03x07 - Yucatan Sinkhole, Shark Biology and Ocean Defenses

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- [Narrator] Coming up next

on "Jonathan Bird's Blue World,"

the incredible world of
sharks and their diversity.

Then a look at how animals in
the ocean defend themselves.

But first, a deep and spooky sink hole

in the Yucatan with a
mysterious false bottom.

All of this today on
"Jonathan Bird's Blue World."

Meet Jonathan Bird,

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one of the world's top underwater
nature cinematographers,

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traveling the world on assignment

for all the major networks.

He is an Emmy Award-winning authority

on the underwater world.

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In freshwater or salt
reefs, wrecks, or caves,

Jonathan documents the
world beneath the waves.

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Welcome to the Blue World.

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- [Jonathan] This murky pond in the middle

of a Yucatan jungle might not
look like an exciting place

to go for a dive,

but as I'm about to find out,

there's something spectacular
lurking beneath the surface.

My day begins at Phantom Divers
in Playa del Carmen, Mexico.

They'll be taking me on a
dive to explore the sinkhole.

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We load the van and set
off on our adventure

through the jungle on miles of dirt roads.

Finally, we arrive at Muyal-Ha.

It looks like a pond,

but it's actually an
incredibly deep sinkhole.

I can only imagine what
I'll find down there.

A sinkhole is a formation
that used to be a cave,

but a section of the roof
fell in creating an opening

to the sunlight above.

Then it filled with water
and sediment at the bottom.

My guide is cave diver Carlos Estrabeau,

but everyone calls him Charlie.

We discuss the dive and he
tells me what to expect.

Most importantly, it's deep.

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Charlie shows me his favorite
way to get in the water,

but with scuba gear, I
think I'll go off the dock.

We swim out into the
middle of the sinkhole

and it's time to descend.

As we descend, I can see
that the water's not as clear

as I was expecting,

because this is a sinkhole
and not a cave system,

the water's a bit stagnant.

At the center of the
sinkhole is a pile of debris,

including tree branches

that have fallen in from
the surrounding forest.

There are a lot of leaves

like this partially decomposed one.

The light in from the
opening above shows that

the interior of the sinkhole

is much larger than the opening above.

Around the sides of the cavern,

stalactites hang from the ceiling,

including some really big ones.

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As I descend deeper
markings on the wall show

where the water level used to be

when the water table
was lower in the past.

There isn't much living down here,

but a few fish survive in areas
where the light penetrates.

Charlie directs me to a
small opening in the wall.

It's a little cave that
just goes back a few meters.

He leads me into investigate.

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On the ceiling is a type of
rock formation called popcorn

where the calcium carbonate

has formed little round bumps.

Deeper in the cave,

Charlie shows me the bones
and teeth of an animal

that probably died in here
thousands of years ago

when the cave was dry.

The bones are encrusted with calcium

and I will not move them
since they've been unmoved

for so long.

Heading out of the cave, I
follow Charlie into the light

and then deeper down into
the depths of the sinkhole.

When we reach what
appears to be the bottom,

I'm in for a huge surprise.

I can't believe my eyes,

when Charlie sinks into the
bottom like it's a cloud.

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It's not the bottom at all,

but a layer of sulfur

that hangs near the bottom
in the motionless water.

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I sink down with Charlie
into the cloudy layer.

Underneath it turns reddish yellow,

where more than 100 feet deep.

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On the bottom are thousands of years worth

of decomposing leaves,

which are the source of the sulfur cloud.

No fish can live down here.

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As I rise up out of the cloud,

I find more wispy cloud layers

to explore along the edge of the wall.

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It is really fun to swim through them

like an airplane soaring
through the clouds in the sky.

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With our scuba tanks getting low,

it's now time to head back to the surface.

The dim light from above is
a welcome site as I ascend.

Man, that was wild.

The water in the bottom
of the sinkhole is so calm

that that sulfur layer just lays there

and it it looks like sand.

I mean it looks like the
bottom of the sinkhole,

but then you realize you
could put your hand in it

and your whole body will
just go right down into it

like a cloud.

