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00:00¶¶
00:30Life began in the sea.
00:37The water carries oxygen so that creatures can breathe
00:40and microscopic organisms to provide them with food.
00:44It's a rich world, it covers three-quarters of the planet
00:47and the fish are masters of it.
00:57The world of water is a varied one.
01:00But the fish, by developing into thousands of different forms,
01:03exploit almost every part of it.
01:06Collecting different kinds of food requires different shaped bodies
01:09and some are quite unexpected.
01:11They've developed a multitude of different ways of propelling themselves through the water.
01:17They've developed a multitude of different ways of propelling themselves through the water.
01:23They've developed a multitude of different ways of propelling themselves through the water.
01:28They've developed a multitude of different ways of propelling themselves through the water.
01:42And their leader is through the water.
01:44If I went straight in to the waters, I walked across the water.
01:47We decided to know that sea-like haç”»an level is expert.
01:48And to seeÃvel is roughly explained into the water.
01:51elephants
02:00Watch or cry, innovation is a spirit of water.
02:03And earthquakes have good feelings of satisfaction.
02:04You may have been trying to create some facts on the soil.
02:06In size, they vary enormously.
02:18There are giants.
02:19A grouper like this can grow to be twice as long as a man.
02:22And others are so tiny that they can slip inside a big fish's mouth
02:26and pick its teeth for it.
02:36Fish have developed some surprising ways of finding their way about
02:42in this varied underwater world.
02:45The four-eyed fish has eyes divided horizontally
02:48so that it can look above the surface and below it at the same time.
02:54On the other hand, the cave fish,
02:55which normally lives in eternal blackness, has no eyes at all.
03:02How did this astounding variety come about?
03:05What were the earliest fishes like
03:07whose descendants now exploit the resources of the seas,
03:10the lakes and the rivers, in such a multitude of different ways?
03:13The answer may lie with one of the simplest organisms in the sea.
03:18It's a tiny, insignificant little blob of jelly.
03:37And amazing, indeed fantastic though it is,
03:41there are good reasons to suppose that it was a creature like this
03:45that gave rise to a line which led not only to the fish
03:49but through them to the amphibians, reptiles, mammals and man.
03:53It's called, not very attractively, but quite accurately, a sea squirt.
03:59And to know why, you have to look at it in water.
04:06Its structure is very simple indeed.
04:09Just a U-shaped tube enclosed in jelly.
04:12It sucks water in at the top,
04:16passes it through a grid inside the body
04:18that filters out the food particles
04:20and then squirts it out at the side.
04:24When it first hatches, however, it's rather different.
04:28And here is the clue that links it to fish.
04:30It has a tail with a thin, flexible rod in it.
04:34Little bunches of muscle are attached to the rod
04:36so that the animal can swim by beating it from side to side.
04:42In front, it has some sensory pits
04:45so it has some perception of its surroundings.
04:48We know that this is a very ancient body pattern
04:50because a fossil creature with both these characters
04:53has been found in rocks 530 million years old.
04:57Here again, those bunches of muscles attached to a rod.
05:01It's larger but built on the same principles as the young sea squirt.
05:05And a creature very like this still survives today.
05:09The Lancelot.
05:16This tiny sliver of flesh has no jaws,
05:19just a mouth surrounded by tentacles.
05:22The bunches of muscles attached to the rod in its back
05:24enable it to swim with an S-shaped wriggle,
05:28each bend pushing against the water
05:30so that the creature moves forward.
05:32Here it's filmed in slow motion.
05:34It's an action that's going to appear again and again
05:37in what is to come.
05:40Lancelots live half buried in the bottom of the sea
05:42with just their heads projecting above the gravel
05:44so that they can filter feed.
05:53There's another creature that has the same kind of lifestyle
05:56and is built on similar lines
05:58and it swims in the same way as the Lancelot.
06:05It's a lamprey.
06:06And later, in some species,
06:08it will change from filter feeding
06:10to a parasitic way of life
06:12using a rasping sucker at its head end.
06:14It extracts oxygen from the water
06:17and continues to suck it in at the mouth
06:20and expel it through gill slits on the neck.
06:23Its close relative, the hagfish,
06:26lives in the sea,
06:27sometimes burying itself in the mud on the bottom,
06:30sometimes fastening itself to fish with its teeth
06:32and eating with a sucker-like mouth.
06:38So, judging from the design of their bodies
06:40and the way they move them,
06:42there does seem to be a connection
06:43between the young sea squirt, the Lancelot
06:45and the hagfish and the lamprey.
06:48But although the hagfish looks like a fish,
06:51it's not one.
06:52It has no strengthening to that rod in the back,
06:55no real backbone and no jaws.
06:58Of course, it could be that the reason
07:05that the lamprey and the hagfish
07:07haven't got any jaws
07:08is not that they are primitive creatures
07:11that never developed them,
07:13but they are degenerate ones that lost them.
07:17The way to find the answer to that
07:19is to look in the rocks.
07:20The earliest fossils of shells and corals
07:24appear about 600 million years ago.
07:26And then, for 200 million years,
07:29there's no sign whatever of any backbone animals.
