0:00:33 > 0:00:35Life began in the sea.
0:00:35 > 0:00:38The water carries oxygen so that creatures can breathe,
0:00:38 > 0:00:42and microscopic organisms to provide them with food.
0:00:42 > 0:00:45It's a rich world, it covers three quarters of the planet,
0:00:45 > 0:00:48and the fish are masters of it.
0:00:55 > 0:00:57The world of water is a varied one.
0:00:57 > 0:01:01But the fish, by developing into thousands of different forms,
0:01:01 > 0:01:04exploit almost every part of it.
0:01:04 > 0:01:08Collecting different food requires different-shaped bodies.
0:01:08 > 0:01:11And some are quite unexpected.
0:01:35 > 0:01:41They've developed a multitude of different ways of propelling themselves through the water.
0:02:13 > 0:02:16In size, they vary enormously.
0:02:16 > 0:02:17There are giants.
0:02:17 > 0:02:20A grouper like this can grow to be twice as long as a man.
0:02:20 > 0:02:27Others are so tiny that they can slip inside a big fish's mouth and pick its teeth for it.
0:02:36 > 0:02:41Fish have developed some surprising ways of finding their way about
0:02:41 > 0:02:43in this varied underwater world.
0:02:44 > 0:02:46The four-eyed fish has eyes divided horizontally
0:02:46 > 0:02:52so that it can look above the surface and below it at the same time.
0:02:52 > 0:02:54On the other hand, the cave fish,
0:02:54 > 0:02:57which normally lives in eternal blackness, has no eyes at all.
0:03:00 > 0:03:04How did this astounding variety come about?
0:03:04 > 0:03:05What were the earliest fishes like,
0:03:05 > 0:03:08whose descendents now exploit the resources
0:03:08 > 0:03:12of the seas, lakes and rivers in such a multitude of different ways?
0:03:12 > 0:03:17The answer may lie with one of the simplest organisms in the sea.
0:03:30 > 0:03:35It's a tiny, insignificant little blob of jelly.
0:03:35 > 0:03:39And amazing, indeed, fantastic though it is,
0:03:39 > 0:03:45there are good reasons to suppose that it was a creature like this
0:03:45 > 0:03:48that gave rise to a line which led not only to the fish,
0:03:48 > 0:03:53but through them to the amphibians, reptiles, mammals and man.
0:03:53 > 0:03:58It's called, not very attractively but quite accurately, a sea squirt.
0:03:58 > 0:04:02And to know why, we have to look at it in water.
0:04:06 > 0:04:08Its structure is very simple indeed.
0:04:08 > 0:04:11Just a U-shaped tube enclosed in jelly.
0:04:12 > 0:04:14It sucks water in at the top,
0:04:14 > 0:04:17passes it through a grid inside the body
0:04:17 > 0:04:19that filters out the food particles,
0:04:19 > 0:04:22and then squirts it out at the side.
0:04:23 > 0:04:27When it first hatches, however, it's rather different.
0:04:27 > 0:04:29And here is the clue that links it to fish.
0:04:29 > 0:04:32It has a tail with a thin, flexible rod in it.
0:04:32 > 0:04:35Little bunches of muscle are attached to the rod
0:04:35 > 0:04:37so that the animal can swim
0:04:37 > 0:04:39by beating it from side to side.
0:04:41 > 0:04:43In front, it has some sensory pits,
0:04:43 > 0:04:47so it has some perception of its surroundings.
0:04:47 > 0:04:49We know that this is a very ancient body pattern.
0:04:49 > 0:04:55A fossil creature with both these characters has been found in rocks 530 million years old.
0:04:55 > 0:05:00Here again, those bunches of muscles attached to a rod.
0:05:00 > 0:05:04It's larger, but built on the same principles as the young sea squirt.
0:05:04 > 0:05:09And a creature very like this still survives today - the lancelet.
0:05:15 > 0:05:17This tiny sliver of flesh has no jaws,
0:05:17 > 0:05:20just a mouth surrounded by tentacles.
0:05:20 > 0:05:23The bunches of muscles attached to the rod in its back
0:05:23 > 0:05:26enable it to swim with an S-shaped wriggle,
0:05:26 > 0:05:30each bend pushing against the water so the creature moves forward.
0:05:30 > 0:05:32Here it's filmed in slow motion.
0:05:32 > 0:05:38It's an action that's going to appear again and again in what is to come.
0:05:38 > 0:05:41Lancelets live half-buried in the bottom of the sea
0:05:41 > 0:05:43with just their heads projecting above the gravel,
0:05:43 > 0:05:46so that they can filter-feed.
0:05:52 > 0:05:55Another creature has the same kind of lifestyle
0:05:55 > 0:06:00and is built on similar lines, and it swims in the same way as the lancelet.
0:06:03 > 0:06:05It's a lamprey.
0:06:05 > 0:06:11And later, in some species, it will change from filter-feeding to a parasitic way of life,
0:06:11 > 0:06:13using a rasping sucker at its head end.
0:06:14 > 0:06:16It extracts oxygen from the water,
0:06:16 > 0:06:21and continues to suck it in at the mouth and expel it through gill slits on the neck.
0:06:23 > 0:06:26Its close relative, the hagfish, lives in the sea,
0:06:26 > 0:06:28sometimes burying itself in the mud,
0:06:28 > 0:06:34sometimes fastening itself to fish with its teeth and eating with a sucker-like mouth.
0:06:36 > 0:06:40So, and judging from the design of their bodies and the way they move them,
0:06:40 > 0:06:44there does seem to be a connection between the young sea squirt, the lancelet
0:06:44 > 0:06:47and the hagfish and the lamprey.
0:06:47 > 0:06:51But although the hagfish looks like a fish, it's not one.
