Q & A

Question & Answer Session

Question 7


Becky emailed me and asked "I know your web site is designed for children, but I think its great!  I have to research the class chondrichthyes which includes sharks and rays.  Here are some questions that I havnt been able to find:

- What are 2 reasons why sharks must swim continuously and bony fish can remain stationary?

- What special adaptations do sharks have to detecting prey?

great questions, like everything there's a simple answer and a long answer, here's the short answer and the explanation for it is below :

2 reasons sharks must swim continuously ?

1. Not all sharks need to swim continually to survive , only some. But all sharks will sink unless they are moving forward. Their fins act like an airplanes wing as they swim they provide lift. - like a fighter plane

2. Bony fish have swim bladders (balloons) that allow them to hover motionless - like an airship or blimp.

Special adapations to find food ?

1. lateral line, senses pressure changes and vibrations in the water.

2. Amazing sense of smell detects a drop of blood in millions of gallons of water.

3. Ampullae of lorenzini, electrical detectors that can detect the tiny amounts of electricity given off by living things at close range, even if buried in sand.

4. Excellent eyes, like a cat's that can see in very low light and low visibility.

5. Backward pointing teeth with a shape designed to catch the type of food that it likes best.

Well there's more and this is going to be incorporated into a whole section. Also look at Q&A1 which talks a lot about adaptation.

IN MORE DETAIL. - NOTE A FEW TERMS ARE USED "BOUYANCY", "BERNOULLI EFFECT" and an explanation of how an aircraft's wing works. In the main text I've jumped over some of these, these background knowledge subjects are explained in more detail at the end.

Well Becky that's a couple of excellent questions. Like most of the Q&A sessions (since I'm getting more and more questions) I'm going to answer this one just with text and I hope I'll get around later to adding graphics and writing a better response.

In the meantime I hope the information I emailed you back below will help.

Like much of this site it will grow and improve continually as time goes on.


Here goes Becky :

Sharks don't have to swim constantly , not to live anyway.

Many sharks spend their life pretty stationary e.g. Whitetip reef sharks and nurse sharks sleep during the day on the bottom and can suck water over their gills without moving forward.

Some sharks cannot breathe unless they are moving, these are called Ram ventilators e.g. Great white, Mako, Oceanic white tip, blue shark. Mostly open ocean sharks.
All sharks however will sink if they are not moving forward, this is not fatal in the case of sharks that live on the bottom and aren't ram ventilators such as the nurse shark.
the reason for this is that bony fish have a thing called a swim bladder.



This is a balloon inside it that it can fill with gas to create bouyancy and counteract its own weight. the swim bladder has muscles around it that it can use to contract the size of the swim bladder (and therefore reduce the volume, so the fish displaces less water has less bouyancy - so now the weight is larger than the bouyancy so the fish sinks). Think of a bony fish as working just like a submarine!

If they want to go up, they relax the muscles in their swim bladder, which increases in volume, displaces more water so the bouyancy pushing up is higher than the weight and up it goes.
(There are two types of swim bladder in bony fish, some are joined to the stomach so excess gas can vent off and they can take air from the surface, this is good since they can change depth quickly, the other type has a swim bladder with that is totally closed, it fills the swim bladder with gas that diffuses out of the blood, so if it goes too far up too quick it cannot reabsorb the gas again in time and it gets decompression illness (like a diver) e.g. Snappers, if they're dragged up from depth too quickly they can be injured, so for catch and release some fishermen insert a hollow needle carefully to bleed off the gas and release the fish - a high % survive to be caught again).

Now sharks don't have a swim bladder. their skeletons are made from cartilage rather than bone , so they're more flexible, they also have a HUGE liver full of oil. This means that the oil provides a lot of the shark's bouyancy (oil floats on water remember) about 70% of the sharks bouyancy comes from this. however this is not all of the bouyancy needed to keep the shark floating.

The shark's fins provide the rest of the bouyancy just like an airplane's wings, the water flowing over them create lift due to the shape of the fins in cross section (they are an aerofoil - as water flows over the top curve of the fin/wing it accelerates to arrive at the same time as the water flowing the shorter route, as a fluid accellerates it's pressure drops so the wing is "sucked" up by low pressure above it and higher pressure below) - This is called the "Bernouilli" effect after the Italian scientist who discovered it - its the reason airplanes fly!

S o the fins make up the rest of the bouyancy, keeping it afloat. Some sharks, notably the sand tiger can take up some of the balance of this bouyancy by taking air into their stomachs allowing them to hang almost motionless barely swimming!.
So 2 reasons why sharks must swim to avoid sinking :

1. They have no swim bladder, only a liver which through its oil provides 70% of the bouyancy needed to float.

2. The other 30% of their lift comes from water flowing over their pectoral fins and providing lift, no water flowing over them = no lift and now the sharks weight is larger than its bouyancy and it sinks.

Special adaptations for detecting prey :

1. Smell , sharks can detect tiny amounts of blood in the water, one figure I remember but cannot seem to find is 4 parts per billion. This is their most long range sense (though vibration is close)

2. Vibration they have a lateral line, a line of pores leading directly to nerves so that vibrations can be detected from hundreds of metres away, fish struggling make low frequency vibrations

3. sight, sharks have an excellent sense of sight, they have a semi reflective membrane inside the front of their eyes and on their retinas (like cats,) in low light, light enters the eye, hits the retina to make an image, reflect off the retina, hits the front inside and reflects back again on the retina so amplifying the amount of light hitting the retina

4. Electrical sense, sharks have sensors in their nose and around the head called, ampulae of lorenzini, that up close allows the sharks to detect the tiny electrical signals given off by the nerves of prey, e,g, many sharks use this to detect the electrical signals from animals buried in the sand!

More on adaptation - CLICK HERE


Mor information on - Bouyancy and How an airplane flies

Ever wondered how steel sinks but a steel ship can float ? Its because if something pushes aside more water that it weighs it will float so if the steel is shaped so that it pushes away a greater weight of water than the weight of the steel, the water tries to push back with the same force as the weight of water pushed away.

So if an object pushes aside 1 cubic metre (1000 litres - or 500 large family sized coke bottle) it will displace an amount of water that is 1000kg (or 1 tonne) in weight, (fresh water weighs 1kg for every litre - so a 2 litre coke bottle weights about 2kg - for American Students, 1 gallon of water (3.8L) weighs about 8 and a half pounds).

So before this vessel would sink you could load it with 1 tonne of weight!! (Less actually since you must take into account the weight of the vessel too)

A submarine has tanks that can be flooded to change its bouyancy and either float or sink depending whether those "ballast tanks" are filled with air or water.

If the bouyancy is the same as the weight the sub (or the fish) neither floats nor sinks and therefore hovers.

This is one of the greatest thrills of scuba diving, by filling our bouyancy compensators (like a lifejacket) with just enough air to be "Neutrally bouyant" (neither float nor sink) we can be effectively weightless and hover over the edge of a sea wall that drops to 3000ft yet we can fly over the edge and not sink!! THAT IS PARADISE!!

A fish works the same way as the submarine....

Sharks work more like airplanes.

What does this mean ? What is pressure ? Simple think of air as being made up of millions of little balls, every time they hit a surface they create a force. This mulitplied by the area is pressure. So if there is a higher pressure under the wing there are more little balls of air hitting the bottom of the wing and pusing up than there are hitting the top of the wing pushing down so the airplane (or if those balls are water molecules), or the shark "flies" through the water.