The Ocean Where Life Begins Under the Sea

It's the birthplace of life on our planet. The source of more than half of the Earth's oxygen. Yet uninhabitable for 80% of the world's species. Millions of years of evolution have left only the strongest and strangest to survive in this wondrous but challenging environment. And it's not all down to size. Microscopic life forms live alongside the mighty, the menacing, and even the mysterious. An eclectic community of creatures all learning how to endure in this sometimes hostile home. From the lovers, to the fighters, the devious, to the delightful. This is life under the sea. The size of the Earth's oceans is simply astounding. A billion cubic kilometres of water. Deeper than our highest mountain. Covering 70% of our planet. Filled with predators of all shapes and sizes. In this sometimes perilous world under the waves, there's only one reason important enough to cross oceans for. To fight for. Or even die for. And that's survival of the species. In this competitive environment, it's all about passing on the best genes. When finding a mate becomes an art form. And courtship can just as easily turn to cannibalism. Some pair up for life. Others for mere moments. But no matter the method, the goal is still the same. The creation of new life. This is parenthood in the ocean. Off the southern coast of Australia, two giant cuttlefish engage in a spectacular inking display. A battle between bulls to determine who has the right to sire the next generation. They're amongst tens of thousands that have gathered to breed. It's an intense fight and for good reason. These cephalopods will only live for a few more months. This is a once-in-a-lifetime chance to pass on their genes. The males dominate at this congregation. They are larger and outnumber the females seven to one. To attract a mate, he turns his mantle into a canvas, displaying his colour range. Females aren't easy to impress, rejecting up to 70% of advances. Sometimes smaller males succeed where larger ones fail. Mimicking the movements of a female to get close to the cow, slipping past bigger bulls unchallenged. If she's receptive, the pair will mate head-to-head, linking their eight arms and two feeding tentacles. Once attached, the male slips his sperm packet into a receptacle under the female's beak. She'll store it until her eggs are ready for fertilisation. Until then, she'll mate with multiple partners. Other males will try to flush out any of their competitors' spermatophores. Generally, only the last suitor will be successful. For the others, all the drama may have ultimately been for nothing. As other lives are about to begin, theirs will soon come to an end. Another cephalopod takes extra care when approaching its mate. He's right to be cautious. There's a chance she might eat him. Octopuses are normally solitary creatures, treating others of their species with suspicion, as potential competition or even food. They only come together when it's time to reproduce. The male approaches carefully, displaying his interest. The female seems responsive. He tentatively reaches out. One of his eight arms doubles as a reproductive organ, his long limbs helping to keep him at a safe distance as he delivers packets of spermatophores into the female. If the situation turns sour, some species have been known to sacrifice their mating arms to escape. Today, there's no need for such drastic action. He departs with life and limb. Cephalopods are amongst the small group of ocean animals that reproduce through physical contact. But in their case, it comes at a cost. These soft-bodied creatures are classified as semelparous, meaning they mate just once. With so much at stake, some cephalopods are determined to breed for success, no matter the cost. The bobtail squid wastes no time on courtship. The male restrains the female with his tentacled arms. The pair can stay in a tight bind for up to three hours. The exhausting ritual is costly for the tiny squid. It takes them around 30 minutes to recover and regain energy to swim, making them easy targets for predators. Their own safety is sacrificed for the survival of the species. The bigfin reef squid is 10 times larger than the bobtail, but it has a more delicate approach to courtship. The male tries to entice with his body posture and patterns in a wonderful display of flashes. If the light show isn't well received, he can choose to accentuate the colour of his reproductive organs instead. A translucent billboard advertising his internal wares. Another mating ritual is even harder to ignore. Hundreds of crimson-coloured crabs have gathered along the coast of Christmas Island. These crustaceans were born in the sea and returned to the shore to reproduce. Breeding takes place in the rocky terraces above the ocean. When the male red crabs head back inland, the females stay behind. Their work in the water has only just begun. It might not always occur en masse, but mating for marine-based crustaceans is sometimes just as much of a spectacle. Male spider crabs can be very possessive when they find the perfect partner. He'll capture and carry her around for up to a week prior to breeding. Mating can only occur just after the seasonal molt, when the new exoskeletons haven't yet hardened. To make sure they don't miss this narrow window of opportunity, some males will grab a female before she's even completed her shedding. Parading her around on the seabed, ensuring