Secrets of the Coral Reef - Inside the Ocean’s Underwater Cities

Water, the liquid that oceans are made of, and it fills endless depths. Only few will venture out into the endless open ocean of this vast underwater world. Most of the ocean inhabitants live in the city, as it were, like human societies. Very close together, with friendly neighbours and nasty co-tenants. While dangerous robbers lurk around at the edge of town. To assert yourself here, you have to be equipped with all kinds of tricks and clever strategies, or lie and cheat. Coral reefs are the largest structures in the world. They're giant submerged metropolises. Millions of different inhabitants live in these cities, a universe with coral palaces and soaring towers. Every neighbourhood has a chief. The entire street belongs to this guy. No trespassing, OK? A typical large city, with all the noise and ruckus. Even this little porcupine fish bleats around. It creates this sound with its swim bladder. Two small harlequin shrimps are nibbling on this blue sea star. If someone else shows up, he will catch hell. Another harlequin shrimp busy pulling a bite out of a sea star. More eels in disagreement. A scorpionfish shuffles across the ocean floor. Everything and everybody makes a lot of noise. A boxer crab threatens everybody that comes by, holding two poisonous anemones in its claws. At least the anemone reef looks peaceful. Or does it? The clownfishes are having a huge squabble. Who does this anemone belong to? We can hardly see this fish, but he's easy to hear. A crocodilefish. Does this giant pufferfish hear something? It's a ray that's getting away. A dispute amongst the members of the cichlid family. Who is stronger, and especially who is louder? Two walkman fish are noisily trying to woo a female. Big city noise everywhere, especially when a large structure is being torn down. Everybody lives in their own way in this noisy metropolis. Some hover above it all, but others are more down-to-earth and literally live in the coral sand. These goatfishes, for example. They plough through the ocean floor and catch smaller animals between their teeth. They use their barbels like a dowser. With them, they can detect even the weakest electric fields that point them towards small prey. These gobies also dredge through the sand and feed on what they find in there. There's another way of finding prey in the sand. This blue-spotted stingray uses its wings like shovels, and there's always a chance of stealing something. While one works, others just stand by and feed on what's left over, maybe a piece of crab or a mussel. You have to be very familiar with this area to find the best morsels. This triggerfish uses a strong stream of water. The fish blows away the sand to unearth whatever is hidden inside it. Others are quick to steal whatever they can. The triggerfish is a commanding fish, and most reef inhabitants are easily intimidated by him, except for these strange visitors that don't seem to show fear. In any case, it's best to cover them up with sand. It's not easy for a small fish in a big city. There's always somebody with bad intentions, and you have to be extra careful not to get swallowed, like these moonrasses, for example. So it's always safer for the smallest among the reef inhabitants to show up in schools. It's difficult to keep an eye out for only one little fish, and as a result, the predator often grabs blindly at nothing and comes up empty. To show up in a big mob is always safest. Others are easily distracted at the sight of so much prey all at once. This easygoing giant pufferfish would never have a chance here anyway. Being part of a school of fish is always the best protection from predators. The slow ones are being ignored. No danger here. The school distributes the danger. If there are 1,000 fishes, any of them can be the target, and 999 are going to be just fine. But what about loners, or those that live as a couple? Seahorses, for example, camouflage themselves to look like the coral branches they're holding onto. Even more difficult to see. Where is the animal here? Only the eye betrays this trumpetfish's camouflage. They do everything to not look like themselves. Ornate ghost pipefishes. They're inseparable, a couple. The female is already carrying eggs in her extended belly. Before long, the male will fertilise her eggs. They are still regular fishes, despite their unique costume. This one looks like torn-off seagrass. The spiny waspfish rocks back and forth in the current. Not a monster, but ingenious camouflage. A spider crab that's decorated its head with a sponge. This will deter any predator. Another great camouflage. A barrel decorated with rocks. Only the movement of the gills betrays this stonefish. Even a smack with a fin doesn't bother him. The fish knows it will not hurt him. It only shoves him a little further to the side. Similarly tough are its cousins, the scorpionfishes. They lie there as if dead, especially when others around them freak out. And even when moray eels fight with lionfishes for food, the scorpionfish lies in wait for its chance. And then immediately becomes completely motionless again. The only difference is that it raises its