Original subtitles

Every day when the tide retreats, a secret world is exposed.

A magical and intriguing place

full of remarkable and unusual characters.

The rock pool is a cornucopia of life.

It's full of diverse animals.

Some we're familiar with, some we're not.

There's a nice little cushion star,

there's a shrimp,

there's a dog whelk, there's a top shell.

There's a little pipefish wiggling about.

There's a porcelain crab.

All of these animals have their own ecological parts to play

in the life of the rock pool.

But this unique environment

experiences some of the most extreme conditions in the natural world.

My name's Professor Richard Fortey and like everybody else,

I just love rootling around in rock pools.

But I'm a palaeontologist, so for me, rock pools are more

than just a collection of wonderful and interesting animals.

They also provide a window into the past.

Part the weeds on any rock pool, and you open the curtains

onto a life and death drama that has been played out

for hundreds of millions of years.

Some of the creatures that live here have outlived the dinosaurs

and have evolved truly extraordinary adaptations to survive.

I want to show you how rock pool creatures

have stood the test of time.

We have created our own rock pool laboratory

deep in the heart of the National Marine Aquarium in Plymouth.

Here, under the guidance of some of Britain's leading marine biologists,

we will take a closer look at rock pool creatures

and reveal just how they have evolved to cope

with the ever-changing tide.

Life in rock pools is more complicated than we thought.

I think it's far more complicated than we thought.

We will investigate how they compete for food and space...

There are specialised tentacles simply for fighting.

..and reveal the surprising behaviour they use

to fight off predators.

It's really very agitated.

It becomes like a sort of animated mushroom.

This is The Secret Life Of Rock Pools.

It's high tide and the little world of the rock pool

is connected to the greater world of the ocean beyond.

It's a good place to be. There's normal salinity,

there's plenty of oxygen and above all,

there's nutrients coming in from beyond.

And yet, it's not going to last for long.

In a few hours, everything will change.

As the tide falls, life becomes very different for the creatures here.

The exposed shore is now subject to unpredictable changes.

Changes that depend on the weather, the time of year,

and the time of day.

Here, temperatures can range from freezing to baking,

oxygen levels fluctuate and salinity can increase or decrease,

causing body tissues to dehydrate or swell with water.

But before any of these changes even begin to come into play,

there is a more immediate problem.

There is now less room for everyone to live

and resources are diminished.

Everything is dictated by competition.

Finding a good position becomes a matter of life or death

for all the creatures here.

For anemones, it is important to have a good spot

to catch the most food.

Anemones appear sedentary, but they do move around very slowly.

To find, secure and defend the best spot, they have a secret weapon.

And to shed some light on their lives,

Dr Mark Briffa of the University of Plymouth has come into the lab.

So, Mark, sea anemones are beautiful creatures,

but most people might think that they're pretty inactive.

They just sit there waiting for food to come along.

Yes, they are relatively slow-moving animals, but they are animals,

and that means they have to consume food.

And one of the things sea anemones have to do before they can consume it

is to capture their food.

Can you see the feeding tentacles?

There are six rows of tentacles on the top of the animal,

192 in total. And just by looking at them for a small amount of time,

you can see that the tentacles are moving about

and these tentacles are there to trap food and bring it in

towards this structure in the middle of the animal.

This is the oral disc. Otherwise known as a mouth.

A mouth, yes.

And they ingest the food through their oral disc, or their mouth.

They have two different types of cell which will help them trap food.

Nematocysts are stinging cells common to all anemones

and jellyfish.

When stimulated, they fire a venomous dart

attached to a thread into their prey.

We can look at the use of the tentacles to trap food

by taking a small piece of food, this is a little piece of limpet,

and dropping it over the ring of tentacles.

They kind of close in on it and pull it down.

Oh, it likes that. It likes that a lot.

We've got a very hungry anemone here.

It's closing all of the tentacles, all six rings.

It's closing them in to push the food back down towards its mouth.

And they're not just for trapping prey either.

In this species of sea anemone, there are specialised tentacles

simply for fighting.

And these specialised tentacles appear as little blue beadlets

in a ring around the outside of the six feeding tentacles.

Hence the name "beadlet".

That's where it gets its name from, yeah.

And they will use them in combat with rival anemones

who are of the same species and therefore require exactly

the same resources in terms of a good place in the rock pool.

Using a specialist time-lapse camera,

we can speed these battles up to see what's really happening.

Twisting their flexible bodies,

anemones take aggressive swipes at each other,

tearing off ribbons of skin.

Losers have no choice but to find another place to settle.