0:06:51 > 0:06:54It has no strengthening to that rod in the back,
0:06:54 > 0:06:57no real backbone and no jaws.
0:07:02 > 0:07:07Of course, it could be that the reason that the lamprey and the hagfish haven't got any jaws
0:07:07 > 0:07:11is not that they are primitive creatures that never developed them,
0:07:11 > 0:07:15but they are degenerate ones that lost them.
0:07:15 > 0:07:19The way to find the answer to that is to look in the rocks.
0:07:19 > 0:07:25The earliest fossils of shells and corals appear about 600 million years ago.
0:07:25 > 0:07:31And then, for 200 million years, there's no sign whatever of any backboned animals.
0:07:31 > 0:07:33But then, suddenly, they appear.
0:07:33 > 0:07:37Some of the finest specimens have been found in these ancient rocks
0:07:37 > 0:07:40at the mouth of the Severn, in the west of England.
0:07:42 > 0:07:44And these creatures have no jaws either.
0:07:44 > 0:07:49They have scales down their flanks and a head covered by a heavy bony shield.
0:07:49 > 0:07:52They must have swum by wriggling this body
0:07:52 > 0:07:55and pushing their head along the bottom.
0:07:55 > 0:07:58And at the front, between two small eyes, there is a nostril.
0:07:58 > 0:08:02In fact, it's a kind of lamprey in armour.
0:08:05 > 0:08:09At the time of which we are talking, about 400 million years ago,
0:08:09 > 0:08:13the face of the Earth was not at all like what it is today.
0:08:13 > 0:08:17The relationships of the continents, the ocean basins, the coastlines, all were very different.
0:08:17 > 0:08:23Only in a few places can you today get a clear picture of what those ancient shores were like.
0:08:23 > 0:08:26And here, in Western Australia, in the Kimberly Ranges,
0:08:26 > 0:08:27there's one of them.
0:08:27 > 0:08:31And the best place to see it is from the air.
0:08:41 > 0:08:44Rising above the parched and sandy scrub,
0:08:44 > 0:08:48there are strangely shaped outcrops of rock.
0:08:52 > 0:08:56Those bluffs owe their curious shape not to the erosion of wind and rain
0:08:56 > 0:09:01but to the labours, millions of years ago, of coral polyps.
0:09:08 > 0:09:10We are flying over an ancient seabed,
0:09:10 > 0:09:15with the original coast and the land behind it, now a rocky plateau,
0:09:15 > 0:09:17stretching away in the distance.
0:09:17 > 0:09:20Once, this plain was covered by a shallow blue lagoon
0:09:20 > 0:09:24in which corals built their great constructions of limestone.
0:09:24 > 0:09:28Over the millennia, rivers eroded the continent nearby,
0:09:28 > 0:09:29washed down the sand and mud
0:09:29 > 0:09:32and deposited it over the sea floor.
0:09:32 > 0:09:37So the lagoons slowly silted up and the sea retreated.
0:09:37 > 0:09:41Then the continent rose, rain and sun eroded the mudstones
0:09:41 > 0:09:47and eventually the coral reefs were exposed once more as cliffs on a sun-baked plain.
0:09:56 > 0:10:00And here I am walking on the ancient seabed.
0:10:00 > 0:10:04The surface of the sea would have been near the top of those reefs.
0:10:04 > 0:10:08So here I would have been about 200 feet down.
0:10:08 > 0:10:13And the sediments that lay on the bottom of that ancient sea
0:10:13 > 0:10:18are still here, turned into sandstones and mudstones.
0:10:18 > 0:10:21And in them are the remains of the creatures
0:10:21 > 0:10:23that lived in those seas.
0:10:24 > 0:10:27Here is one that I picked up only a few minutes ago.
0:10:27 > 0:10:31It's the scale of a huge fish.
0:10:31 > 0:10:38And this is the flank of a smaller fish with many scales on it.
0:10:38 > 0:10:40And this, which is perhaps the least impressive of all,
0:10:40 > 0:10:45is actually the most interesting, because this is a fossil skull.
0:10:45 > 0:10:49There is the line of its lower jaw.
0:10:49 > 0:10:54And if this nodule is treated with acids, the matrix will be eroded away
0:10:54 > 0:10:58and expose the perfectly preserved bones of the skull.
0:11:02 > 0:11:04These creatures, 400 million years old,
0:11:04 > 0:11:09were a considerable advance on the lancelets and lampreys
0:11:09 > 0:11:11because they had true jaws.
0:11:11 > 0:11:14And on their edges, the scales grew particularly long and sharp
0:11:14 > 0:11:18so that the fish could bite and cut.
0:11:20 > 0:11:23Jaws armed with teeth
0:11:23 > 0:11:26enabled the fish to become very effective food-gatherers,
0:11:26 > 0:11:29and so grow into large and powerful creatures.
0:11:29 > 0:11:31And some of them became monsters.
0:11:33 > 0:11:36Judging from the size of these gigantic teeth,
0:11:36 > 0:11:40the shark was about 45 feet long.
0:11:40 > 0:11:45It's extinct, but its relatives are very much alive.
0:12:09 > 0:12:12Sensitive pits in the front of the head, nostrils,
0:12:12 > 0:12:15enable them to detect their prey from great distances.
0:12:15 > 0:12:19The hammerhead shark is said to be particularly sensitive.
0:12:19 > 0:12:23And this may explain the grotesque shape of its head.
0:12:23 > 0:12:26There's a nostril at the end of each side of the hammer.
0:12:26 > 0:12:30And the fish habitually swings its head from side to side.
0:12:30 > 0:12:33So when the scent is equally strong in both nostrils,
0:12:33 > 0:12:37then it must know that its prey lies straight ahead.
0:12:38 > 0:12:42That rod in the back has now been strengthened with cartilage.
