0:00:07 > 0:00:11Every day when the tide retreats, a secret world is exposed.
0:00:11 > 0:00:14A magical and intriguing place
0:00:14 > 0:00:17full of remarkable and unusual characters.
0:00:22 > 0:00:25The rock pool is a cornucopia of life.
0:00:25 > 0:00:28It's full of diverse animals.
0:00:28 > 0:00:30Some we're familiar with, some we're not.
0:00:30 > 0:00:33There's a nice little cushion star,
0:00:33 > 0:00:35there's a shrimp,
0:00:35 > 0:00:38there's a dog whelk, there's a top shell.
0:00:39 > 0:00:42There's a little pipefish wiggling about.
0:00:42 > 0:00:45There's a porcelain crab.
0:00:45 > 0:00:48All of these animals have their own ecological parts to play
0:00:48 > 0:00:50in the life of the rock pool.
0:00:51 > 0:00:53But this unique environment
0:00:53 > 0:00:58experiences some of the most extreme conditions in the natural world.
0:00:59 > 0:01:02My name's Professor Richard Fortey and like everybody else,
0:01:02 > 0:01:06I just love rootling around in rock pools.
0:01:06 > 0:01:09But I'm a palaeontologist, so for me, rock pools are more
0:01:09 > 0:01:12than just a collection of wonderful and interesting animals.
0:01:12 > 0:01:15They also provide a window into the past.
0:01:17 > 0:01:21Part the weeds on any rock pool, and you open the curtains
0:01:21 > 0:01:24onto a life and death drama that has been played out
0:01:24 > 0:01:26for hundreds of millions of years.
0:01:28 > 0:01:31Some of the creatures that live here have outlived the dinosaurs
0:01:31 > 0:01:35and have evolved truly extraordinary adaptations to survive.
0:01:38 > 0:01:40I want to show you how rock pool creatures
0:01:40 > 0:01:42have stood the test of time.
0:01:46 > 0:01:49We have created our own rock pool laboratory
0:01:49 > 0:01:52deep in the heart of the National Marine Aquarium in Plymouth.
0:01:56 > 0:02:00Here, under the guidance of some of Britain's leading marine biologists,
0:02:00 > 0:02:03we will take a closer look at rock pool creatures
0:02:03 > 0:02:06and reveal just how they have evolved to cope
0:02:06 > 0:02:08with the ever-changing tide.
0:02:09 > 0:02:12Life in rock pools is more complicated than we thought.
0:02:12 > 0:02:14I think it's far more complicated than we thought.
0:02:14 > 0:02:17We will investigate how they compete for food and space...
0:02:17 > 0:02:21There are specialised tentacles simply for fighting.
0:02:21 > 0:02:24..and reveal the surprising behaviour they use
0:02:24 > 0:02:26to fight off predators.
0:02:26 > 0:02:28It's really very agitated.
0:02:28 > 0:02:30It becomes like a sort of animated mushroom.
0:02:31 > 0:02:34This is The Secret Life Of Rock Pools.
0:02:48 > 0:02:52It's high tide and the little world of the rock pool
0:02:52 > 0:02:56is connected to the greater world of the ocean beyond.
0:02:56 > 0:02:59It's a good place to be. There's normal salinity,
0:02:59 > 0:03:02there's plenty of oxygen and above all,
0:03:02 > 0:03:06there's nutrients coming in from beyond.
0:03:06 > 0:03:09And yet, it's not going to last for long.
0:03:09 > 0:03:12In a few hours, everything will change.
0:03:16 > 0:03:22As the tide falls, life becomes very different for the creatures here.
0:03:22 > 0:03:27The exposed shore is now subject to unpredictable changes.
0:03:27 > 0:03:30Changes that depend on the weather, the time of year,
0:03:30 > 0:03:31and the time of day.
0:03:33 > 0:03:37Here, temperatures can range from freezing to baking,
0:03:37 > 0:03:42oxygen levels fluctuate and salinity can increase or decrease,
0:03:42 > 0:03:47causing body tissues to dehydrate or swell with water.
0:03:47 > 0:03:51But before any of these changes even begin to come into play,
0:03:51 > 0:03:53there is a more immediate problem.
0:03:54 > 0:03:58There is now less room for everyone to live
0:03:58 > 0:04:00and resources are diminished.
0:04:01 > 0:04:04Everything is dictated by competition.
0:04:05 > 0:04:08Finding a good position becomes a matter of life or death
0:04:08 > 0:04:10for all the creatures here.
0:04:12 > 0:04:15For anemones, it is important to have a good spot
0:04:15 > 0:04:17to catch the most food.
0:04:17 > 0:04:22Anemones appear sedentary, but they do move around very slowly.
0:04:25 > 0:04:30To find, secure and defend the best spot, they have a secret weapon.
0:04:32 > 0:04:33And to shed some light on their lives,
0:04:33 > 0:04:38Dr Mark Briffa of the University of Plymouth has come into the lab.
0:04:39 > 0:04:44So, Mark, sea anemones are beautiful creatures,
0:04:44 > 0:04:48but most people might think that they're pretty inactive.
0:04:48 > 0:04:50They just sit there waiting for food to come along.
0:04:50 > 0:04:55Yes, they are relatively slow-moving animals, but they are animals,
0:04:55 > 0:04:57and that means they have to consume food.
0:04:57 > 0:05:01And one of the things sea anemones have to do before they can consume it
0:05:01 > 0:05:03is to capture their food.
0:05:03 > 0:05:05Can you see the feeding tentacles?
0:05:05 > 0:05:08There are six rows of tentacles on the top of the animal,
0:05:08 > 0:05:13192 in total. And just by looking at them for a small amount of time,
0:05:13 > 0:05:16you can see that the tentacles are moving about
0:05:16 > 0:05:20and these tentacles are there to trap food and bring it in
0:05:20 > 0:05:22towards this structure in the middle of the animal.
0:05:22 > 0:05:25- This is the oral disc. - Otherwise known as a mouth.
0:05:25 > 0:05:26A mouth, yes.
0:05:26 > 0:05:30And they ingest the food through their oral disc, or their mouth.
0:05:30 > 0:05:34They have two different types of cell which will help them trap food.
0:05:34 > 0:05:38Nematocysts are stinging cells common to all anemones
0:05:38 > 0:05:39and jellyfish.
0:05:40 > 0:05:43When stimulated, they fire a venomous dart
0:05:43 > 0:05:46attached to a thread into their prey.
0:05:47 > 0:05:51We can look at the use of the tentacles to trap food
0:05:51 > 0:05:56by taking a small piece of food, this is a little piece of limpet,
0:05:56 > 0:06:00and dropping it over the ring of tentacles.
0:06:00 > 0:06:03They kind of close in on it and pull it down.
0:06:03 > 0:06:05Oh, it likes that. It likes that a lot.
0:06:05 > 0:06:07We've got a very hungry anemone here.
0:06:07 > 0:06:10It's closing all of the tentacles, all six rings.
0:06:10 > 0:06:15It's closing them in to push the food back down towards its mouth.
0:06:15 > 0:06:18And they're not just for trapping prey either.
0:06:18 > 0:06:21In this species of sea anemone, there are specialised tentacles
0:06:21 > 0:06:23simply for fighting.
0:06:24 > 0:06:29And these specialised tentacles appear as little blue beadlets
0:06:29 > 0:06:33in a ring around the outside of the six feeding tentacles.
0:06:33 > 0:06:35Hence the name "beadlet".
0:06:35 > 0:06:37That's where it gets its name from, yeah.
0:06:37 > 0:06:41And they will use them in combat with rival anemones
0:06:41 > 0:06:45who are of the same species and therefore require exactly
0:06:45 > 0:06:49the same resources in terms of a good place in the rock pool.
0:06:51 > 0:06:54Using a specialist time-lapse camera,
0:06:54 > 0:06:58we can speed these battles up to see what's really happening.
0:07:04 > 0:07:06Twisting their flexible bodies,
0:07:06 > 0:07:09anemones take aggressive swipes at each other,
0:07:09 > 0:07:11tearing off ribbons of skin.
0:07:27 > 0:07:31Losers have no choice but to find another place to settle.