That was just weird. I've
never seen anything like that.

So cool.

My dive in Muyal-Ha sinkhole
was an exciting combination

of creepy and mysterious.

It's another example of the wonders

of the blue world in a place

where you would never expect it.

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- [Narrator] Next on
"Jonathan Bird's Blue World,"

a look at the amazing
ways animals in the ocean

cleverly defend themselves
against predators.

But first, everything you ever
wanted to know about sharks.

- [Jonathan] Before any
animals existed on land,

sharks ruled the oceans.

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They've been around for
over 250 million years.

Many people think that because
they're ancient animals,

they're in some way primitive.

But as it turns out,

sharks are highly evolved
in sophisticated animals

with amazing adaptations for survival.

The fact that sharks have survived

so long demonstrates the
incredible effectiveness

of their anatomy.

And let's face it, 250 million
years worth of evolution

has got to count for something.

Sharks live in the oceans
across many habitats.

Some like the Caribbean reef shark

live on shallow tropical coral reefs.

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Others like the blue shark are pelagic,

living far from land in the open ocean.

The Greenland shark lives
in the freezing waters

of the Arctic,

while the tiger shark prefers the tropics.

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Sharks are fish,

but there are several key things

that differentiate them
from other kinds of fish.

Sharks are everywhere, but what are they?

Sharks and their close relatives, the rays

differ from the bony
fishes in several ways.

First, sharks and rays have a soft,

flexible skeleton made of cartilage.

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The cartilage skeleton makes
the shark more flexible

than similarly sized bony fishes.

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Also, sharks and rays
have no swim bladder.

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A bony fish uses an organ
called a swim bladder

to maintain neutral buoyancy,

so it can hover like a hot air balloon.

The fish adds or removes
gas from its swim bladder

through its bloodstream

to increase or decrease its buoyancy.

The shark's lack of a swim bladder

means that unlike bony fishes,
the shark tends to sink.

To stay off the bottom,
sharks have to keep moving.

While the shark uses
its tail fin in a back

and forth motion to
provide forward thrust,

its pectoral fins work like airplane wings

to provide lift.

Like an airplane wing,

as long as the fins move
forward through the water,

they provide lift to keep the shark up.

Not too long ago, scientists
thought all sharks

had to swim at all times, not
just to stay off the bottom,

but to keep water moving over their gills.

For some species like the
hammerhead, this is true.

Hammerheads must keep
swimming at all times

to force water through their gills.

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Ironically, if they stop
swimming, they'll drown,

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but many species of sharks
sometimes stop swimming

and rest on the bottom,

gulping water to ventilate their gills.

This is a lemon shark,
common in the Caribbean,

taking a break, resting on the sand,

it gulps water to breathe.

Nurse sharks also spend a lot
of time resting on the bottom.

Bottom dwelling species of
sharks like the wobbegong

actually live their entire
lives on the bottom.

They're camouflage to look
like a rock covered in algae,

and they hunt by being very patient

and waiting for an unsuspecting
fish to come close.

With such a long evolutionary history,

sharks have had plenty of
time to refine their senses.

For example, most sharks have
an incredible sense of smell.

Experiments on sharks smell have suggested

that some sharks can
detect one drop of blood

dissolved in as much as one
million gallons of water.

Sharks also have senses we
can't even begin to experience.

Sharks have an electrosensory
system that allows them

to detect the extremely
minute electrical currents

generated by the muscles
of a swimming fish.

The snout of a shark is
covered in tiny pores

called ampullae of Lorenzini,

which convert electrical
impulses in the water

to an electrical signal
in the shark's nerves.

Therefore, the shark can feel

extremely tiny electrical
currents in the water,

a skill it uses to hunt
even in complete darkness.

Speaking of darkness,

many people believe that
sharks have poor eyesight.

Not true.

In fact, not only do most
sharks have excellent eyesight,

but they also have super night vision.

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That's because they have a tiny mirror

called the tapetum lucidum
located behind the retina

that reflects light back
through the retina a second time

increasing its sensitivity.

Many sharks like to hunt
at dawn, dusk and night

because they can see their
prey much better than the prey

can see them.