no other rivals can get their claws onto her. Until the time is right to breed. Getting carried away in this crazy courtship of crabs. It's true, there are plenty of fish in the sea. More than 20,000 species, in fact. But finding a mate can still be a long and involved process. Many end up in a vicious cycle of fending off rivals. Others need to prove they are a worthy partner. Their persistent approach can play out for days, while a few have turned courtship into an art. The delicate mandarinfish is only tiny, but it's big on demonstrative displays. Most vibrant marine animals get their colour from the foods they eat. But this scaleless fish is one of only two species that can produce its own shade of blue. With something so special to flaunt, it relies on beauty to attract a mate. At dusk, females gather at the reef to watch the larger male perform a courtship dance. The more elaborate the moves, the more likely he is to win over the audience. If all goes to plan, at least one will show her appreciation by joining him centre stage. She rests on his pectoral fin, and cheek to cheek, they rise up through the water column, releasing a cloud of sperm and egg in perfect harmony. Another underwater Romeo hopes to lure a partner by creating a masterpiece in the sand. The male white-spotted pufferfish is the Michelangelo of the sea. He spends days, sometimes weeks, designing a detailed crop circle that he hopes will become a nest. He goes to so much trouble to prove how fine the sand is, the perfect spot to lay some eggs. This finger-long fish furiously flaps his fins along the silt, carving out a circular pattern no less than two metres wide. When the design is done, it's hoped a female will come to appreciate this work of art. Not all fish go to so much effort to find the mate. Some have evolved to simply change sex if there's a shortage of suitable partners. Many reef residents can transition from male to female and vice versa as their communities require. Most parrotfish start their lives as females. Their skin is a dark, dull colour. As they form large schools, they far outnumber the blue-green males. And that can be a problem at breeding time. Once they reach sexual maturity, the biggest in the group will transform into a male to boost the chances of successful reproduction. For clownfish, the change occurs the other way. They all start out as males. When it's time to mate, the bigger of the colony will change into females, the dominant sex. Once they've formed a couple, they pair for life in their anemone home. If she dies, the male will quickly adapt, transitioning into another female to attract a new mate. There's no space for sentimentality in survival of the species. Some animals will aggressively pursue potential partners. Not surprisingly, sharks are amongst them. Once a female white tip has sent out chemical cues that she's fertile, the males will stalk her relentlessly. At first, she resists. But these are pursuit predators, skilled at hunting their quarry. When the male catches up, he sinks his teeth into the female's flank to ensure she doesn't escape. Even the generally more placid species get riled up at breeding time. This tawny nurse shark has latched onto his mate's large pectoral fin and won't let go. The duo continues to twist and turn until he gets into the right position for reproduction. More than one male can fertilise the same litter. If all goes well, she'll produce up to eight pups in six months' time. The females will repeat this process every two years, a rough and tumble path to parenthood. But they're not the only animal that takes a battering to breed. Elephant seals spend the majority of their life at sea. But twice a year, they haul themselves up onto land to molt and to mate. Males arrive early to compete for sections of the beach. These three-tongued bulls throw their weight about to decide dominance on the sand. The battles can be brutal. A lot is at stake. The victor wins exclusive breeding rights. He'll defend his patch for months, mating hundreds of times. The females are willing participants. By mating with the dominant male, the strongest genes will be passed on to the single pup they produce. The bull will fast while he's out of the water, unwilling to risk abandoning his hard-won harem to find food. He'll survive on his extensive blubber reserve, losing more than a third of his body weight every season. It's the price they have to pay to ensure the well-being of their species. Some ocean animals go to extraordinary lengths to mate. Green sea turtles have been known to travel thousands of kilometres to breed. Returning to the waters which they first entered as hatchlings. Researchers believe they achieve this phenomenal feat by navigating via the Earth's magnetic field. Each part of the coastline puts out its own unique signature. Their internal compass recognises the signals from any part of the shoreline they have passed before. For the males, it's an annual journey. The females only return every three to four years. Their part of the breeding process is far more taxing. The male uses the enlarged claws on his front flippers to grasp onto her carapace. His reproductive organ is in his tail, which is always much larger than a female's. They can remain in this position for hours, which is a problem for air-breathing reptiles. Adult green turtles can easily weigh over 100 kilograms each. During mating, the female is responsible for bearing the weight