07:32But then, suddenly, they appear.
07:35And some of the finest specimens
07:36have been found in these very ancient rocks
07:38at the mouth of the Seven in the west of England.
07:43And these creatures have no jaws either.
07:46They have scales down their flanks
07:48and a head covered by a heavy bony shield.
07:51And they must have swum by wriggling this body
07:53and pushing their head along the bottom.
07:55And at the front, between two small eyes,
07:58there is a nostril.
08:00In fact, it's a kind of lamprey in armour.
08:06At the time of which we're talking,
08:08about 400 million years ago,
08:10the face of the Earth was not at all
08:12like what it is today.
08:14The relationships of the continents,
08:16the ocean basins, the coastlines,
08:18all were very different.
08:19Only in a few places can you today
08:21get a clear picture
08:22of what those ancient shores were like.
08:25And here, in Western Australia,
08:27in the Kimberley Ranges,
08:28there's one of them.
08:29And the best place to see it
08:30is from the air.
08:31Rising above the parched and sandy scrub,
08:45there are strangely shaped outcrops of rock.
08:48Those bluffs owe their curious shape
08:55not to the erosion of wind and rain,
08:58but to the labours, millions of years ago,
09:00of coral polyps.
09:02We are flying over an ancient seabed,
09:12with the original coast and the land behind it,
09:14now a rocky plateau,
09:16stretching away in the distance.
09:18Once, this plain was covered
09:20by a shallow blue lagoon
09:21in which corals built
09:23their great constructions of limestone.
09:25Over the millennia,
09:27rivers eroded the continent nearby,
09:29washed down the sand and mud,
09:31and deposited it over the seafloor.
09:34So the lagoons slowly silted up,
09:36and the sea retreated.
09:38Then the continent rose,
09:40rain and sun eroded the mudstones,
09:43and eventually the coral reefs
09:44were exposed once more
09:46as cliffs on a sun-baked plain.
09:57And here, I'm walking
09:59on the ancient seabed.
10:01The surface of the sea
10:03would have been close
10:04to the top of those reefs over there,
10:06so here I would have been
10:07about 200 feet down.
10:10And the sediments that lay
10:12on the bottom of that ancient sea
10:14are still here,
10:16turned into sandstones and mudstones.
10:20And in them are the remains
10:22of the creatures that lived in those seas.
10:24Here is one that I picked up
10:27only a few minutes ago.
10:29It's the scale of a huge fish.
10:33And this is a flank of a smaller fish
10:36with many scales on it.
10:39And this, which is perhaps
10:41the least impressive of all,
10:43is actually the most interesting,
10:44because this is a fossil skull.
10:47There is the line of its lower jaw.
10:49And if this nodule is treated with acids,
10:53the matrix will be eroded away
10:55and expose the perfectly preserved bones
10:58of the skull.
11:03These creatures, 400 million years old,
11:06were a considerable advance
11:08on the lancelets and the lampreys,
11:10for these had true jaws.
11:12And on their edges,
11:14the scales grew particularly long and sharp,
11:16so that the fish could bite and cut.
11:22Jaws, armed with teeth,
11:24enabled the fish to become
11:25very effective food gatherers
11:27and so grow into large and powerful creatures.
11:30And some of them became monsters.
11:34Judging from the size of these gigantic teeth,
11:37the shark was about 45 feet long.
11:40It's extinct,
11:43but its relatives are very much alive.
11:46Sensitive pits in the front of the head,
12:12nostrils,
12:13enable them to detect their prey
12:15from great distances.
12:17The hammerhead shark
12:18is said to be particularly sensitive,
12:20and this may explain
12:22the grotesque shape of its head.
12:24There's a nostril
12:25at the end of each side of the hammer,
12:27and the fish habitually swings its head
12:30from side to side,
12:31so that when the scent is equally strong
12:33in both nostrils,
12:34then it must know
12:35that its prey lies straight ahead.
12:37That rod in the back
12:41has now been strengthened with cartilage,
12:43and the entire skeleton of sharks
12:45is built from this soft, light material.
12:50They still swim like the lancelets
12:52with sideways beats of their body,
12:54which are restricted mostly
12:55to the back half and to the tail.
12:58The thrust created
12:59tends to drive the nose downwards,
13:01and to compensate for that,
13:03sharks have a pair of horizontal fins
13:05on either side at the front,
13:07like the veins of a submarine.
13:09But these fins are stiff and inflexible.
13:12The shark can't twist them vertically
13:14to act as brakes.
13:15Indeed,
13:16a charging shark can't stop,
13:18only swerve to one side.
13:20Nor can it swim backwards.
13:22Furthermore,
13:23since its body is heavier than water,
13:24if it stopped swimming,
13:26a shark would sink.
13:33The Wobbegong,
13:35a shark from Australian waters,
13:37has a tendency to do just that.
13:40It's largely abandoned the effort
13:42of perpetually swimming
13:43to keep in mid-water,
13:44and has settled on the sea floor
13:46where it leads a more restful life.
13:54The transition from continuous swimming
14:00in the open sea
14:01to a life more or less permanently
14:03on the bottom
14:04can be seen in a series of fishes.
