Built by the Sea: Evolution’s Strangest Creations | 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 ocean is evolution's first frontier. Here, over countless millennia, creatures have changed and crawled out of the water, slowly escaping the competitive oxygen-poor expanse. Those that stayed behind have an interesting story. One that's often stranger than fiction. They've adapted how they move, how they look and even how they've armed themselves to survive against all odds. This is where the prehistoric meets the present. The Earth's ancestors and wildest oddities. An ancient curiosity drifts on the current. A creature without a brain, blood or heart. Yet it's outlived the dinosaurs. For at least 500 million years, through five mass extinctions, sea jellies have prevailed. Their success is even more surprising given their makeup. 95% of their mass is water. The remainder, a small cluster of muscles, proteins and nerve cells. Sea jellies are found in every ocean. From the coastal shallows to the darkest depths. Lighting the way with their bioluminescence. There are 2,000 known species. Some as small as a pinhead. Others as impressive as the lion's mane. Its 12,000 tentacles can trail up to 37 metres. Longer than the largest whale. But it's not the deadliest. The box jelly can kill a human in minutes. Many of its relatives, however, are a haven for juvenile fish seeking sanctuary from predators. While these gelatinous creatures are unusual, they still need the basics in life. Oxygen is absorbed from the water through their thin outer membrane. Food is captured via stinging cells and fed into their mouth, which is under the bell. They move by contracting muscles, expelling water from their bodies, pulsing through the ocean, guided by the currents. Simple survivors in a sea of complex creatures. Not every ocean animal makes motion seem so effortless. The odd-shaped sunfish lacks a tail. Instead, it moves by synchronising the flapping of its fins. These awkward swimmers are one of the largest boned fish in the world. Adults can weigh more than two tonnes and measure close to four metres across. Over their lifetime, they undergo the greatest growth spurt of any animal on the planet. When they're fully matured, sunfish can be 60 million times their birth size. They're also prolific breeders, producing the most eggs of any vertebrate, 300 million at a time. The sunfish draws its name from its habit of lying on the surface, solar charging before heading to the cold depths to hunt for fish, jellies and squid. A light-powered animal also designed for the dark. The ocean floor is home to some of the most unusual life forms on Earth. Alien-like animals eking out an existence amongst the sand and silt. The Dumbo octopus is one of them. This curious cephalopod moves by flapping ear-like fins in the same way as the cartoon elephant it was named after. It's one of 13 species of umbrella octopuses which have arms connected by a web of skin. Some can grow to more than a metre. Most are less than a third of that size, surviving on worms and other invertebrates. A comical creature flying through the ocean's depths. Closer to the coast, a pint-sized predator is hiding in the seabed. The pajama squid is no bigger than a thumb but it's packed with poison. If under threat, it can secrete a toxic slime, warning others to stay away. This striped attacker hunts at night-time, snatching small crustaceans with its tentacles. Rows of suckers hold the victim tight. A cute but accomplished killer with the misfortune of being misnamed. Despite appearances, the pajama squid is actually a tiny cuttlefish. It's not the only bottom dweller with an identity crisis. The flounder looks like a work in progress, caught in the midst of evolutionary change. With both eyes on the same side of its head, its face could be straight out of a Picasso painting. But it hasn't always been this way. In their larval stage, these fry are free swimmers with perfect symmetry. As they develop, the bones in their skull bend and shift as one eye forces its way to the opposite side of the head. The mouth, however, is yet to make the same transition, still retaining its sidewards gape. Despite their anatomic anomalies, flounders have perfected camouflage to hunt and hide, helping these flattened fish to flourish. Some animals aren't shy about flaunting their bizarre bodies. The dragon seamoth drags its oversized pectoral fins as it searches for worms amongst the rocks. This weird winged fish has lost the ability to swim. Instead, it scuttles along the seabed, bumping into boulders as it hunts. Its fanned out fins are just a defence strategy designed to scare off predators. At barely the size of a hand span, the little dragon relies on bony plates for protection, helping it to camouflage amongst the rocky rubble. Not all ocean floor oddities are quite so drab. Some of the seabed's strangest and most striking creatures look more like flora than fauna. Colourful colonies of coral polyps form into elegant sea pens. Fields of feather duster worms extend their antenna to be fed by the currents. Amongst this crop of filter feeders is the sea apple. It's a type of sea cucumber, a soft bodied invertebrate filled with water. Their tentacled arms are coated in mucus. The food is fed into a centralised mouth. Every last morsel is extracted before reaching out for more. To prevent this colourful cucumber from floating away, hundreds of tubular feet keep it