0:12:42 > 0:12:47And the entire skeleton of sharks is built from this soft, light material.
0:12:49 > 0:12:52They still swim like the lancelets, with sideways beats of their body
0:12:52 > 0:12:57which are restricted mostly to the back half and to the tail.
0:12:57 > 0:13:00The thrust created tends to drive the nose downwards,
0:13:00 > 0:13:05and to compensate for that, sharks have a pair of horizontal fins on either side at the front,
0:13:05 > 0:13:07like the vanes of a submarine.
0:13:07 > 0:13:11But these fins are stiff and inflexible.
0:13:11 > 0:13:14The shark can't twist them vertically to act as brakes.
0:13:14 > 0:13:18Indeed, a charging shark can't stop, only swerve to one side.
0:13:18 > 0:13:20Nor can it swim backwards.
0:13:20 > 0:13:27Furthermore, since its body is heavier than water, if it stopped swimming, a shark would sink.
0:13:32 > 0:13:35The wobbegong, a shark from Australian waters,
0:13:35 > 0:13:38has a tendency to do just that.
0:13:38 > 0:13:42It's largely abandoned the effort of perpetually swimming
0:13:42 > 0:13:43to keep in mid-water,
0:13:43 > 0:13:48and has settled on the sea floor where it leads a more restful life.
0:13:56 > 0:14:00The transition from continuous swimming in the open sea
0:14:00 > 0:14:03to a life more or less permanently on the bottom
0:14:03 > 0:14:05can be seen in a series of fishes.
0:14:08 > 0:14:10The dogfish is very shark-like.
0:14:12 > 0:14:15The angel shark is rather more flattened,
0:14:15 > 0:14:18with wide side fins and a rather smaller tail.
0:14:24 > 0:14:27The ray has flattened its body to an extreme degree,
0:14:27 > 0:14:31dispensing with that rear engine, the powerful thrashing tail,
0:14:31 > 0:14:33and expanding the lateral fins
0:14:33 > 0:14:38so their ripples can take over the job of propelling the fish through the water.
0:14:38 > 0:14:42And it spends most of its time lying on the bottom.
0:14:56 > 0:15:01A light dusting of gravel does wonders for camouflage.
0:15:06 > 0:15:09The sawfish shark is another bottom-liver.
0:15:09 > 0:15:13It uses its extraordinary blade like a double-edged scythe,
0:15:13 > 0:15:17excavating in the sand and gravel for shells and crabs
0:15:17 > 0:15:19and sometimes flailing through a shoal of fish,
0:15:19 > 0:15:23slashing them so that they fall injured and can be eaten.
0:15:25 > 0:15:28So, bodies with cartilaginous skeletons
0:15:28 > 0:15:30developed into two main shapes.
0:15:30 > 0:15:32Long ones, like sharks,
0:15:32 > 0:15:36and wide ones, like rays and skates.
0:15:51 > 0:15:55But having learned, as it were, to live on the bottom,
0:15:55 > 0:15:57some rays took off again.
0:15:57 > 0:16:01Undulating side fins are effective motors for mid-water swimming,
0:16:01 > 0:16:04provided that speed is not needed.
0:16:04 > 0:16:07So they are suitable for fish like the manta ray
0:16:07 > 0:16:12that drifts through these surface waters, filter-feeding on plankton.
0:16:12 > 0:16:16The blades on either side of the manta's head help to channel the food-bearing water
0:16:16 > 0:16:19into the slot-like mouth.
0:16:19 > 0:16:22The manta cannot swim much faster than this,
0:16:22 > 0:16:25but it wouldn't help its feeding even if it did.
0:16:25 > 0:16:28For the water can't flow through the sieve in the gill slits
0:16:28 > 0:16:31any faster than it's doing now.
0:16:40 > 0:16:45Filter-feeding in the surface of the ocean is clearly a very effective way of life.
0:16:45 > 0:16:49It doesn't require much energy, there's an unlimited supply of food,
0:16:49 > 0:16:53and some of the fish that have taken to it have become very large indeed.
0:16:53 > 0:16:59The basking shark grows to a length of 15 metres - 45 feet.
0:16:59 > 0:17:03Only one fish today is any bigger - the whale shark.
0:17:03 > 0:17:05And that too is a filter-feeder.
0:17:12 > 0:17:16And there, clinging under its tail, is a primitive jawless lamprey,
0:17:16 > 0:17:21sucking at its flesh, a reminder of the fish's remote past.
0:17:21 > 0:17:24A close relative of the earliest swimmers.
0:17:28 > 0:17:31Another filter-feeder - the paddlefish.
0:17:31 > 0:17:35But this is only very distantly related to the sharks and rays.
0:17:36 > 0:17:39400 million years ago, right at the beginning of fish history,
0:17:39 > 0:17:45a group started constructing their skeletons not of cartilage but of solid bone.
0:17:45 > 0:17:50And the ancestors of the paddlefish were among them.
0:17:50 > 0:17:53And another of these primitive bony fish, the sturgeon.
0:17:53 > 0:17:56Not only does it have bone in its internal skeleton,
0:17:56 > 0:18:00it also has heavy bony scales in its skin.
0:18:17 > 0:18:21It's the eggs of this fish that are made into caviar.
0:18:28 > 0:18:30It still swims very like a shark,
0:18:30 > 0:18:33with sweeps of its hind body and tail.
0:18:33 > 0:18:37And the tail looks shark-like too.
0:18:41 > 0:18:43Soon after the bony fish first appeared,
0:18:43 > 0:18:49they spread from the seas up the rivers to colonise the fresh waters of the world.
0:18:49 > 0:18:54It was an invasion that was to have revolutionary consequences.
0:18:59 > 0:19:03The waters of rivers and lakes are shallow compared to the sea,
0:19:03 > 0:19:06and often, as a consequence, they get quite warm.