14:09The dogfish is very shark-like.
14:13The angel shark, however,
14:15is rather more flattened
14:16with wide side fins
14:17and a rather smaller tail.
14:25The ray has flattened its body
14:27to an extreme degree,
14:29dispensing with that rear engine
14:31the powerful flashing tail
14:32and expanding the lateral fins
14:35so that their ripples
14:36can take over the job
14:37of propelling the fish
14:38through the water.
14:39And it spends most of its time
14:41lying on the bottom.
14:54A light dusting of gravel
14:59does wonders for camouflage.
15:07The sawfish shark
15:09is another bottom liver,
15:10and it uses its extraordinary blade
15:13like a double-edged scythe,
15:15excavating in the sand and gravel
15:16for shells and crabs,
15:18and sometimes flailing
15:19through a shoal of fish,
15:21slashing them
15:22so that they fall injured
15:23and can be eaten.
15:24So bodies with cartilaginous skeletons
15:29developed into two main shapes,
15:31long ones like sharks
15:33and wide ones like rays and skates.
15:36But having learned, as it were,
15:54to live on the bottom,
15:56some rays took off again.
15:59Undulating side fins
16:00are very effective motors
16:01for mid-water swimming,
16:03provided that speed is not needed.
16:05So they're very suitable
16:07for fish like the manta ray
16:08that drifts through
16:09these surface waters,
16:11filter-feeding on plankton.
16:13The blades on either side
16:15of the manta's head
16:16help to channel
16:17the food-bearing water
16:18into the slot-like mouth.
16:20The manta cannot swim
16:22much faster than this,
16:23but it wouldn't help
16:24its feeding very much
16:25even if it did,
16:26for the water can't flow
16:27through the sieve
16:28in the gill slits
16:29any faster than it's doing now.
16:41Filter feeding
16:42in the surface of the ocean
16:43is clearly
16:44a very effective way of life.
16:47It doesn't require much energy,
16:48there's an unlimited supply of food,
16:50and some of the fish
16:51that have taken to it
16:52have become very large indeed.
16:55The basking shark
16:56grows to a length
16:57of 15 metres,
16:5945 feet.
17:00Only one fish today
17:02is any bigger,
17:03the whale shark,
17:04and that too
17:05is a filter feeder.
17:06And there,
17:14clinging under its tail,
17:15is a primitive,
17:16jawless lamprey,
17:18sucking at its flesh,
17:19a reminder
17:20of the fish's remote past,
17:22a close relative
17:22of the earliest swimmers.
17:29Another filter feeder,
17:31the paddlefish,
17:33but this is only
17:34very distantly related
17:35to the sharks and rays.
17:37400 million years ago,
17:39right at the beginning
17:40of fish history,
17:41a group started
17:42constructing their skeletons
17:43not of cartilage,
17:45but of solid bone,
17:46and the ancestors
17:47of the paddlefish
17:48were among them.
17:50And another of these
17:51primitive bony fish,
17:53the sturgeon.
17:54Not only does it have bone
17:56in its internal skeleton,
17:57it also has heavy,
17:59bony scales
18:00in its skin.
18:05It's the eggs of this fish
18:20that are made into caviar.
18:29It still swims
18:31very like a shark
18:32with sweeps
18:33of its hind body
18:34and tail.
18:35And the tail
18:35looks shark-like too.
18:42Soon after the bony fish
18:44first appeared,
18:45they spread from the seas
18:47up the rivers
18:47to colonise the fresh waters
18:49of the world.
18:50It was an invasion
18:51that was to have
18:52revolutionary consequences.
18:54The waters of rivers
19:01and lakes
19:02are shallow
19:03compared to the sea
19:04and often,
19:05as a consequence,
19:05they get quite warm.
19:07And the warmer water
19:08becomes,
19:09the less oxygen
19:10it can hold
19:11dissolved in it.
19:12That presents
19:13a serious problem
19:14to any fish living there.
19:16How are they to breathe?
19:19This is one of them,
19:20the polypterus,
19:21and this is its solution.
19:23It gulps air
19:25and then absorbs
19:27the gaseous oxygen
19:28from a pouch
19:29that leads off its gut.
19:31In other words,
19:31it has developed
19:32a very simple lung.
19:35But an air-filled pouch
19:36within the body
19:37brings another
19:38incidental advantage.
19:39It gives buoyancy.
19:41So the bony fish
19:42acquired a swim bladder,
19:44a controllable bag of air
19:46inside the body.
19:47Now the elements
19:52of the modern fish
19:53had been assembled.
19:55A swim bladder
19:55for buoyancy,
19:57a backbone
19:57with muscles attached
19:58for strength,
19:59and gills for breathing.
20:02And for further
20:03precision and control,
20:04there is
20:05the lateral line,
20:06a row of tiny pits
20:07that are sensitive
20:08to pressures
20:08and currents in the water.
20:10And so the modern
20:11bony fish,
20:12like this trout,
20:13is very finely tuned
20:14to its world.
20:25This perfection of senses
20:26and control of movement
20:28is critical
20:29when a pike
20:30is on the hunt
20:30for roach.