anchored to the coral, which should make it ripe for the picking of any predator. But its bright blue body is a warning that it has toxin on board. A poisonous fruit that's strange to the core. It's not the only oddity amongst this family of bottom dwellers. There are over 1,200 types of sea cucumbers. While most are slug-like in appearance, these headless, eyeless creatures are more closely related to sea stars. Sea cucumbers breathe from their bottoms. Water is sucked in through their sphincters. The oxygen extracted by internal structures known as respiratory trees. They are also the ocean's cleaners. Using their tentacled mouths, they sift through sediment, searching for anything edible, removing rotten debris. In the process, they ingest large amounts of sand. The vacuumed grains are then filtered out, leaving a trail of processed substrate. It's estimated that up to half of the ocean's sand will have passed through a sea cucumber at some point. An odd yet efficient existence. While the cucumber is cleansing the seabed for a meal, another echinoderm has turned its world upside down to feed. The sea star has a mouth underneath its body, giving it easier access to the food in the substrate. Waste is released from the top, away from where they eat. It's an unusual setup, but the sea star stands out for a more extraordinary evolutionary advantage. They can jettison an arm and part of their central hub to create a new animal. By doing so, these masters of regeneration can reproduce asexually if no partners are about. It's also a handy strategy if they're under attack, severing a limb to escape. Regrowth takes time, up to a year in some cases. Nonetheless, these star qualities allow them to live for up to 35 years, outlasting many others in the marine world. A close relative appears more sinister than star-like, unfurling intricate arms up to a metre long to ensnare its food. Basket stars mostly come out at night, waiting for zooplankton to snag on their hooked appendages. While most echinoderms use their tubular feet to move, the basket star puts its twisting, turning arms to work instead, crawling about on coral, one of the ocean's eeriest animals. Some sea creatures manage to combine colourful, creepy and complex. The peacock mantis shrimp is an eye-catching killer. With the fastest punch on the planet. In addition to its tough-guy exterior, the mantis shrimp has the most sophisticated eyes in the animal kingdom. Perched on stalks, they rotate independently, covering all directions. Their retinas have more photoreceptors than any other animal, helping them to see ultraviolet and polarised light. Images are processed in the eye rather than the brain, giving them exceptional optics in an instant. Once in their sight, there's no escaping these miniature marine marvels. Compared to the mantis shrimp, the nudibranch has drawn the evolutionary short straw. This group of marine mollusks is known for their vibrant appearance, but ironically, they're colourblind. Most species lack any sort of eyesight, relying on scent receptors on their tentacles to sniff out food. These sea slugs are named for their most prominent part. Nudibranch means naked gills, openly displaying their breathing apparatus on their backs. Their soft bodies have no protective cover, but their bright colours suggest danger to other animals. Some are toxic, many are not, developing their deterrent hues to bluff their way free from predators. A few have also learnt to adopt the defence mechanisms of their food, retaining the stinging cells of sea jellies for their own use if under attack. Sometimes devious can be delightful too. The flatworm looks like a type of nudibranch, and that's part of its survival strategy. By imitating the potentially poisonous mollusk, it hopes to be left alone by reef fish. But this unsegmented worm is not related to the nudie. Its design is even simpler. It lacks respiratory or circulatory organs, relying instead on its large flattened surface to access and diffuse oxygen. A basic yet beautiful creature that moves with style. Not all residents of the reef have been as lucky with their locomotion. The pipefish is a relative of the seahorse, sharing many of the same features but without the curves of its cousin. Its miniature fins are barely visible and provide little assistance with swimming. Some of the 200 species have prehensile tails to help hold themselves still in the current. Others prefer to stay amongst the slender seagrasses and weeds, hoping to be mistaken for twigs. The straight and narrow can have its advantages underwater. In protected parts of the ocean, vast forests stretch towards the surface. These are amongst the most productive ecosystems on the planet. Kelp is a giant seaweed, capable of growing more than half a metre a day. These otherworldly gardens are a haven for all sorts of sea creatures, including dragons. In cool coastal waters, a member of the seahorse clan is drifting in disguise. The plant-like appendages of the weedy sea dragon blend beautifully with its green underwater habitat. At 45 centimetres, it's larger than its leafy cousin, but just as slow swimming as others in the family. A set of tiny fins on its head is used to keep balance. Another on its back propels it forward. Moving with the current, its body resembles swaying seaweed. Hunting for tiny crustaceans that cross its path, this ocean oddity manages to combine the majestic with the mythical. The velvet fish is