Izawwlgood wrote:btilly wrote:Note that the top of the tail is consistently larger than the bottom. That is to provide the extra downwards push which is needed to keep them from sinking.
Erm, citation needed on that point.
I linked to a picture that shows you diagrams of what various shark tails look like. Click on that link, and look at the picture and you have your citation. Finding additional verification is as simple as http://images.google.com/images?client= ... a=N&tab=wi.
Izawwlgood wrote:It's my understanding that a DOWNWARD push will push you DOWN, making you sink. And it is also my understanding that sharks are neutrally buoyant. I could be wrong.
Repeat after me, For every action there is an equal and opposite reaction. If you push down, you are pushed up.
Izawwlgood wrote:Aha!Locomotion and buoyancy
Sharks swim by moving their caudal fin from side to side in a sweeping motion, which propels them forward through the water. The large upper lobe of the caudal fin of most sharks provides most of the forward thrust. Sharks, like makos, which sometimes need to swim at high speed, also have a well-developed lower caudal fin lobe for greater thrust. As a shark moves through the water, it angles the pectoral fins to change direction.
Sharks are slightly heavier than water, so they naturally tend to sink. Buoyancy or lift is provided in two ways. First, sharks store large quantities of oil in their liver. Because oil is less dense than water, storing this oil decreases the overall density of the shark, and increases its buoyancy. Second, as a shark swims, its pectoral fins provide lift, in much the same way the wings of an airplane does. If a shark stops swimming it will sink, but its stored oil and relatively light skeleton help it to float and decreases the amount of energy that must be expended on swimming.
It seems sharks are actually slightly negative, that their fins provide upward thrust, and they increase their buoyancy by storing oil in their livers. Neat. Sharks are cool. I see nothing about being less efficient swimmers for this, just that they can't stop swimming.
The fins and tails provide upwards thrust on the shark by providing downwards thrust on the water. As the diagram by headprogrammingczar indicates, any need to push in a direction other than forwards represents wasted energy, and wasted energy means you need to put out more energy to swim. Sharks don't generally waste that much energy, but it still is an inefficiency that some other marine mammals don't have to deal with.