We may barely give anemones a second glance,

but their remarkable fighting behaviour has allowed them

to colonise the most sought-after locations in the rock pool

and has helped them thrive on our beaches

for around 540 million years.

Other creatures have dealt with the lack of space very differently.

They have left the pools altogether, taking up residence

on the rocks, where they are exposed at low tide.

Around 530 million years ago,

molluscs developed hard shells to house their soft body parts,

creating a microclimate into which they could retreat.

And one of the first animals to do this is still with us.

It's a living fossil. The chiton.

The chiton has a number of plates which allow it to shuffle around

and grip tightly to the surface of the rock.

But an even more effective way of doing this is under a single shell,

and the mollusc that has done this most successfully is still

with us in every rock pool and every rocky shore.

It's the limpet.

Apart from clinging steadfastly to rocks,

limpets play an important part in the ecology of the rocky shore.

They too have to compete for resources.

Professor Stephen Hawkins, of the University of Southampton,

is a limpet expert.

You might think that a limpet is a limpet is a limpet,

but actually we've got three British species.

We do have three British species, we have...

These are all patella vulgata.

And nearby, we also have patella depressa,

which is a Southern species of limpet and it goes from Senegal

up to North Wales. And underneath here, I know there's

a patella aspera - they have a nice hat of seaweeds on top of them.

And there it is. And there it is.

I'm told that they vary in conicality according to where

they are on the shore. Yes, and also with age.

I think as they get bigger and older, they tend to get more conical

and it makes quite a lot of sense to be conical like this,

because the circumference is where water gets lost

when the tide's out, so there's more of an animal contained,

there's more biomass contained, within a more conical limpet

than, say, a flatter one, or a younger one.

What about hiding under these weeds?

Well, different species have different habits.

Patella vulgata actually likes to shelter under seaweed.

They can actually munch away at the bases of the seaweeds

and even, by feeding at night, sort of gnaw away on the ends.

'So what physiological adaptation allows limpets to hang on

'like the proverbial limpet?'

They've got a big extensive foot. You can see on this animal here

and essentially it's a very complicated

biological suction device. That's how it works.

I mean, I notice most of the limpets seem to be on the rocks,

but are they in the rock pools as well?

You do get...

some limpets in rock pools.

The juveniles settle out of the plankton

and actually settle in rock pools. Is that one there? That's one there.

And they get attracted by chemicals in the pink algae that are in

this rock pool. It's a very good indication

of what's a nice place to settle,

and the limpets use these rock pools as a nursery ground

for the first year or so of their lives.

And then they move up onto the barer rock surfaces?

They move up. And they're quite vagrant when they're young, but once

they get to 15-20mm, then they start homing on a fairly regular basis.

Surprisingly, limpets are territorial.

They create a depression in the rock known as a home scar.

As the tide starts to go down, they return to this place

and hunker securely down.

Territorial fights are common, and losers are prised off the rock.

I think limpets are really neat,

and they're a great experimental animal to work with.

They're so interesting and they move, but they don't move too far,

and you can do all sorts of things with them.

I'm afraid I'm rather fond of limpets.

Well, I've become a fan too.

As the tide covers them, limpets leave their home scars

and begin to feed.

Limpets are very important grazers on the seashore.

However, there is intense competition.

To investigate further, we have to go back into the laboratory.

Stephen, you were up early this morning collecting us some limpets.

Yes, I collected these this morning at low tide just as the tide

was about to come over them,

so we should be able to stimulate them to set off on their

foraging excursions, to go off feeding, if we put them in the tank.

It doesn't take long before they sense they're surrounded with water.

Yes, when the tide is out they're on a home scar, which they create

in the rock, which their shell fits really well.

One of the limpets is still on its home scar,

but the other is off and raised up.

Little tentacles coming out?

Yes, they have these fantastic sensory tentacles all the way around

the edge of the shell.

The big ones, the primary tentacles, actually match with those rays

you can see on the shell, and there's smaller tentacles

in between, and that gives lots of information about the physical

and biological environment when they're out foraging.

And foraging means scraping algae and other things

off the surface of the rock.

That's right, they feed by scraping the rock surface

using their radula, yes.

The radula of the limpet is a ribbon-like tongue

covered in teeth.

It moves back and forth scraping algal slime from the rocks.

The limpet's radula is tipped with haematite - an extremely hard

material that allows the limpets to graze on hard surfaces.

So what's the consequence of that?

'Stephen's research has shown that limpets have a profound effect

'on the ecology of the seashore.'

When they're off foraging, and this is where I fenced the rock

to keep limpets out, and all the rest of the area here is where

limpets were able to forage freely, and just six months later... Wow.

Yes, it's amazing, isn't it?