dorsal fins with the poisonous barbs inside. It's completely still, but now also armed. This large anglerfish virtually blends into the background tangle of algae and leaves. Right now, the anglerfish is on high alert. It ejects a large cloud in order to throw off predators. It's not the time to feed or catch anything. Its angling rod is lying flat on the fish's back. The white tip at the end of the rod is used to lure prey. The anglerfish feels threatened by something. Finally, it gives up its camouflage and moves further over, using its fins to float. As soon as the fish retracts the fins, it sinks to the ocean floor. Anglerfishes come in all kinds of shapes and sizes. Their trick is ingenious, using the angling rod as a lure. And every curious little fish becomes dinner. Many animals, including ocean inhabitants, communicate with a mixture of sounds and behaviour. Everybody has to look out for their own survival in this metropolis of ocean inhabitants. But it seems maybe not quite everybody does it by themselves. The partner goby has come to an agreement with two pistol shrimps. The goby keeps an eye on things while the shrimps keep maintaining their home. Larger rocks at the entrance are ideal for securing it. They're an experienced team. The shrimps keep in constant contact with the goby and even massage the fish's belly. The goby will get nervous without these gestures from the shrimps. The pistol shrimps need a cool and collected bouncer at their entrance. Teamwork. This is where the anemones settle with their poisonous tentacles. Whoever gets caught by those will pay for it dearly or lose their life. No wonder that there is virtually no one who likes to settle here. The little clownfishes are the only ones that know how to deal with these tentacles. They cover themselves with the slime of the anemone's tentacles and are protected from the poison. The clowns are safe in here and in return will protect their anemone as well as they can. This seems like a sensible deal. But on occasion, the anemone will pull the tentacles inside its column-shaped body, which means the clownfish's home is gone. When this happens, they panic and look for another anemone that they can hide in for the time being. Unfortunately, this one is already occupied. And the next one as well. And this means having to fight its current owner. There's much to lose for the clowns. Without the anemone, they can't procreate. The clownfishes attach their eggs to the bottom of the column-shaped foot of their anemone, hidden behind the poisonous tentacles. Both male and female tend to the eggs. They fan fresh water on them. The eyes of the babies are already clearly visible. The clowns are very protective of their eggs. It takes the two of them to protect their offspring and only in conjunction with the anemone. Everybody in the reef has to take special care of their young. If they don't, it means their species will simply die out. So it's not surprising that everybody gives it their best effort. This triggerfish has hidden its offspring in a small hollow in the sandy ocean floor. And this means having to constantly protect this open egg site. On occasion, they blow and fan the eggs, but mostly they keep an eye out for predators. This means grave danger. One of the sea urchins is wandering straight towards the nest. The eggs have to be protected at all cost, but how? The triggerfish seems to consider the alternatives. But then it blows and then rams the sea urchin. Again and again until it breaks. The sea urchin's guts spill out into the open. A few more hits for good measure. The danger has been averted. Everything else is easy. Two little damselfishes cling to a coral branch, a pretty and obviously inseparable pair. They appear to have something important to do. And there it is. The female has been laying eggs and glued them to a coral branch. Over and over again, both of them sweep past the eggs to keep them clean. Two very busy parents in the middle of town. Not far from them, the same kind of damselfish, but this one is feeding on the eggs, not tending to them. This damselfish's priorities have changed. Its partner is gone, and it's impossible to bring the eggs through by itself. So it feeds on its own eggs to at least recycle the nutrients. There are many different strategies to give one's offspring a good start. Some are surprisingly easy. The mass mating of the black sea breams. The males are so generous with their sperm that often a cloud of it will engulf the entire wedding party. Countless couples and constantly in motion. A predator would have a hard time to find its bearings in this schooling behaviour. The fish's instincts cause them to mass mate in order to ensure the survival of their species. Cardinal fishes have their own clever means of child protection. It's not time yet. The couple are still flirting and courting with each other, going around and around. We'll leave them to it and take a detour to a couple not quite as gentle with each other. Monkfishes live in groups, and their nests are very close to one another. As a result, all the fishes are alerted to a predator nearing their nests. Hermit crabs love to feast on the eggs of the monkfishes, so the