0:07:47 > 0:07:50We may barely give anemones a second glance,
0:07:50 > 0:07:53but their remarkable fighting behaviour has allowed them
0:07:53 > 0:07:56to colonise the most sought-after locations in the rock pool
0:07:56 > 0:07:59and has helped them thrive on our beaches
0:07:59 > 0:08:02for around 540 million years.
0:08:04 > 0:08:08Other creatures have dealt with the lack of space very differently.
0:08:08 > 0:08:12They have left the pools altogether, taking up residence
0:08:12 > 0:08:15on the rocks, where they are exposed at low tide.
0:08:17 > 0:08:20Around 530 million years ago,
0:08:20 > 0:08:25molluscs developed hard shells to house their soft body parts,
0:08:25 > 0:08:29creating a microclimate into which they could retreat.
0:08:29 > 0:08:33And one of the first animals to do this is still with us.
0:08:33 > 0:08:35It's a living fossil. The chiton.
0:08:36 > 0:08:40The chiton has a number of plates which allow it to shuffle around
0:08:40 > 0:08:43and grip tightly to the surface of the rock.
0:08:43 > 0:08:47But an even more effective way of doing this is under a single shell,
0:08:47 > 0:08:50and the mollusc that has done this most successfully is still
0:08:50 > 0:08:53with us in every rock pool and every rocky shore.
0:08:53 > 0:08:54It's the limpet.
0:08:59 > 0:09:02Apart from clinging steadfastly to rocks,
0:09:02 > 0:09:06limpets play an important part in the ecology of the rocky shore.
0:09:07 > 0:09:10They too have to compete for resources.
0:09:15 > 0:09:18Professor Stephen Hawkins, of the University of Southampton,
0:09:18 > 0:09:20is a limpet expert.
0:09:20 > 0:09:24You might think that a limpet is a limpet is a limpet,
0:09:24 > 0:09:26but actually we've got three British species.
0:09:26 > 0:09:29We do have three British species, we have...
0:09:29 > 0:09:32These are all patella vulgata.
0:09:33 > 0:09:36And nearby, we also have patella depressa,
0:09:36 > 0:09:39which is a Southern species of limpet and it goes from Senegal
0:09:39 > 0:09:43up to North Wales. And underneath here, I know there's
0:09:43 > 0:09:47a patella aspera - they have a nice hat of seaweeds on top of them.
0:09:47 > 0:09:49- And there it is.- And there it is.
0:09:49 > 0:09:52I'm told that they vary in conicality according to where
0:09:52 > 0:09:55- they are on the shore. - Yes, and also with age.
0:09:55 > 0:10:00I think as they get bigger and older, they tend to get more conical
0:10:00 > 0:10:03and it makes quite a lot of sense to be conical like this,
0:10:03 > 0:10:06because the circumference is where water gets lost
0:10:06 > 0:10:09when the tide's out, so there's more of an animal contained,
0:10:09 > 0:10:12there's more biomass contained, within a more conical limpet
0:10:12 > 0:10:16than, say, a flatter one, or a younger one.
0:10:16 > 0:10:19What about hiding under these weeds?
0:10:19 > 0:10:21Well, different species have different habits.
0:10:21 > 0:10:25Patella vulgata actually likes to shelter under seaweed.
0:10:25 > 0:10:29They can actually munch away at the bases of the seaweeds
0:10:29 > 0:10:34and even, by feeding at night, sort of gnaw away on the ends.
0:10:34 > 0:10:38'So what physiological adaptation allows limpets to hang on
0:10:38 > 0:10:40'like the proverbial limpet?'
0:10:40 > 0:10:45They've got a big extensive foot. You can see on this animal here
0:10:45 > 0:10:47and essentially it's a very complicated
0:10:47 > 0:10:50biological suction device. That's how it works.
0:10:50 > 0:10:54I mean, I notice most of the limpets seem to be on the rocks,
0:10:54 > 0:10:56but are they in the rock pools as well?
0:10:56 > 0:10:57You do get...
0:10:57 > 0:10:59some limpets in rock pools.
0:10:59 > 0:11:02The juveniles settle out of the plankton
0:11:02 > 0:11:05- and actually settle in rock pools. - Is that one there?- That's one there.
0:11:05 > 0:11:09And they get attracted by chemicals in the pink algae that are in
0:11:09 > 0:11:11this rock pool. It's a very good indication
0:11:11 > 0:11:13of what's a nice place to settle,
0:11:13 > 0:11:16and the limpets use these rock pools as a nursery ground
0:11:16 > 0:11:18for the first year or so of their lives.
0:11:18 > 0:11:20And then they move up onto the barer rock surfaces?
0:11:20 > 0:11:23They move up. And they're quite vagrant when they're young, but once
0:11:23 > 0:11:29they get to 15-20mm, then they start homing on a fairly regular basis.
0:11:31 > 0:11:34Surprisingly, limpets are territorial.
0:11:34 > 0:11:38They create a depression in the rock known as a home scar.
0:11:38 > 0:11:42As the tide starts to go down, they return to this place
0:11:42 > 0:11:44and hunker securely down.
0:11:48 > 0:11:53Territorial fights are common, and losers are prised off the rock.
0:11:55 > 0:11:58I think limpets are really neat,
0:11:58 > 0:12:01and they're a great experimental animal to work with.
0:12:01 > 0:12:05They're so interesting and they move, but they don't move too far,
0:12:05 > 0:12:07and you can do all sorts of things with them.
0:12:07 > 0:12:09I'm afraid I'm rather fond of limpets.
0:12:09 > 0:12:11Well, I've become a fan too.
0:12:13 > 0:12:17As the tide covers them, limpets leave their home scars
0:12:17 > 0:12:19and begin to feed.
0:12:19 > 0:12:22Limpets are very important grazers on the seashore.
0:12:22 > 0:12:25However, there is intense competition.
0:12:25 > 0:12:28To investigate further, we have to go back into the laboratory.
0:12:30 > 0:12:35Stephen, you were up early this morning collecting us some limpets.
0:12:35 > 0:12:38Yes, I collected these this morning at low tide just as the tide
0:12:38 > 0:12:40was about to come over them,
0:12:40 > 0:12:44so we should be able to stimulate them to set off on their
0:12:44 > 0:12:47foraging excursions, to go off feeding, if we put them in the tank.
0:12:54 > 0:12:59It doesn't take long before they sense they're surrounded with water.
0:12:59 > 0:13:02Yes, when the tide is out they're on a home scar, which they create
0:13:02 > 0:13:04in the rock, which their shell fits really well.
0:13:04 > 0:13:06One of the limpets is still on its home scar,
0:13:06 > 0:13:09but the other is off and raised up.
0:13:09 > 0:13:11Little tentacles coming out?
0:13:11 > 0:13:14Yes, they have these fantastic sensory tentacles all the way around
0:13:14 > 0:13:15the edge of the shell.
0:13:15 > 0:13:20The big ones, the primary tentacles, actually match with those rays
0:13:20 > 0:13:23you can see on the shell, and there's smaller tentacles
0:13:23 > 0:13:27in between, and that gives lots of information about the physical
0:13:27 > 0:13:29and biological environment when they're out foraging.
0:13:29 > 0:13:33And foraging means scraping algae and other things
0:13:33 > 0:13:35off the surface of the rock.
0:13:35 > 0:13:38That's right, they feed by scraping the rock surface
0:13:38 > 0:13:39using their radula, yes.
0:13:41 > 0:13:44The radula of the limpet is a ribbon-like tongue
0:13:44 > 0:13:45covered in teeth.
0:13:45 > 0:13:49It moves back and forth scraping algal slime from the rocks.
0:13:50 > 0:13:54The limpet's radula is tipped with haematite - an extremely hard
0:13:54 > 0:13:57material that allows the limpets to graze on hard surfaces.
0:14:02 > 0:14:05So what's the consequence of that?
0:14:05 > 0:14:09'Stephen's research has shown that limpets have a profound effect
0:14:09 > 0:14:11'on the ecology of the seashore.'
0:14:11 > 0:14:14When they're off foraging, and this is where I fenced the rock
0:14:14 > 0:14:17to keep limpets out, and all the rest of the area here is where
0:14:17 > 0:14:21- limpets were able to forage freely, and just six months later...- Wow.
0:14:21 > 0:14:22Yes, it's amazing, isn't it?
