0:00:52 > 0:00:56The waters that cover most of the planet are in constant movement.
0:00:56 > 0:00:59As the moon circles around the spinning Earth,
0:00:59 > 0:01:04so the pull of its gravity causes the oceans to rise and fall,
0:01:04 > 0:01:05and twice every day,
0:01:05 > 0:01:09the sea surges up and down the coasts of the continents.
0:01:18 > 0:01:20In the bay of Fundy in North America,
0:01:20 > 0:01:24the particular shape of the coast and the slope of the seabed,
0:01:24 > 0:01:28produces the highest tides of all, rising 50 feet.
0:01:39 > 0:01:43Living in this in-between World, which is neither sea nor land,
0:01:43 > 0:01:46demands very special talents.
0:01:54 > 0:01:56This is a battle ground.
0:02:09 > 0:02:12In many places, the sea is forcing the land to retreat,
0:02:12 > 0:02:15cutting back its cliffs and leaving islands and towers
0:02:15 > 0:02:19as markers of the territory that the land has lost.
0:02:19 > 0:02:21The debris is swept away
0:02:21 > 0:02:25and strewn on beaches farther down the coast as sand and gravel.
0:02:29 > 0:02:32In some places, the land is advancing.
0:02:32 > 0:02:37In the tropics, mangroves are moving out into the sea, gathering mud
0:02:37 > 0:02:40and building new territory for land-living creatures.
0:02:43 > 0:02:44Even in the mouths of rivers,
0:02:44 > 0:02:47where fresh water, laden with sediment
0:02:47 > 0:02:49mingles with the salt water of the sea,
0:02:49 > 0:02:53new land is being created of a sort.
0:02:56 > 0:02:58HE PANTS AND GROANS
0:02:58 > 0:03:02I'm in an estuary in the west of England.
0:03:02 > 0:03:04You might think that this mud
0:03:04 > 0:03:07is not the most attractive stuff in which to live.
0:03:07 > 0:03:10Certainly, any animals that do live in it
0:03:10 > 0:03:12have to face some severe problems.
0:03:12 > 0:03:16For one thing, part of their time, they're out of water like this,
0:03:16 > 0:03:19part of the time they're underwater.
0:03:19 > 0:03:21The saltiness of the water, too, varies.
0:03:21 > 0:03:26Fresh water comes down from the land, the tides bring in salt water.
0:03:26 > 0:03:32And then there's the nature of this extraordinarily sticky mud itself.
0:03:32 > 0:03:35It is so glutinous that little oxygen gets into it
0:03:35 > 0:03:40but the rewards for enduring these unpromising conditions are high.
0:03:44 > 0:03:48Edible particles deposited every day on the surface of the mud
0:03:48 > 0:03:53are cautiously sucked up by the searching siphon of Scrobicularia,
0:03:53 > 0:03:56a little mollusc whose main body, enclosed in a shell,
0:03:56 > 0:03:58is hidden within the mud for safety.
0:03:58 > 0:04:02A tiny crustacean, Corophium, half an inch long,
0:04:02 > 0:04:05grazes on the bacteria which proliferate in millions,
0:04:05 > 0:04:08breaking down the rotting organic matter in the mud.
0:04:10 > 0:04:12Ragworms live in burrows
0:04:12 > 0:04:14and will tackle Corophium, algae, bacteria,
0:04:14 > 0:04:16almost anything that's around.
0:04:25 > 0:04:28The puddles are flecked with floating mucus.
0:04:28 > 0:04:32It's produced by spire shells, no bigger than grains of wheat.
0:04:32 > 0:04:37The mucus attracts bacteria, and the spire shells eat the lot.
0:04:50 > 0:04:54The peacock worm fans out its tentacles from the top of its tube
0:04:54 > 0:04:57to gather food particles before they settle.
0:05:09 > 0:05:12Beating threads on each filament of the fan
0:05:12 > 0:05:15transport the catch down to the mouth at the centre.
0:05:19 > 0:05:23While it feeds, it also disgorges a cement of mud and mucus
0:05:23 > 0:05:26and builds up the margin of its tube.
0:05:32 > 0:05:34The cockle lies with its shell agape,
0:05:34 > 0:05:38filtering the water by sucking it in through one siphon...
0:05:40 > 0:05:43..and blowing it out through another.
0:05:45 > 0:05:49Mussels use the same technique, collecting within their shells
0:05:49 > 0:05:51substantial quantities of the abundant
0:05:51 > 0:05:54and surprisingly nutritious drifting particles.
0:05:59 > 0:06:03When the tide goes out, they clamp their shells tightly together
0:06:03 > 0:06:06to keep in their moisture and to keep out their attackers,
0:06:06 > 0:06:09but some creatures know how to deal with that.
0:06:18 > 0:06:21Each oyster-catcher has its favourite technique
0:06:21 > 0:06:22for dealing with mussels.
0:06:22 > 0:06:25It's usually the same as that used by its parents,
0:06:25 > 0:06:27and has been learned from them,
0:06:27 > 0:06:29though a bird needs several years of practice
0:06:29 > 0:06:31before it becomes really expert.