protective parents are all getting nervous. The hermit crab is still far enough away and currently busy with something else. The hermit crab's best defence is its house that it carries around for protection. The patience of the monkfishes is wearing thin. Now everyone attacks in order to bowl the crab away from their nests. They're clearly telling this hermit crab to take a hike, but they should have stayed with their offspring because there's new danger approaching. A gaggle of rainbow wrasses takes advantage of this unique opportunity and ravages the monkfish nests. The predators are quick, brutal and in the majority. Most of the monkfish eggs will be devoured. An attack of this kind would never happen to the cardinal fishes because they are mouth brooders and don't have to watch over a nest that's out in the open. The males carry the eggs in their mouths, up to 20,000 of them. A mobile nursery. They're safe here in their father's mouth, unless, of course, he gets eaten. During the night, when it's dark, the danger is greatest. These underwater cities have a very active nightlife with special attractions. Basket starfishes come out and sit in the current. It's a night-active type of brittle star. With their arms full of tiny little hooks, they filter plankton out of the water to feed. A very elegant feather star. It's on the move to find a better spot to filter plankton. This Spanish dancer is even more graceful and elegant than the brittle stars. It's a type of nudibranch. The giant pufferfish is relaxed and spends the night dozing off on the sandy bottom. Others prefer the safety of their quiet bedroom. A surgeonfish. Everybody settles in for the night as comfortably as possible. The parrotfish tucks itself away in its mucous cocoon for protection against predators. Everybody's sleeping arrangements are different. The poor triggerfish has to stand guard by its nest all night long. There's no time to rest. Some of the predators are quite soft-footed. A cone snail has spotted a small fish. The snail is fast and the prey disappears in the blink of an eye. The night is still young and still full of danger and terror. These lionfishes pose danger to the cardinalfishes. Lionfishes suck in their prey, even if they choke on it. They're hanging around, waiting for their chance. This poor cardinal is sideways in the lionfishes' mouth, eggs and all. Lionfishes aren't the most elegant predators with all their attachments and frayed fins. But this is all part of their camouflage. They're harder to see and blend into the background. Outside the reef, the lionfishes hunt as a school. They drive their prey towards one another with their fanned-out fins. They prefer to attack from above, cornering their prey. This is not a different location, just different light. These special blue lights totally change the appearance of the reef. Now we can see that the coral polyps are bright red and they clearly stand out from their environment. A new and fascinating perspective on the communication of many ocean inhabitants. And suddenly, an eruption of sorts. It looks like smoke, but in reality, it's a type of reproduction. A carpet anemone is ejecting its sperm. In the blue light, it looks like a dramatic eruption. A hermit crab has a magic lantern. The blue light brings out these colours. An entire group of lionfishes on a food foray. The animals don't seem to know any fear. It seems that they're well aware of the poison they carry in their back spines. They even use the light of our dive torches to their advantage. There's no nightly rest to be had here. With the first daylight, new life comes into the reef city. They all seem to start going again, ponderously with armoured fins. As pedestrians, and as light and quiet as possible. Mother and daughter, elegant and synchronous. Some have to hustle along on ten legs. This spider crab runs straight into a brittle star gathering. A giant smash-up in the early morning. Wildly flopping around, everybody tries to get away. As in every city, there are also shopping opportunities here. This moray eel knows her shops well. She rushes to her favourite store every morning. The same with this blue-spotted ray. But they haven't opened yet. It leans on one fin as if to say, it's waiting. Similar problems in a branch further up. These sweet lips are also waiting to be tended to. Here they come. A little late this morning. The small, cleaner wrasses are starting to open their shop and are hungry. They're ready to rid their customers of parasites. This ray is also being served. It likes to have its gills cleaned. This is a barter business. One side gets cleaned, the other fed. A very fair deal. Clownfishes seem to be excluded from this business because they can't leave their anemone. In their case, the cleaner fishes make house calls. This snake eel is also otherwise engaged and the cleaner shrimps are also making house calls for him. Finally, it's the moray's turn. It has a lot of nutritious freeloaders on its skin and in its gills. All of a sudden, the cleaners leave. Why? This titan triggerfish is no better off. The treatment has stopped before he was finished. The cleaner fishes have new customers, clients that have never