0:19:06 > 0:19:08And the warmer water becomes,
0:19:08 > 0:19:11the less oxygen it can hold dissolved in it.
0:19:11 > 0:19:15That presents a serious problem to any fish living there.
0:19:15 > 0:19:17How are they to breathe?
0:19:17 > 0:19:22This is one of them, the polypterus. And this is its solution.
0:19:23 > 0:19:29It gulps air and then absorbs the gaseous oxygen from a pouch that leads off its gut.
0:19:29 > 0:19:32In other words, it has developed a very simple lung.
0:19:33 > 0:19:38But an air-filled pouch within the body brings another incidental advantage.
0:19:38 > 0:19:42It gives buoyancy. So the bony fish acquired a swim bladder.
0:19:42 > 0:19:46A controllable bag of air inside the body.
0:19:50 > 0:19:53Now the elements of the modern fish have been assembled.
0:19:53 > 0:19:55A swim bladder for buoyancy,
0:19:55 > 0:19:58a backbone with muscles attached for strength,
0:19:58 > 0:20:01and gills for breathing.
0:20:01 > 0:20:04And for further precision and control, there is the lateral line,
0:20:04 > 0:20:08a row of tiny pits that are sensitive to pressures and currents in the water.
0:20:08 > 0:20:12And so the modern bony fish, like this trout,
0:20:12 > 0:20:14is very finely tuned to its world.
0:20:23 > 0:20:27This perfection of senses and control of movement
0:20:27 > 0:20:30is critical when a pike is on the hunt for roach.
0:20:43 > 0:20:46With buoyancy provided by the swim bladder,
0:20:46 > 0:20:52the fins can be used entirely for fine adjustments of its position as it hovers.
0:21:47 > 0:21:52At normal speed, it's almost impossible to see what happens, it's so fast.
0:22:00 > 0:22:02Slowed down, it's possible to see
0:22:02 > 0:22:04the enormous acceleration and accuracy.
0:22:04 > 0:22:07The actual bite only lasts a split second.
0:22:07 > 0:22:09And the prey goes straight in.
0:22:29 > 0:22:34I'm standing on the brink of one of the most densely populated parts of the sea.
0:22:34 > 0:22:38I'm on the edge of a coral reef at a low tide.
0:22:38 > 0:22:41A few feet out there, the bottom sinks dramatically,
0:22:41 > 0:22:45and there you will find an abundance of life of all kinds.
0:22:45 > 0:22:51Microscopic plants, invertebrates, corals, and, of course, a multitude of fish
0:22:51 > 0:22:55that come there to harvest this rich source of food.
0:22:55 > 0:23:00Each kind of fish has its own particular place in this mosaic, its own particular food,
0:23:00 > 0:23:04and each has, in consequence, developed its own way of swimming,
0:23:04 > 0:23:07its own way of using its fins.
0:23:19 > 0:23:25The huge number of fish that swarm on the reef, harvesting the great variety of food it offers,
0:23:25 > 0:23:28causes considerable social problems.
0:23:28 > 0:23:35Each species has its own particular niche on the reef and is designed accordingly.
0:23:35 > 0:23:39Many are slim for slipping through the tangle of coral.
0:23:39 > 0:23:43Others, like the cowfish, have a rigid box of bony plates
0:23:43 > 0:23:47and can stop dead with precise control from its fins.
0:23:48 > 0:23:53The trigger sticks its fin-free front half between coral branches to feed.
0:23:53 > 0:23:57The angelfish picks off small morsels from the surface of corals,
0:23:57 > 0:24:00once again with perfect control.
0:24:05 > 0:24:08And this butterfly fish has elongated jaws
0:24:08 > 0:24:10that enable it to probe into narrow crevices
0:24:10 > 0:24:14with the accuracy of forceps.
0:24:15 > 0:24:19For turning sharply, banking steeply, or simply flapping along,
0:24:19 > 0:24:23most coral fishes have been able to abandon the S-shaped wriggle.
0:24:23 > 0:24:28They've deployed their fins and adjusted their bodies to live in this particular world.
0:24:39 > 0:24:45No shark could do this, but then they are adapted to a different kind of life.
0:24:51 > 0:24:57The puffer fish doesn't wriggle its body but it does flex its fins, and to great effect.
0:24:57 > 0:25:00The S-shape action is now being used there.
0:25:06 > 0:25:09And fins have another important role, as flags.
0:25:09 > 0:25:15In such a mixed and dense crowd, it's very much to the advantage of every individual fish
0:25:15 > 0:25:19to proclaim its presence and identity from among the throng.
0:25:19 > 0:25:24So rivals will be aware that this particular food patch has got an owner.
0:25:24 > 0:25:27The same markings will also serve, when the time comes,
0:25:27 > 0:25:29to attract a mate of the right species.
0:25:33 > 0:25:39The sharks and rays have eyes that, though they see shapes, are largely blind to colour.
0:25:39 > 0:25:43It's hardly surprising that they are largely drab-coloured creatures.
0:25:43 > 0:25:47But the bony fish have excellent colour vision.
0:25:47 > 0:25:51And so they are able to signal to one another with stripes and spots and blotches,
0:25:51 > 0:25:54and in the most wonderful variety of colours.
0:25:55 > 0:25:58The coral fish can risk making themselves conspicuous
0:25:58 > 0:26:01because the reef is full of crevices and corners
0:26:01 > 0:26:04where they can dart to safety if danger threatens.
0:26:11 > 0:26:15Away from the reef, however, the sea is a dangerous place.
0:26:15 > 0:26:19For there, there is nowhere to hide, except among your fellows.
0:26:25 > 0:26:28And these are designed for a very different way of life.
0:26:28 > 0:26:34Fast swimming, fast feeding in the open sea with plankton at the base of the food chain.