Just six months later there's a really dense growth of seaweeds,

bladderwrack, fucoids covering the rock in the absence

of the limpet grazing, so basically the limpets,

through their radulae, really control the algae.

Limpets are synonymous with rock pools, but through millions of years

of evolution, they have pushed back the boundaries and have left

the pools to colonise the rocks along the shore.

The rising tide not only gives limpets an opportunity to feed -

it brings with it danger.

Starfish.

Starfish belong to a phylum of animals called the echinoderms,

which first appear in the fossil record more than 500 million years ago.

Starfish have macabre eating habits.

Using their strong, sticky tube feet, they force open the shells

of molluscs and then, pushing their stomach out

through their mouth, they digest the animal inside.

Limpets have been locked in an arms race with starfish for millions

of years, and have evolved their own way of dealing with them.

So, what are we looking for here?

What happens, usually, is that the limpets get agitated

when they sense a predator in the area and then,

when the starfish is in contact with the limpet, the limpet tends

to raise up, and then it will often stamp down on the starfish

and maybe drive it off.

Let's see if that behaviour happens.

In a rock pool, there is nothing quite as sinister

as a marauding starfish.

Small limpets have no choice but to flee.

A lucky escape.

Large limpets, however, stand their ground.

'Using the edge of the shell, a limpet can push the starfish away

'to prevent it climbing on top.'

Look at that!

'Continually scraping at the arm can damage the tube feet,

'deterring an attack.'

I don't think I'd like to be approached by a great battery

of wiggly tube feet, if I was a limpet.

Didn't think they did this behaviour. There he goes, look at that.

It's really very agitated.

It becomes like an animated mushroom in the end, doesn't it?

Well, we can't say that rock pools lack drama.

I'll never look at limpets in the same way again.

Beneath that implacable shell hides a strong personality.

Unseen by us, their battles with starfish have been fought

beneath the waves for millennia.

But it is not just limpets that have to face predators.

All the creatures in the rock pool must be

constantly on their guard, and have evolved many different ways

of dealing with potential attackers from land and sea.

The majority of molluscs living around rock pools rely on

their hard shells for protection against predators,

but not all of them do so.

This orange blob is actually a sea slug, and it's protected because

it absorbs toxins from other sources, in this case the sponges

that it eats. So in spite of its vulnerable appearance,

it's actually rather well protected against predators.

It's pressure from predators that has encouraged them to evolve

these incredible defence mechanisms.

This is a lemon sea slug.

Even more remarkable is the sea slug elysia.

It sucks in rock pool algae

and keeps the photosynthetic cells alive, providing energy

direct from the sun.

This rare Celtic sea slug, found in rock pools around the UK,

is unusual in that it is descended from terrestrial slugs.

It is often found grazing out of the water as well as under it.

At high tide, the rock pools are sometimes visited

by another mollusc.

But this mollusc couldn't be more different

from its relative, the sea slug.

It is a fast-moving killer -

the cuttlefish.

Cuttlefish belong to a class of molluscs called cephalopods

and are widely considered

to be the most intelligent of all invertebrates.

I think they are the ultimate rock pool predator.

At the Marine Biological Association in Plymouth

I have a great opportunity to get up close and personal

with these magnificent animals.

The ones here are kept for fisheries research

and are easily trained to take food from my hand.

Wonderfully accurate vision.

Tentacles at the front,

two of which are modified to grasp the prey.

They also have remarkable colour-changing abilities.

This colour change is often used for camouflage,

allowing them to sneak up on unsuspecting prey.

They're hiding in little tubes down there,

which are obviously there for their comfort, so they can lurk.

And when food comes around, well, out they pop.

Let's see if we can get the bigger one to come up this time.

Come on, you know you want it!

Whoa! It inked me!

Cuttlefish are one of the largest predators to visit the rock pool.

Some rock pool predators though

are far less conspicuous but no less deadly.

This is a dog whelk.

A fearsome predator in the rock pools.

Unlike its limpet cousins,

this carnivore has devised an ingenious way

of hunting other molluscs.

And one of its favourite prey are mussels.

Mussels are filter feeders,

sieving off the abundant food that drifts in the upper ocean.

They attach themselves to the rock surface

by strong threads which they secrete through their muscular foot.

These threads enable them to cling to the rocks

despite the relentless pounding of the ocean waves.

However, the stationary mussel is an easy target for prowling dog whelks.

Their radula is specially modified to drill through the shells

to reach the soft flesh of the mussel.

It's a gruesome attack.

Mussels, however, can turn the tables on a dog whelk.

Sensing a nearby attack, others in the colony

start to produce more and more sticky threads.