20:44With buoyancy provided
20:46by the swim bladder,
20:48the fins can be used
20:49entirely for fine
20:50adjustments of its position
20:51as it hovers.
20:52as it hovers.
21:22At normal speed,
21:50it's almost impossible
21:51to see what happens.
21:52it's so fast.
22:01Slowed down,
22:02it's possible to see
22:03the enormous acceleration
22:04and accuracy.
22:06The actual bite
22:06only lasts
22:07a split second,
22:08and the prey
22:09goes straight in.
22:22I'm standing on the brink
22:32of one of the most
22:33densely populated parts
22:35in this world.
22:35I'm on the edge
22:36of a coral reef
22:37at a low tide.
22:39A few feet out there,
22:40the bottom sinks dramatically,
22:42and there you'll find
22:43an abundance of life
22:45of all kinds.
22:46Microscopic plants,
22:48invertebrates,
22:49corals,
22:50and of course,
22:51a multitude of fish
22:52that come there
22:54to harvest
22:55this rich source of food.
22:57Each kind of fish
22:58has its own particular place
22:59in this mosaic,
23:00its own particular food,
23:02and each has,
23:03in consequence,
23:03developed its own way
23:04of swimming,
23:05its own way
23:06of using its fins.
23:20The huge number
23:21of fish
23:22that swarm on the reef,
23:23harvesting the great variety
23:25of food that it offers,
23:26causes considerable
23:27social problems.
23:30Each species
23:31has its own particular niche
23:33on the reef,
23:34and is designed accordingly.
23:35Many are slim
23:37for slipping through
23:38the tangle of coral.
23:41Others,
23:41like the cowfish,
23:42have a rigid box
23:43of bony plates
23:44and can stop dead
23:45with precise control
23:46from its fins.
23:49The trigger
23:50sticks its fin-free
23:52front half
23:52between coral branches
23:54to feed.
23:55The angelfish
23:56picks off small morsels
23:57from the surface of corals,
23:59once again
23:59with perfect control.
24:05And this butterfly fish
24:07has elongated jaws
24:09that enable it
24:10to probe
24:10into narrow crevices
24:11with the accuracy
24:13of forceps.
24:16For turning sharply,
24:17banking steeply,
24:19or simply flapping along,
24:20most coral fishes
24:21have been able
24:22to abandon
24:23the S-shaped wriggle.
24:24They've deployed
24:25their fins
24:26and adjusted
24:26their bodies
24:27to live
24:27in this particular world.
24:29No shark
24:41could do this,
24:42but then they are adapted
24:43to a different kind of life.
24:52The pufferfish
24:53doesn't wriggle its body,
24:55but it does flex its fins
24:57and to great effect.
24:58The S-shape action
24:59is now being used there.
25:07And fins have another
25:09important role
25:10as flags.
25:11In such a mixed
25:12and dense crowd,
25:13it's very much
25:14to the advantage
25:15of every individual fish
25:16to proclaim its presence
25:17and identity
25:18from among the throng,
25:20so that rivals
25:21will be aware
25:21that this particular
25:22food patch
25:23has got an owner.
25:25And the same markings
25:26will also serve,
25:27when the time comes,
25:28to attract a mate
25:29of the right species.
25:34The sharks and rays
25:36have eyes that,
25:37though they see shapes
25:38very well,
25:38are largely blind to colour.
25:40It's hardly surprising,
25:41therefore,
25:42that they are,
25:43by and large,
25:43drab-coloured creatures.
25:45But the bony fish
25:46have excellent colour vision,
25:47and so they are able
25:49to signal to one another
25:50with stripes and spots
25:52and blotches
25:52and in the most wonderful
25:54variety of colours.
25:56The coral fish
25:58can risk making themselves
25:59conspicuous
26:00because the reef
26:01is full of crevices
26:02and corners
26:02where they can dart
26:03to safety
26:04if danger threatens.
26:05away from the reef,
26:14however,
26:14the sea
26:15is a dangerous place,
26:17for there
26:17there is nowhere
26:18to hide
26:19except among
26:20your fellows.
26:26And these are designed
26:27for a very different
26:28way of life.
26:30Fast swimming,
26:31fast feeding
26:32in the open sea
26:32with plankton
26:34at the base
26:34of the food chain,
26:35and it's that
26:36wriggling body action
26:37that pushes them along.
26:52Open water fish
26:54often form huge shoals,
26:56and this may be
26:57for safety's sake.
26:58The drifting,
26:59darting multitudes
27:00of fish
27:00may tend to baffle
27:02and confuse predators,
27:04and if you meet
27:04a shark on your own,
27:06it'll go for you,
27:07but if you are with others,
27:09your chances are much better.
27:21From the plankton
27:22to the small fish
27:23and on up the food chain
27:25to the big fish.
27:28In the open ocean,
27:29spring speed
27:29is of great value
27:31and since water
27:32is very dense,
27:33800 times more so
27:34than air,
27:35streamlining
27:36is of the greatest
27:37importance to fish.