0:06:31 > 0:06:33Some hunt in the shallow waters
0:06:33 > 0:06:36looking for mussels that have not yet shut their shells.
0:06:40 > 0:06:43Others pick up unattached shells and carry them off
0:06:43 > 0:06:46away from the main flock so they've got a little privacy.
0:06:46 > 0:06:49And there they skilfully place the mussel in such a position
0:06:49 > 0:06:52that they can cut it open along its hinge.
0:07:04 > 0:07:08Other individual birds regularly resort to brute force.
0:07:08 > 0:07:11They hammer their way in through the shell itself.
0:07:21 > 0:07:26As the tide retreats still further, spire shells are exposed,
0:07:26 > 0:07:30as many as 35,000 buried within a single square yard.
0:07:30 > 0:07:33All these mud feeders together constitute a rich prize,
0:07:33 > 0:07:36and there are abundant claimants.
0:07:50 > 0:07:53Sandpipers, on migration, depend on them,
0:07:53 > 0:07:57but at all times of the year, wading birds come to the estuaries to feed.
0:08:00 > 0:08:04The godwit, equipped with long legs and a long bill,
0:08:04 > 0:08:05can wade in water several inches deep
0:08:05 > 0:08:09and collect food before it can be reached by other birds.
0:08:10 > 0:08:13The curlew prefers to work out of water.
0:08:13 > 0:08:16Its long bill enables it to probe deep into the mud for a worm,
0:08:16 > 0:08:20and serves equally well as a pair of forceps.
0:08:24 > 0:08:27The dunlin is a smaller bird and goes for smaller prey,
0:08:27 > 0:08:29ragworms and insect larvae.
0:08:29 > 0:08:32It feels for its food with its short bill.
0:08:55 > 0:08:58The ringed plover, with a very short bill,
0:08:58 > 0:09:02can only collect food from the surface and locates it by sight.
0:09:02 > 0:09:03It usually works alone
0:09:03 > 0:09:06so that its prey won't be disturbed by pattering feet
0:09:06 > 0:09:09and withdraw before being spotted.
0:09:11 > 0:09:13The scything action of the avocet
0:09:13 > 0:09:15collects creatures that live in the liquid mud.
0:09:24 > 0:09:26Their bills are very sensitive.
0:09:26 > 0:09:28As soon as they close on something edible,
0:09:28 > 0:09:31the bird can juggle it up into its mouth.
0:10:11 > 0:10:15The quantities of food taken by wading birds from estuaries
0:10:15 > 0:10:16is enormous.
0:10:16 > 0:10:21Some species consume every day about a third of their own weight in food.
0:10:21 > 0:10:23In a year, a single oyster-catcher
0:10:23 > 0:10:27can consume the flesh over half a ton of cockles,
0:10:27 > 0:10:31and many an estuary supports tens of thousands of wading birds,
0:10:31 > 0:10:34so these places are rich indeed.
0:10:37 > 0:10:41As the river brings down more and more particles of mud,
0:10:41 > 0:10:44so the flats grow bigger and higher,
0:10:44 > 0:10:49and on their surface they develop a slimy skin,
0:10:49 > 0:10:54and that's formed by microscopic plants, algae.
0:10:54 > 0:10:57They start the process of consolidation.
0:10:57 > 0:11:02But soon, bigger plants get root, like this glasswort,
0:11:02 > 0:11:05and now the process really speeds up.
0:11:08 > 0:11:11As the high tide brings in more mud particles,
0:11:11 > 0:11:13they clog around the stems of the glasswort
0:11:13 > 0:11:17and don't swill back to the sea when the tide falls.
0:11:17 > 0:11:20So with each new tide, the flats grow higher and higher.
0:11:23 > 0:11:27Glasswort is a plant of the cold estuaries of Europe.
0:11:27 > 0:11:32In the tropics, the colonisers of mud are not small plants but trees.
0:11:32 > 0:11:34Mangroves.
0:11:36 > 0:11:41This mud is the pulverised remains of rocks eroded from the Himalayas
0:11:41 > 0:11:44that has been carried down by the Ganges for 1,000 miles
0:11:44 > 0:11:47and dumped on the edge of the Bay of Bengal.
0:11:47 > 0:11:51This is the biggest intertidal forest of all, the Sunderbans,
0:11:51 > 0:11:534,000 square miles of it,
0:11:53 > 0:11:56and here roam many animals that usually live in dry-land forests.
0:11:58 > 0:11:59Axis deer.
0:12:11 > 0:12:14Woodpeckers. The Indian golden-banded.
0:12:20 > 0:12:22And wild boar.
0:12:26 > 0:12:29But mangrove forests can also harbour creatures
0:12:29 > 0:12:31that live nowhere else at all.
0:12:31 > 0:12:35The proboscis monkey eats almost nothing but mangrove leaves.
0:12:35 > 0:12:38It developed that specialism on the island of Borneo,
0:12:38 > 0:12:40and has never spread overseas,
0:12:40 > 0:12:42trapped by its own specialised requirements.