of both of them, struggling to swim with the male on board. She needs to reach the surface every few minutes for air. If she runs out of energy, she's at risk of drowning. It's an exhausting ritual, and one she will repeat multiple times during breeding season. But her hardest work is still a few weeks away. One marine animal faces the ultimate test of endurance to mate. Humpbacks travel halfway across the world to breed. Every year, they leave their feeding grounds in polar waters to make their record-breaking trek to the tropics. Some cover more than 8,000 kilometers each way, the longest migration of any mammal. These 40-ton whales leave with bellies full of krill. They won't eat for the next six months, relying on reserves to see them through. Despite their epic journey, the males still have energy to burn. Once they detect a female, some will engage in a heat run. Multiple males will chase her down, vying for the right to mate. She will only breed once every two to three years, her chosen partner often staying close to protect her and ward off other whales. Other humpbacks have a more haunting way to woo a female. They produce one of the most complex songs in the animal kingdom. Each is unique to the singer and can last more than 30 minutes, played on repeat for hours on end, until a female responds. It's this kind of dedication that decides who is worthy to pass on their genes. For some underwater animals, reproduction is more fantasy-like than physical. Spawning is a carefully choreographed event for the vast coral colonies that make up the world's reefs. It's a synchronized snow globe release of sperm and eggs from the hermaphrodite coral. These tiny reproductive cells known as gametes are only viable for a few hours. To give them the best chance of survival, this annual spectacle must be perfectly timed. Just after a full moon and only when temperatures are ideal, in an upward release, the sperm and egg find each other and become larvae. They float on the currents for weeks before settling on something solid. Only once it's in a new permanent home will the larvae metamorphosize into a polyp. Most hard corals grow no more than two centimetres a year. It can take up to 10,000 years to create a large reef system from these minuscule life forms. Like coral, giant clams are a fixture on the seabed. Each of these mollusks produces both eggs and sperm, but never together. For the best chance of genetic diversity, this mass spawner releases its gametes in separate batches of up to 500 million at a time. The sperm is sent out first, carried away in the current to connect with eggs from other clams before its own are released. Fire urchins clump together to mate, creating dense clouds of gametes that hover above their spines. The vast majority of sea creatures reproduce by spawning, but of the hundreds of millions of larvae produced this way, only a handful will survive. For months they will float through the water column as planktonic life forms, one of the most basic elements in the marine food chain. To increase the odds of success, many fish spawn in large aggregations. A school of colourful antheas has gathered above the coral to mate. At just the right moment, the males rush towards the females, so eggs and sperm are released together. This coordinated breeding is timed to coincide with peak conditions, when large currents will carry the larvae far away from the dangers of the reef. It's thought these mass spawnings deter total predation, allowing as many larvae as possible to escape unscathed. While millions of potential young are left to drift on the high seas, some spawners keep a much tighter hold on their eggs. And it's often the dads keeping watch, caring for the unhatched in unconventional ways, or even giving birth. No other male on earth can do for his offspring what the seahorse does. After the female deposits her eggs in his large front pouch, he fertilises and incubates them for up to 25 days. During this time, the male provides nutrition and bacterial protection to his unhatched young, preparation for delivering them into the underwater world. While not as famous as his seahorse cousin, the male leafy sea dragon is also left looking after his clutch. But instead of being tucked away, these eggs are on display. Up to 250 are attached to the male's tail, cushioned on a special patch which supplies oxygen to the developing fry. It takes around eight weeks for the eggs to turn from bright pink to orange, a sure sign they're almost ready to be released. Role reversals don't end on the reef. Emperor penguins spend at least half their lives in the sea and the remainder on the ice sheets of Antarctica, where they split their parenting duties. Come winter, these monogamous birds go looking for their mate. The female lays her one egg for the season and returns to the sea to hunt, leaving her partner in the bitterly cold conditions to keep the egg warm. He cradles it on his feet, covered in a fold of skin where it stays a toasty 38 degrees Celsius. For several months, he battles the elements, enduring minus 50 degree days with no food to sustain him. The colony huddles together, constantly adjusting position to share the relative warmth of being in the centre of the crowd. But the extreme conditions still take their toll. The males can lose up to half their body weight while waiting for their offspring to emerge. A huge personal sacrifice from these dedicated dads. But caring for