27:39Both hunters
27:39and the fish they pursue
27:40have developed
27:41very similar shapes,
27:43pointed in front
27:44and tapering
27:45to a two-bladed
27:46symmetrical tail
27:47at the back.
27:48Barracuda,
27:49among the most voracious
27:50and swift
27:51of the bony fish.
27:52And this is a hunter's eye
28:00view of a fish
28:00that escapes
28:01not by swimming fast,
28:03but in a quite
28:03different way.
28:07It's a flying fish.
28:09Its front pair of fins
28:10are greatly enlarged
28:12so that with a flick
28:13of its tail,
28:14it launches itself
28:15into the air
28:16and out
28:16of a hunter's sight.
28:17This is the flight
28:26in slow motion.
28:27The fish is already
28:28swimming fast
28:29when it comes
28:29to the surface
28:30and it takes off
28:31helped by the beating tail
28:33which has a specially
28:34enlarged lower lobe.
28:35The front fins
28:36are then spread
28:37to assist the glide.
28:42Occasionally,
28:42they dip their tails
28:43into the surface
28:44to give themselves
28:45an additional boost
28:46and so they can
28:47sometimes fly
28:48for several hundred metres.
28:54Some fish
28:55have sought safety
28:55by going not upwards
28:56but downwards.
28:59These eggs
28:59that float
29:00in astronomic numbers
29:01on the surface
29:02of the sea
29:02during the summer
29:03have come from
29:04one of the bottom dwellers.
29:06Only one in 100,000
29:08will survive
29:09but those that do
29:10will pass through
29:11a most extraordinary
29:12transformation
29:13before they become adult.
29:16after about a week
29:21they hatch
29:22into what looks like
29:23a fairly normal
29:23kind of fish fry.
29:25They hang near the surface
29:27where it's warm
29:28and there's a lot
29:28of oxygen
29:29feeding on microorganisms.
29:31Each is not much bigger
29:32than a pinhead.
29:33each one still contains
29:49a tiny bag of yolk
29:50that will sustain it
29:51for a day or two more.
29:55The young fish
29:56has eyes
29:57on either side
29:57of its head
29:58but they won't remain
29:59that way for long.
30:00Its body deepens
30:01as it begins to feed
30:03and its stomach swells
30:04and it develops
30:05a swim bladder.
30:12Its eyes are beginning
30:14to look a little lopsided.
30:15One is higher
30:16than the other.
30:22Now they've developed pigment
30:23but only on one flank
30:25and they swim
30:26on their sides
30:27with that coloured flank
30:28upwards.
30:29These are going
30:30to be flat fish
30:30turbot.
30:32Place, sole and flounder
30:33also go through
30:35such a transformation.
30:40And they finally
30:41settle on the bottom.
30:48One eye is now
30:49on the edge
30:50of the fish.
30:54Now the transformation
30:55is complete
30:56and the fish
30:57has lost
30:58that swim bladder
30:59for buoyancy
31:00is a positive
31:01hindrance
31:01on the sea bottom.
31:03A bony fish
31:04has joined
31:05the skates
31:06and the rays
31:06on the sea floor
31:07by the simple
31:08if drastic expedient
31:10of lying on its side
31:11and moving one eye
31:13right round its body.
31:17Many other bony fish
31:19have abandoned
31:19the swim bladder
31:20and settled down.
31:21Each has found
31:22its own way
31:23of adapting to life
31:24where skill in swimming
31:25is of less importance
31:27than an ability
31:28to merge
31:28into the background
31:29of the sea floor
31:30and so fins
31:31can be used
31:32for all kinds
31:33of other purposes.
31:34This looks like
31:35a rock
31:36lying on the bottom
31:37but it has a gill
31:38an eye
31:40and an upturned mouth.
31:43It's a stonefish
31:44a hunter
31:44that relies on invisibility
31:46to catch its prey
31:47unawares
31:48and its fins
31:49are coloured
31:50and shaped
31:51to help its camouflage.
31:52The anglerfish
31:56uses its fins
31:57not for swimming
31:58but for walking
31:59and the front spine
32:02of its dorsal fin
32:03is a fishing rod.
32:07With that lure
32:08it attracts
32:09unsuspecting creatures
32:10within range
32:11of its mouth.
32:12The bearded ghoul
32:20uses its fins
32:21for defence.
32:23Without a membrane
32:24between them
32:24they are no longer
32:25of any use
32:25as stabilisers
32:26when swimming
32:27and instead
32:28they are sharp
32:29and tipped with poison.
32:31Very effective protection.
32:33It's just what you need
32:34if you're lying
32:35on the bottom.
32:35The gurnard
32:43uses some of the rays
32:44of its front pair
32:45of fins
32:46as delicate legs
32:47for finding food
32:49in the gravel.
32:49fish like those
33:08live in comparatively
33:10shallow waters
33:11100 feet
33:1230 meters
33:13something like that.
33:15Their world
33:16is a heavily
33:17inhabited one
33:18and also quite a bright one
33:19because the water
33:20is shallow enough
33:21to receive light
33:22from the sun.
33:23It's also one
33:24that's comparatively
33:25familiar to us.