0:12:50 > 0:12:54Mangroves themselves are distributed widely through the tropics,
0:12:54 > 0:12:57for they have evolved from many different plant families
0:12:57 > 0:13:00and today there are some 40 different species of them.
0:13:01 > 0:13:05The flowers of this pioneering mangrove are pollinated by the wind.
0:13:05 > 0:13:09The seed, however, doesn't immediately leave the parent tree.
0:13:09 > 0:13:12It starts to grow while it is still attached,
0:13:12 > 0:13:17producing a green shoot a foot long with a sharp end to it.
0:13:19 > 0:13:21If it falls when the tide is in,
0:13:21 > 0:13:23it floats horizontally in the buoyant salt water
0:13:23 > 0:13:26and may be carried for miles before being stranded.
0:13:26 > 0:13:30If the tide is out, it stabs the mud and stays in that position
0:13:30 > 0:13:32when the tide returns.
0:13:32 > 0:13:37It puts out rootlets from the bottom and leaves from the top,
0:13:37 > 0:13:40and within a few days, it's firmly established.
0:13:42 > 0:13:45Just as in cold-water estuaries,
0:13:45 > 0:13:48there's a lot of organic matter in this mud.
0:13:48 > 0:13:52But because it's so sticky, it isn't stirred up,
0:13:52 > 0:13:54so there's little oxygen in it,
0:13:54 > 0:13:58and the process of rotting produces within the mud itself
0:13:58 > 0:14:01an acid, smelly, poisonous chemical.
0:14:01 > 0:14:04Hydrogen sulphide.
0:14:04 > 0:14:09So these roots don't go down far into the mud.
0:14:09 > 0:14:14Instead, they support the trees by their sheer number.
0:14:14 > 0:14:16But what about the other things
0:14:16 > 0:14:18that normal roots do for normal trees,
0:14:18 > 0:14:23like gathering nutrients and water and oxygen?
0:14:23 > 0:14:27Well, these roots deal with the nutrient problem like this.
0:14:34 > 0:14:38It has this cluster of very fine roots
0:14:38 > 0:14:42which don't go more than an inch or so below the surface of the mud,
0:14:42 > 0:14:45but it is on the surface of the mud
0:14:45 > 0:14:48that the bulk of the nutrients are found.
0:14:48 > 0:14:50As for water, there's plenty of it here,
0:14:50 > 0:14:52the trouble is that it's salty.
0:14:52 > 0:14:57Some mangroves have a special membrane around the cells
0:14:57 > 0:15:01in the root hairs which filters off the salt.
0:15:01 > 0:15:06Others actually absorb the salt but then excrete it from the leaves,
0:15:06 > 0:15:10or concentrate it in the leaf and then the leaves are shed.
0:15:10 > 0:15:15And oxygen, well, there are several different solutions to that problem.
0:15:15 > 0:15:19This mangrove has pores actually in these prop roots
0:15:19 > 0:15:23which absorb the oxygen directly.
0:15:23 > 0:15:26And this one has roots which actually grow upwards,
0:15:26 > 0:15:31so keeping pace with the rising surface of the accumulating mud.
0:15:31 > 0:15:34But it's not only plants in the mangrove swamps
0:15:34 > 0:15:36that have difficulty in getting oxygen.
0:15:36 > 0:15:38So do animals,
0:15:38 > 0:15:43and this time, low tide, is a period of particular difficulty.
0:15:43 > 0:15:46Many of the molluscs, like cockles and mussels elsewhere,
0:15:46 > 0:15:49simply shut their shells to keep what moisture they have
0:15:49 > 0:15:53and wait for the food-and oxygen-bearing water to return.
0:15:53 > 0:15:56For them, this is a period of inactivity,
0:15:56 > 0:15:59but for other creatures, it's just the opposite.
0:16:12 > 0:16:14The mudskipper, of course, is a fish.
0:16:14 > 0:16:16There are several different kinds.
0:16:16 > 0:16:18This one lives near high-water mark,
0:16:18 > 0:16:21and is the sort that spends most time out of water.
0:16:22 > 0:16:26It has to keep its skin moist for it absorbs oxygen through it.
0:16:26 > 0:16:30It also keeps its mouth full of water swilling over its gills.
0:16:33 > 0:16:37It feeds on the little crabs that graze on the mud.
0:16:41 > 0:16:44And having got one, it needs another mouthful of water.
0:16:53 > 0:16:56A second kind lives close to low-water mark,
0:16:56 > 0:16:59so it is only out of water for an hour or so each day.
0:16:59 > 0:17:03It sifts the liquid mud for small crustaceans and worms.
0:17:14 > 0:17:17In between these two kinds lives the largest of the three.
0:17:17 > 0:17:20It is a vegetarian, collecting algae
0:17:20 > 0:17:22and other microscopic plants from the mud.
0:17:29 > 0:17:32And it, too, nips back every now and again for a wet.