If they make contact, it can spell doom for the dog whelk,

which will starve to death.

The hard shell of molluscs like the dog whelk

persist long after

the soft parts of the animal itself have decayed away.

But these empty shells don't go to waste.

In the rock pool, when one species dies or moves on

another takes over.

Empty shells are put to good use

by one of my favourite rock pool creatures, hermit crabs.

Hermit crabs use shells as a very effective defence against predators

and their bodies have evolved to fit them perfectly.

Unlike other crabs, their abdomen has become soft and asymmetrical

and their back legs are very reduced,

allowing them to fit inside shells.

The asymmetry of their claws

also allows them to close up the entrance to the shell

as a defence against predators.

The crab's shell must not only be tough enough to withstand an attack,

but must also afford it some camouflage.

Dr Mark Briffa from Plymouth University has returned to our lab

to demonstrate how crabs go about choosing shells.

Well, nothing in nature is wasted and that goes for our shells too,

because here they are with a new occupant.

That's right. The common European hermit crab.

What's happened in hermit crab evolution

is that they've become adapted to occupy this free resource.

So they're taking advantage of somebody else's hard work,.

That's right. The snail has put all the effort

into growing these shells,

which means that the hermit crab doesn't have to.

And how big do they grow?

Well, these are the sorts

that are the size of hermit crabs that you'll find in rock pools.

This guy...

is the same species.

My goodness!

These tiny little guys and this monster. They're all adults.

Shall I pop that one in here so we can see him come out?

You can see the contrast in sizes.

Sort of orders of magnitude,

bigger than these tiny little intertidal specimens.

Pushing some of the smaller guys out of the way.

This is really as big as the common European hermit crab will get.

So these shells are obviously a protection. That's right.

But are the crabs even choosier

about which types of shells they pick up?

The crabs are incredibly choosy about what they want.

They'll spend a lot of time and effort

deciding whether to change shells,

whether a potential new shell is a good one.

They're also known to be particular about the colour of the shell,

at least in terms of its contrast against the background.

We can run a little experiment here,

So, what I have are two containers with a dark-coloured substrate

and I have some Littorina obtusata shells.

These are called citrina and dark reticulata.

The only thing that's really different about them is the colour.

What I'm going to do is place these shells,

so you can see straight away that, to our eyes at least,

the citrina shells really stand out

and the dark reticulata shells don't stand out so much.

So, I'm going to take four crabs in the citrina shells.

And give them the option to move into the empty black shells.

Now, the other half of the experiment

is to take four crabs in dark reticulata shells.

So, I'll find those.

If you fish out four crabs in dark reticulata shells.

One, two, three four.

We'll put them into here,

and these guys have the option of moving into citrina shells.

So, these crabs can move into shells that blend in.

These crabs can move into shells that stand out.

Very particular about moving into new shells.

They want to make sure that a new shell is absolutely better

than the shell they're coming out of.

I think he's going to come out.

There he goes. He's swapped shells. There we go.

He's gone from yellow into dark.

And I can count here

that three of the crabs are in dark shells on this stage,

blending in well with the background.

Except for that stubborn one there,

which, of course, has stayed obstinately in a yellow shell.

Standing right out against the background.

Maybe it hasn't made its decision yet,

maybe it hasn't spotted that the shell stands out.

It might have just got it wrong. What it shows overall,

if we had run this experiment lots and lots of times,

the overall trend would be

that significantly more crabs would be in the darker-coloured shells.

And that just goes to show how important

blending into the background, or crypsis, is for these animals.

'And it's not just camouflage

'that's important as a defence against predators.

'The shell must also fit well

'if it is to give the crab the best chance of survival.

'Because of this, there is intense competition for shells.'

OK, so here we are.

These crabs have been isolated for about 16 hours.

We're going to use this tank as an arena to stage a fight in.

And this large crab is in a shell that's too small for it.

It's uncomfortable. It doesn't fit in there very well.

It's trying to withdraw but the claws are still exposed.

So let's put him in there.

This is a smaller crab

and you can see he's withdrawing right into that shell.

He's shaking around in a rather loose coat.

He's got a very, very spacious house.

So we've got a big crab in a shell that's too small

and a little crab in a shell that is just right for the big crab.

'The large crab adopts intimidation tactics

'in an attempt to make the smaller crab leave it's shell.'

It's a little bit like a war of attrition.

Who can keep going for the longest.

Will the attacker wear itself out with the shell rapping

before the defender decides to give up.

Here we go.

Eviction! So the attacking crab just evicted the defending crab.

The attacking crab has gone into the shell

that it's just pulled the defending crab out of,

and it's trying to keep the defending crab...

