Rock Pool Survivors


Rock Pool Survivors

Professor Richard Fortey reveals the hidden secrets of the fascinating world of rock pools, from how limpets fend off starfish to how hermit crabs choose their shells.


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

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A magical and intriguing place,

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full of remarkable and unusual characters.

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SEAGULLS CALL

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The rock pool is a cornucopia of life.

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It's full of diverse animals.

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

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But this unique environment experiences some of the most

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extreme conditions in the natural world.

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My name's Professor Richard Fortey.

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I just love rootling around in rock pools.

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

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just a collection of wonderful and interesting animals.

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They also provide a window into the past.

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Part the weeds on any rock pool

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and you open the curtains onto a life and death drama

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that has been played out for hundreds of millions of years.

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Some of the creatures that live here have outlived the dinosaurs,

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and have evolved truly extraordinary adaptations to survive.

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I want to show you how rock pool creatures have stood

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the test of time.

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This is the intertidal zone -

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the land between the high and low tide marks.

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Here, animals have to cope with extreme fluctuations in moisture,

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temperature and salinity, as well as predators on land and in sea.

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It is a hostile place in which to survive.

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

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and this has a profound effect on all living things,

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even the sea weeds.

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For more than a billion years,

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life on Earth was dominated by very simple single-celled organisms.

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Slime, if you like.

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This rock's covered in it.

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But those organisms included photosynthesising blue-green

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bacteria called cyanobacteria that formed living films and breathed

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oxygen into the atmosphere, thereby transforming the early Earth.

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

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multi-celled organisms - algae.

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Doing the same job, still photosynthetic,

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

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Of course, most people know it simply as seaweed.

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

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the sheer variety and volume of them is staggering.

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A quarter of the total global energy captured by photosynthesis is

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fixed here in the intertidal zone.

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

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Constantly changing salinity and exposure can have a dramatic effect

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on their survival, and determine where they colonise the beach.

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Distinct patterns from upper to lower shore can be seen.

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This is known as zonation.

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The intertidal zone can be divided into four vertical zones...

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Each zone is exposed to moisture, temperature and salinity

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in different ways, and this dictates what can survive.

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Seaweed produce eggs and sperm.

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After 24 hours, the fertilised eggs develop into embryos

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which are extremely sensitive to the fluctuating levels of salinity

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in each zone.

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Exposure to rainwater can have a dramatic effect.

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It's dead.

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The rainwater penetrates the cells by osmosis,

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causing them to swell and burst.

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And this is what determines where different species of seaweed

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colonise the beach.

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The environments in the different intertidal zones play

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a vital role in controlling where an organism can survive.

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Rock pool animals can go without food for a long time,

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they can survive changes in salinity, they're extremely tough.

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All this means they've evolved a whole series of adaptations

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to cope with life in the in-between zone.

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

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The exposed shore is now subject to unpredictable changes.

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Changes that depend on the weather,

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the time of year, and the time of day.

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Here, temperatures can range from freezing to baking,

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oxygen levels fluctuate,

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and salinity can increase or decrease,

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causing body tissues to dehydrate or swell with water.

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

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there is a more immediate problem.

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There is now less room for everyone to live

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and resources are diminished.

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Everything is dictated by competition.

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Finding a good position becomes a matter of life or death

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for all the creatures here.

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For anemones, it is important to have a good spot

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to catch the most food.

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

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

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'And to shed some light on their lives,

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

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So, Mark, sea anemones are beautiful creatures,

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

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they just sit there waiting for food to come along.

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

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and that means that they have to consume food,

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and one of the things that sea anemones have to do

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before they can consume it is to capture their food.

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Can you see the feeding tentacles?

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There are six rows of tentacles on the top of the animal, 192 in total,

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and just by looking at them for a small amount of time,

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you can see that the tentacles are moving about,

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

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towards this structure in the middle of the animal.

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This is the oral disc.

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-Otherwise known as a mouth.

-A mouth, yeah.

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'Nematocysts are stinging cells

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'common to all anemones and jellyfish.'

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'When stimulated, they fire a venomous dart

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'attached to a thread into their prey.'

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

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

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and dropping it over the ring of tentacles.

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

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

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It's closing all six rings,

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it's pushed the food back down towards its mouth.

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And they're not just for trapping prey, either.

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In this species of sea anemone,

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there are specialised tentacles simply for fighting...

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..and these specialised tentacles appear as little blue beadlets

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in a ring around the outside of the six rings of the feeding tentacles,

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and they will use them in combat with rival anemones

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of the same species, and therefore require exactly the same resources

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in terms of a good place in the rock pool.

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'Using a specialist time-lapse camera,

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

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'Twisting their flexible bodies, anemones take aggressive swipes

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'at each other, tearing off ribbons of skin.'