33:26For one thing
33:27all the sea fish
33:28that we eat
33:29come from it
33:29and for another
33:30hundreds of thousands
33:31of people
33:32regularly visit it
33:33wearing aqualungs.
33:35But in fact
33:36it is only
33:38a tiny proportion
33:39of the seas
33:40of the world.
33:42Most of the oceans
33:43are very, very much
33:44deeper than that
33:45and to visit
33:46those deep waters
33:47you can't go down
33:49an aqualung.
33:50You have to use
33:50something like this
33:51a submersible.
34:08These craft work
34:09on the sea floor
34:10helping in the drilling
34:11for oil
34:12and they give
34:13a splendid view
34:14of what's happening
34:14at depth
34:15both to oil engineers
34:17and to fish watchers.
34:35There's a highly sensitive
34:36television camera
34:37mounted on the outside
34:38of the hull
34:39with a monitor screen
34:41in the cockpit
34:41and spotlights
34:43to illuminate places
34:44that the sun's rays
34:45have never reached.
34:49As we go down
34:50it gets darker
34:52and darker
34:53and the pressure
34:54increases too
34:55very quickly.
34:56By the time
34:57we are 500 feet
34:58the loading
34:59on this viewing dome
35:01here
35:01will be about
35:0270 tonnes
35:03and it also
35:05gets colder
35:06and colder
35:07and at one point
35:09and the precise depth
35:11varies according
35:12to where we are
35:12in the world
35:13between say
35:1420 metres
35:15which is about
35:1660 feet
35:16and 150 metres
35:18450 feet
35:19it suddenly
35:20gets very much
35:22colder indeed
35:23and drops to about
35:245 degrees
35:25above freezing.
35:26That point is called
35:28the thermocline
35:29and it's a kind of
35:30frontier
35:31in the ocean
35:32separating two
35:34very different worlds
35:36between which
35:36there is very
35:37little traffic.
35:38Above
35:39there is the
35:40sunlit
35:41warm waters
35:42near the surface
35:43which have
35:43their own
35:44circulation
35:44and below
35:46the thermocline
35:47there is the
35:48black
35:48near freezing
35:49world
35:50of the ocean
35:51depths
35:51and there
35:52there live
35:53very different
35:54fish indeed.
36:00This is part
36:01of the world
36:02that man
36:02is only just
36:03beginning to explore
36:04until a few
36:06years ago
36:06most of our
36:07knowledge of
36:08these creatures
36:08came from
36:09specimens that
36:10had been hauled
36:11up in dredges
36:11but as they
36:13came up
36:13the great changes
36:14in pressure
36:15and temperature
36:15usually distorted
36:17their bodies
36:17and they quickly
36:18died.
36:19Only now
36:20from such craft
36:21as the submersibles
36:22are we beginning
36:23to get an accurate
36:24idea of what life
36:25is really like
36:26in the deeper
36:27parts of the oceans.
36:30A shark
36:31built on the same
36:32pattern as its
36:33relatives above
36:34like most of
36:35the inhabitants
36:36of these oxygen
36:37poor waters
36:38it moves
36:39comparatively
36:39slowly
36:40probably never
36:41meeting a boundary
36:42or a barrier
36:43in the endless
36:43deep sea.
36:48A red prawn
36:49doubtless food
36:51for some big fish.
36:52fish.
37:00An extraordinary
37:01relative of the
37:02prawn
37:02another crustacean
37:03called an ostracod
37:05fossils of species
37:06very like these
37:07have been found
37:08in extremely
37:09ancient rocks
37:10so we know
37:11that they were here
37:12long before the
37:13fish arrived.
37:15This fish
37:15still uses that
37:17antique way
37:18of swimming
37:18with S-shaped
37:19undulations
37:20dimensions of its
37:21body.
37:32A fang tooth
37:34one of the
37:34hunters of this
37:35lightless world.
37:37Our knowledge of the
37:39fish of these depths
37:39is still very
37:40fragmentary.
37:41Many species
37:42have never been
37:43filmed and we
37:44know them only
37:44from a few
37:45mangled specimens
37:46and still photographs.
37:49This bait
37:50on a line
37:51is suspended
37:52from a rod
37:53dangling in front
37:54of an upper jaw
37:55lined with needle
37:56teeth.
37:57It's another kind
37:58of anglerfish.
38:01There are so few
38:02animals at these
38:03depths that when
38:04a meal arrives
38:05a hunter must make
38:06quite sure of
38:07catching it.
38:07This angler
38:15normally looks
38:16like this
38:17but when it's
38:17had its meal
38:18its stomach
38:19becomes hugely
38:20distended.
38:22The gulper
38:23is little more
38:24than a swimming
38:25mouth also
38:26with a stomach
38:27capable of
38:28great extension.
38:29The bigger
38:29the stomach
38:30can go
38:30the wider
38:31the choice
38:32of prey
38:32and meals
38:33may be few
38:34and far
38:34between.
38:35Many fish
38:37produce lights
38:38in this blackness
38:39some as face
38:41patches or dots
38:41along the sides.
38:43At night
38:44many of them
38:44move up to
38:45shallower water
38:46where of course
38:47it's still dark.