0:26:34 > 0:26:38And it's that wriggling body action that pushes them along.
0:26:51 > 0:26:54Open-water fish often form huge shoals.
0:26:54 > 0:26:57And this may be for safety's sake.
0:26:57 > 0:27:02The drifting, darting multitudes of fish may tend to baffle and confuse predators.
0:27:02 > 0:27:06And if you meet a shark on your own, it'll go for you.
0:27:06 > 0:27:10But if you are with others, your chances are much better.
0:27:20 > 0:27:27From the plankton to the small fish and on up the food chain to the big fish.
0:27:27 > 0:27:30In the open ocean, speed is of great value.
0:27:30 > 0:27:33And since water is very dense, 800 times more so than air,
0:27:33 > 0:27:37streamlining is of the greatest importance to fish.
0:27:37 > 0:27:42Both hunters and the fish they pursue have developed very similar shapes.
0:27:42 > 0:27:46Pointed in front and tapering to a two-bladed symmetrical tail at the back.
0:27:46 > 0:27:51Barracuda. Among the most voracious and swift of the bony fish.
0:27:57 > 0:27:59And this is a hunter's-eye view
0:27:59 > 0:28:01of a fish that escapes not by swimming fast,
0:28:01 > 0:28:04but in a quite different way.
0:28:06 > 0:28:12It's a flying fish. Its front pair of fins are greatly enlarged so that with a flick of its tail,
0:28:12 > 0:28:16it launches itself into the air and out of the hunter's sight.
0:28:23 > 0:28:25This is the flight in slow motion.
0:28:25 > 0:28:29The fish is already swimming fast when it comes to the surface,
0:28:29 > 0:28:34and it takes off, helped by the beating tail which has a specially enlarged lower lobe.
0:28:34 > 0:28:38The front fins are then spread to assist the glide.
0:28:40 > 0:28:45Occasionally they dip their tails into the surface to give themselves an additional boost.
0:28:45 > 0:28:50And so they can sometimes fly for several hundred metres.
0:28:53 > 0:28:57Some fish have sought safety by going not upwards but downwards.
0:28:57 > 0:29:02These eggs, that float in astronomic numbers on the surface of the sea during the summer,
0:29:02 > 0:29:05have come from one of the bottom dwellers.
0:29:05 > 0:29:08Only one in 100,000 will survive.
0:29:08 > 0:29:12But those that do will pass through a most extraordinary transformation
0:29:12 > 0:29:14before they become adult.
0:29:18 > 0:29:24After about a week, they hatch into what looks like a fairly normal kind of fish fry.
0:29:24 > 0:29:28They hang near the surface where it's warm and there's a lot of oxygen,
0:29:28 > 0:29:30feeding on micro-organisms.
0:29:30 > 0:29:33Each is not much bigger than a pinhead.
0:29:46 > 0:29:52Each one still contains a tiny bag of yolk that will sustain it for a day or two more.
0:29:54 > 0:29:57The young fish has eyes on either side of its head,
0:29:57 > 0:29:59but they won't remain that way for long.
0:29:59 > 0:30:03Its body deepens as it begins to feed, and its stomach swells.
0:30:03 > 0:30:06And it develops a swim bladder.
0:30:11 > 0:30:14Its eyes are beginning to look a little lopsided.
0:30:14 > 0:30:17One is higher than the other.
0:30:20 > 0:30:24Now they've developed pigment, but only on one flank,
0:30:24 > 0:30:27and they swim on their sides with that coloured flank upwards.
0:30:27 > 0:30:30These are going to be flatfish.
0:30:30 > 0:30:35Turbot, plaice, sole and flounder also go through such a transformation.
0:30:39 > 0:30:41And they finally settle on the bottom.
0:30:46 > 0:30:50One eye is now on the edge of the fish.
0:30:53 > 0:30:55Now the transformation is complete
0:30:55 > 0:30:58and the fish has lost that swim bladder,
0:30:58 > 0:31:02for buoyancy is a positive hindrance on the sea bottom.
0:31:02 > 0:31:06A bony fish has joined the skates and the rays on the sea floor
0:31:06 > 0:31:10by the simple if drastic expedient of lying on its side
0:31:10 > 0:31:13and moving one eye right round its body.
0:31:16 > 0:31:20Many other bony fish have abandoned the swim bladder and settled down.
0:31:20 > 0:31:25Each has found its own way of adapting to life where skill in swimming is less important
0:31:25 > 0:31:29than an ability to merge into the background of the sea floor.
0:31:29 > 0:31:33So fins can be used for all kinds of other purposes.
0:31:33 > 0:31:37This looks like a rock lying on the bottom,
0:31:37 > 0:31:41but it has a gill, an eye and an upturned mouth.
0:31:41 > 0:31:47It's a stonefish, a hunter that relies on invisibility to catch its prey unawares.
0:31:47 > 0:31:52And its fins are coloured and shaped to help its camouflage.
0:31:54 > 0:31:59The angler fish uses its fins not for swimming but for walking.
0:31:59 > 0:32:03And the front spine of its dorsal fin is a fishing rod.
0:32:06 > 0:32:11With that lure, it attracts unsuspecting creatures within range of its mouth.
0:32:18 > 0:32:21The bearded ghoul uses its fins for defence.
0:32:21 > 0:32:23Without a membrane between them,
0:32:23 > 0:32:26they are no longer of any use as stabilisers when swimming,
0:32:26 > 0:32:30and instead they are sharp and tipped with poison.
0:32:30 > 0:32:31Very effective protection.
0:32:31 > 0:32:35It's just what you need if you are lying on the bottom.
0:32:41 > 0:32:45The gurnard uses some of the rays of its front pair of fins
0:32:45 > 0:32:48as delicate legs for finding food in the gravel.