0:17:37 > 0:17:39It guards its grazing rights with vigour,
0:17:39 > 0:17:41building walls around its territory.
0:17:51 > 0:17:53And when neighbours meet, there's trouble.
0:18:04 > 0:18:08On clear mud, their territories form a patchwork of walled ponds.
0:18:08 > 0:18:10These flats are very flat,
0:18:10 > 0:18:12so when a male starts to advertise for a mate,
0:18:12 > 0:18:14he has to be a bit of a gymnast.
0:18:27 > 0:18:30When a female is finally enticed into his private pond,
0:18:30 > 0:18:33he can continue his courtship at close quarters
0:18:33 > 0:18:35in a more conventionally fish fashion,
0:18:35 > 0:18:40with flexed fins, waggling tail and enormous excitement.
0:19:13 > 0:19:16They'll spawn in a burrow at the bottom of the pond.
0:19:20 > 0:19:23This crab is too big to be intimidated by mudskippers,
0:19:23 > 0:19:26even when it does wander through their territories.
0:19:35 > 0:19:39Its scissoring mouthparts not only sort out its food
0:19:39 > 0:19:40but help it to breathe.
0:19:40 > 0:19:42On top of its shell, there is a puddle of water,
0:19:42 > 0:19:44and as its mouthparts move,
0:19:44 > 0:19:48they circulate this into a gill chamber within the shell,
0:19:48 > 0:19:51out again and up to the reservoir on the top.
0:19:51 > 0:19:54Eventually, the oxygen in the water is exhausted
0:19:54 > 0:19:56and the crab has to return to the sea,
0:19:56 > 0:19:58tip it off and get a fresh supply.
0:20:02 > 0:20:04Close by the edge of the sea,
0:20:04 > 0:20:07the tiny soldier crabs feed with frantic haste.
0:20:07 > 0:20:11No one else will steal their mud,
0:20:11 > 0:20:13but they have to eat an enormous quantity
0:20:13 > 0:20:17to extract the few particles necessary to keep alive.
0:20:17 > 0:20:21They have to work at it pretty well non-stop and have no time to waste.
0:20:30 > 0:20:33High up, beyond the reach of all but the highest tides,
0:20:33 > 0:20:35lives the large mangrove crab.
0:20:35 > 0:20:38It keeps moist by boring its hole
0:20:38 > 0:20:41as much as six feet deep to reach permanent water.
0:20:41 > 0:20:45The lure that tempts it out is a newly fallen mangrove leaf.
0:20:49 > 0:20:51And quickly back to safety.
0:20:56 > 0:20:59Among the air-absorbing roots of the mangroves,
0:20:59 > 0:21:01fiddler crabs are busy.
0:21:01 > 0:21:03The females collect mud with both pincers,
0:21:03 > 0:21:07working with the same frantic speed as the soldier crabs.
0:21:09 > 0:21:12The males need to munch just as much mud as the females,
0:21:12 > 0:21:14but they have to work with one hand only,
0:21:14 > 0:21:18for one of their claws is so big that it's useless for feeding.
0:21:20 > 0:21:24They use it instead to wave at passing females.
0:21:30 > 0:21:33But it's also a weapon to brandish at rivals.
0:21:38 > 0:21:40A less well-equipped male
0:21:40 > 0:21:43gets a nasty hammering even before he can get out of his hole.
0:21:55 > 0:21:57The claw is long enough to reach down into the burrow
0:21:57 > 0:22:01to give his opponent a tweak where he's least expecting it.
0:22:08 > 0:22:11The purpose of the wave is to encourage a female
0:22:11 > 0:22:13to follow a male into his burrow.
0:22:26 > 0:22:29Is it possible perhaps just to take a moment or so off
0:22:29 > 0:22:31from munching mud?
0:22:34 > 0:22:37At low tide, there's lots for birds to eat on the mangrove mud,
0:22:37 > 0:22:40just as there is on estuaries elsewhere.
0:22:40 > 0:22:42Terns hawk for fish
0:22:42 > 0:22:45that are easier to catch now in the shallowing waters.
0:22:47 > 0:22:49Kingfishers pounce on the fiddler crabs.
0:22:57 > 0:23:01Great white heron stalk and stab.
0:23:19 > 0:23:23The returning tide signals "all change" for everyone.
0:23:29 > 0:23:32This African mangrove snail crops the algae growing on the mud,
0:23:32 > 0:23:36but it mustn't stay there when the tide comes in, for it would
0:23:36 > 0:23:38be attacked by fish.
0:23:38 > 0:23:40It takes refuge up in the trees.
0:23:40 > 0:23:44Its speediest climb is barely faster than the rise of the tide,
0:23:44 > 0:23:46so it has to set off in good time
0:23:46 > 0:23:49and have some sort of internal alarm clock
0:23:49 > 0:23:51that tells it when it should do so.
0:24:04 > 0:24:07The soldier crabs are so well adapted
0:24:07 > 0:24:10to their life scavenging on the exposed mud
0:24:10 > 0:24:12that they have become breathers of air,
0:24:12 > 0:24:14and without it they will drown.