No, the defending crab has now gone into the shell

that the attacking crab vacated.

But not putting up much of a fight, I have to say.

Well, you're not going to hang around without a shell

if you can possibly avoid it.

Hermit crabs are an evolutionary marvel,

perfectly adapted to recycle the discards of another species

as a defence against predators.

As well as competing for space and avoiding attack,

other creatures have evolved remarkable adaptations

to deal with the ever-changing environment.

Well, the tide's now really out

and this place has become quite a hostile environment.

Everything's drying out.

You'd think that any organism with any sense

would have retreated out to sea with the ebbing tide,

and yet, hiding away here,

is something really extraordinary.

It's a fish.

A blenny.

Not just one, but several, hiding away in a crack in the rocks.

They have chosen not to retreat with the tide,

but to stay and risk life as a fish out of water.

Gulping air, they absorb oxygen

through blood vessels in their oesophagus.

It will be six hours or more before the sea returns

and they can resume their normal fishy lives.

By staying put when the tide retreats,

a blenny does not leave its territory

and does not have to compete for a new one

every time the tide returns

and it also avoids larger predators it might encounter at sea.

But they must return to their chosen rock crevice

before the tide retreats.

Timing it wrong could be fatal.

Anticipating tidal change

is a problem all rock pool creatures face.

Dr David Wilcockson of the University of Aberystwyth

is going to show me how animals are adapted to cope with this.

So, the tide is out,

and the question is

how do the organisms on the beach know when it's coming in?

That's actually is a very good question,

because all organisms, including ourselves, have biological clocks

which enable us to anticipate changes in our environment,

such as night and day, and in this case,

the incoming and outgoing of the tides.

And this organism we have buzzing around in these tanks

is a marine equivalent of the woodlouse.

It's an animal called Eurydice pulchra.

And Eurydice has a very, very good 12.4-hour or tidal clock,

whereas ours is run on a 24-hour basis.

But they come out of the sand and swim when the tide is in

and feed and breed,

and then what they'll do before the tide goes out

is actually bury back into the sand

so they maintain their position on the shore,

their preferred position on the shore.

'And, as we have seen, maintaining the best position on the shore

'is essential for survival.

'To best illustrate tidal rhythms,

'David has devised a unique experiment.'

So, what we have here, Richard, is activity monitors,

and in each tube is a little bit of sand and some seawater

and there's an individual Eurydice in each of these tubes

and they're all inactive at the moment

because currently they're expecting it to be low water.

When they expect high water, they'll start to swim

and across each tube is a little infrared beam.

And when they swim through that beam the beam is broken

and the beam break is recorded on the computer.

We can actually turn those recordings into plots

so we can visualise their activity,

and this is a plot from one individual Eurydice.

And you can see these black bars here represent beam breaks,

or activity periods.

And these bouts of activity are occurring every 12.4 hours.

On the nail.

A very precise 12.4-hour rhythm.

So we can actually show they have a tidal rhythm,

and the important thing is

that this will continue in the absence of any tides.

The tide outside has now risen

and there is a definite change in activity of our subjects.

Well, there's an amazing sight.

It's been a few hours since we looked at them last

and we can see now that they think it's high tide,

or they're expecting it to be high tide,

and they're zooming up and down, crossing the infrared beam.

I can see the numbers going up. That's right.

And those beam breaks are being recorded on the monitor here.

So, in nature, this is when they'd be feeding and on the hunt,

but obviously this internal clock needs some controls on it.

I mean, are there things in the natural environment

that help set those controls?

There are. What happens is that each individual animal,

its clock will be slightly different to the next one.

Their clocks drift out of phase with the natural cycle,

if we remove it from its natural environment.

So the incoming and outgoing tide actually resets their clock.

It re-synchronises their clock.

Life in rock pools is more complicated than we thought.

It's far more complicated than we thought, yes.

The ability to anticipate the changing tide is essential.

Knowing when to feed, breed or hide is vital for any creature

living on the shore.

As the tide changes, so do conditions on the beach,

and this has a profound on all living things - even the seaweeds.

For more than a billion years, life on Earth was dominated by very

simple single-celled organisms slime, if you like.

This rock's covered in it.

But those organisms included photosynthesizing blue-green

bacteria called cyanobacteria that form living films and breathed

oxygen into the atmosphere, thereby transforming the early Earth.

And about 1.3 billion years ago, they were joined by much larger

multi-celled organisms algae.

Doing the same job, still photosynthetic,

but these today dominate what we see on the beach and in the rock pools.

Of course, most people know it simply as seaweed.