0:33:06 > 0:33:10Fish like those live in comparatively shallow waters,
0:33:10 > 0:33:13100 feet, 30 metres, something like that.
0:33:13 > 0:33:18Their world is a heavily inhabited one and also quite a bright one
0:33:18 > 0:33:21because the water is shallow enough to receive light from the sun.
0:33:21 > 0:33:25It's also one that's comparatively familiar to us.
0:33:25 > 0:33:28For one thing, all the sea fish that we eat come from it.
0:33:28 > 0:33:34For another, hundreds of thousands of people regularly visit it wearing aqualungs.
0:33:34 > 0:33:40But in fact it is only a tiny proportion of the seas of the world.
0:33:40 > 0:33:44Most of the oceans are very, very much deeper than that.
0:33:44 > 0:33:48And to visit those deep waters, you can't go down in an aqualung,
0:33:48 > 0:33:52you have to use something like this - a submersible.
0:34:07 > 0:34:11These craft work on the sea floor, helping in the drilling for oil.
0:34:11 > 0:34:17They give a splendid view of what's happening at depth, both to oil engineers and to fish watchers.
0:34:34 > 0:34:38There's a highly sensitive television camera on the outside of the hull,
0:34:38 > 0:34:41with a monitor screen in the cockpit
0:34:41 > 0:34:43and spotlights to illuminate places
0:34:43 > 0:34:46that the sun's rays have never reached.
0:34:48 > 0:34:52As we go down, it gets darker and darker.
0:34:52 > 0:34:55And the pressure increases too, very quickly.
0:34:55 > 0:35:03By the time we are 500 feet, the loading on this viewing dome here will be about 70 tons.
0:35:03 > 0:35:07And it also gets colder and colder.
0:35:07 > 0:35:12At one point, and the precise depth varies according to where we are in the world,
0:35:12 > 0:35:15between, say, 20 metres, which is about 60 feet,
0:35:15 > 0:35:18and 150 metres, 450 feet,
0:35:18 > 0:35:25it suddenly gets very much colder indeed and drops to about five degrees above freezing.
0:35:25 > 0:35:28That point is called the thermocline,
0:35:28 > 0:35:32and it's a kind of frontier in the ocean,
0:35:32 > 0:35:36separating two very different worlds between which there is very little traffic.
0:35:36 > 0:35:44Above, there is the sunlit, warm waters near the surface which have their own circulation,
0:35:44 > 0:35:50and below the thermocline, there's the black, near-freezing world of the ocean depths.
0:35:50 > 0:35:54And there, there live very different fish indeed.
0:35:59 > 0:36:03This is part of the world that man is only just beginning to explore.
0:36:03 > 0:36:07Until a few years ago, most of our knowledge of these creatures
0:36:07 > 0:36:10came from specimens hauled up in dredges.
0:36:10 > 0:36:14But, as they came up, changes in pressure and temperature
0:36:14 > 0:36:17usually distorted their bodies and they quickly died.
0:36:17 > 0:36:23Only now, from such craft as the submersibles, are we beginning to get an accurate idea
0:36:23 > 0:36:27of what life is really like in the deeper parts of the oceans.
0:36:29 > 0:36:33A shark, built on the same pattern as its relatives above.
0:36:33 > 0:36:36Like most of the inhabitants of these oxygen-poor waters,
0:36:36 > 0:36:39it moves comparatively slowly.
0:36:39 > 0:36:44Probably never meeting a boundary or a barrier in the endless deep sea.
0:36:47 > 0:36:51A red prawn, doubtless food for some big fish.
0:36:59 > 0:37:03An extraordinary relative of the prawn, another crustacean, called an ostracod.
0:37:03 > 0:37:09Fossils of species very like these have been found in extremely ancient rocks,
0:37:09 > 0:37:14so we know that they were here long before the fish arrived.
0:37:14 > 0:37:20This fish still uses that antique way of swimming with S-shaped undulations of its body.
0:37:31 > 0:37:35A fangtooth, one of the hunters of this lightless world.
0:37:36 > 0:37:39Our knowledge of the fish at these depths is still very fragmentary.
0:37:39 > 0:37:42Many species have never been filmed,
0:37:42 > 0:37:47and we know them only from a few mangled specimens and still photographs.
0:37:48 > 0:37:53This bait on a line is suspended from a rod dangling in front of an upper jaw
0:37:53 > 0:37:56lined with needle teeth.
0:37:56 > 0:37:58It's another kind of angler fish.
0:38:00 > 0:38:03There are so few animals at these depths that when a meal arrives,
0:38:03 > 0:38:06a hunter must make quite sure of catching it.
0:38:13 > 0:38:15This angler normally looks like this,
0:38:15 > 0:38:20but when it's had its meal, its stomach becomes hugely distended.
0:38:21 > 0:38:24The gulper is little more than a swimming mouth,
0:38:24 > 0:38:27also with a stomach capable of great extension.
0:38:27 > 0:38:31The bigger the stomach can go, the wider the choice of prey,
0:38:31 > 0:38:34and meals may be few and far between.
0:38:36 > 0:38:41Many fish produce lights in this blackness, some as face patches or dots along the sides.
0:38:41 > 0:38:48At night, many of them move up to shallower water where, of course, it's still dark.
0:38:57 > 0:39:01With special light-sensitive cameras and a little illumination,
0:39:01 > 0:39:04you can just make out the fish that are producing these lights.
0:39:04 > 0:39:07They are called flashlight fish.
0:39:24 > 0:39:28With no illumination at all, they become disembodied green spots again,
0:39:28 > 0:39:32mysteriously circulating in the blackness.
0:39:34 > 0:39:39The light is produced by bacteria which live in this one patch of skin
0:39:39 > 0:39:43and glow as a normal by-product of the chemistry of their life processes.