0:24:14 > 0:24:17As the tide advances, each constructs itself a little igloo
0:24:17 > 0:24:19which traps a bubble of air
0:24:19 > 0:24:22with which the crab can breathe while the tide is in.
0:24:40 > 0:24:43The mudskippers' territorial walls, built with such labour,
0:24:43 > 0:24:46are breached by the incoming wavelets.
0:24:49 > 0:24:52Higher up, the mudskippers shelter in burrows.
0:25:00 > 0:25:04The incoming tide brings new creatures into the swamps.
0:25:04 > 0:25:08Shoals of fish arrive, searching for morsels that may have been deposited
0:25:08 > 0:25:10by the river while the tide was out.
0:25:15 > 0:25:19In the swamps of South-East Asia, archer fish feed on insects
0:25:19 > 0:25:21that have fallen on the surface.
0:25:28 > 0:25:31Uniquely, they also have a way of collecting insects
0:25:31 > 0:25:32from above the water.
0:25:34 > 0:25:36There's a groove in the roof of their mouth,
0:25:36 > 0:25:40so that a sudden thrust of the tongue produces a spurt of droplets
0:25:40 > 0:25:42like a water pistol.
0:25:49 > 0:25:53When there is a crowd, a marksman can't be sure of getting his prize.
0:26:05 > 0:26:09So in company, it may be better to try a direct assault.
0:26:32 > 0:26:36The larger fish are themselves food for otters,
0:26:36 > 0:26:39but these hunters have broad appetites
0:26:39 > 0:26:44and will enthusiastically tackle snails, crabs and even mussels.
0:27:00 > 0:27:04They are great travellers, swimming for many miles up into fresh water
0:27:04 > 0:27:08or down into the sea and even out to offshore islands,
0:27:08 > 0:27:11and they have an enormous appetite for play.
0:27:23 > 0:27:28The largest of all living reptiles is found among mangroves -
0:27:28 > 0:27:34the estuarine crocodile, a monster that grows to 23 feet long.
0:28:18 > 0:28:22Like its ancestors that lived when dinosaurs dominated the earth,
0:28:22 > 0:28:25it's an ocean-going creature,
0:28:25 > 0:28:29and, as a consequence, it's the most widely distributed of all crocodiles
0:28:29 > 0:28:33living from the Bay of Bengal through northern Australia to the Pacific,
0:28:33 > 0:28:39even reaching such isolated mangrove swamps as those on the islands of Fiji.
0:28:41 > 0:28:45As the mangroves establish themselves farther out into the sea,
0:28:45 > 0:28:48the mudflats they've built grow higher and higher.
0:28:48 > 0:28:50Rainwater washes them clean of salt,
0:28:50 > 0:28:56and eventually they become dry fertile forest, beyond the reach of the sea.
0:28:58 > 0:29:01The banks of mud and sand that the rivers lay down around their mouths,
0:29:01 > 0:29:04even when they are not big enough to rise above water,
0:29:04 > 0:29:09protect the land against the attacks of the sea, for tall waves can't travel
0:29:09 > 0:29:11across shallow water.
0:29:11 > 0:29:15But if a current sweeping down the coast carries away the sediment
0:29:15 > 0:29:18and scours the sea floor clean,
0:29:18 > 0:29:21then waves arrive at the coast full of power.
0:29:48 > 0:29:51Where the land dips steeply into the sea,
0:29:51 > 0:29:55then the territory between the tides is not miles across but condensed
0:29:55 > 0:29:57into a narrow band.
0:29:57 > 0:30:00The creatures that live here, like all intertidal creatures,
0:30:00 > 0:30:04are constantly threatened by two dangers.
0:30:04 > 0:30:08At the high-water mark, there are physical problems of being dried out,
0:30:08 > 0:30:12and at the low-water mark, there are biological problems
0:30:12 > 0:30:17of animals that creep up from the sea to prey upon the intertidal creatures.
0:30:17 > 0:30:20The interplay of those two sets of problems
0:30:20 > 0:30:25produces a series of horizontal bands along the coast,
0:30:25 > 0:30:28each dominated by the particular species
0:30:28 > 0:30:33which best deals with the problems at that particular level.
0:30:33 > 0:30:37Such bands can be seen on coasts all over the world,
0:30:37 > 0:30:41but here on the north-west coast of America, they are strikingly clear.
0:30:41 > 0:30:45The bottom band of all is only fully exposed
0:30:45 > 0:30:48when the moon and the sun are in such an alignment that they pull together
0:30:48 > 0:30:52and the tide withdraws a long way from the edge of the dry land.
0:30:52 > 0:30:56The organisms here only tolerate a brief exposure to the air
0:30:56 > 0:31:00for they have no special devices to prevent themselves from being dried out.
0:31:05 > 0:31:10The sea urchin, in water, gnaws away at encrusting algae.
0:31:12 > 0:31:17But out of water, it can do nothing but simply hang on to the rocks.