38:58With special
38:59light sensitive
39:00cameras
39:01and a little
39:01illumination
39:02you can just
39:03make out the fish
39:04that are producing
39:05these lights
39:05they're called
39:06flashlight fish.
39:25With no
39:26illumination
39:26at all
39:27they become
39:28disembodied green
39:29spots again
39:30mysteriously
39:31circulating
39:31in the blackness.
39:35The light
39:36is produced
39:37by bacteria
39:37which live
39:38in this one
39:39patch of skin
39:39and glow
39:40as a normal
39:41byproduct
39:42of the chemistry
39:42of their life
39:43processes.
39:47The light
39:48may serve
39:48the flashlight
39:49fish in
39:49several ways.
39:50It may be
39:51assigned to
39:51other members
39:52of the show.
39:53It may baffle
39:54predators.
39:55After fish
39:55switches off
39:56its light
39:57it immediately
39:57darts away
39:58to a different
39:58position
39:59or it may
40:00simply be
40:00a method
40:01of finding
40:01your way
40:02around
40:02in the blackness.
40:14The problem
40:14of finding
40:15the way
40:15in the dark
40:16faces other
40:17fish too.
40:18They live
40:19in turbid waters
40:20and under
40:21a thick carpet
40:22of floating
40:23vegetation.
40:23Not an easy
40:24place to find
40:25them.
40:35This somewhat
40:37surprising piece
40:38of apparatus
40:39is the latest
40:41device developed
40:43and designed
40:44by research
40:44workers who
40:46are interested
40:46in electric
40:47fishes.
40:49This plastic
40:50tube has got
40:51two leads
40:52which come up
40:53through this
40:53cable.
40:54along here
40:55to this
40:55extraordinary
40:56hat.
40:57There they
40:58go into
40:59this amplifier
40:59and on the
41:01brim of the
41:01hat there's
41:03a loudspeaker
41:04here and
41:05very thoughtfully
41:06a counterweight
41:08here so that
41:08when I put
41:09the hat on
41:11my head
41:11it doesn't
41:12flop over
41:13one ear.
41:14And then
41:14if I turn
41:15on the
41:16amplifier
41:16the speaker
41:18is next
41:19to my ear
41:20I have
41:20one hand
41:21free
41:21and the
41:23wires
41:24will pick
41:24up the
41:25signals
41:26of those
41:27electric
41:27fishes
41:27and I
41:28can hear
41:28them
41:28as a
41:29series
41:29of clicks.
41:30The fish
41:44produce their
41:45electricity
41:45from stacks
41:46of plate
41:47shaped cells
41:48embedded in
41:49jelly that
41:49lie in a
41:50column along
41:51each flank
41:51like batteries.
41:53Each fish
41:54sends out a
41:54particular kind
41:55of discharge
41:56and each
41:57makes a
41:57different sound
41:58on the
41:58loudspeakers.
41:59so these
42:00may be
42:01another kind
42:02of call
42:02sign like
42:03the Morse
42:03code of
42:04the flashlight
42:04fish, a
42:05way of
42:06proclaiming
42:06identity.
42:08They also
42:09certainly help
42:10the fish
42:10in navigation.
42:12Each creates
42:13in the water
42:14around it
42:14a weak
42:15electric field.
42:16Any other
42:17solid object
42:17in the water
42:18causes a
42:19change in
42:20that field
42:20and the
42:21fish have
42:22special
42:22pores
42:23spaced out
42:24over their
42:24bodies which
42:25detect such
42:26alterations.
42:27As a result
42:28they're able
42:29to find
42:29their way
42:29about in
42:30total darkness
42:31and they
42:32can swim
42:32just as
42:33accurately
42:33backwards
42:34as they
42:35can forwards.
42:47Electricity
42:48has evolved
42:48independently
42:49in many
42:50fishes
42:50from South
42:51America
42:51and Africa
42:52in rivers,
42:53lakes and
42:54also in
42:55the sea.
42:56In some
42:56the tail
42:57muscles are
42:58used,
42:58others
42:59the head
42:59area and
43:00even the
43:00eye
43:01muscles.
43:09A straight
43:10knife-shaped
43:11body is
43:12characteristic
43:12of all
43:13these fish
43:13and it
43:14may be
43:14that it's
43:15important to
43:15keep the
43:16body stiff
43:17in this
43:17position
43:17in order
43:18to maintain
43:19an accurate
43:19output of
43:20navigational
43:21signals.
43:21And the
43:22fins do
43:23the
43:23manoeuvring.
43:25Some
43:25frequently
43:26rest wedged
43:27between
43:28plant stems.
43:28arms.
43:58Most of the discharges produced by those electric fish are extremely weak.
44:08You couldn't possibly detect them without special amplifying equipment.
44:12But that is very much not the case with all of them.
44:16This is the most powerful electric fish of all, the famous electric eel from South America.
44:23This has two kinds of electricity.
44:26Not only does it have batteries which produce the discharges used for navigation,
44:31but it's also capable of delivering a massive electric shock,
44:35which it stuns its prey with, and which is quite sufficient to throw me on my back,
44:41if I were not wearing rubber gloves.