0:39:46 > 0:39:49The light may serve the flashlight fish in several ways.
0:39:49 > 0:39:53It may be a sign to other members of the shoal. It may baffle predators.
0:39:53 > 0:39:58After a fish switches off its light, it immediately darts away to a different position.
0:39:58 > 0:40:03Or it may simply be a method of finding your way around in the blackness.
0:40:13 > 0:40:17The problem of finding the way in the dark faces other fish too.
0:40:17 > 0:40:22They live in turbid waters and under a thick carpet of floating vegetation.
0:40:22 > 0:40:25Not an easy place to find them.
0:40:33 > 0:40:39This somewhat surprising piece of apparatus...
0:40:39 > 0:40:48is the latest device developed and designed by research workers interested in electric fishes.
0:40:48 > 0:40:52This plastic tube has got two leads which come up through this cable
0:40:52 > 0:40:56along here to this extraordinary hat.
0:40:56 > 0:40:59There, they go into this amplifier,
0:40:59 > 0:41:03and on the brim of the hat, there's a loudspeaker here
0:41:03 > 0:41:07and, very thoughtfully, a counterweight here,
0:41:07 > 0:41:13so that when I put the hat on my head, it doesn't flop over one ear.
0:41:13 > 0:41:18And then, if I turn on the amplifier, the speaker is next to my ear,
0:41:18 > 0:41:26I have one hand free and the wires will pick up the signals of those electric fishes
0:41:26 > 0:41:30and I can hear them as a series of clicks.
0:41:30 > 0:41:32LOW HUMMING
0:41:42 > 0:41:44The fish produce their electricity
0:41:44 > 0:41:48from stacks of plate-shaped shells embedded in jelly
0:41:48 > 0:41:51that lie in a column along each flank, like batteries.
0:41:51 > 0:41:55Each fish sends out a particular kind of discharge.
0:41:55 > 0:41:58And each makes a different sound on the loudspeakers.
0:41:58 > 0:42:01So these may be another kind of call sign,
0:42:01 > 0:42:03like the Morse code of the flashlight fish,
0:42:03 > 0:42:07a way of proclaiming identity.
0:42:07 > 0:42:10They also certainly help the fish in navigation.
0:42:11 > 0:42:15Each creates in the water around it a weak electric field.
0:42:15 > 0:42:19Any other solid object in the water causes a change in that field,
0:42:19 > 0:42:26and the fish have special pores, spaced out over their bodies, which detect such alterations.
0:42:26 > 0:42:30As a result, they are able to find their away about in total darkness.
0:42:30 > 0:42:34And they can swim just as accurately backwards as they can forwards.
0:42:36 > 0:42:38HIGH-PITCHED HUMMING
0:42:44 > 0:42:46PULSING
0:42:46 > 0:42:49Electricity has evolved independently in many fishes.
0:42:49 > 0:42:54From South America and Africa, in rivers, lakes and also in the sea.
0:42:54 > 0:42:57In some, the tail muscles are used.
0:42:57 > 0:43:01Others, the head area and even the eye muscles.
0:43:08 > 0:43:12A straight, knife-shaped body is characteristic of all these fish,
0:43:12 > 0:43:16and it may be important to keep the body stiff in this position
0:43:16 > 0:43:21in order to maintain an accurate output of navigational signals.
0:43:21 > 0:43:24And the fins do the manoeuvring.
0:43:24 > 0:43:27Some frequently rest, wedged between plant stems.
0:43:34 > 0:43:35HARSHER AMPLIFIED TONE
0:44:03 > 0:44:06Most of the discharges produced by those electric fish
0:44:06 > 0:44:07are extremely weak.
0:44:07 > 0:44:11You couldn't possibly detect them without special amplifying equipment.
0:44:11 > 0:44:13But that is very much not the case with all of them.
0:44:14 > 0:44:18This is the most powerful electric fish of all,
0:44:18 > 0:44:22the famous electric eel from South America.
0:44:22 > 0:44:24This has two kinds of electricity.
0:44:24 > 0:44:29Not only does it have batteries which produce the discharges used for navigation,
0:44:29 > 0:44:33but it's also capable of delivering a massive electric shock,
0:44:33 > 0:44:39which it stuns its prey with and which is quite sufficient to throw me on my back,
0:44:39 > 0:44:43if I were not wearing rubber gloves.
0:44:43 > 0:44:47And I can demonstrate that electric shock
0:44:47 > 0:44:49by tapping him near his head and tail
0:44:49 > 0:44:53with these electrodes, which will then, if he gives a shock,
0:44:53 > 0:44:55light up these bulbs.
0:44:55 > 0:44:59And the more powerful the shock, the more bulbs he will light.
0:45:02 > 0:45:04There's a rapid output of volts,
0:45:04 > 0:45:08peaking in this case at about 400. Four bulbs were lit.
0:45:08 > 0:45:14Of all backboned animals, fish are the only ones to produce electricity in their bodies.
0:45:18 > 0:45:23So the bony fish, one way or another, have managed to colonise all the waters of the world,
0:45:23 > 0:45:29from the black depths of the sea to inland rivers and lakes, even lakes like this.
0:45:29 > 0:45:32Lake Magadi in the Rift Valley of East Africa
0:45:32 > 0:45:39is, I think, just about the most hostile environment that I know for land animals, let alone fish.
0:45:52 > 0:45:56It's a lake not of water but of solid soda and potash,
0:45:56 > 0:45:58solidified by the baking African sun
0:45:58 > 0:46:02from solutions bubbling up from volcanic rocks far below.
0:46:04 > 0:46:11And here, at last, is somewhere you might think you'll get a place to cool your feet.
0:46:11 > 0:46:17You might get a nice refreshing drink of clear, cool water.