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Sharks are also the only
fish to have eyelids.

They're called nictitating membrane

and sharks use them to protect their eyes

when something gets too close.

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Sharks never need to go to a dentist,

since they don't really
care about cavities,

they have many rows of teeth.

As old teeth break or become too dull,

they fall out and new
ones rotate into place.

For the entire life of a shark,

it never runs out of new teeth

and it never worries
about the teeth it has.

And you may find this hard to believe,

but sharks are really important

in the ecosystems of the ocean.

You'll never see a fish
that needs glasses.

That's because a nearsighted fish

would quickly be devoured by a shark.

So sharks help keep
fish populations healthy

by weeding out the sick and the weak.

Healthy fish can get away from sharks,

so they reproduce and make more fish.

As top predators,

sharks have an important role to play,

and because they do not reproduce quickly,

sharks are vulnerable to overfishing.

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And while many people
are afraid of sharks,

most do not represent a
threat to people at all.

The world's largest shark,

the whale shark grows larger than a bus,

but eats only plankton and small fish.

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Even mean looking sharks

with pointy teeth like
the sand tiger shark

prefer to eat fish and
never attack people.

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The more we learn about sharks,

the more we discover how
little we know about them.

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Even the idea of a feeding
frenzy isn't really true,

I get up close to sharks all
the time while they're feeding,

and I've never been bitten.

This group of over 30
Caribbean reef sharks

is competitively feeding
on a frozen chum ball.

They're all around me,

but they're not interested in biting me.

They know that there's food
available and I'm not it,

so I just sit back and
enjoy watching the action.

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And when all the bait is
gone, the action is over,

and I am perfectly fine.

Far from being the horrific
monsters of legends,

sharks are actually highly evolved

and sophisticated animals
with a range of senses

and abilities unmatched
in the animal world.

The study of sharks may teach
us many things about our world

and maybe even about ourselves.

- [Narrator] Don't go away.

Jonathan is about to
travel across the world

looking at astonishing
marine animal defenses.

- [Jonathan] The ocean is a
tough environment to live,

to survive every animal must eat,

and that means that
other animals get eaten

and nobody wants to get eaten.

So animals in the ocean have
come up with a lot of ways

to defend themselves.

They have spines, venom,
speed, maneuverability,

and even a smokescreen.

Come check out this fish.

The lionfish has pretty obvious defenses.

Dozens of spines sticking
out in all directions.

They're not only sharp but venomous.

Very few animals can eat a lionfish,

especially when it's fully grown.

A sting from one of these
spines is extremely painful.

The stone fish has much
less obvious spines,

but they're considerably more venomous.

A sting from one of these spines

will send a grown man to the hospital.

Fortunately, these well camouflaged fish

pose no threat to divers
if we don't touch them.

Stingrays only have a single spine.

It's located at the base of the tail

and can be pointed and jabbed
at a threat from above.

Since sharks love to eat stingrays,

the venomous spine is used to jab sharks

that come too close.

Every once in a while,
an unfortunate swimmer

steps on a stingray and gets stung,

but for the most part,

stingrays are not at all
aggressive towards people.

In the Cayman Islands,
they're downright friendly.

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Fish aren't the only animals
in the ocean with spines.

Sea urchins have many sharp
spines with brittle tips.

If an animal gets skewered
by one of these spines,

the end breaks off below the skin

and causes pain for months.

Long spine urchins are so well protected

that sometimes small fish
even use them for safety,

hiding between the spines

where no predator would
dare try to get them.

Spines work so well for urchins

that some of their close relatives,

the sea stars have spines too.

The spines on the crown of thorn sea star

are every bit as sharp and
painful as those on an urchin.

Believe me, I learned that the hard way.

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Sea cucumbers are closely
related to sea urchins

and sea stars,

but most sea cucumbers have
no spines at all for defense.

The sea cucumber has an even more amazing

and disgusting way of
getting rid of predators.

When attacked,

many species of sea cucumbers
eviscerate from their mouth

a gooey mess called Cuvierian tubules.