With more than 9,000 species of seaweed in the UK alone,

the sheer variety and volume of them is staggering.

A quarter of the total global energy captured by photosynthesis

is fixed here in the intertidal zone.

So seaweeds are the basis of a rich and complex food chain.

Seaweeds show a distinct pattern of colonisation,

from the upper to the lower shore.

This is known as zonation,

and gives us a visual indication of how conditions vary.

But what causes zonation and why is it important?

Professor Colin Brownlee of the Marine Biological Association

is going to demonstrate.

These seaweeds grow on the rocks as you know, and they produce eggs

and sperm, just like animals, and they're produced into the

sea water and the eggs are fertilised by the sperm and what they want

to do is sink to the rock surface and then attach where they can grow.

We can analyse a sample of sea water under the microscope.

So what can we see here?

The large round cells are the eggs.

Each one of those is a single egg

and all those small little creatures swimming around them are the sperm.

Over the next day, the fertilised eggs develop into embryos.

So this is what they look like after about 24 hours.

They're no longer spherical eggs.

It's a pear-shaped embryo and it's protruded a little rhizoid.

It's a tiny little root.

All seaweeds get a chance to colonise the beach,

but one factor deciding survival is rainfall.

Remarkably, fresh rain water can kill some seaweed embryos.

And we can actually try to demonstrate that now,

just by adding some dilute sea water to this dish.

It's wobbling a bit as I add it.

It's twitching.

That's me dropping water onto it.

Good Lord! That's dead.

That is one seaweed less.

Fresh water penetrates the cells by osmosis,

causing them to swell and burst.

And the difference between different species and how they swell

really determines where they can survive on that shore.

So there is a constant bombardment of microscopic seaweed

embryos upon the shore,

and order is only maintained by relative salinity.

Seaweeds provide food and shelter for many other creatures.

They are the basis upon which much life in the rock pool depends.

The rock pools highest up the beach are exposed to the air

for the longest.

Sunshine causes evaporation that, in turn, causes the salinity

of the pools to increase and the temperature to rise.

Oxygen can fluctuate throughout the day and night.

Here, everything is pushed to its very limit.

Most active animals choose to abandon the highest rock pools.

But one of our rock pool favourites has evolved a unique biology

that has allowed it to survive further up the shore.

Crabs are arthropods, which means they have jointed legs

as well as this hard exoskeleton which encloses their body.

It allows them to tolerate conditions that other animals

might find difficult.

Of course, arthropods have been around for a very long time.

In fact, my own favourite organism, the trilobite,

is an arthropod, sadly long extinct.

They died out about 250 million years ago,

but their relatives, the crustaceans, prospered.

And it has to be said that if you have an exoskeleton,

you also have one particular drawback.

If you want to grow, you have to moult and then you're vulnerable.

Prior to moulting, a crab secretes enzymes to separate the old shell

from the underlying skin.

The crab then absorbs sea water, causing the old shell to come apart

at a seam that runs around the body.

The carapace then opens up like a lid

and the crab extracts itself from the old shell.

At this time, crabs are extremely vulnerable and will usually hide,

but moulting is also essential for another function.

Crabs don't just need to moult when they want to get larger,

they also have to moult when they want to mate.

To investigate this further I have invited Dr David Wilcockson

back into the lab.

David, what's going on?

Well, Richard the female crab has to be soft,

she has to have just moulted in order for the male to transfer sperm.

So that's a soft-shell crab stage.

Yes. So she's very vulnerable at this stage. She's quite immobile.

She's very soft and makes a nice meal for a predator,

so the male crab will actually guard her.

He'll embrace her like this,

and the female opens her abdomen, that's the flap

on the underside of her body, and the male also opens his abdomen,

and they transfer sperm through a pair of modified legs from the male.

The male will detect the fact that she's coming into moult

because she releases pheromones and he'll stand over her and cradle her.

When she moults, he then flips her over and they copulate,

and he'll continue to guard her for quite a number of days,

maybe up to ten days or more.

So if anybody finds a pair of crabs like this,

the message is "put them back".

Absolutely, yes.

So I can actually show you a neat trick here in the lab

with the pheromone from females.

I've got here some water that I've collected from an aquarium

that's contained female crabs that are coming into moult

and I've got an inanimate stone from an aquarium.

If I drop that into a tank over here, we might be able to see

the male crab responses.

So he's scented the stone and he's up on tip toes.

He's started to cradle.

He's definitely attracted to it.

Oh, dear. Poor crab.

Fancy having a piece of stone for a partner.

Of course, this is all driven by the pheromone, this behaviour.

This strong reaction to the scent a female gives off

when she moults changes the male from potential attacker to defender.

