons, Sharks , Rays and the third group of Chondricthyes called "Chimaera" (funny fish that look like rabbits) all have skeletons made up of cartilage instead of bone. This stuff is really flexible and sharks too are very flexible because of it.
Osteithyes * - Have bones and sometimes cartilage
Chondricthyes - Don't have a bone in they're body
*(remember these are the BONY fish, in latin anything with the word "Ost" in it means bone e.g. An Osteopath is a doctor who specialises in bones)
Sharks don't have a swim bladder -
OK, so what's a swim bladder ? - A swim bladder is a balloon full of gas inside a fish. It acts like the ballast tanks on a submarine or like a Diver's life jacket. The fish can change the size of the swim bladder by tightening muscles.
This allows a fish to hover motionless in the water and to move straight up and straight down in the water.
Bony fish are a big like airships or blimps, they have a gasbag inside that lets them float motionless. The fish's own weight would normally make it sink, but the air in the swim bladder lets it float without moving, acting a little like a lifejacket.
The fish can squash the bladder with its muscles to make it smaller (less bouyant) to let the fish sink, or relax those muscles and make the bladder bigger so he goes up in the water. Divers can do similar by putting air in or letting air out of their lifejackets to make them, float or sink or get just enough air in so that the diver doesn't float or sink.
Click on the picture to see MPG video of a fish hovering
Sharks don't have a swim bladder, they sink to the bottom if they don't swim, but this is not a problem, we'll explain later.
Sharks have big pectoral fins, these are the fins at the front, below the head. These pectoral fins are shaped like an airplane's wings. When an airplane moves forward quickly the air flowing over the wings generates "lift", it pulls the plane up. You can see this by blowing over a piece of paper.
Well when a shark swims forward through the water, the water flows over the fins, just like the air flowing over an airplane's wings. Without moving forward sharks would sink, the "lift" generated by their pectoral fins keeps them afloat.
Sharks also have a very large liver (an internal organ that processes waste in the blood and many other things - Click here to find out what the liver does) , this liver is filled with oil, which is lighter than water (oil floats on water). This helps make the shark less heavy in the water and provides bouyancy. This works very well since oil is "incompressible" - this means that oil won't squash. Take a bicycle pump, put your finger on the end and push the plunger down.
You see that the plunger moves in and you can feel the pressure on your finger get bigger. Air does squash. As a fish goes deeper his swim bladder compresses (squashes) so he must let more gas into the bladder, it takes time for the fish to get gas out of the water. Also, as he goes up he must let air out of the bladder, some fish can't do this, they must reabsorb the gas, this means it takes time for the fish to move upwards. Well a shark can go up or down in the water as much and as fast as it likes since its not got any air/gas inside it.
So sharks always have to swim or they'll sink and die ??
- Yes and no - Its a common myth that sharks have to swim or they'll die, but its not true for all sharks.
All sharks must move forward or they'll sink - As always there is an exception the Sand Tiger (Or sometimes known as Grey Nurse or Raggy Tooth Sharks) have learned that they can take some air into their stomachs to allow them to hover almost motionless (they still have to move forward a little or have a current flowing at them)
Not all sharks need to move forward to breathe - Some sharks are "ram ventilators" meaning that they must move forward to force water over their gills (the organ behind their mouth that allows them to breathe water).
However many sharks (probably a majority) don't need to move forward, they can suck water in over their gill.
Examples of Ram Ventilators :
- Great white shark
- Blue Shark
most of the big open water sharks
Examples of sharks that can breathe without moving:
- Catsharks and dogfish (all of the little bottom dwelling sharks)
Actually some bony fish are ram ventilators and must move all the time, most of the big tuna (Blue fin, Yellow fin) are ram ventilators, though they do have a swim bladder.
So what's this Gill Cover / 5-7 gill slit business ?
Well first of all let's ask - "what are gills?"
Take a deep breath, hold it , now let it out slowly - Feels good doesn't it. Humans are mammals, we breathe air, we can't breathe underwater. Air is made mostly of two gases -
- Oxygen (~20%)- The stuff our body needs to "burn" the fuel that is the food we eat, its also the gas needed for anything to burn, try (with a grown up) lighting a candle, then putting a glass jar over the top of it, in a few moments the flame will die as its used up all the oxygen. Things can only burn if there is fuel and oxygen. When you eat food, its energy is released, in effect your body is burning the food slowly.
- Nitrogen (~80%) - a gas that mostly does nothing (inert). But if air was all oxygen then it would harm our bodies, too much oxygen can be as bad as too little. So nitrogen acts as a filler gas or "diluent" gas.
- Tiny amounts of other gases e.g. Argon, Carbon Dioxide, Neon
So how do fish breathe when they live in water?
Simple - breathe water! But to do that they need special organs. We have lungs that allow us to breathe air. The lungs are basically bags full of air that have millions of little blood vessels in them, the Oxygen in the air can pass across the thin membrane of the lung into our blood where it goes to all our tissues.
Well gills work in a similar way, instead of being a bag surrounded by small blood vessels, the blood vessels all hang out in the water, the water passes over these blood vessels and oxygen passes into the blood stream.
This doesn't work for us , we can't just breathe water since there's not as many blood vessels in our lungs as a fish has in its gills, equally a fish can't use its gills to breathe air since out of the water all the little blood vessels clump together.
So back to the gill slit question -
Bony fish have a cover over their gills, with just one exit for the water, out the back of the gill cover. Sharks however have slits in the side of their heads between 5 and 7 of them. Most sharks have 5 gill slits, a few have 6 and 7.
A bony fish has scales, what are scales and what do sharks have if they don't have scales ?
Most fish don't have skin, (though many, like catfish do), instead of skin they have scales, these are a series of overlapping bony plates, they can be very small or very big, they all overlap from nose to tail and help protect the fishes body.
Sharks don't have scales, they have a tough skin, out of that skin grows millions of little teeth. These form a protective covering to the shark's skin and they are literally tiny teeth, often the same shape, made out of the same stuff (dentine) all facing the same way from head to tail. This is why a shark's skin feels smooth one way and rough the other way. These are called "dermal denticles" which literally means "skin teeth"
You can read more about the science behind sharks in other pages. There'll be a link here too sometime!