0:14:22 > 0:14:26Just six months later there's a really dense growth of seaweeds,
0:14:26 > 0:14:30bladderwrack, fucoids covering the rock in the absence
0:14:30 > 0:14:33of the limpet grazing, so basically the limpets,
0:14:33 > 0:14:36through their radulae, really control the algae.
0:14:38 > 0:14:42Limpets are synonymous with rock pools, but through millions of years
0:14:42 > 0:14:45of evolution, they have pushed back the boundaries and have left
0:14:45 > 0:14:48the pools to colonise the rocks along the shore.
0:14:51 > 0:14:56The rising tide not only gives limpets an opportunity to feed -
0:14:56 > 0:14:58it brings with it danger.
0:14:59 > 0:15:01Starfish.
0:15:03 > 0:15:07Starfish belong to a phylum of animals called the echinoderms,
0:15:07 > 0:15:12which first appear in the fossil record more than 500 million years ago.
0:15:15 > 0:15:18Starfish have macabre eating habits.
0:15:19 > 0:15:23Using their strong, sticky tube feet, they force open the shells
0:15:23 > 0:15:27of molluscs and then, pushing their stomach out
0:15:27 > 0:15:30through their mouth, they digest the animal inside.
0:15:37 > 0:15:42Limpets have been locked in an arms race with starfish for millions
0:15:42 > 0:15:46of years, and have evolved their own way of dealing with them.
0:15:48 > 0:15:50So, what are we looking for here?
0:15:50 > 0:15:54What happens, usually, is that the limpets get agitated
0:15:54 > 0:15:57when they sense a predator in the area and then,
0:15:57 > 0:16:00when the starfish is in contact with the limpet, the limpet tends
0:16:00 > 0:16:04to raise up, and then it will often stamp down on the starfish
0:16:04 > 0:16:06and maybe drive it off.
0:16:06 > 0:16:08Let's see if that behaviour happens.
0:16:21 > 0:16:25In a rock pool, there is nothing quite as sinister
0:16:25 > 0:16:26as a marauding starfish.
0:16:50 > 0:16:54Small limpets have no choice but to flee.
0:17:03 > 0:17:04A lucky escape.
0:17:09 > 0:17:12Large limpets, however, stand their ground.
0:17:17 > 0:17:22'Using the edge of the shell, a limpet can push the starfish away
0:17:22 > 0:17:24'to prevent it climbing on top.'
0:17:24 > 0:17:25Look at that!
0:17:25 > 0:17:29'Continually scraping at the arm can damage the tube feet,
0:17:29 > 0:17:31'deterring an attack.'
0:17:32 > 0:17:35I don't think I'd like to be approached by a great battery
0:17:35 > 0:17:38of wiggly tube feet, if I was a limpet.
0:17:38 > 0:17:41- Didn't think they did this behaviour. - There he goes, look at that.
0:17:41 > 0:17:43It's really very agitated.
0:17:43 > 0:17:46It becomes like an animated mushroom in the end, doesn't it?
0:17:48 > 0:17:50Well, we can't say that rock pools lack drama.
0:18:01 > 0:18:04I'll never look at limpets in the same way again.
0:18:04 > 0:18:08Beneath that implacable shell hides a strong personality.
0:18:11 > 0:18:16Unseen by us, their battles with starfish have been fought
0:18:16 > 0:18:18beneath the waves for millennia.
0:18:21 > 0:18:24But it is not just limpets that have to face predators.
0:18:26 > 0:18:29All the creatures in the rock pool must be
0:18:29 > 0:18:33constantly on their guard, and have evolved many different ways
0:18:33 > 0:18:37of dealing with potential attackers from land and sea.
0:18:37 > 0:18:41The majority of molluscs living around rock pools rely on
0:18:41 > 0:18:45their hard shells for protection against predators,
0:18:45 > 0:18:47but not all of them do so.
0:18:47 > 0:18:54This orange blob is actually a sea slug, and it's protected because
0:18:54 > 0:18:59it absorbs toxins from other sources, in this case the sponges
0:18:59 > 0:19:04that it eats. So in spite of its vulnerable appearance,
0:19:04 > 0:19:07it's actually rather well protected against predators.
0:19:10 > 0:19:14It's pressure from predators that has encouraged them to evolve
0:19:14 > 0:19:17these incredible defence mechanisms.
0:19:21 > 0:19:24This is a lemon sea slug.
0:19:35 > 0:19:39Even more remarkable is the sea slug elysia.
0:19:39 > 0:19:42It sucks in rock pool algae
0:19:42 > 0:19:46and keeps the photosynthetic cells alive, providing energy
0:19:46 > 0:19:47direct from the sun.
0:19:53 > 0:19:58This rare Celtic sea slug, found in rock pools around the UK,
0:19:58 > 0:20:02is unusual in that it is descended from terrestrial slugs.
0:20:02 > 0:20:06It is often found grazing out of the water as well as under it.
0:20:14 > 0:20:17At high tide, the rock pools are sometimes visited
0:20:17 > 0:20:19by another mollusc.
0:20:19 > 0:20:22But this mollusc couldn't be more different
0:20:22 > 0:20:25from its relative, the sea slug.
0:20:25 > 0:20:27It is a fast-moving killer -
0:20:27 > 0:20:28the cuttlefish.
0:20:31 > 0:20:35Cuttlefish belong to a class of molluscs called cephalopods
0:20:35 > 0:20:37and are widely considered
0:20:37 > 0:20:39to be the most intelligent of all invertebrates.
0:20:39 > 0:20:43I think they are the ultimate rock pool predator.
0:20:46 > 0:20:49At the Marine Biological Association in Plymouth
0:20:49 > 0:20:53I have a great opportunity to get up close and personal
0:20:53 > 0:20:55with these magnificent animals.
0:20:57 > 0:21:00The ones here are kept for fisheries research
0:21:00 > 0:21:03and are easily trained to take food from my hand.
0:21:05 > 0:21:06Wonderfully accurate vision.
0:21:06 > 0:21:09Tentacles at the front,
0:21:09 > 0:21:12two of which are modified to grasp the prey.
0:21:16 > 0:21:19They also have remarkable colour-changing abilities.
0:21:22 > 0:21:26This colour change is often used for camouflage,
0:21:26 > 0:21:30allowing them to sneak up on unsuspecting prey.
0:21:36 > 0:21:38They're hiding in little tubes down there,
0:21:38 > 0:21:42which are obviously there for their comfort, so they can lurk.
0:21:42 > 0:21:45And when food comes around, well, out they pop.
0:21:45 > 0:21:49Let's see if we can get the bigger one to come up this time.
0:21:49 > 0:21:52Come on, you know you want it!
0:21:53 > 0:21:55Whoa! It inked me!
0:22:01 > 0:22:05Cuttlefish are one of the largest predators to visit the rock pool.
0:22:08 > 0:22:10Some rock pool predators though
0:22:10 > 0:22:14are far less conspicuous but no less deadly.
0:22:19 > 0:22:21This is a dog whelk.
0:22:21 > 0:22:24A fearsome predator in the rock pools.
0:22:26 > 0:22:28Unlike its limpet cousins,
0:22:28 > 0:22:32this carnivore has devised an ingenious way
0:22:32 > 0:22:34of hunting other molluscs.
0:22:35 > 0:22:38And one of its favourite prey are mussels.
0:22:41 > 0:22:43Mussels are filter feeders,
0:22:43 > 0:22:47sieving off the abundant food that drifts in the upper ocean.
0:22:48 > 0:22:51They attach themselves to the rock surface
0:22:51 > 0:22:55by strong threads which they secrete through their muscular foot.
0:22:57 > 0:23:00These threads enable them to cling to the rocks
0:23:00 > 0:23:03despite the relentless pounding of the ocean waves.
0:23:05 > 0:23:11However, the stationary mussel is an easy target for prowling dog whelks.
0:23:12 > 0:23:17Their radula is specially modified to drill through the shells
0:23:17 > 0:23:20to reach the soft flesh of the mussel.
0:23:20 > 0:23:22It's a gruesome attack.
0:23:31 > 0:23:36Mussels, however, can turn the tables on a dog whelk.
0:23:36 > 0:23:39Sensing a nearby attack, others in the colony
0:23:39 > 0:23:42start to produce more and more sticky threads.
0:23:42 > 0:23:45If they make contact, it can spell doom for the dog whelk,
0:23:45 > 0:23:48which will starve to death.