These sticky stringy organs presumably

give a predator something
unappealing to eat,

and they're often accompanied
by the secretion of a poison.

This combination prevents sea cucumbers

from being a very popular
food item in the ocean.

The sea cucumber can retract the tubules

and replace the lost ones in a few weeks

and be ready to eviscerate again.

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Some forms of defense are
just simple common sense.

These fish are staying
within the protection

of a branching coral.

They feed on plankton that
the current brings by,

but they don't venture very far

from the safety of the coral.

Other fish are too big to feed on plankton

and must go out of the
protection of the reef

if they wanna find food.

Their defense is to keep an eye peeled

and be ready to zoom into the
reef if a threat comes along.

Being alert is a key defense mechanism.

When it comes to a good defense,

nothing beats being really fast.

These sailfish can swim 60 miles per hour,

their top predators in the open ocean.

And even though sharks could
theoretically eat them,

sharks can't catch them.

The sailfish are attacking
a small school of sardines.

Schooling is a defensive move
used by fish in open water

where there's no place to hide.

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This big school of jacks stays
together in a large group,

which helps protect the fish

because it makes it harder for a predator

to focus on a single fish.

It's sort of a safety in numbers strategy,

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but what happens at night?

Many fish sleep safely at night

by hiding in holes in the reef.

For the most part, they're safe.

The parrot fish creates a
mucus cocoon around itself

every night,

the bubble protects the fish from annoying

parasitic isopods, the equivalent
of an underwater mosquito.

This sleeping parrot
fish has a nice bubble

that's doing its job.

Look closely, and you can
see the isopod on the bubble

like a fly on a screen door.

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Nearby on the nighttime reef,

an octopus is out on the prowl
hunting for sleeping fish.

It sticks its tentacles into the reef

to startle a sleeping fish

and hopes the fish will come
out into its web of arms

it has wrapped around the coral head.

But how does the octopus defend
itself against bigger fish

and fearsome underwater photographers

with bright video lights?

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It squirts an ink cloud.

The ink cloud is a distraction

to focus the predator's attention

while the octopus makes a quick getaway.

On a reef in the Philippines,

a tiny octopus barely
larger than a bottle cap

is hunting for a meal.

It's a blue ring to octopus,

one of the most dangerous
animals in the world,

packing a venom 10,000 times
more deadly than cyanide.

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One bite from the blue ring octopus

will k*ll a man in only minutes.

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Reaching into cracks and holes,

the octopus hunts for
shrimp or small fish to eat.

Goby boldly protects its burrow.

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To a nearby scorpion fish,

the octopus looks like
a nice bite-sized snack.

The octopus doesn't see the
well camouflage scorpion fish

watching and waiting to make its move.

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The scorpion fish strikes

and gets more than he bargained for.

Within only a fraction of a
second, the octopus bites back

and the scorpion fish spits it out.

The scorpion fish will
have a sore mouth for days

if it even survives.

It turns out that the blue ring octopus

does not make a very good snack after all.

Some animals are not
blessed with spines, venom,

or even the ability to move quickly.

Crabs and lobsters have
a tough exoskeleton

that makes them very hard to eat,

as well they have powerful
claws that can pinch and crush.

When a threat comes near,

a lobster doesn't hesitate
to put up those claws

and use them.

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A hermit crab ups the defense
by using a snail shell

as a portable house to provide
an extra layer of protection.

Some hermit crabs take it
even another step further,

placing venomous stinging
anemones on their shell

to further increase their
defensive positions.

This crab might be slow,
but she is very hard to eat.

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Even a lowly scallop has a defense.

It has no legs for walking
or fins for swimming,

but when it's arch enemy,
the sea star approaches

and makes a move,

the scallop opens its shells wide

and claps them together,

squirting a jet of
water out of its mantle.

Whelks are another predator
that tried to take advantage

of the scallop, but they're
given the squirt treatment too.

Take that whelks.

No matter how vulnerable an
animal in the ocean might look,

you can bet that it has
a strategy to survive.

Spines, venom, speed, a smokescreen,

or maybe even a squirt g*n,

there are many ways that
animals have learned

to defend themselves in the blue world.

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