Not all sharks need to swim forward all the time. There are plenty of kinds that are sedentary and spend most of their time on the seabed breathing, awaiting food, and no doubt philosophising. Some other kinds seem to spend the idle part of their day in apparent sleep in sheltered spots where the water is reasonably oxygenated.

On the other hand active pelagic sharks such as whale sharks, great whites, threshers and tiger sharks are constantly active and need a constant supply of oxygen to support their metabolism. Water contains only minute quantities of oxygen; the amount of oxygen that you absorb from a casual breath probably would keep a great white going for several minutes.

That, incidentally, is the major argument in favour of the reason that there is no selection pressure on whales and seals for the ability to breathe water; no matter how great the nuisance of having to surface for air, having to breathe huge quantities of water is worse. Large tunny, sharks, manta and the like have to swim continuously if they are to keep enough oxygen passing through their gills to keep them alive.

Some kinds, such as whale sharks, basking sharks, and manta also keep swimming because they are plankton feeders. They keep swimming, not only for air, but for food as well.

I haven't checked this, but I seem to remember hearing that they sink if they stop swimming because they don't have inbuilt buoyancy from a swim-bladder. Mackerel are the same. This could be the cause for confusion?

Another reason I heard of long ago was that

sharks lack a active  mechanism to pump water

by their gills, as other fish do.

So I wondered about this question too , after I had

learned about sharks lurking at sea bed.

Georg

 Pete,

the swim bladder question is complex, evolutionary speaking. As far as I know, no cartilaginous fish has or had anything like that. It seems to be generally agreed that the cartilaginous condition is secondary, but this does not mean that modern sharks are descendants of bony fish that ever had swim bladders! As far as I can tell, the sharks and the ray-finned fishes diverged before the appearance of swim bladders.

On the other hand, as you point out, swim bladders in many species of mackerel are reduced, or essentially absent. This too appears to be a derived condition. As far as I know, reduced swim bladders in ray-finned fishes either appear in bottom dwellers, or in fish that manage their attitude or depth dynamically with their fins while swimming. This seems to have advantages of manoeuvrability and insensitivity to sudden large changes of depth, which can be fatal to fish with large swim bladders. More or less continuously swimming fish such as mackerel can afford the energetic cost of doing without a swim bladder or buoyant oil. None the less it is an unusual adaptation; tunney are related to mackerel and generally swim continuously, but they seem to have roughly average swim bladders for their size.

Georg

That story about sharks lacking the gill ventilation mechanism, though traditional, is not strictly correct. It certainly is not universally true. Plenty of shark relatives (rays and related fishes) breathe quite actively while parked stationary on the sea bed or seaweed. One can watch them in aquaria, their gills pumping much like the gill coverts of bony fishes. Many true sharks with sedentary habits, like “cat sharks”, behave in similar ways. The sharks with the least adequate stationary breathing ability are the cruising species I mentioned before, such as the great white. How readily such sharks really would suffocate if they were held stationary in the water, I cannot guess. Specimens that I read of, that did suffocate, all had been caught and handled under stressful circumstances. However, I cannot imagine non-stressful circumstances in the wild under which they could not cruise, so the question is perhaps a bit academic. Note that in this field however, as in many others, I do not pretend to speak with any authority.