0:23:52 > 0:23:56The hard shell of molluscs like the dog whelk
0:23:56 > 0:23:59persist long after
0:23:59 > 0:24:01the soft parts of the animal itself have decayed away.
0:24:01 > 0:24:05But these empty shells don't go to waste.
0:24:08 > 0:24:12In the rock pool, when one species dies or moves on
0:24:12 > 0:24:14another takes over.
0:24:16 > 0:24:18Empty shells are put to good use
0:24:18 > 0:24:23by one of my favourite rock pool creatures, hermit crabs.
0:24:24 > 0:24:28Hermit crabs use shells as a very effective defence against predators
0:24:28 > 0:24:31and their bodies have evolved to fit them perfectly.
0:24:33 > 0:24:36Unlike other crabs, their abdomen has become soft and asymmetrical
0:24:36 > 0:24:39and their back legs are very reduced,
0:24:39 > 0:24:42allowing them to fit inside shells.
0:24:43 > 0:24:45The asymmetry of their claws
0:24:45 > 0:24:48also allows them to close up the entrance to the shell
0:24:48 > 0:24:50as a defence against predators.
0:24:52 > 0:24:56The crab's shell must not only be tough enough to withstand an attack,
0:24:56 > 0:24:59but must also afford it some camouflage.
0:25:00 > 0:25:05Dr Mark Briffa from Plymouth University has returned to our lab
0:25:05 > 0:25:10to demonstrate how crabs go about choosing shells.
0:25:10 > 0:25:13Well, nothing in nature is wasted and that goes for our shells too,
0:25:13 > 0:25:17because here they are with a new occupant.
0:25:17 > 0:25:19That's right. The common European hermit crab.
0:25:19 > 0:25:22What's happened in hermit crab evolution
0:25:22 > 0:25:27is that they've become adapted to occupy this free resource.
0:25:27 > 0:25:30So they're taking advantage of somebody else's hard work,.
0:25:30 > 0:25:32That's right. The snail has put all the effort
0:25:32 > 0:25:33into growing these shells,
0:25:33 > 0:25:36which means that the hermit crab doesn't have to.
0:25:36 > 0:25:37And how big do they grow?
0:25:37 > 0:25:40Well, these are the sorts
0:25:40 > 0:25:43that are the size of hermit crabs that you'll find in rock pools.
0:25:43 > 0:25:45This guy...
0:25:46 > 0:25:48is the same species.
0:25:48 > 0:25:49My goodness!
0:25:49 > 0:25:55These tiny little guys and this monster. They're all adults.
0:25:55 > 0:25:57Shall I pop that one in here so we can see him come out?
0:25:57 > 0:26:01You can see the contrast in sizes.
0:26:01 > 0:26:03Sort of orders of magnitude,
0:26:03 > 0:26:07bigger than these tiny little intertidal specimens.
0:26:07 > 0:26:10Pushing some of the smaller guys out of the way.
0:26:11 > 0:26:15This is really as big as the common European hermit crab will get.
0:26:18 > 0:26:20- So these shells are obviously a protection.- That's right.
0:26:20 > 0:26:23But are the crabs even choosier
0:26:23 > 0:26:26about which types of shells they pick up?
0:26:26 > 0:26:30The crabs are incredibly choosy about what they want.
0:26:30 > 0:26:33They'll spend a lot of time and effort
0:26:33 > 0:26:35deciding whether to change shells,
0:26:35 > 0:26:38whether a potential new shell is a good one.
0:26:38 > 0:26:43They're also known to be particular about the colour of the shell,
0:26:43 > 0:26:48at least in terms of its contrast against the background.
0:26:48 > 0:26:49We can run a little experiment here,
0:26:49 > 0:26:53So, what I have are two containers with a dark-coloured substrate
0:26:53 > 0:26:58and I have some Littorina obtusata shells.
0:26:58 > 0:27:01These are called citrina and dark reticulata.
0:27:01 > 0:27:05The only thing that's really different about them is the colour.
0:27:05 > 0:27:08What I'm going to do is place these shells,
0:27:08 > 0:27:11so you can see straight away that, to our eyes at least,
0:27:11 > 0:27:14the citrina shells really stand out
0:27:14 > 0:27:18and the dark reticulata shells don't stand out so much.
0:27:18 > 0:27:21So, I'm going to take four crabs in the citrina shells.
0:27:24 > 0:27:29And give them the option to move into the empty black shells.
0:27:29 > 0:27:31Now, the other half of the experiment
0:27:31 > 0:27:34is to take four crabs in dark reticulata shells.
0:27:34 > 0:27:36So, I'll find those.
0:27:36 > 0:27:39If you fish out four crabs in dark reticulata shells.
0:27:39 > 0:27:41One, two, three four.
0:27:41 > 0:27:43We'll put them into here,
0:27:43 > 0:27:46and these guys have the option of moving into citrina shells.
0:27:46 > 0:27:50So, these crabs can move into shells that blend in.
0:27:50 > 0:27:53These crabs can move into shells that stand out.
0:27:53 > 0:27:55Very particular about moving into new shells.
0:27:55 > 0:27:58They want to make sure that a new shell is absolutely better
0:27:58 > 0:28:00than the shell they're coming out of.
0:28:00 > 0:28:02I think he's going to come out.
0:28:02 > 0:28:05- There he goes. - He's swapped shells. There we go.
0:28:05 > 0:28:07He's gone from yellow into dark.
0:28:07 > 0:28:08And I can count here
0:28:08 > 0:28:12that three of the crabs are in dark shells on this stage,
0:28:12 > 0:28:15blending in well with the background.
0:28:15 > 0:28:16Except for that stubborn one there,
0:28:16 > 0:28:19which, of course, has stayed obstinately in a yellow shell.
0:28:19 > 0:28:21Standing right out against the background.
0:28:21 > 0:28:23Maybe it hasn't made its decision yet,
0:28:23 > 0:28:27maybe it hasn't spotted that the shell stands out.
0:28:27 > 0:28:29It might have just got it wrong. What it shows overall,
0:28:29 > 0:28:32if we had run this experiment lots and lots of times,
0:28:32 > 0:28:34the overall trend would be
0:28:34 > 0:28:38that significantly more crabs would be in the darker-coloured shells.
0:28:38 > 0:28:41And that just goes to show how important
0:28:41 > 0:28:44blending into the background, or crypsis, is for these animals.
0:28:46 > 0:28:47'And it's not just camouflage
0:28:47 > 0:28:50'that's important as a defence against predators.
0:28:50 > 0:28:52'The shell must also fit well
0:28:52 > 0:28:56'if it is to give the crab the best chance of survival.
0:28:56 > 0:29:00'Because of this, there is intense competition for shells.'
0:29:00 > 0:29:01OK, so here we are.
0:29:01 > 0:29:04These crabs have been isolated for about 16 hours.
0:29:04 > 0:29:08We're going to use this tank as an arena to stage a fight in.
0:29:08 > 0:29:12And this large crab is in a shell that's too small for it.
0:29:12 > 0:29:15- It's uncomfortable. - It doesn't fit in there very well.
0:29:15 > 0:29:19It's trying to withdraw but the claws are still exposed.
0:29:19 > 0:29:21So let's put him in there.
0:29:21 > 0:29:23This is a smaller crab
0:29:23 > 0:29:26and you can see he's withdrawing right into that shell.
0:29:26 > 0:29:28He's shaking around in a rather loose coat.
0:29:28 > 0:29:30He's got a very, very spacious house.
0:29:30 > 0:29:34So we've got a big crab in a shell that's too small
0:29:34 > 0:29:39and a little crab in a shell that is just right for the big crab.
0:29:56 > 0:29:58'The large crab adopts intimidation tactics
0:29:58 > 0:30:02'in an attempt to make the smaller crab leave it's shell.'
0:30:09 > 0:30:11It's a little bit like a war of attrition.
0:30:11 > 0:30:13Who can keep going for the longest.
0:30:13 > 0:30:17Will the attacker wear itself out with the shell rapping
0:30:17 > 0:30:20before the defender decides to give up.
0:30:21 > 0:30:23Here we go.
0:30:23 > 0:30:27Eviction! So the attacking crab just evicted the defending crab.