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

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'We may barely give anemones a second glance,

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'but their remarkable fighting behaviour has allowed them

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

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'and has helped them thrive on our beaches

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'for around 540 million years.'

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

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'They have left the pools altogether, taking up residence

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

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'Around 530 million years ago, molluscs developed hard shells

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'to house their soft body parts,

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'creating a microclimate into which they could retreat.'

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

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It's a living fossil. The chiton.

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

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and grip tightly to the surface of the rock.

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

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and the mollusc that has done this most successfully

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is still with us in every rock pool and on every rocky shore.

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It's the limpet.

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'Professor Stephen Hawkins of the University of Southampton

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'is a limpet expert.'

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I'm told that they vary in conicality

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according to where they are on the shore.

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Yes, and also with age.

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

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

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because the circumference is where water gets lost when the tide's out.

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'Retaining moisture is vital if the limpet is to survive

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'the drying effects of the sun.'

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They've got a big extensive foot, you can see on this animal here,

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and essentially, it's a very complicated

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biological suction device. That's how it works.

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'This adaptation allows them to hold on to the rock

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'and resist the force of the waves.'

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'Surprisingly, limpets are territorial.

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

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'As the tide starts to go down

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'they return to this place and hunker securely down.'

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

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'As the tide covers them,

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'limpets leave their home scars and begin to feed.'

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'Limpets are very important grazers on the seashore.

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'However, there is intense competition.'

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'To see exactly what impact this has,

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'we have to go to the laboratory.'

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I collected these this morning at low tide

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just as the tide was about to come over them,

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

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

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

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Little tentacles coming out?

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Yes, they have these fantastic sensory tentacles

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all the way round the edge of the shell.

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

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

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

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and biological environment when they're out foraging.

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And foraging means scraping algae and other things

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off the surface of the rock.

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That's right. They feed by scraping the rock surface

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using their radula, yes.

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'The radula of the limpet is a ribbon-like tongue

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'covered in teeth.

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

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'The limpet's radula is tipped with haematite,

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'an extremely hard material that allows the limpets

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'to graze on hard surfaces.'

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

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on the ecology of the seashore.

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

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to keep limpets out,

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and all the rest of the area here is where limpets were able

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to forage freely, and just six months later...

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Wow.

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

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

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bladder wrack, fucoids, covering the rock in the absence

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of the limpet grazing, so basically,

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the limpets, through their radulae, really control the algae.

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'So although limpets appear to be immobile and stuck to the rocks,

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'in fact, they have a much more complicated life cycle

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'that plays an important part in the ecology of the intertidal zone.'

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'The rising tide gives animals an opportunity to hunt for food,

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'but this also means they can become the hunted.'

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'Starfish belong to a phylum of animals called the echinoderms,

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'which first appeared in the fossil record

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'more than 500 million years ago.'

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'Starfish have macabre eating habits.

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'Using their strong sticky tube feet,

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'they force open the shells of molluscs and then,

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'pushing their stomach out through their mouth,

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'they digest the animal inside.'

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

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

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So what are we looking for here?

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What happens, usually, is that the limpets get agitated

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when they sense a predator in the area and then,

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when the starfish is in contact with the limpet,

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the limpet tends to raise up

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and then it will often stamp down on the starfish and maybe drive it off.

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'In a rock pool,

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'there is nothing quite as sinister as a marauding starfish.'

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'Small limpets have no choice but to flee.'

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'A lucky escape.'

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'Large limpets, however, stand their ground.'

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

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'to prevent it climbing on top.'

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Look at that!

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

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'deterring an attack.'

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

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of wiggly tube feet, if I was a limpet.

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There he goes. Look at that.

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It's really very agitated.

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

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'Unseen by us,

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'there are many battles being fought beneath the waves.'

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'Over time, predators and prey have developed a range of adaptations

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'to attack and defend.'

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This is a dog whelk.

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A fearsome predator in the rock pools.

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'This carnivore has devised an ingenious way

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'of hunting other molluscs...

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'..and one of its favourite prey are mussels.'

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'Mussels are filter feeders sieving off the abundant food

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'that drifts in the upper ocean.'

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'They attach themselves to the rock surface by strong threads

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'which they secrete through their muscular foot.'

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'These threads enable them to cling to the rocks,

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'despite the relentless pounding of the ocean waves.'

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However, the stationary mussel

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is an easy target for prowling dog whelks.

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Their lethal weapon is a radula.

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A short, horny ribbon containing many rows of teeth,

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which are used like a file in combination with an acid secretion,

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to drill through the shells and tear the flesh of the mussel.

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It's a gruesome attack.

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

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Sensing a nearby attack, others in the colony

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start to produce more and more sticky threads.

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

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which will starve to death.

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The hard shell of molluscs like the dog whelk

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persist long after the soft parts of the animal itself have decayed away,

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but these empty shells don't go to waste.

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

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another takes over.