This is vital if crabs are ever going to mate.

So what happens next is the question?

So the male crab has transferred his sperm to the female

and the female will now produce eggs which are fertilised,

and if I can just disturb them gently...

She's not very keen to be disturbed.

Not keen at all.

So we can see, I'm trying not to take them apart,

the female abdomen is open there,

and underneath there, she will produce a very big mass of eggs.

We call them berried females.

The eggs females carry look like small round berries.

They can carry this egg mass for several months.

They eventually develop into free-swimming larvae,

which feed among the plankton for up to three years

before settling on the seabed as juvenile crabs.

So you may find what you think is a berried female on the shore

but actually it may not be,

and I've got something really quite interesting I found earlier.

Sounds intriguing.

This is a shore crab again...

If I can just pick it up,

and on the underside we have what appears to be an egg mass.

It looks like a berried female but in actual fact,

if we look more closely, this is a male crab,

and this egg mass is a parasitic barnacle.

How on Earth do you know that?

We know it's a barnacle only because of its larval stage.

It has the same larvae as all the other types of barnacle.

It's only a millimetre or two long?

Very small, yes, and the larvae actually penetrate into the body of

the crab and sends out rootlets into the body parts and absorbs nutrients.

It takes over the crab.

It's like something out of Alien.

It's converting the crab into a machine

for producing more parasitic barnacles.

That's exactly what that egg mass is.

After the invasion of sacculina, the crab is unable to perform

its normal function of moulting.

Sacculina interferes with the male crab's hormones.

It becomes sterile and even begins to behave like a female.

It's hard to believe that such a frankly disgusting creature

is related to the most abundant things you find on practically every

shore, the regular barnacle.

It's very different, yes.

Crabs are a rock pooling favourite but few can imagine how complex

and interesting their world really is.

As well as their fascinating reproductive cycle,

crabs have evolved to thrive in some of the most extreme environments

on the rocky shore.

To explore the lengths some animals will go in order to survive

in the most extreme conditions of the rock pool,

I'm going on a little adventure.

Something I've never done before night rock pooling

with Dr John Spicer of Plymouth University.

I think at night it's more exciting

because things happen much more quickly.

There's a lot going on and if you've got a little bit of patience,

you can see so much, even in a small pool like this.

It is a busy, violent, exciting place.

One of the really nice things about night is the fact that

even animals which would normally be submerged,

because it's quite humid at night, will come out and they will scuttle

across the rocks.

For instance if you look really carefully, you can see

the shadows of little crabs as they run from crevice to crevice.

They're no longer in the water but as long as they're damp,

their gills are such that they can still breathe

when they're out of water.

Although this is good news, this is water when the water's gone.

It's a little refuge, a little sea.

It's still got its own problems.

At night, the plants are no longer photosynthesizing.

The plants and animals are using the oxygen

and so the oxygen declines throughout the night.

Sometimes down to zero.

So you might finish up with somewhere that's

extremely unpleasant for life.

It's one of the prices of staying hydrated.

You have to put up with real severe extremes of oxygen,

so if I take some water from this rock pool, which should be

quite low in oxygen, we can take it back to the laboratory,

and we can actually use it in the laboratory

and see the response of some of the creatures that live here

to that low oxygen water.

There's plenty of water in there!

Just don't spill it.

So, Richard, what I'm going to do is actually here in the laboratory

set up an experiment with two artificial rock pools.

And which species are we using?

Now, the animal we will use is an animal called Palaemon elegans,

which sounds very grand.

It's a common glass shrimp, where you can see all the inner working parts.

It's very beautiful.

And you're pouring the water in slowly

cos you don't want to alter the oxygen.

That's right. The oxygen will alter by itself.

If we do it nice and slowly, it should take a while to alter.

Now we'll bring the shrimp across.

Here they are - Palaemon elegans. Relatives of the common prawn.

This one used to be fished commercially off Britain.

At least, its big brother did.

These particular shrimps you only find intertidally in tide pools.

They're the ones you often see almost colourless against the sand.

Yeah. They can change colour within a few minutes.

Quite busy at the moment.

They're normally extremely active animals.

We just have to leave them to settle down a bit down now,

and see what happens.

Because it is found in the higher rock pools, where the biggest

changes in oxygen occur, the glass shrimp has evolved a unique

behaviour that we can see in the laboratory.

You'll notice that it's now crawled onto the rock here

and by lying on the side, it has a partial immersion response.

It's not totally out of the water,

so it's actually making oxygen come into the water by beating its

back limbs and also inside its gill chamber, it's got a little device

called a scaphognathite, which is beating and oxygenating the water.