0:30:27 > 0:30:29The attacking crab has gone into the shell
0:30:29 > 0:30:32that it's just pulled the defending crab out of,
0:30:32 > 0:30:35and it's trying to keep the defending crab...
0:30:35 > 0:30:37No, the defending crab has now gone into the shell
0:30:37 > 0:30:40that the attacking crab vacated.
0:30:40 > 0:30:42But not putting up much of a fight, I have to say.
0:30:42 > 0:30:45Well, you're not going to hang around without a shell
0:30:45 > 0:30:47if you can possibly avoid it.
0:30:48 > 0:30:51Hermit crabs are an evolutionary marvel,
0:30:51 > 0:30:55perfectly adapted to recycle the discards of another species
0:30:55 > 0:30:57as a defence against predators.
0:30:59 > 0:31:02As well as competing for space and avoiding attack,
0:31:02 > 0:31:06other creatures have evolved remarkable adaptations
0:31:06 > 0:31:10to deal with the ever-changing environment.
0:31:10 > 0:31:12Well, the tide's now really out
0:31:12 > 0:31:15and this place has become quite a hostile environment.
0:31:15 > 0:31:17Everything's drying out.
0:31:17 > 0:31:20You'd think that any organism with any sense
0:31:20 > 0:31:24would have retreated out to sea with the ebbing tide,
0:31:24 > 0:31:28and yet, hiding away here,
0:31:28 > 0:31:30is something really extraordinary.
0:31:32 > 0:31:33It's a fish.
0:31:34 > 0:31:36A blenny.
0:31:36 > 0:31:41Not just one, but several, hiding away in a crack in the rocks.
0:31:41 > 0:31:43They have chosen not to retreat with the tide,
0:31:43 > 0:31:48but to stay and risk life as a fish out of water.
0:31:48 > 0:31:50Gulping air, they absorb oxygen
0:31:50 > 0:31:53through blood vessels in their oesophagus.
0:31:53 > 0:31:55It will be six hours or more before the sea returns
0:31:55 > 0:31:59and they can resume their normal fishy lives.
0:32:07 > 0:32:10By staying put when the tide retreats,
0:32:10 > 0:32:13a blenny does not leave its territory
0:32:13 > 0:32:16and does not have to compete for a new one
0:32:16 > 0:32:18every time the tide returns
0:32:18 > 0:32:21and it also avoids larger predators it might encounter at sea.
0:32:23 > 0:32:25But they must return to their chosen rock crevice
0:32:25 > 0:32:27before the tide retreats.
0:32:27 > 0:32:29Timing it wrong could be fatal.
0:32:35 > 0:32:36Anticipating tidal change
0:32:36 > 0:32:40is a problem all rock pool creatures face.
0:32:41 > 0:32:45Dr David Wilcockson of the University of Aberystwyth
0:32:45 > 0:32:48is going to show me how animals are adapted to cope with this.
0:32:49 > 0:32:51So, the tide is out,
0:32:51 > 0:32:52and the question is
0:32:52 > 0:32:56how do the organisms on the beach know when it's coming in?
0:32:56 > 0:32:59That's actually is a very good question,
0:32:59 > 0:33:03because all organisms, including ourselves, have biological clocks
0:33:03 > 0:33:07which enable us to anticipate changes in our environment,
0:33:07 > 0:33:09such as night and day, and in this case,
0:33:09 > 0:33:11the incoming and outgoing of the tides.
0:33:11 > 0:33:14And this organism we have buzzing around in these tanks
0:33:14 > 0:33:18is a marine equivalent of the woodlouse.
0:33:19 > 0:33:21It's an animal called Eurydice pulchra.
0:33:21 > 0:33:28And Eurydice has a very, very good 12.4-hour or tidal clock,
0:33:28 > 0:33:31whereas ours is run on a 24-hour basis.
0:33:31 > 0:33:34But they come out of the sand and swim when the tide is in
0:33:34 > 0:33:36and feed and breed,
0:33:36 > 0:33:38and then what they'll do before the tide goes out
0:33:38 > 0:33:40is actually bury back into the sand
0:33:40 > 0:33:42so they maintain their position on the shore,
0:33:42 > 0:33:45their preferred position on the shore.
0:33:45 > 0:33:49'And, as we have seen, maintaining the best position on the shore
0:33:49 > 0:33:51'is essential for survival.
0:33:51 > 0:33:53'To best illustrate tidal rhythms,
0:33:53 > 0:33:57'David has devised a unique experiment.'
0:33:57 > 0:34:00So, what we have here, Richard, is activity monitors,
0:34:00 > 0:34:05and in each tube is a little bit of sand and some seawater
0:34:05 > 0:34:08and there's an individual Eurydice in each of these tubes
0:34:08 > 0:34:10and they're all inactive at the moment
0:34:10 > 0:34:14because currently they're expecting it to be low water.
0:34:14 > 0:34:17When they expect high water, they'll start to swim
0:34:17 > 0:34:22and across each tube is a little infrared beam.
0:34:22 > 0:34:25And when they swim through that beam the beam is broken
0:34:25 > 0:34:29and the beam break is recorded on the computer.
0:34:29 > 0:34:32We can actually turn those recordings into plots
0:34:32 > 0:34:34so we can visualise their activity,
0:34:34 > 0:34:38and this is a plot from one individual Eurydice.
0:34:38 > 0:34:42And you can see these black bars here represent beam breaks,
0:34:42 > 0:34:44or activity periods.
0:34:44 > 0:34:47And these bouts of activity are occurring every 12.4 hours.
0:34:47 > 0:34:48On the nail.
0:34:48 > 0:34:51A very precise 12.4-hour rhythm.
0:34:51 > 0:34:53So we can actually show they have a tidal rhythm,
0:34:53 > 0:34:55and the important thing is
0:34:55 > 0:34:59that this will continue in the absence of any tides.
0:35:06 > 0:35:08The tide outside has now risen
0:35:08 > 0:35:12and there is a definite change in activity of our subjects.
0:35:12 > 0:35:15Well, there's an amazing sight.
0:35:15 > 0:35:18It's been a few hours since we looked at them last
0:35:18 > 0:35:20and we can see now that they think it's high tide,
0:35:20 > 0:35:22or they're expecting it to be high tide,
0:35:22 > 0:35:26and they're zooming up and down, crossing the infrared beam.
0:35:26 > 0:35:28- I can see the numbers going up. - That's right.
0:35:28 > 0:35:32And those beam breaks are being recorded on the monitor here.
0:35:32 > 0:35:35So, in nature, this is when they'd be feeding and on the hunt,
0:35:35 > 0:35:41but obviously this internal clock needs some controls on it.
0:35:41 > 0:35:43I mean, are there things in the natural environment
0:35:43 > 0:35:45that help set those controls?
0:35:45 > 0:35:48There are. What happens is that each individual animal,
0:35:48 > 0:35:53its clock will be slightly different to the next one.
0:35:53 > 0:35:56Their clocks drift out of phase with the natural cycle,
0:35:56 > 0:36:00if we remove it from its natural environment.
0:36:00 > 0:36:03So the incoming and outgoing tide actually resets their clock.
0:36:03 > 0:36:05It re-synchronises their clock.
0:36:05 > 0:36:08Life in rock pools is more complicated than we thought.
0:36:08 > 0:36:11It's far more complicated than we thought, yes.
0:36:11 > 0:36:15The ability to anticipate the changing tide is essential.
0:36:15 > 0:36:20Knowing when to feed, breed or hide is vital for any creature
0:36:20 > 0:36:22living on the shore.
0:36:24 > 0:36:28As the tide changes, so do conditions on the beach,
0:36:28 > 0:36:33and this has a profound on all living things - even the seaweeds.
0:36:34 > 0:36:39For more than a billion years, life on Earth was dominated by very
0:36:39 > 0:36:44simple single-celled organisms slime, if you like.
0:36:44 > 0:36:46This rock's covered in it.
0:36:46 > 0:36:50But those organisms included photosynthesizing blue-green
0:36:50 > 0:36:56bacteria called cyanobacteria that form living films and breathed
0:36:56 > 0:37:01oxygen into the atmosphere, thereby transforming the early Earth.
0:37:01 > 0:37:05And about 1.3 billion years ago, they were joined by much larger
0:37:05 > 0:37:08multi-celled organisms algae.
0:37:08 > 0:37:11Doing the same job, still photosynthetic,
0:37:11 > 0:37:16but these today dominate what we see on the beach and in the rock pools.