also prepared to go with the flow. This rare species spends its life nestled amongst the greenery, in areas with large tidal movements. It's named for the soft appearance of its skin, but it's topped with venomous spines for protection. Its huge dorsal fins are better suited to camouflage than cruising, mimicking the motion of weeds in the water, ready to ambush passing fish or crabs. When it does swim, the velvet fish is clumsy and lopsided, moving just far enough to reposition for a better feeding ground. Rocking gently to hide the killer within. Other predators are more difficult to disguise. A quirk of nature making them instantly recognisable. Hammerheads are odd-looking sharks. All nine species have the cephalofoil shape to help with their hunting. They feed mainly on the seabed, detecting electrical impulses of rays, octopus and crabs. Sensory organs are spread across their broad head, making them more effective at picking up signals. Their nostrils are far apart, allowing them to easily discern the direction of scent. Wideset eyes offer 360-degree vision vertically, spying animals above and below. They respond quickly, turning twice as fast as most other sharks. Their unusual design providing stability and also helping to pin down prey. Odd looks are a small price to pay for superior survival skills. Some sharks are strange both inside and out, displaying few of the traits associated with their class. Carpet sharks are a wide-ranging group of fish named for their patterned bodies. All 39 species have two spineless dorsal fins. Most have small mouths and many lack the predatory behaviours of their more infamous relatives. Even amongst this eclectic bunch of animals, the zebra shark is unusual. It has the spots of a leopard, but stripes when it hatches and its tail makes up almost half its length. But what really makes this species unique is its ability to reproduce without a mate. In extreme circumstances, when males are nowhere to be found, the female can develop embryos without fertilisation. This astonishing evolutionary process is by no means the preferred method of reproduction. It reduces genetic diversity and adaptability. But it's a useful backup plan if the zebra shark is ever at threat of extinction. A much smaller carpet shark shows off its special skills. The epaulette has evolved to walk out of water, expanding its hunting grounds to include reef pools. Using its fins as feet, this slim-bodied fish can move itself over the coral to reach animals trapped by the falling tide. If it gets stranded, it can adapt to survive. Out of the water, the epaulette can sustain itself for up to an hour by drawing on its oxygen reserves, slowing its breathing and heart rate while powering down its brain. By suppressing all non-essential activity, it needs less oxygen while waiting for the incoming waves until it's no longer a fish out of water. The mudskipper has taken topside exploring one step further. It can virtually walk on water. These flexible fish hatch in the sea but spend almost 90% of their time in estuaries. As they grow, their pelvic and pectoral fins become weight-bearing. Working like shoulders to drag them across the mud and skip over water. Their breathing has evolved to allow them up to 36 hours of surface time. Water is stored in special cavities near the mudskipper's head, continually keeping their gills moist. If supplies run low, this extraordinary fish can breathe through its skin and the lining of its mouth like a frog. Its movements also mimic the amphibian as it makes its way through the mudflats, regularly returning to the sea to top up its reserves. Walking also works underwater for weak swimmers. Frogfish are known for their rod and lure hunting style along with their method of motion. With no swim bladder, these colourful anglers move about on the seabed. For a slow stroll, their two pectoral fins give them propulsion. To pick up the pace, the pelvic fins are also employed. Boosted by expulsion of water from their small gill openings. An awkward yet strangely effective gait. The sea is filled with odd movers. Creatures that have adapted to quirks of evolution as they try to make their way underwater. The wonderpuss has perfected fluid motion. This vibrant cephalopod will slide, slink and glide its way over the seabed. Like all octopuses, this hunter has complex chromatophores or ink sacs to change its colour. But its head-to-body ratio is unique. Its arms can grow up to seven times the size of its mantle. Ideal for probing for prey. A smooth operator that has all the right moves. Another long-limbed sea creature prefers to hop, skip and jump its way over the ocean floor. This orangutan crab is well hidden by its furry facade and diminutive stature. Its body is barely bigger than a thumbnail. If its camouflaged coat gets too shaggy, this tiny crustacean sheds excess baggage by giving itself a trim. Its lightweight frame now ready to carry it on the current and move in leaps and bounds. Even the most static of sea animals can let loose when the time is right. By day, feather stars or crinoids are often clamped in place on reefs, presenting like a plant. But these echinoderms are capable of breaking free. Walking over the seabed to reposition. Even a venomous waspfish won't stand in its way. At night time, this filter feeder shows it's a star, putting on an enchanting display. It's all designed to find a better position to catch tiny prey. Feather stars can only