0:46:17 > 0:46:19And yet in fact...
0:46:19 > 0:46:27this water is so...hot that actually it's really quite difficult to bear.
0:46:27 > 0:46:32And when you taste it, the water is sickeningly salty.
0:46:32 > 0:46:35This is actually one of the hot volcanic springs
0:46:35 > 0:46:40where water bubbles up from deep below the surface of the ground,
0:46:40 > 0:46:47bringing up a solution of soda and salt to trickle down and crystallise out in the lakes.
0:46:47 > 0:46:53You could hardly imagine a less likely place to find a fish.
0:46:59 > 0:47:03And yet there it is. A species of tilapia.
0:47:03 > 0:47:07The water here can be as high as 43 degrees centigrade,
0:47:07 > 0:47:09110 degrees Fahrenheit.
0:47:09 > 0:47:14Algae grow here and the fish survive by feeding on it.
0:47:14 > 0:47:17Yet another niche, a most unlikely one,
0:47:17 > 0:47:21has been filled by the incredibly adaptable fishes.
0:47:22 > 0:47:27At the other extreme, there's one fish in the coldest waters on Earth.
0:47:34 > 0:47:38Sea water freezes below the temperature of fresh water.
0:47:38 > 0:47:43The ice fish from the seas of the Antarctic has developed a substance in its blood
0:47:43 > 0:47:47which keeps it liquid even when the sea water above it freezes solid.
0:47:47 > 0:47:50It has, in fact, a kind of antifreeze.
0:47:54 > 0:47:59But if one wanted to pick out of the 30,000 or so species of fish alive today
0:47:59 > 0:48:03the king of them all, my vote would go to this, the salmon.
0:48:09 > 0:48:13In the acuteness of its senses, the skilfulness of its navigation,
0:48:13 > 0:48:20the strength and athleticism of its body, this surely must be a paragon among fish.
0:48:20 > 0:48:22In the Pacific, there are several different kinds.
0:48:22 > 0:48:27They spend much of their time in the ocean, feeding on plankton and small fish.
0:48:27 > 0:48:31But in the summer, they assemble off the North American coast
0:48:31 > 0:48:34and then they begin to battle their way up the rivers.
0:48:35 > 0:48:38Even falls don't stop them.
0:48:38 > 0:48:42The flexible rod that first appeared in the young sea squirt
0:48:42 > 0:48:46is here marvellously muscled and strengthened with a jointed column of bone.
0:48:46 > 0:48:50So with a thrash of its hind end and fins,
0:48:50 > 0:48:54the movement first developed by the earliest ancestors of the fish,
0:48:54 > 0:48:57the salmon can swim and leap up the fastest torrents.
0:48:57 > 0:49:02And its lateral line can sense the details of the surge.
0:49:16 > 0:49:20The salmon's sense of smell is almost unbelievably acute.
0:49:20 > 0:49:22This river is not just any river.
0:49:22 > 0:49:25It is the precise one in which all these fish were hatched.
0:49:25 > 0:49:30Each has retained a memory of the taste and smell of these waters.
0:49:30 > 0:49:34And this has drawn them back across thousands of miles of ocean
0:49:34 > 0:49:38so that they may complete their lives where they began them.
0:49:48 > 0:49:49During the past few days,
0:49:49 > 0:49:53their bodies have been changing with astonishing speed.
0:49:53 > 0:49:59These pink salmon have developed a high humped back with thin bodies,
0:49:59 > 0:50:02and their lower jaws have become hooked.
0:50:02 > 0:50:06The front teeth of the males have developed
0:50:06 > 0:50:09into long and powerful fangs.
0:50:09 > 0:50:11Hopeless for feeding,
0:50:11 > 0:50:15but then the time of feeding is long since over.
0:50:15 > 0:50:18Their teeth are for battle.
0:50:46 > 0:50:51The males fight for a scrape scooped in the gravel of the river bed.
0:50:51 > 0:50:56When one takes possession, a female will join him and lie alongside.
0:50:56 > 0:51:01Then, as she sheds her eggs into the gravel, his milt will fertilise them.
0:51:37 > 0:51:40And now they're totally spent.
0:51:40 > 0:51:45They don't even have enough energy to heal their battered, wounded bodies.
0:51:45 > 0:51:52Their scales fall off and the once-powerful muscles, the flesh, dwindles and shrivels.
0:51:52 > 0:51:55And they die. All of them.
0:51:55 > 0:51:58Not a single one of the millions of fish
0:51:58 > 0:52:01which fought their way up this river
0:52:01 > 0:52:03ever goes back to the sea.
0:52:03 > 0:52:08But their eggs remain, and will stay here throughout the winter, safe in the gravel,
0:52:08 > 0:52:15until, in the spring, they'll hatch and the fry will be swept down the river to the ocean.
0:52:15 > 0:52:20There they will feed and grow until, at the appointed time, two years hence exactly,
0:52:20 > 0:52:25as far as these pink salmon are concerned, the fully-grown fish will once again
0:52:25 > 0:52:29beat its way powerfully upriver to the place where it was born.
0:52:48 > 0:52:52The salmon is the master both of salt water and fresh,
0:52:52 > 0:52:56but one part of the world is closed even to it - the land.
0:52:56 > 0:53:01And yet a few fish can survive even there for a short time.
0:53:01 > 0:53:04The walking catfish travels over land
0:53:04 > 0:53:07using bony fins and a sideways wriggle.
0:53:07 > 0:53:09But it's not the first fish to do that.
0:53:09 > 0:53:15One managed it some 350 million years ago, and that was a momentous move.
0:53:15 > 0:53:21Because from that fish developed frogs and lizards, birds and mammals
0:53:21 > 0:53:23and, ultimately, ourselves.
0:53:51 > 0:53:55Subtitles by Red Bee Media Ltd