0:31:17 > 0:31:21Alongside them, giant sea anemones droop their tentacles,
0:31:21 > 0:31:24and many withdraw them, for in air there is nothing to feed on.
0:31:39 > 0:31:43Sea squirts can only filter for their food spasmodically.
0:31:44 > 0:31:50Starfish are meat-eaters, and this species feeds on mussels.
0:31:50 > 0:31:54It envelops them with its adhesive arms, slowly wrenches apart their shells,
0:31:54 > 0:31:56and feeds on the flesh within.
0:31:56 > 0:32:01Below low-water mark, they kill any mussel that tries to establish itself,
0:32:01 > 0:32:06but like so many of these low-level creatures, they can't feed out of water.
0:32:06 > 0:32:10So a little higher up, where the rocks are regularly exposed to air for longer,
0:32:10 > 0:32:14conditions favour the mussels, and they form a dense band,
0:32:14 > 0:32:18cropped at the lower edge by starfish, but beyond their reach higher up.
0:32:24 > 0:32:28The massed mussels provide shelter between them for lots of other creatures -
0:32:28 > 0:32:32small starfish, too small to tackle a mussel,
0:32:32 > 0:32:36worms and crustaceans, winkles and other molluscs.
0:32:44 > 0:32:47The mussels hold on to the rocks with bundles of threads,
0:32:47 > 0:32:50but they can't withstand the pull of the roughest of the waves
0:32:50 > 0:32:53and in winter storms, sheets of them may be ripped away.
0:33:08 > 0:33:12In more exposed places where the waves beat with a particular ferocity,
0:33:12 > 0:33:16mussels give way to goose-necked barnacles which clasp the rock
0:33:16 > 0:33:18with a long fleshy foot.
0:33:29 > 0:33:34They feed by holding out stiff, fan-like arms which catch particles
0:33:34 > 0:33:39from the waves, not when they crash in, but as their waters flow gently back.
0:33:59 > 0:34:04On the most exposed promontories, the mussels are ousted by a plant:
0:34:04 > 0:34:07An odd-looking alga known as a sea palm
0:34:07 > 0:34:10which lives only on these north western coasts of North America.
0:34:13 > 0:34:17The crown of leaves at the top of its rubbery stem is a device that enables
0:34:17 > 0:34:22the sea palm to harness the power of the waves and use it to attack
0:34:22 > 0:34:23the mussels.
0:34:23 > 0:34:27The plants, perhaps surprisingly, are annual.
0:34:27 > 0:34:32In the spring, an individual plant may achieve the difficult feat of
0:34:32 > 0:34:38getting hold of an individual mussel in the mussel bed, as this one has done.
0:34:39 > 0:34:42When it's mature, it will produce spores,
0:34:42 > 0:34:46but only when it's out of water as it is now.
0:34:46 > 0:34:51So instead of the spores being distributed widely as those of other
0:34:51 > 0:34:57plants are, the spores of the sea palm trickle down the grooves
0:34:57 > 0:35:00in these leaves and into the mussel bed here.
0:35:02 > 0:35:05When the first storms of the autumn comes,
0:35:05 > 0:35:12they may catch underneath the fronds of this plant and rip it up,
0:35:12 > 0:35:16but the holdfast grips the mussels so firmly that the mussels
0:35:16 > 0:35:20come away with it, revealing the bare rock,
0:35:20 > 0:35:26and that means that the offspring of other nearby plants
0:35:26 > 0:35:30can get a hold on the bare rock.
0:35:30 > 0:35:36So by the sacrifice of one palm growing on a mussel one year,
0:35:36 > 0:35:43next year, there will be a whole grove of palms growing firmly on the bedrock.
0:35:54 > 0:35:59But mussels do require a certain amount of immersion every day
0:35:59 > 0:36:02if they are not to dry out and die,
0:36:02 > 0:36:05and this line marks exactly that.
0:36:05 > 0:36:08Above it, no mussel can live.
0:36:08 > 0:36:13The creatures that can are these - barnacles.
0:36:13 > 0:36:17Clamped tightly to the rocks, they conserve very effectively indeed
0:36:17 > 0:36:19the moisture within their shells.
0:36:19 > 0:36:22They manage to collect the minute quantities of food they require to grow
0:36:22 > 0:36:27and reproduce from the relatively infrequent submersions at high tide,
0:36:27 > 0:36:31which in some cases may only occur for an hour once a month.
0:36:57 > 0:37:01So each level on a rocky shore is dominated by the organisms
0:37:01 > 0:37:05that best deal with the precise combination of pounding by the waves,
0:37:05 > 0:37:09exposure to the air, and attack by deep-water predators.
0:37:09 > 0:37:12None, in the long run, can claim permanent occupation,
0:37:12 > 0:37:15for the attacks of the waves are unceasing.
0:37:43 > 0:37:48With unfailing accuracy, the sea picks out the softer parts of the rocks
0:37:48 > 0:37:50and cuts its way into them.
0:37:50 > 0:37:53Water at great pressure is driven into joints and cracks
0:37:53 > 0:37:57until it penetrates a cliff and forms a blowhole.