0:37:16 > 0:37:20Of course, most people know it simply as seaweed.
0:37:23 > 0:37:28With more than 9,000 species of seaweed in the UK alone,
0:37:28 > 0:37:32the sheer variety and volume of them is staggering.
0:37:32 > 0:37:37A quarter of the total global energy captured by photosynthesis
0:37:37 > 0:37:40is fixed here in the intertidal zone.
0:37:40 > 0:37:45So seaweeds are the basis of a rich and complex food chain.
0:37:48 > 0:37:51Seaweeds show a distinct pattern of colonisation,
0:37:51 > 0:37:53from the upper to the lower shore.
0:37:56 > 0:37:59This is known as zonation,
0:37:59 > 0:38:02and gives us a visual indication of how conditions vary.
0:38:04 > 0:38:07But what causes zonation and why is it important?
0:38:10 > 0:38:15Professor Colin Brownlee of the Marine Biological Association
0:38:15 > 0:38:17is going to demonstrate.
0:38:18 > 0:38:23These seaweeds grow on the rocks as you know, and they produce eggs
0:38:23 > 0:38:26and sperm, just like animals, and they're produced into the
0:38:26 > 0:38:30sea water and the eggs are fertilised by the sperm and what they want
0:38:30 > 0:38:34to do is sink to the rock surface and then attach where they can grow.
0:38:36 > 0:38:39We can analyse a sample of sea water under the microscope.
0:38:41 > 0:38:43So what can we see here?
0:38:43 > 0:38:45The large round cells are the eggs.
0:38:45 > 0:38:47Each one of those is a single egg
0:38:47 > 0:38:51and all those small little creatures swimming around them are the sperm.
0:38:54 > 0:38:58Over the next day, the fertilised eggs develop into embryos.
0:39:00 > 0:39:04So this is what they look like after about 24 hours.
0:39:04 > 0:39:09They're no longer spherical eggs.
0:39:09 > 0:39:13It's a pear-shaped embryo and it's protruded a little rhizoid.
0:39:13 > 0:39:15It's a tiny little root.
0:39:16 > 0:39:20All seaweeds get a chance to colonise the beach,
0:39:20 > 0:39:23but one factor deciding survival is rainfall.
0:39:23 > 0:39:28Remarkably, fresh rain water can kill some seaweed embryos.
0:39:28 > 0:39:31And we can actually try to demonstrate that now,
0:39:31 > 0:39:36just by adding some dilute sea water to this dish.
0:39:36 > 0:39:38It's wobbling a bit as I add it.
0:39:40 > 0:39:42It's twitching.
0:39:42 > 0:39:44That's me dropping water onto it.
0:39:45 > 0:39:47- Good Lord!- That's dead.
0:39:47 > 0:39:49That is one seaweed less.
0:39:52 > 0:39:54Fresh water penetrates the cells by osmosis,
0:39:54 > 0:39:56causing them to swell and burst.
0:39:59 > 0:40:03And the difference between different species and how they swell
0:40:03 > 0:40:07really determines where they can survive on that shore.
0:40:09 > 0:40:12So there is a constant bombardment of microscopic seaweed
0:40:12 > 0:40:14embryos upon the shore,
0:40:14 > 0:40:18and order is only maintained by relative salinity.
0:40:26 > 0:40:30Seaweeds provide food and shelter for many other creatures.
0:40:30 > 0:40:35They are the basis upon which much life in the rock pool depends.
0:40:38 > 0:40:42The rock pools highest up the beach are exposed to the air
0:40:42 > 0:40:44for the longest.
0:40:44 > 0:40:48Sunshine causes evaporation that, in turn, causes the salinity
0:40:48 > 0:40:51of the pools to increase and the temperature to rise.
0:40:54 > 0:40:57Oxygen can fluctuate throughout the day and night.
0:41:00 > 0:41:03Here, everything is pushed to its very limit.
0:41:05 > 0:41:09Most active animals choose to abandon the highest rock pools.
0:41:10 > 0:41:15But one of our rock pool favourites has evolved a unique biology
0:41:15 > 0:41:18that has allowed it to survive further up the shore.
0:41:20 > 0:41:26Crabs are arthropods, which means they have jointed legs
0:41:26 > 0:41:31as well as this hard exoskeleton which encloses their body.
0:41:31 > 0:41:35It allows them to tolerate conditions that other animals
0:41:35 > 0:41:36might find difficult.
0:41:36 > 0:41:40Of course, arthropods have been around for a very long time.
0:41:40 > 0:41:46In fact, my own favourite organism, the trilobite,
0:41:46 > 0:41:50is an arthropod, sadly long extinct.
0:41:50 > 0:41:54They died out about 250 million years ago,
0:41:54 > 0:41:57but their relatives, the crustaceans, prospered.
0:41:59 > 0:42:02And it has to be said that if you have an exoskeleton,
0:42:02 > 0:42:04you also have one particular drawback.
0:42:04 > 0:42:09If you want to grow, you have to moult and then you're vulnerable.
0:42:11 > 0:42:15Prior to moulting, a crab secretes enzymes to separate the old shell
0:42:15 > 0:42:16from the underlying skin.
0:42:18 > 0:42:23The crab then absorbs sea water, causing the old shell to come apart
0:42:23 > 0:42:26at a seam that runs around the body.
0:42:26 > 0:42:28The carapace then opens up like a lid
0:42:28 > 0:42:31and the crab extracts itself from the old shell.
0:42:33 > 0:42:39At this time, crabs are extremely vulnerable and will usually hide,
0:42:39 > 0:42:43but moulting is also essential for another function.
0:42:43 > 0:42:47Crabs don't just need to moult when they want to get larger,
0:42:47 > 0:42:49they also have to moult when they want to mate.
0:42:49 > 0:42:53To investigate this further I have invited Dr David Wilcockson
0:42:53 > 0:42:54back into the lab.
0:42:54 > 0:42:56David, what's going on?
0:42:56 > 0:42:59Well, Richard the female crab has to be soft,
0:42:59 > 0:43:03she has to have just moulted in order for the male to transfer sperm.
0:43:03 > 0:43:06So that's a soft-shell crab stage.
0:43:06 > 0:43:10Yes. So she's very vulnerable at this stage. She's quite immobile.
0:43:10 > 0:43:14She's very soft and makes a nice meal for a predator,
0:43:14 > 0:43:16so the male crab will actually guard her.
0:43:16 > 0:43:19He'll embrace her like this,
0:43:19 > 0:43:21and the female opens her abdomen, that's the flap
0:43:21 > 0:43:26on the underside of her body, and the male also opens his abdomen,
0:43:26 > 0:43:31and they transfer sperm through a pair of modified legs from the male.
0:43:31 > 0:43:35The male will detect the fact that she's coming into moult
0:43:35 > 0:43:40because she releases pheromones and he'll stand over her and cradle her.
0:43:40 > 0:43:45When she moults, he then flips her over and they copulate,
0:43:45 > 0:43:48and he'll continue to guard her for quite a number of days,
0:43:48 > 0:43:50maybe up to ten days or more.
0:43:50 > 0:43:53So if anybody finds a pair of crabs like this,
0:43:53 > 0:43:56the message is "put them back".
0:43:56 > 0:43:57Absolutely, yes.
0:43:57 > 0:44:01So I can actually show you a neat trick here in the lab
0:44:01 > 0:44:04with the pheromone from females.
0:44:04 > 0:44:11I've got here some water that I've collected from an aquarium
0:44:11 > 0:44:14that's contained female crabs that are coming into moult
0:44:14 > 0:44:18and I've got an inanimate stone from an aquarium.
0:44:18 > 0:44:22If I drop that into a tank over here, we might be able to see
0:44:22 > 0:44:23the male crab responses.
0:44:36 > 0:44:41So he's scented the stone and he's up on tip toes.
0:44:43 > 0:44:45He's started to cradle.
0:44:45 > 0:44:47He's definitely attracted to it.
0:44:47 > 0:44:49Oh, dear. Poor crab.
0:44:49 > 0:44:51Fancy having a piece of stone for a partner.
0:44:51 > 0:44:55Of course, this is all driven by the pheromone, this behaviour.
