Form and Function of Sharks 45
the pull of the tail across the midline, increasing the power output at the
beginning of each propulsive stroke.
Relying on an oil-filled liver rather than a gas-filled bladder might seem
an inferior solution to the buoyancy challenge, but oil has distinct advan-
tages over gas. Bony fishes control their buoyancy by filling or emptying
their swim bladder as they move up and down in the water. Gas secretion
and absorption take time, and this limits how fast a bass or a perch can
move up and down. Oil, unlike gas, is incompressible and provides constant
buoyancy regardless of depth and pressure. Sharks can therefore move up
and down rapidly and repeatedly over tens or hundreds of meters without
having to make adjustments in their buoyancy control mechanism. Vertical
movements of 500 to 1,000 m (1,600–3,200 ft) are known in hammerhead,
thresher, Basking, Whale, and White sharks that have been followed using
telemetry tags. Few if any bony fishes move up and down as much. Sharks
gain at least one additional advantage from their oil-filled livers. They can
use the oil as an energy reserve, metabolizing it when they are starving.
Some sharks have huge livers that make up as much as 30% of their
body weight. Deep-sea squaloid sharks that live under tremendous water
pressure, and some open-water species such as Whale, Basking, Tiger, and
White sharks have large livers and are almost neutrally buoyant. A Bask-
ing Shark that weighs 1,000 kg (2,200 lb) on land weighs only 3.3 kg (7.25
lb) in water, or about 0.3% of its weight on land, owing to the buoyancy
provided by its large, oil-filled liver. Remove the liver from a dead shark,
and the shark sinks rapidly. But so much oil in one place subjected Basking
Sharks to intense hunting, and they were speared out of existence in many
places. The species is now considered endangered.
The batoid skates and rays have mostly lost their tail fins over evolu-
tionary time, so they rely more on various movements of their pectoral fins
to propel them. (Torpedo rays are an exception. They have modified their
pectoral fins to produce strong electric pulses and move around using their
tail fins.) A typical ray swims by passing waves of muscle contraction down
both pectoral fins, front to back, rather than by flapping the fins. Mylioba-
tid rays spend more time in the water column than most other rays, either
to move from place to place (Spotted Eagle Rays) or to feed (plankton-
eating manta rays). Myliobatids flap their pectorals up and down. The ani-
mation of Mr. Ray in Finding Nemo was a very accurate portrayal of the
graceful way in which a Spotted Eagle Ray swims.
Watching a manta ray “fly” through the water, diving and turning with
ease and grace is a beautiful sight. Engineers have built robots that attempt
to mimic the flapping flight and maneuverability of manta rays in an ef-
fort to design more efficient autonomous (self-propelled) underwater ve-