0:38:01 > 0:38:06On the southernmost tip of Australia, storms of great ferocity sweeping up
0:38:06 > 0:38:10from the south, with the full force of the Antarctic gales behind them,
0:38:10 > 0:38:15beat away at sandstone cliffs which have lines of weakness that run
0:38:15 > 0:38:21horizontally and vertically, so the rock is cut away in huge blocks.
0:38:57 > 0:39:01The sea, having demolished the cliffs, then works on the debris.
0:39:01 > 0:39:05During storms, it picks up the boulders and hurls them at the cliff face.
0:39:05 > 0:39:09At calmer times, it rolls the rocks over the seabed and casts them up
0:39:09 > 0:39:11on shingle banks.
0:39:11 > 0:39:15Every movement chips and grinds the fragments until they are reduced
0:39:15 > 0:39:20to sand grains, and now even a gentle current can pick them up and carry
0:39:20 > 0:39:24them for miles down the coast, eventually to abandon them in banks
0:39:24 > 0:39:28and strands in the lee of islands or in sheltered bays.
0:40:42 > 0:40:48Every wave of every tide stirs up the surface of the sand, so plants find
0:40:48 > 0:40:54it impossible to get any grip on it as they can on rocky shores or mudflats.
0:40:54 > 0:41:02So a beach like this looks as lifeless as any part of the margins of the land.
0:41:02 > 0:41:07But if the sand grains are not too small and compacted, then each
0:41:07 > 0:41:12will retain around it a thin film of moisture even when the tide is out,
0:41:12 > 0:41:16and in that microscopic space, animals can live.
0:41:19 > 0:41:23These translucent boulders are, in fact, sand grains,
0:41:23 > 0:41:28and the tiny snake-like animal - a worm that could sit on a pinhead.
0:41:43 > 0:41:46All these inhabitants of the sand are, necessarily,
0:41:46 > 0:41:51adept at writhing, gliding and crawling as they search for the few
0:41:51 > 0:41:56edible fragments trapped between the grains, or pursue one another.
0:42:08 > 0:42:13This one is only a temporary lodger in the sand. It is the larva of a mollusc.
0:42:17 > 0:42:21A hydra lives here. It's like the one that's common in freshwater ponds,
0:42:21 > 0:42:25but it has one elongated tentacle with which it anchors itself.
0:42:27 > 0:42:31A nematode worm produces glue from a gland on its tail
0:42:31 > 0:42:33which helps it to maintain its position.
0:42:42 > 0:42:46And this is another larva that at the beginning of its life floats in the sea
0:42:46 > 0:42:50but settles down into the sand to continue its development.
0:42:50 > 0:42:54It builds a tiny tube of mucus which it carries about with it
0:42:54 > 0:42:57and clings to with bristles on its flanks.
0:43:05 > 0:43:11When it grows up, it does the same thing on a larger scale, above the sand.
0:43:11 > 0:43:13It's a worm called the sand mason.
0:43:15 > 0:43:19Now it not only builds a tube, but it adds long tassels to the top.
0:43:19 > 0:43:23These slow down the water so that suspended food particles fall
0:43:23 > 0:43:26and can be gathered by the waving tentacles.
0:43:26 > 0:43:29In this shifting sand, the tubes need constant renewal,
0:43:29 > 0:43:34and this is how the sand mason does it, speeded up 125 times.
0:44:17 > 0:44:21Although plants can't grow on these perpetually moving sands,
0:44:21 > 0:44:26those dislodged from the rocky parts of the coast by waves are washed up here,
0:44:26 > 0:44:29and there are plenty of creatures on the beach waiting for them.
0:44:41 > 0:44:43These are sand-hoppers.
0:44:43 > 0:44:48They hide below the surface to avoid being baked and dried out by the sun,
0:44:48 > 0:44:50but now there is food to be had.
0:45:07 > 0:45:10On many beaches, their numbers are astronomic.
0:45:10 > 0:45:15There can be as many as 25,000 of them in one square yard of beach sand.
0:45:28 > 0:45:31The sand-hoppers favour rotting vegetation.
0:45:31 > 0:45:35Rotting flesh attracts crabs.
0:45:43 > 0:45:47The remains of a squid is a banquet for ghost crabs.
0:46:08 > 0:46:11Occasionally, when there is a chance, it may be better to cut off a length
0:46:11 > 0:46:15and haul it away to consume it in the privacy of a burrow.
0:46:20 > 0:46:23The crabs and the shrimps live close to the high-tide mark.
0:46:23 > 0:46:28But the incoming waters also bring with them another team of scavengers.
0:46:29 > 0:46:33This periscope on a South African beach belongs to a mollusc -
0:46:33 > 0:46:35a plough snail.
0:46:40 > 0:46:44It inflates its plough-like foot by pumping in water,
0:46:44 > 0:46:48and it uses it not so much as a ploughshare as a surfboard.
0:46:48 > 0:46:52The waters pick it up and wash it swiftly inshore, together with its
0:46:52 > 0:46:54potential food...
