Against expectations, monitors show that sharks aren’t sinkers
Most bony fish have built in floaties. These built-in balloons can be filled with gas, usually oxygen, in order to control the fish’s buoyancy. That way the fish can maintain a comfortable depth in the ocean without needing to expend too much energy swimming to remain there. Cartilaginous fish, like a shark, don’t have a swim bladder, and we’ve been slowly piecing together how they replicate this function and avoid sinking to the bottom of the sea.
The recent theory was that sharks’ anatomy helped lighten their load, but that they then needed to swim constantly avoid sinking. A lighter skeleton of cartilage and low-density oils in the liver helped, but at best achieved neutral buoyancy, and would still require considerable effort on the part of the shark or ray. New data gathered from shark movement, however, seems to contradict this basic concept.
Monitors were attached to two species of deep-diving sharks so that depth, speed and tail exertion could all be measured. The expectation was that as sharks began to sink to lower depths, we’d see their tail activity perk up to move up again. Instead they found the opposite— tails seemed to be working harder to move down, and could be nearly effortless during ten-minute ascensions. This seems to indicate that the sharks have positive, not neutral or negative, buoyancy. While explanations for why this would be beneficial to the shark were offered, no physiological explanations were offered as to how this was achieved.
Source: Deep-sea sharks show surprising buoyancy by Chris Cesare, Science Shot