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Empty shells are put to good use

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

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Hermit crabs use shells as a very effective defence against predators,

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and their bodies have evolved to fit them perfectly.

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Unlike other crabs, their abdomen has become soft

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and asymmetrical, and their back legs are very reduced,

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allowing them to fit inside shells.

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The asymmetry of their claws also allows them to close up

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the entrance to the shell as a defence against predators.

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

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it must also afford it some camouflage.

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So these shells are obviously a protection.

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That's right.

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But are the crabs even choosier

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about which types of shells they pick up?

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The crabs are incredibly choosy about what they want.

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They'll spend a lot of time and effort deciding

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whether to change shells, whether a potential new shell is a good one.

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I mean, they're also known to be particular about the colour of the

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shell, at least in terms of if it's contrast against the background.

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We can run a little experiment here, so what I have are two containers

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with a dark coloured substrate,

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and I have some littorina obtusata shells.

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These are called citrina and dark reticulata.

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

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What I'm going to do is place these shells, so you can see straight away

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that, to our eyes at least, the citrina shells really stand out,

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

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

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

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Now, the other half of the experiment

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is to take four crabs in dark reticulata shells.

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So I'll find those.

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If you fish out four crabs in dark reticulata shells.

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One, two, three, four. There we go.

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We'll put them into here,

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

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So these crabs can move into shells that blend in,

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and these crabs can move into shells that stand out.

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Very particular about moving into new shells.

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

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than the shell they're coming out of.

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I think he's going to come out. There he goes.

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Swapped shells, there we go. Gone from yellow into dark,

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and I can count here that three of the crabs are in dark shells.

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And blending in well with the background.

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'Whereas the ones in the dark shells stay where they are.'

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What it shows overall if we'd run this experiment lots and lots

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of times, the overall trend would be that significantly more crabs

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would be in the darker coloured shells, and that just goes to show

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how important blending into the background is for these animals.

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'Choosing their shells carefully is a matter of survival

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'for the hermit crab,

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'as this affords it the camouflage and protection it needs

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'to hide from roaming predators.'

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'Anticipating tidal change

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'is a problem all rock pool creatures face.'

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'Dr David Wilcockson of the University of Aberystwyth

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

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So the tide is out,

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and the question is how do the organisms on the beach

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know when it is coming in?

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That's actually a very good question,

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because all organisms, including ourselves,

0:22:090:22:12

have biological clocks which enable us to anticipate changes

0:22:120:22:16

in our environment such as night and day,

0:22:160:22:19

and in this case, the incoming and outgoing of the tides,

0:22:190:22:21

and this organism we have buzzing around in these tanks

0:22:210:22:25

is the marine equivalent of the woodlouse.

0:22:250:22:28

It's an animal called Eurydice pulchra,

0:22:300:22:33

and Eurydice has a very good 12.4 hour, or tidal clock,

0:22:330:22:38

whereas ours is run on a 24-hour basis.

0:22:380:22:41

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

0:22:410:22:45

and feed and breed, and then what they'll do

0:22:450:22:47

before the tide goes out is actually bury back into the sand

0:22:470:22:51

so they maintain their preferred position on the shore.

0:22:510:22:54

'Maintaining the best position on the shore

0:22:550:22:57

'is essential for survival.'

0:22:570:22:59

'To best illustrate tidal rhythms,

0:23:000:23:02

'David has devised a unique experiment.'

0:23:020:23:05

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

0:23:050:23:09

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

0:23:090:23:13

and there is an individual Eurydice in each of these tubes

0:23:130:23:16

and they are all inactive at the moment,

0:23:160:23:18

because currently they are expecting it to be low water.

0:23:180:23:21

When they expect high water, they will start to swim,

0:23:230:23:26

and across each tube is a little infrared beam,

0:23:260:23:29

and when they swim through that beam, the beam is broken

0:23:290:23:33

and the beam break is recorded on the computer.

0:23:330:23:37

I see.

0:23:370:23:38

We can actually turn those recordings into plots,

0:23:380:23:40

so we can visualise the activity,

0:23:400:23:42

and this is a plot from one individual Eurydice,

0:23:420:23:47

and you can see these black bars here

0:23:470:23:49

represent beam breaks or activity periods,

0:23:490:23:52

and these bouts of activity are occurring every 12.4 hours.

0:23:520:23:55

On the nail.

0:23:550:23:57

A very precise 12.4 hour rhythm,

0:23:570:23:59

so we can actually show they have a tidal rhythm,

0:23:590:24:01

and the important thing is that this rhythm will continue

0:24:010:24:04

in the absence of any tides.

0:24:040:24:06

'The tide outside has now risen, and there is a definite change

0:24:150:24:19

'in activity of our subjects.'

0:24:190:24:20

Well, there's an amazing sight.