the unhatched is not all down to males. On a much warmer coastline, the female Christmas Island crab has remained by the sea. She's been brooding her eggs for 13 days in a beach burrow built by the male. Now she is ready to drop up to 100,000 of them into the water. Conditions for this must be perfect. The right lunar phase is critical to the protection of her eggs. In its last quarter, the moon creates a gentler tide, giving the offspring the best odds of survival in the sea. In these final moments of parenthood, nothing is left to chance. Clownfish are prolific breeders. The female produces up to 1,500 eggs every two weeks, laying them on a flat patch close to her and enemy home and the protection of its stinging tentacles. The male must guard and care for them. For up to 10 days, he will fan the eggs to oxygenate them while keeping predators away. But occasionally, for the good of the group, he will eat the odd one himself. By removing damaged or infertile eggs, he keeps the brood pure. Male anemonefish will even care for a clutch they have no biological link to, going beyond their own parental instincts to invest in their species' survival. Some fertilised eggs are not as diligently tended. After mating, the female reef squid deposits clutches of finger-shaped capsules amongst the fronds of a sunken palm, watched on by her partner. Each sack contains up to 13 eggs. These cephalopod parents won't be around to see them hatch. They only have weeks left to live. Best hope for their offspring is to remain hidden from predators. The female's final act is to coat the eggs in a gelatinous substance, offering extra protection and nourishment before they're abandoned for good. A broadclub cuttlefish has found an even safer place to leave her clutch. Her choice of nursery has its own defence mechanism. Fire coral is a rare stinging species that causes pain on contact. Most fish know to avoid it, which makes it an inspired choice for a female cuttle to deposit her leathery eggs in. They harden as they make contact with the coral's frame. They'll stay like this for 40 days, giving them the best chance of hatching unharmed. A flamboyant cuttlefish has found the perfect hiding place for her eggs. A wayward coconut will become their cocoon. After laying, the female pushes them through a hole in the shell, where they attach to the inside wall. This is where her parental care ends. Her brood is now safe from predators, as it develops its camouflage capabilities ready for the outside world. It's been several weeks since the green turtles have mated. The female has remained around her natal beach, preparing to retrace the path of her own mother more than 20 years earlier. For an animal designed to move in the water, hauling up over sand is hard-going. She needs to find a spot above the high tide mark to prepare a nest. Over several hours, she will use her flippers to dig a pit up to one metre deep. Only when she is satisfied there's no risk of collapse will she begin to deposit approximately 100 eggs. They are soft-shelled and leathery to survive the impact of the drop. Covering the cavity with sand is essential, not just for protection from predators, but also to maintain the perfect temperature for development. Around 30 degrees is ideal, but the heat of the sand has another long-term impact on the hatchlings. It determines their sex. The higher it is, the more likelihood of an all-female brood. Just a degree lower will result in a majority of males. For the female green turtle, her work is almost done. With the little energy she has left, she must drag herself back to the sea for some much-needed nourishment. For her, it's an exhausting journey, but not as hazardous as the future trek for her hatchlings. New life is beginning to emerge. The vast majority of ocean creatures have no parental guidance when they arrive into the underwater world. Some will come equipped to hunt and to hide, but many are hapless fry, a staple for hungry fish. Mangroves become a makeshift nursery for the young and defenceless. These shallow ecosystems offer protection from large predators and the chance for underdeveloped gills to strengthen in the oxygenated water. Bacteria-rich mud provides a ready food supply without the need to leave the sanctuary of the roots. Wherever they start out in life, fish fry have a habit of sticking together. As their swim bladders develop and they leave shelter to actively feed, schooling is their best chance of surviving. It's a simple numbers game. It's much harder to be picked off individually when lost in a crowd. Even fish that mature to a life of solitude will swim together while young. But there are other evolutionary tactics to help them live through their most vulnerable phase. A juvenile rock mover wrasse looks nothing like an adult and for good reason. At this young stage of development, it has little chance of fleeing predators. Instead, it hopes to survive by blending in. Its elongated dorsal fins and body pattern are designed to appear like drifting seaweed, hoping to avoid attention until it is bigger and faster. A juvenile batfish takes its disguise to the next level. Rather than opting for uninteresting, it's trying for potentially poisonous, mimicking the appearance of a toxic flatworm. Vastly different to the broad silvery fins of its adult form. But it's not just other species that pose a threat. In the competitive world of coral reefs, those that share a similar colour, shape