Today, Jonathan investigates sharks that tiltwhen they swim!
Welcome to Jonathan Bird’s Blue World!
The underwater world is full of mysteriesto be solved.
One such mystery involves the Great Hammerheadshark.
Divers rarely see Great Hammerheads.
They are known for being shy loners,
unwilling to approach people.
But a few years ago,
divers began visiting a place near
Bimini Island in the Bahamas,
where Great Hammerheads have been caught byfishermen for years.
They found a special place with lots of sharks.
Over time, the sharks got used to divers
and started coming in close for a snack,
giving people an intimate look at the Great Hammerhead shark.
I have been to Bimini several times and
I am always amazed by how huge Great Hammerheads are.
Not only does the head reach a meter or more wide,
but the tall dorsal fin is
a striking feature of this unique and majestic shark.
And in spite of the aggressive appearance,
Great Hammerheads almost never attack people.
They much prefer to eat stingrays in the sand.
But diving with Great Hammerheads has
allowed divers to notice something unusual—
the shark often swims tilted to the side at an angle.
I also noticed this behavior in the Ocean Voyager Exhibit at the Georgia Aquarium.
My friend Andy Casagrande—
one of Discover Channel’s primary Shark Week cinematographers—
wanted to help investigate the mystery of the leaning Hammerhead.
He attached a GoPro camera
to the huge dorsal fin of a Great Hammerhead,
to film the shark for an extended period of time.
His amazing footage shed light on the behavior
of these sharks…and it has to do with their anatomy.
The hammerhead shark never stops swimming.
But contrary to popular belief, many sharksdo.
Most species of sharks, like the lemon shark,
can rest on the bottom
and move water over their gills by simply gulping.
Biologists call it “Buccal Pumping”.
A few species of sharks,
like the Whale shark and the White shark,
are known as Obligate Ram Ventilators.
They can only breathe by ramming oxygenated water
into their gills through their mouths as they swim.
If they stop swimming, they’ll suffocate!
The Great Hammerhead is also an Obligate Ram Ventilator,
and has to keep swimming all the time.
Sharks such as the Blue shark keep swimming all the time too,
but for a totally different reason,
because sharks sink if they stop swimming.
鱼类 包括鲨鱼 比海水要重
Fish, including sharks, are heavier than water,
so they sink.
Bony fish have an organ called a swim bladder
that they use to adjust their buoyancy.
By adjusting how much gas is in its swim bladder,
fish can keep itself perfectly neutral,
so it doesn’t sink or float.
Since sharks have no swim bladder,
they sink to the bottom when they stop swimming.
But some sharks like the Blue shark
live in the open ocean where the bottom is thousands
of feet below where it’s dark and cold.
They can’t rest on the bottom.
So they have adapted to this environment
by developing very large pectoral fins
to act as airplane wings.
As the shark cruises through the water,
the fins provide lift to keep the shark aloft,
and prevent it from sinking into the dark,cold abyss.
The bigger the pectoral fins, the more lift they provide,
and the slower the shark can swim without sinking,
which saves energy.
The Great Hammerhead on the other hand has
small pectoral fins
and a big head called a cephalofoil.
The cephalofoil seems to accomplish several things
for the shark, including expanded vision,
a larger surface area for electro-receptors,
and additional front end lift like a mini-wing.
But it also has mass, and scientists think this mass,
combined with small pectoral fins,
is the reason that hammerheads are so much
more maneuverable than other similarly-sized sharks.
But while those small pectoral fins help the shark
turn quickly to catch prey,
they have a disadvantage: not much lift.
And this is where the tilted swimmingcomes in.
Andy Casagrande’s dorsal fin footage showed
that when Great Hammerheads are on the move,
they are almost always swimming
at a tilted angle of more than 45 degrees!
2015年 尼古拉斯·佩恩 吉尔·艾欧斯拉夫斯基
In 2015, Nicholas Payne, Gil Iosilevskii
and other five researchers got together to investigate
this strange behavior.
They put sensors on Great Hammerhead sharks
to measure the roll angles.
Then they used wind tunnel tests
to model the effect and figured out
what the sharks are doing.
The sharks roll on their sides to use
that huge dorsal fin as a wing
and provide lift while swimming distances.
It allows the shark to generate more lift,
which means it can swim a bit slower
without sinking, and save energy.
In fact, swimming tilted to the side saves the shark
about 8% swimming effort.
So when the shark needs to go somewhere,
it tilts to the side and saves energy.
But when it needs to hunt,
it swims straight up and down
to take advantage of the increased maneuverability.
The giant dorsal fin serves two purposes:
a keel when upright,
and a wing when tilted!
At this point, no other shark is known to use this technique,
and it explains a mysterious
habit that divers observed in Great Hammerhead sharks.
Sometimes just being in the ocean creates
new understanding of the life that lives there.
As divers, our time in the ocean is fleeting,
lasting just an hour or so per dive.
But it affords us the opportunity to catch a glimpse
of the lives of the animals that live there.
This is one of the things that keeps me
coming back over and over,
for my short glimpses into the Blue World.