0:24:220:24:23

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

0:24:230:24:26

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

0:24:260:24:28

or they're expecting it to be high tide,

0:24:280:24:30

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

0:24:300:24:34

I can see the numbers going up.

0:24:340:24:35

That's right, and those beam breaks are being recorded

0:24:350:24:38

on the monitor here.

0:24:380:24:40

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

0:24:400:24:43

That's right, yeah.

0:24:430:24:44

But obviously this internal clock needs some controls on it.

0:24:440:24:49

I mean, are there things in the natural environment

0:24:490:24:52

that help set those controls?

0:24:520:24:53

There are. What happens is that each individual animal,

0:24:530:24:57

its clock will be slightly different to the next one.

0:24:570:25:01

-Their clocks drift out of phase...

-Right.

0:25:010:25:04

..with the natural cycle, if we remove it

0:25:040:25:06

from its natural environment.

0:25:060:25:08

So the incoming and outgoing tide

0:25:080:25:10

actually re-synchronises their clock.

0:25:100:25:13

Life in rock pools is more complicated than we thought.

0:25:130:25:16

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

0:25:160:25:19

Creatures of the rock pool provide one of the most sensitive

0:25:260:25:29

barometers to monitor the way our natural environment is changing.

0:25:290:25:34

'Like all intertidal animals, barnacles have to deal with

0:25:360:25:40

'fluctuating conditions on both a daily and seasonal basis.'

0:25:400:25:43

'However, recent research suggests that barnacles and other creatures

0:25:460:25:50

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

0:25:500:25:54

'Changes that we may be responsible for.'

0:25:540:25:57

'Nova Mieskowska of the Marine Biological Association

0:25:590:26:03

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

0:26:030:26:08

We've found over the many decades that we've been studying barnacles

0:26:090:26:13

all around the UK, but especially down in the southwest here,

0:26:130:26:17

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

0:26:170:26:20

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

0:26:200:26:24

These warm water barnacles have become a lot more abundant,

0:26:240:26:27

especially over the last 20, 25 years since climate change

0:26:270:26:30

really started to take hold.

0:26:300:26:33

Their northern limits are in Scotland

0:26:330:26:34

for the warm water barnacles,

0:26:340:26:36

and they go all the way down south,

0:26:360:26:38

past the Mediterranean and slightly into north Africa

0:26:380:26:41

whereas the cold water barnacles,

0:26:410:26:43

these are the ones that are slightly whiter.

0:26:430:26:46

Here's one. You can see. This is Semibalanus Balanoides here.

0:26:460:26:49

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

0:26:490:26:52

Their northern limits go way up into the Arctic Circle,

0:26:520:26:56

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

0:26:560:26:59

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

0:26:590:27:02

where there has been a big trimming northwards

0:27:020:27:04

because it's just plainly too warm for them to live there any more.

0:27:040:27:08

And we're even seeing the effects here in the southwest.

0:27:080:27:11

We've seen a massive decline

0:27:110:27:12

in the survival of these cold water barnacles.

0:27:120:27:16

And have we got their natural predators

0:27:160:27:17

dotted around on the surface?

0:27:170:27:19

Yes. You can see that we've got some marauding dog whelks,

0:27:190:27:23

and these dog whelks do preferentially

0:27:230:27:25

eat the cold water barnacles Semibalanus Balanoides,

0:27:250:27:28

so it will be very interesting to see whether,

0:27:280:27:30

when we lose these for good in the southwest,

0:27:300:27:33

whether the dog whelks will actually be able to change

0:27:330:27:35

and then feed entirely on the warm water barnacle or not.

0:27:350:27:39

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

0:27:390:27:42

Yes.

0:27:420:27:43

It is alarming to think that we might be responsible

0:27:460:27:48

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

0:27:480:27:53

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

0:27:560:27:59

on Earth, rock pool animals have outlived many other species

0:27:590:28:04

they shared the seas with.

0:28:040:28:06

'As a palaeontologist, I marvel to think that their ancestors

0:28:060:28:10

'lived alongside fossil species I have studied,

0:28:100:28:13

'but whose lives I can only really imagine.'

0:28:130:28:15

'And rock pool animals may well outlive us.

0:28:170:28:21

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

0:28:210:28:25

'for they are masters of an ever-changing environment.'

0:28:250:28:28

Subtitles by Red Bee Media Ltd

0:28:520:28:55

Palaeontologist Professor Richard Fortey reveals the hidden secrets of the fascinating and surprising world of rock pools.

The constantly changing environment of a rock pool creates one of the most hostile places to live on earth. Dramatic fluctuations in moisture, temperature and salinity all have to be overcome by the rock pool inhabitants in order to survive.

Professor Fortey is joined by some of the UK's leading marine biologists to take a detailed look at how some of these remarkable rock pool creatures cope with their ever-changing world - from how limpets fend off marauding starfish to how hermit crabs choose their shells.


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