0:46:57 > 0:46:59..a stranded jellyfish.
0:47:08 > 0:47:12The plough snails detect its presence from the taste of decay
0:47:12 > 0:47:15in the surrounding water and advance on it with great speed.
0:47:52 > 0:47:57To avoid being swept up the beach and being stranded, they eat fast,
0:47:57 > 0:48:02and then, while there is some food still left, they burrow into the sand.
0:48:02 > 0:48:04There they wait for the tide to turn
0:48:04 > 0:48:08so that they can ride back on their surfboards to deeper water and safety.
0:48:15 > 0:48:21Very few sea creatures venture above the limit of the highest tide and survive.
0:48:21 > 0:48:26One group of animals is compelled to do so by the nature of their ancestry,
0:48:26 > 0:48:31and on this one beach in Costa Rica, they stage an astonishing invasion.
0:48:32 > 0:48:34Turtles.
0:48:34 > 0:48:37They are ridleys, the smallest of the sea-going turtles,
0:48:37 > 0:48:39only a couple of feet long.
0:48:40 > 0:48:43Turtles are descended from land-living reptiles,
0:48:43 > 0:48:48and, like all reptiles, they lay eggs that only develop and hatch in air.
0:48:48 > 0:48:52So every year, adult females, having mated at sea, must make
0:48:52 > 0:48:53their way onto dry land.
0:48:58 > 0:49:03They arrive at a rate of up to 5,000 an hour.
0:49:03 > 0:49:06They use only one or two of the many thousands of beaches
0:49:06 > 0:49:08that seem to be suitable.
0:49:08 > 0:49:12And what is more, they only choose to do so on just a few nights in the year
0:49:12 > 0:49:14between August and November.
0:49:22 > 0:49:24Efficient though their flippers are in water,
0:49:24 > 0:49:28on land they are barely strong enough to lift the turtle clear of the sand.
0:49:28 > 0:49:31It has to drag itself up the beach.
0:49:33 > 0:49:36This mass breeding may be an advantage to the turtle.
0:49:36 > 0:49:39For since it only occurs on a few nights of the year,
0:49:39 > 0:49:43their eggs can't support a large permanent population of predators,
0:49:43 > 0:49:47as they might if the turtles were to lay over several months.
0:49:47 > 0:49:50Yet, even so, for reasons that we still don't understand,
0:49:50 > 0:49:54less than one in a hundred of the eggs produces a hatchling
0:49:54 > 0:49:56which reaches the sea.
0:49:57 > 0:49:59Each female lays a hundred or so.
0:50:12 > 0:50:16That done, she carefully fills in the hole.
0:50:33 > 0:50:38A few coatimundi and vultures come down from the forest to plunder,
0:50:38 > 0:50:41but they make little impact on the millions of eggs that are laid.
0:50:50 > 0:50:53Next night, many thousands more ridleys arrive.
0:51:02 > 0:51:08On other beaches, more secretly, other very different turtles are laying.
0:51:10 > 0:51:17This is the largest of all the marine turtles.
0:51:17 > 0:51:22This magnificent creature is the giant leatherback turtle.
0:51:22 > 0:51:25And it's a most mysterious animal.
0:51:25 > 0:51:29It's a solitary wanderer of the oceans.
0:51:29 > 0:51:32Individuals turn up almost anywhere in the tropics
0:51:32 > 0:51:35but they go much farther than that.
0:51:35 > 0:51:38They've been recorded as far south as Argentina,
0:51:38 > 0:51:42and as far north as the British Isles and North America.
0:51:42 > 0:51:46But it's a creature of mystery, because although we know what it feeds on,
0:51:46 > 0:51:50which is sea urchins and fish and, oddly enough, jellyfish,
0:51:50 > 0:51:53we know very little else about it.
0:51:53 > 0:51:58We don't know how long they live. We don't know how the male finds females.
0:51:58 > 0:52:03We don't know indeed how females navigate to find traditional nesting sites
0:52:03 > 0:52:04like this one.
0:52:04 > 0:52:10Indeed we didn't know where the main nesting sites were until 25 years ago.
0:52:10 > 0:52:15And then it was discovered that some nested on the Suriname coast of
0:52:15 > 0:52:20South America and some nested here, on the east coast of Malaysia.
0:52:20 > 0:52:25Of course, the people here have always known about the turtles
0:52:25 > 0:52:28and have always plundered those eggs.
0:52:28 > 0:52:32Today, however, there are more people than ever here,
0:52:32 > 0:52:36and the eggs are plundered more seriously,
0:52:36 > 0:52:42so undoubtedly, this huge and extraordinary creature is in danger.
0:52:43 > 0:52:47But maybe the leatherback turtle has other breeding grounds
0:52:47 > 0:52:48that we don't know about.
0:52:48 > 0:52:54Maybe it goes to small, tiny coral islands in the emptiness of the ocean
0:52:54 > 0:52:58to find beaches far away from man.
0:52:58 > 0:53:03That, indeed, is where we ourselves will be going in the next programme.