If they were totally out of the water, the gills would collapse

and it suffocates. If it stays in the water, it suffocates.

This partial immersion response...

Is a way of keeping alive in a time of crisis.

That's exactly right.

Its closely-related, deeper-water relative

Palaemon serratus doesn't do this behaviour.

So this guy is able to survive for longer in shallower

and more challenged rock pools than its close relative.

That's right. And we see that on the shore.

You go the middle, lower shore, you get both species.

The high shore and it's only this one.

So it's a tough little beast.

It is this sort of ingenious behaviour

that makes rock pool creatures so resilient

to the extremes of the intertidal environment.

They've risen to every challenge that has been thrown at them.

Through geological history, some of these animals have survived

global catastrophes that have wiped out much of life on Earth.

With their physiological and behavioural adaptations,

they have had what it takes to endure.

But - what does the future hold for them?

The tide's almost in but there's still some animals that can survive

having been out of water for more than half the day.

And the last toughest ones are barnacles,

and when the sea finally splashes over them,

they'll feed at last, extracting all their nutrients

in just an hour or two in the whole day.

Like all intertidal animals, barnacles have to deal with

fluctuating conditions on both a daily and seasonal basis.

However recent research suggests that barnacles and other

creatures have to cope with changes over a much bigger timescale.

Changes that we may be responsible for.

Nova Mieskowska of the Marine Biological Association

has been analysing long-term data on barnacles here in Devon.

Well, if you don't know barnacles very well, it has to be said that

they do all look rather alike but when you do know barnacles well,

like you, you know that there are subtle differences between species

and that those species' differences are connected to climate change.

Is that right?

Yes, we've found over the many decades that we've been studying

barnacles all around the UK but especially down in the South West

here that the warm water barnacles, which you can see here

with the slightly more greenish tinges, they're kite shaped.

These warm water barnacles have become a lot more abundant,

especially over the last 20, 25 years

since climate change really started to take hold.

Their northern limits are in Scotland for warm water barnacles

and they go and they go all the way down south

past the Mediterranean and slightly into north Africa,

whereas the cold water barnacles these are the ones

that are slightly whiter.

This is the one Semibalanus balanoides here.

I can see it now. You have to get your eye in, don't you.

Their northern limits go way up into the Arctic Circle,

but their southern limits have been cut back and back, further north.

They used to be in northern Spain in the Bay of Biscay,

where there has been a big trimming northwards

because it's plainly too warm for them to live there anymore.

We're even seeing the effects here in the South West.

We've seen a massive decline in the survival

of these cold water barnacles.

Do they get bigger than this or is this their usual size?

This is quite a representative size, however

when you find shores further up north with just the cold water

species, Semibalanus balanoides can grow quite large.

In its first six months, it can grow significantly bigger than

the warm water ones do.

So given an opportunity, it can increase its size quite rapidly.

And have they got their natural predators dotted around the surface?

Yes, you can see we've got some marauding dog whelks

and these dog whelks do preferentially eat the cold water barnacles

Semibalanus balanoides, so it will be very interesting to see whether,

when we lose these for good in the South West,

whether the dog whelks will be able to change

and feed entirely on the warm water barnacle or not.

Well, I guess the story of evolution is often change or die.

Yes.

It is alarming to think that we might be responsible

for affecting the survival of the creatures we know and love so well.

However because they have adapted to one of the toughest places on Earth,

rock pool animals have outlived many other species they shared

the seas with, including my favourites, the trilobites.

As a palaeontologist, I marvel to think that their

ancestors lived alongside fossil species I have studied

but whose lives I can only really imagine.

And rock pool animals may well outlive us.

For if anything has got what it takes to endure, it is them,

for they are masters of an ever-changing environment.

And if we look around, we can see that they have played

an important part in our own evolution.

Here on Oronsay in the Western Isles of Scotland,

the whole hill behind me is a shell midden.

Our ancestors 7,000 years ago or more collected huge numbers

of limpets and some cockles that became a staple part of their diet.

This shows what an extraordinary asset the intertidal zone

has been in the course of human evolution.

So maybe it is our hunter-gatherer past that leaves us

with such a basic curiosity about rock pools.

Now we number in our billions, we are probably the biggest threat

to the seashore, but, as we have seen, the creatures

of the rock pool have what it takes to survive well into the future.

On my journey, I have had just a brief glimpse into the world

of rock pools but have discovered first-hand the marvels they hold.

Throughout hundreds of millions of years,

they have evolved extraordinary adaptations to survive

one of the toughest environments on Earth.

They have also played a part in our own colonisation of the planet.

May they continue to be a source of joy and wonder to us all.

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