44:44And I can demonstrate that electric shock by tapping him near his head and near his tail with these electrodes,
44:52which will then, if he gives a shock, light up these bulbs.
44:56And the more powerful the shock, the more bulbs he will light.
45:03There's a rapid output of volts, peaking in this case at about 400.
45:08Four bulbs were lit.
45:09Of all backbone animals, fish are the only ones to produce electricity in their bodies.
45:14So, the bony fish, one way or another, have managed to colonise all the waters of the world,
45:25from the black depths of the sea to inland rivers and lakes, even lakes like this.
45:31Lake Magadi, in the Rift Valley of East Africa,
45:33is, I think, just about the most hostile environment that I know for land animals, let alone fish.
45:39It's a lake not of water, but of solid soda and potash,
45:57solidified by the baking African sun from solutions bubbling up from volcanic rocks far below.
46:03And here, at last, is somewhere where you might think you'll get a place to cool your feet.
46:13You might get a nice, refreshing drink of clear, cool water.
46:18And yet, in fact, oh, this water is so, so hot.
46:26Actually, it's really quite difficult to bear.
46:28And when you taste it, the water is sickeningly salty.
46:33This is actually one of the hot volcanic springs where water bubbles up from deep below the surface of the brown,
46:41bringing up a solution of soda and salt to trickle down and crystallise out in the lakes.
46:49You can hardly imagine a less likely place to find a fish.
46:58And yet, there it is, a species of tilapia.
47:04The water here can be as high as 43 degrees centigrade, 110 degrees Fahrenheit.
47:10Algae manage to grow here, and the fish survive by feeding on it.
47:15Yet another niche, a most unlikely one, has been filled by the incredibly adaptable fishes.
47:21At the other extreme, there is one fish in the coldest waters on Earth.
47:35Sea water freezes below the temperature of fresh water.
47:39The ice fish from the seas of the Antarctic has developed a substance in its blood
47:44which keeps it liquid even when the sea water above it freezes solid.
47:48It has, in fact, a kind of antifreeze.
47:55But if one wanted to pick out of the 30,000 or so species of fish alive today,
48:01the king of them all, my vote would go to this, the salmon.
48:05In the acuteness of its senses, the skilfulness of its navigation,
48:15the strength and athleticism of its body,
48:17this surely must be a paragon among fish.
48:21In the Pacific, there are several different kinds.
48:24They spend much of their time in the ocean feeding on plankton and small fish.
48:29But in the summer, they assemble off the North American coast
48:32and then they begin to battle their way up the rivers.
48:37Even falls don't stop them.
48:40The flexible rod that first appeared in the young sea squirt
48:43is here marvellously muscled and strengthened with a jointed column of bone.
48:47So, with a thrash of its hind end and fins,
48:51the movement first developed by the earliest ancestors of the fish,
48:55the salmon can swim and leap up the fastest torrents
48:58and its lateral line can sense the details of the surge.
49:02The salmon's sense of smell is almost unbelievably acute.
49:21This river is not just any river.
49:23It is the precise one in which all these fish were hatched.
49:27Each has retained a memory of the taste and smell of these waters.
49:31And this has drawn them back across thousands of miles of ocean
49:35so that they may complete their lives where they began them.
49:49During the past few days,
49:51their bodies have been changing with astonishing speed.
49:55These pink salmon have developed high, humped back
49:59with thin bodies and their lower jaws have become hooked.
50:04The front teeth of the males
50:07have developed into long and powerful fangs,
50:11hopeless for feeding.
50:12But then the time of feeding is long since over.
50:16Their teeth are for battle.
50:18The males
50:48Females fight for a scrape scooped in the gravel of the riverbed.
50:52When one takes possession, a female will join him and lie alongside.
50:57Then, as she sheds her eggs into the gravel, his milk will fertilize them.
51:18And now they're totally spent.
51:41They don't even have enough energy to heal their battered, wounded bodies.
51:46The scales fall off and the once powerful muscles, the flesh, dwindles and shrivels.
51:53And they die. All of them.
51:56Not a single one of the millions of fish which fought their way up this river ever goes back to the sea.
52:04But their eggs remain and they'll stay here throughout the winter, safe in the gravel,
52:09until in the spring they'll hatch and the fry will be swept down the river to the ocean.
52:15There they will feed and grow until at the appointed time, two years hence exactly as far as these pink salmon are concerned,
52:23the fully grown fish will once again beat its way powerfully up river to the place where it was born.
52:30The salmon is the master both of salt water and fresh, but one part of the world is closed even to it, the land.
52:56And yet a few fish can survive even there for a short time.
53:01The walking catfish travels over land using bony fins and a sideways wriggle.
53:07But it's not the first fish to do that.
53:10One managed it some 350 million years ago and that was a momentous move
53:16because from that fish developed frogs and lizards, birds and mammals and ultimately ourselves.
53:24.
53:29.
53:31.
53:33.
53:34.
53:35.
53:37.
53:39.
53:40.
53:41.
53:42.
53:59.
54:01.
54:02.
54:03.
54:04.
54:05.
54:28.
54:29.
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