come to see them before, and they're now being preferred over old customers. The point is that these new clients will like the service here so much that they will come back again. The regular customers have to be patient and wait until the new clients are finished. Cleaner fishes operate solely according to economic rules. This guy has also come for the first time and quite enjoys their service. Sometimes it's too much of a good thing, though. This small black-spotted puffer just wants to look around and is not asking for a cleaning at this time. It swims away quickly and clearly annoyed. It's time to go back to the regular customers before they get impatient and leave for good. Digging deep, where the gills are dirtiest. One last time through the mouth and it's as good as new and the teeth squeaky clean. Now this male moray can start looking for a suitable female. She is clearly not interested in male visitors. This next dugout also does not look encouraging. These two have already found each other. A strange pair with the triggerfish as roommate. They seem to get along just fine. It's difficult to cut in as a stranger. It would also be well advised to stay away from this giant triggerfish. This, on the other hand, might be a different situation. It's important to be patient. This female moray does not seem to be too reluctant. She's showing her teeth, which is not a sign of aggression. Quite the opposite. She finally agrees to a date. The two of them are getting to know each other. Moray eels are very gentle with their mates. And they're good for another surprise, as partners of a phenomenal hunting party. These types of hunting parties have to find each other first in this big city. The right place for the moray and her partner. The morays cooperate with the groupers. It's not so far-fetched. The grouper hunts in the open and the moray inside the reef. The moray is able to get in between cracks and crevices that the grouper can't get to. Together, they're a strong team. A grouper that's trying to convince this moray to hunt with him. The grouper signals this to the moray by rocking back and forth and shaking for all it's worth. All for nothing. This moray doesn't understand and stays in its cave. Next try. Maybe the moray will react this time and swim with him. Again, no luck. Finally, this moray seems to understand the grouper and is hungry. The grouper tilts its head and the moray accepts his invitation. Together, they swim through the reef. The moray prefers to swim between the dark alleyways and the grouper decides it's safer to stay on the main streets around the reef. This is not the right place. The grouper expected to find scared prey here. The moray takes a break and waits. It's not going to happen this way. Again, another invitation to go. They continue through the reef to another coral head. Also nothing for them here either. The moray is getting ambitious as well now and initiates a hunt on her own. She might be better at finding hunting spots. They're successful in this next coral head and then everything happens very quickly. The fishes are panicked and both hunters are able to profit from that. This coral city offers living space for millions. Many hideouts and crevices provide protection for everybody. My home is my castle. While some escape between sharp corals, others curl up in a ball. Octopuses don't have bones and are therefore able to change shape. Their homes are usually very tight and space-saving. They like to live a bit further apart from their neighbours because they're not necessarily always friends with one another. It might be best to put up a wall of stones between them. The ocean offers many hideouts, which is synonymous with survival for many ocean inhabitants. There can also be hideouts that nature never intended to be there, such as this giant wreck, 240 feet deep. A research submarine on its way into the deep comes across a sunken freighter. It's an artificial world with countless nooks and crevices to offer protection and a home for many ocean inhabitants. The wreck has long been overgrown with all kinds of sea life. The side of the hull has become a living wall made of corals and sponges. Sunfishes, with their odd form of movement, patrol this area. They're looking for jellyfish. In steerage, where the current is strongest, filter feeders have found a new home, such as these sea fan corals. They feed on the tiny plankton particles, a unit that has grown together made of technical man-made and organic structures. Even the steel anchor has grown a second skin made of living beings. It offers these delicate creatures a solid structure to grow on. They have all made a comfortable home in this sunken world. Not even the lights of the stub disturb any of the wreck inhabitants. New life has been created out of the tragedy of this sunken ship. The ingenious, life-renewing cycle of nature. The morays are sharing a home. They seem to be exceptionally friendly towards one another, even with this conger eel as their neighbour. Even the octopus remains unharmed by the morays. A unique environment made of steel that offers many different opportunities for ocean inhabitants to use it. Even this fishing net offers