0:44:55 > 0:44:58This strong reaction to the scent a female gives off
0:44:58 > 0:45:03when she moults changes the male from potential attacker to defender.
0:45:03 > 0:45:06This is vital if crabs are ever going to mate.
0:45:08 > 0:45:10So what happens next is the question?
0:45:10 > 0:45:14So the male crab has transferred his sperm to the female
0:45:14 > 0:45:18and the female will now produce eggs which are fertilised,
0:45:18 > 0:45:22and if I can just disturb them gently...
0:45:25 > 0:45:27She's not very keen to be disturbed.
0:45:30 > 0:45:31Not keen at all.
0:45:31 > 0:45:35So we can see, I'm trying not to take them apart,
0:45:35 > 0:45:39the female abdomen is open there,
0:45:39 > 0:45:46and underneath there, she will produce a very big mass of eggs.
0:45:47 > 0:45:50We call them berried females.
0:45:50 > 0:45:54The eggs females carry look like small round berries.
0:45:56 > 0:45:59They can carry this egg mass for several months.
0:45:59 > 0:46:02They eventually develop into free-swimming larvae,
0:46:02 > 0:46:06which feed among the plankton for up to three years
0:46:06 > 0:46:08before settling on the seabed as juvenile crabs.
0:46:12 > 0:46:16So you may find what you think is a berried female on the shore
0:46:16 > 0:46:18but actually it may not be,
0:46:18 > 0:46:20and I've got something really quite interesting I found earlier.
0:46:20 > 0:46:22Sounds intriguing.
0:46:25 > 0:46:27This is a shore crab again...
0:46:29 > 0:46:30If I can just pick it up,
0:46:30 > 0:46:36and on the underside we have what appears to be an egg mass.
0:46:36 > 0:46:39It looks like a berried female but in actual fact,
0:46:39 > 0:46:43if we look more closely, this is a male crab,
0:46:43 > 0:46:45and this egg mass is a parasitic barnacle.
0:46:45 > 0:46:47How on Earth do you know that?
0:46:47 > 0:46:50We know it's a barnacle only because of its larval stage.
0:46:50 > 0:46:52It has the same larvae as all the other types of barnacle.
0:46:52 > 0:46:55It's only a millimetre or two long?
0:46:55 > 0:46:59Very small, yes, and the larvae actually penetrate into the body of
0:46:59 > 0:47:04the crab and sends out rootlets into the body parts and absorbs nutrients.
0:47:04 > 0:47:06It takes over the crab.
0:47:06 > 0:47:08It's like something out of Alien.
0:47:08 > 0:47:11It's converting the crab into a machine
0:47:11 > 0:47:13for producing more parasitic barnacles.
0:47:13 > 0:47:16That's exactly what that egg mass is.
0:47:16 > 0:47:20After the invasion of sacculina, the crab is unable to perform
0:47:20 > 0:47:23its normal function of moulting.
0:47:24 > 0:47:27Sacculina interferes with the male crab's hormones.
0:47:27 > 0:47:32It becomes sterile and even begins to behave like a female.
0:47:32 > 0:47:37It's hard to believe that such a frankly disgusting creature
0:47:37 > 0:47:41is related to the most abundant things you find on practically every
0:47:41 > 0:47:42shore, the regular barnacle.
0:47:42 > 0:47:44It's very different, yes.
0:47:46 > 0:47:51Crabs are a rock pooling favourite but few can imagine how complex
0:47:51 > 0:47:54and interesting their world really is.
0:47:54 > 0:47:56As well as their fascinating reproductive cycle,
0:47:56 > 0:48:00crabs have evolved to thrive in some of the most extreme environments
0:48:00 > 0:48:02on the rocky shore.
0:48:05 > 0:48:09To explore the lengths some animals will go in order to survive
0:48:09 > 0:48:11in the most extreme conditions of the rock pool,
0:48:11 > 0:48:14I'm going on a little adventure.
0:48:14 > 0:48:18Something I've never done before night rock pooling
0:48:18 > 0:48:21with Dr John Spicer of Plymouth University.
0:48:24 > 0:48:27I think at night it's more exciting
0:48:27 > 0:48:30because things happen much more quickly.
0:48:30 > 0:48:34There's a lot going on and if you've got a little bit of patience,
0:48:34 > 0:48:37you can see so much, even in a small pool like this.
0:48:37 > 0:48:41It is a busy, violent, exciting place.
0:48:43 > 0:48:46One of the really nice things about night is the fact that
0:48:46 > 0:48:48even animals which would normally be submerged,
0:48:48 > 0:48:52because it's quite humid at night, will come out and they will scuttle
0:48:52 > 0:48:53across the rocks.
0:48:53 > 0:48:56For instance if you look really carefully, you can see
0:48:56 > 0:49:00the shadows of little crabs as they run from crevice to crevice.
0:49:00 > 0:49:03They're no longer in the water but as long as they're damp,
0:49:03 > 0:49:06their gills are such that they can still breathe
0:49:06 > 0:49:07when they're out of water.
0:49:07 > 0:49:12Although this is good news, this is water when the water's gone.
0:49:12 > 0:49:16It's a little refuge, a little sea.
0:49:16 > 0:49:19It's still got its own problems.
0:49:19 > 0:49:22At night, the plants are no longer photosynthesizing.
0:49:22 > 0:49:25The plants and animals are using the oxygen
0:49:25 > 0:49:29and so the oxygen declines throughout the night.
0:49:29 > 0:49:31Sometimes down to zero.
0:49:31 > 0:49:34So you might finish up with somewhere that's
0:49:34 > 0:49:36extremely unpleasant for life.
0:49:36 > 0:49:39It's one of the prices of staying hydrated.
0:49:39 > 0:49:43You have to put up with real severe extremes of oxygen,
0:49:43 > 0:49:47so if I take some water from this rock pool, which should be
0:49:47 > 0:49:50quite low in oxygen, we can take it back to the laboratory,
0:49:50 > 0:49:52and we can actually use it in the laboratory
0:49:52 > 0:49:55and see the response of some of the creatures that live here
0:49:55 > 0:49:57to that low oxygen water.
0:49:59 > 0:50:01There's plenty of water in there!
0:50:02 > 0:50:04Just don't spill it.
0:50:15 > 0:50:19So, Richard, what I'm going to do is actually here in the laboratory
0:50:19 > 0:50:23set up an experiment with two artificial rock pools.
0:50:23 > 0:50:25And which species are we using?
0:50:25 > 0:50:29Now, the animal we will use is an animal called Palaemon elegans,
0:50:29 > 0:50:30which sounds very grand.
0:50:30 > 0:50:34It's a common glass shrimp, where you can see all the inner working parts.
0:50:34 > 0:50:36It's very beautiful.
0:50:36 > 0:50:38And you're pouring the water in slowly
0:50:38 > 0:50:40cos you don't want to alter the oxygen.
0:50:40 > 0:50:43That's right. The oxygen will alter by itself.
0:50:43 > 0:50:46If we do it nice and slowly, it should take a while to alter.
0:50:46 > 0:50:49Now we'll bring the shrimp across.
0:50:49 > 0:50:53Here they are - Palaemon elegans. Relatives of the common prawn.
0:50:53 > 0:50:56This one used to be fished commercially off Britain.
0:50:56 > 0:50:58At least, its big brother did.
0:50:58 > 0:51:03These particular shrimps you only find intertidally in tide pools.
0:51:03 > 0:51:08They're the ones you often see almost colourless against the sand.
0:51:08 > 0:51:13Yeah. They can change colour within a few minutes.
0:51:13 > 0:51:14Quite busy at the moment.
0:51:14 > 0:51:16They're normally extremely active animals.
0:51:16 > 0:51:19We just have to leave them to settle down a bit down now,
0:51:19 > 0:51:20and see what happens.
0:51:22 > 0:51:25Because it is found in the higher rock pools, where the biggest
0:51:25 > 0:51:30changes in oxygen occur, the glass shrimp has evolved a unique
0:51:30 > 0:51:33behaviour that we can see in the laboratory.
0:51:33 > 0:51:36You'll notice that it's now crawled onto the rock here
0:51:36 > 0:51:39and by lying on the side, it has a partial immersion response.
0:51:39 > 0:51:41It's not totally out of the water,
0:51:41 > 0:51:45so it's actually making oxygen come into the water by beating its
0:51:45 > 0:51:50back limbs and also inside its gill chamber, it's got a little device
0:51:50 > 0:51:53called a scaphognathite, which is beating and oxygenating the water.