and pattern are often viewed as rivals for food. Many are born with a deliberate disguise. Juvenile emperor angelfish look nothing like the adults. Spotted sweetlips dramatically change as they grow, in the hope of giving the young a fighting chance. Another clutch of fry is ready to hatch. The male pot-bellied seahorse carries as many as 2,000 offspring. After three weeks in his pouch, they're expelled into the outside world. These tiny creatures will struggle to reach adulthood. They'll become part of the ocean's planktonic food, feeding many fish further up the chain. Five in every thousand are expected to survive, a comparatively high number for tiny lifeforms under the sea. The time spent in the pouch has given them the edge over many other species. A year after mating, a white tip reef shark has delivered a litter of five pups. They're predators in their own right, but still at risk when they're young. To stay safe as they develop, the pups will often stick together, seeking shelter in protective crevices and caves. Survival instincts helping these predators to prevail. It's been two months since the green turtle buried her eggs on the beach. The tiny hatchlings have been digging out for two days, breathing air between the sand particles until finally seeing daylight. But the danger is far from over. These reptiles run the risk of attacks from birds as they make their way to the water. Even in the sea, they're not safe. The hatchlings are a bite-sized meal for many predators. The lucky few will make it out to deep water where they disappear for decades, gaining size and strength until it's their turn to mate. Only one in a thousand survive these so-called lost years, returning back to this part of the coast to start the cycle again. The male emperor penguin has completed his parenting duties. For 75 days, he has protected his egg from the worst of the winter weather. Care for the downy chick now passes to the female, who has spent the past few months finding food in the sea. The father, however, is starving. It's his turn to hunt. It will be another five months before his offspring grows waterproof feathers so it too can head for the sea. But it's not the only marine youngster stuck on shore. 11 months after elephant seals mate, the female returns to the beach to give birth. She delivers a single pup weighing over 30 kilograms. For the next four weeks, it will feed continuously, quadrupling its weight in preparation for weaning. The female flicks sand to keep herself cool in the sun, but she won't stay for long. Less than a month after giving birth, she will abandon her pup, mating once more before she returns to the sea. After it's left alone, the youngster will survive on its stored fat, only venturing into the water when all the adults have gone. Sea otter mothers are far more supportive of their growing young. Despite being born in the water, the pup can't swim for the first few months of its life. It remains balanced on its mother's belly, growing rapidly on high-fat milk. If the adult needs a break, she'll swaddle her pup in kelp to keep it afloat, relishing the freedom when they can finally drift apart. Mammals in general are dedicated parents. Compared to all other ocean animals, they invest the most in their offspring's well-being. Manatees and dugongs are no exception. These giant herbivores look virtually identical from the front. The point of difference is at the rear. Manatees have paddle-like tails. The dugong's is more like a dolphin's. Both animals give birth to big babies. Adults can grow in excess of 300 kilograms. A new calf is at least a tenth of that size and over a metre in length. Despite their healthy birth weight, they stay close to their mothers for at least 18 months. Suckling milk from a teat near her pectoral fin until they can graduate to grazing on grass or algae. Even then, the calves will often remain with their family groups for several years. Low reproduction rates means the mothers have the time and energy to devote to each youngster. A year after mating, this female humpback has given birth to a one-ton calf. Next to its massive mother, it looks small. But these newborn whales are longer than a family car. Even at this size, it would not have survived the icy waters of the polar feeding grounds. To build up its bulk, the calf will drink 200 litres of milk a day. This fatty substance has a toothpaste-like consistency, so it doesn't dissolve in the water. The mother produces the milk from her winter reserves. She won't eat for several more months. The duo rests often, conserving energy for the journey ahead. But the calf is curious about its new watery world, wanting to explore. The mother keeps it close. Young humpbacks are a target for great white sharks and killer whales. Play sessions are built into the daily routine as the adult teaches her offspring how to communicate in the ocean. If the calf continues to put on weight, they will soon start the long trek to their icy home. Their close bond will continue for at least a year, when the youngster accompanies its mother back to the tropics, where she will breed once again. No matter what form it takes, the cycle of life is a constant for all ocean animals. Regular reproduction not only creates new life, but it's essential to top up the sea's supply of food. Evolution has ensured every animal is engineered to boost the survival of their species, taking their turn at parenthood under the sea.