a conger eel a place to hide. The original destination of the sub is located a bit deeper, a picturesque underwater city full of ancient amphora architecture. An antique merchant vessel sank here. Only the clay containers, once filled with Greek wine, are still visible. Now they offer homes and protection to many ocean inhabitants. Similar to the reef, hiding places are essential for staying alive down here. The amphorae have a very special shape. They taper towards the bottom, so they could be stored sitting upright in a layer of sand in the hull of the ship. The amphorae and their precious contents were safe during transport. Spiny lobsters welcome these types of caves. Conger eels also feel safe in this mess of amphorae and shards of clay, as if it was made for them and their slender, long bodies. The city of amphorae on the ocean floor provides countless opportunities to observe, disappear and be unseen. Maybe that's why this city seems to be very calm and orderly, even when the conger eel swims above everybody's abode. Nobody panics because they only have to withdraw slowly into their homes and are safe. The same goes for this forkbeard. The conger eel's only chance to make prey down here is to hide himself and ambush somebody. The sunfishes also don't have much luck feeding here, for the same reasons as the conger eel. On the other side of the globe, the Spencer Gulf in southern Australia. Every year, hundreds of giant cuttlefishes meet up to mate. The rocky seabed offers great holes and caves to hide the eggs. The females are smaller than the males and seem to be a bit shy, keeping their arms tucked tightly together. The males, on the other hand, present every bit of themselves openly to impress the females, but also other males that are potential rivals. When the going gets tough, it's always a good idea to hide behind the safety of a cloud of ink... without much success. Escape in reverse. Only the strongest of them have a chance with the females. He's a winner and courts his female by changing his colour to interesting stripes. She has agreed to mate with him and they hug tightly. He deposits his sperm package into her mantle opening. Immediately afterwards, she withdraws into one of the caves on the ocean floor in order to avoid courtship from other males. A clear signal. The male is protecting his female from other rivals. But there's always the unexpected. A so-called sneaker male tries to cut in. This male is much smaller and pretends to be another female by acting like one, very demure and shy. All seems to be fine in the den. He's busy chasing away other rivals from this new supposed female. Especially now that there are two females under his protection. It can't hurt. The two supposed females on the bottom below him are getting closer to each other, the sneaker male and the female. They mate in secret under the protection of the strong male. As soon as the sneaker male is found out, it gets dangerous for him. But he has achieved what he came for and mated with this female. The beatings are nothing compared to the glory of his success. Intelligent strategies and tricks are essential for survival underwater and part of everyday life. A final dive into the magical world of sharks. This spot on a sandbank close to the Bahama Islands has great attraction for these lemon sharks. They come here by the hundreds and nobody exactly knows why. This sandy plateau in the middle of the open ocean is easy for scientists to get to. This experiment will show how sharks react to their own reflection. The metal mirror stands upright on the seabed. The animals that swim towards it will see their own reflection coming closer. How will these lemon sharks react? Now the researchers have to have strong nerves. Things stay surprisingly calm. The foreign object is almost ignored, as if it had been there forever. The sharks either swim past it or underneath it. The scientists expect it a bit more and soon the sharks lose all interest in the mirror. Some sharks even calmly drop to the seabed to rest. The only ones alarmed by this behaviour are the remoras that like to keep moving. They let go of their shark and see if they can't hitch a ride with a more active individual. Shark hopping. This one isn't a good choice either, so they continue to yet another one. All of a sudden something happens to the mirror and the peaceful afternoon on the sandbank changes quickly into the complete opposite. The rope that the mirror is attached to loosens and the mirror falls over. Suddenly and seemingly out of nowhere, several sharks rush to the mirror and show great interest in it. They smell and touch it and they get more and more excited. What happened all of a sudden? None of the scientists have any idea how to explain this change in behaviour. The animals that were calm and collected only a second ago are now completely out of their minds and the situation quickly escalates into a mass attack on the mirror. Do they mistake their own reflection for an opponent? It's time for the divers to leave because it's not safe anymore. The communication of the ocean inhabitants continues to offer new and exciting insights that we will probably never fully understand them.