0:51:53 > 0:51:57If they were totally out of the water, the gills would collapse
0:51:57 > 0:52:00and it suffocates. If it stays in the water, it suffocates.
0:52:00 > 0:52:03This partial immersion response...
0:52:03 > 0:52:06Is a way of keeping alive in a time of crisis.
0:52:06 > 0:52:08That's exactly right.
0:52:08 > 0:52:12Its closely-related, deeper-water relative
0:52:12 > 0:52:14Palaemon serratus doesn't do this behaviour.
0:52:14 > 0:52:20So this guy is able to survive for longer in shallower
0:52:20 > 0:52:23and more challenged rock pools than its close relative.
0:52:23 > 0:52:26That's right. And we see that on the shore.
0:52:26 > 0:52:29You go the middle, lower shore, you get both species.
0:52:29 > 0:52:31The high shore and it's only this one.
0:52:31 > 0:52:33So it's a tough little beast.
0:52:39 > 0:52:42It is this sort of ingenious behaviour
0:52:42 > 0:52:44that makes rock pool creatures so resilient
0:52:44 > 0:52:48to the extremes of the intertidal environment.
0:52:48 > 0:52:51They've risen to every challenge that has been thrown at them.
0:52:53 > 0:52:56Through geological history, some of these animals have survived
0:52:56 > 0:53:00global catastrophes that have wiped out much of life on Earth.
0:53:03 > 0:53:06With their physiological and behavioural adaptations,
0:53:06 > 0:53:09they have had what it takes to endure.
0:53:11 > 0:53:15But - what does the future hold for them?
0:53:15 > 0:53:19The tide's almost in but there's still some animals that can survive
0:53:19 > 0:53:23having been out of water for more than half the day.
0:53:23 > 0:53:27And the last toughest ones are barnacles,
0:53:27 > 0:53:30and when the sea finally splashes over them,
0:53:30 > 0:53:33they'll feed at last, extracting all their nutrients
0:53:33 > 0:53:36in just an hour or two in the whole day.
0:53:39 > 0:53:42Like all intertidal animals, barnacles have to deal with
0:53:42 > 0:53:46fluctuating conditions on both a daily and seasonal basis.
0:53:48 > 0:53:52However recent research suggests that barnacles and other
0:53:52 > 0:53:56creatures have to cope with changes over a much bigger timescale.
0:53:56 > 0:53:59Changes that we may be responsible for.
0:54:01 > 0:54:05Nova Mieskowska of the Marine Biological Association
0:54:05 > 0:54:11has been analysing long-term data on barnacles here in Devon.
0:54:11 > 0:54:15Well, if you don't know barnacles very well, it has to be said that
0:54:15 > 0:54:20they do all look rather alike but when you do know barnacles well,
0:54:20 > 0:54:24like you, you know that there are subtle differences between species
0:54:24 > 0:54:30and that those species' differences are connected to climate change.
0:54:30 > 0:54:31Is that right?
0:54:31 > 0:54:34Yes, we've found over the many decades that we've been studying
0:54:34 > 0:54:39barnacles all around the UK but especially down in the South West
0:54:39 > 0:54:43here that the warm water barnacles, which you can see here
0:54:43 > 0:54:47with the slightly more greenish tinges, they're kite shaped.
0:54:47 > 0:54:50These warm water barnacles have become a lot more abundant,
0:54:50 > 0:54:52especially over the last 20, 25 years
0:54:52 > 0:54:55since climate change really started to take hold.
0:54:55 > 0:54:58Their northern limits are in Scotland for warm water barnacles
0:54:58 > 0:55:00and they go and they go all the way down south
0:55:00 > 0:55:03past the Mediterranean and slightly into north Africa,
0:55:03 > 0:55:07whereas the cold water barnacles these are the ones
0:55:07 > 0:55:09that are slightly whiter.
0:55:09 > 0:55:12This is the one Semibalanus balanoides here.
0:55:12 > 0:55:15I can see it now. You have to get your eye in, don't you.
0:55:15 > 0:55:18Their northern limits go way up into the Arctic Circle,
0:55:18 > 0:55:22but their southern limits have been cut back and back, further north.
0:55:22 > 0:55:25They used to be in northern Spain in the Bay of Biscay,
0:55:25 > 0:55:27where there has been a big trimming northwards
0:55:27 > 0:55:30because it's plainly too warm for them to live there anymore.
0:55:30 > 0:55:33We're even seeing the effects here in the South West.
0:55:33 > 0:55:36We've seen a massive decline in the survival
0:55:36 > 0:55:38of these cold water barnacles.
0:55:38 > 0:55:42Do they get bigger than this or is this their usual size?
0:55:42 > 0:55:44This is quite a representative size, however
0:55:44 > 0:55:47when you find shores further up north with just the cold water
0:55:47 > 0:55:51species, Semibalanus balanoides can grow quite large.
0:55:51 > 0:55:54In its first six months, it can grow significantly bigger than
0:55:54 > 0:55:56the warm water ones do.
0:55:56 > 0:56:00So given an opportunity, it can increase its size quite rapidly.
0:56:00 > 0:56:04And have they got their natural predators dotted around the surface?
0:56:04 > 0:56:07Yes, you can see we've got some marauding dog whelks
0:56:07 > 0:56:11and these dog whelks do preferentially eat the cold water barnacles
0:56:11 > 0:56:15Semibalanus balanoides, so it will be very interesting to see whether,
0:56:15 > 0:56:17when we lose these for good in the South West,
0:56:17 > 0:56:20whether the dog whelks will be able to change
0:56:20 > 0:56:23and feed entirely on the warm water barnacle or not.
0:56:23 > 0:56:26Well, I guess the story of evolution is often change or die.
0:56:26 > 0:56:27Yes.
0:56:30 > 0:56:33It is alarming to think that we might be responsible
0:56:33 > 0:56:37for affecting the survival of the creatures we know and love so well.
0:56:40 > 0:56:45However because they have adapted to one of the toughest places on Earth,
0:56:45 > 0:56:48rock pool animals have outlived many other species they shared
0:56:48 > 0:56:52the seas with, including my favourites, the trilobites.
0:56:54 > 0:56:57As a palaeontologist, I marvel to think that their
0:56:57 > 0:57:02ancestors lived alongside fossil species I have studied
0:57:02 > 0:57:04but whose lives I can only really imagine.
0:57:06 > 0:57:09And rock pool animals may well outlive us.
0:57:12 > 0:57:16For if anything has got what it takes to endure, it is them,
0:57:16 > 0:57:20for they are masters of an ever-changing environment.
0:57:28 > 0:57:32And if we look around, we can see that they have played
0:57:32 > 0:57:34an important part in our own evolution.
0:57:37 > 0:57:41Here on Oronsay in the Western Isles of Scotland,
0:57:41 > 0:57:45the whole hill behind me is a shell midden.
0:57:45 > 0:57:50Our ancestors 7,000 years ago or more collected huge numbers
0:57:50 > 0:57:57of limpets and some cockles that became a staple part of their diet.
0:57:57 > 0:58:01This shows what an extraordinary asset the intertidal zone
0:58:01 > 0:58:04has been in the course of human evolution.
0:58:05 > 0:58:09So maybe it is our hunter-gatherer past that leaves us
0:58:09 > 0:58:11with such a basic curiosity about rock pools.
0:58:13 > 0:58:17Now we number in our billions, we are probably the biggest threat
0:58:17 > 0:58:20to the seashore, but, as we have seen, the creatures
0:58:20 > 0:58:24of the rock pool have what it takes to survive well into the future.
0:58:28 > 0:58:32On my journey, I have had just a brief glimpse into the world
0:58:32 > 0:58:37of rock pools but have discovered first-hand the marvels they hold.
0:58:38 > 0:58:41Throughout hundreds of millions of years,
0:58:41 > 0:58:44they have evolved extraordinary adaptations to survive
0:58:44 > 0:58:47one of the toughest environments on Earth.
0:58:48 > 0:58:52They have also played a part in our own colonisation of the planet.
0:58:54 > 0:59:00May they continue to be a source of joy and wonder to us all.
0:59:18 > 0:59:22Subtitles by Red Bee Media Ltd