maintain flight for a few seconds before parachuting down. A short but sweet performance. But not all of the sea's oddities act alone. Some of the most unlikely creatures have paired up, forming symbiotic relationships to keep themselves safe. Down on the substrate, a goby is on the lookout. It's protecting the entrance of its home, which it shares with another. This burrow was built by the blind pistol shrimp, yet it's happy to have a lodger. During the day, the fish keeps watch as the vision-impaired shrimp maintains the den. They stay connected by the crustacean's long antenna, which rests on the goby's fin. The fish will flick its tail when predators approach, warning the worker to retreat. In return for its services, it enjoys an expertly excavated burrow to hide in. Successful symbiosis on the seafloor. Most larger marine animals are well-equipped to fend for themselves. A few have even developed in unusual ways to help them survive. The narwhal is one of them. These strange-looking whales live in the icy waters of the Arctic. They're known as the unicorns of the sea. Fairy-tale-like creatures with a legendary attachment. It's actually a spiral tooth, protruding up to three metres from the male's mouth. But it's not used for eating. Instead, it's a weapon for fighting and a tool for herding fish. Millions of nerve endings in this elongated tooth help the narwhal to detect subtle changes in water pressure, temperature and salinity. An early warning system for the pod, alerting it to shifts in ice flows. A tooth for fighting and forecasting rather than feeding. It's not the only oversized tusk in the Arctic. Walruses spend two-thirds of their life in the sea in big herds. Both males and females have large canine teeth that grow continuously, reaching up to a metre in an old bull. A worthy weapon, but these pinnipeds hunt with their whiskers, sucking up clams and crustaceans rather than spearing them. The main role of these tusks is social. They signal age, sex and status in the herd. Males will use them to demonstrate dominance, thrusting their heads and taking swings at each other to protect their mating territories. In the water, these two-tonne animals are surprisingly streamlined. But when it's time to haul out, the tusks can give them extra leverage, acting as giant ice picks. The sawfish is armed to hunt. Its rostrum is an extension of its skull, lined with modified scales that act like teeth. Sensitive pores along this snout help it detect electrical signals from prey on the seafloor. It takes just a quick turn of the head to stun or impale its victims. A close relative has a more subtle type of snout. Like the sawfish, the shovelnose is classified as a ray because its mouth and gills are under its body. It's a type of guitarfish, named for its broad head and tapered body. This unusual shape is designed for detecting and digging up fish and crustaceans in the sand. When it's not on the hunt, the light colouration helps it to blend in with the seabed. Some rays are more difficult to miss. Mantas can grow to seven metres across their wingspan and weigh more than two tonnes. But it's not just their size that sets them apart. These giant filter feeders come with lobes at the front. Their unusual accessories are evolutionary extensions of fins, designed to funnel food into their mouths for efficient eating. Its estimated mantas consume more than 10% of their body weight every week, or roughly 19 million pieces of microscopic plankton. It's food on the go for these massive rays that must move to breathe, always with flaps at the ready to flush out more food. Gorging has never looked so graceful. Many marine animals seem at odds with the watery world they inhabit, saved by subtle adaptations that have given them the evolutionary edge. Close to the surface, a prehistoric creature patrols. Its gaping jaws stretching to over a metre, its body the length of a bus, ready to snatch a meal of zooplankton. The basking shark is the second largest of all fish, but targets the smallest food in the sea, spending its summers cruising temperate waters at a steady five kilometres an hour. Mouth wide open, constantly feeding. Every hour it filters through the volume of an Olympic swimming pool. Gill slits almost encircle its head, flushing out unwanted water. A strange but harmless animal out of place in the shallows. Down in some of the darkest, coldest waters on the planet, another strange creature has also been able to thrive. The Greenland shark is thought to live to 400 years, one of the longest lifespans of any animal. Its longevity is attributed to a slow metabolism and sluggish swimming speed of around one kilometre an hour, earning it the nickname sleeper shark. To survive in these sub-zero temperatures, the Greenland shark has a chemical compound which acts as an antifreeze in its body and also makes it toxic, guaranteeing no predator will touch this denizen of the deep. Not all animals find their perfect place. Penguins are caught between two worlds, awkward on land and unable to breathe underwater. But these flightless birds are surprisingly well adapted for life in the sea. Coating their feathers with oil to make them waterproof and keep their bodies warm. Their wings turn into flippers in the ocean. Webbed feet act like rudders, helping to propel them at speeds of up to 35 kilometres an hour. Diving deeper than 100 metres to catch fish and squid. To stay down longer, penguins have learnt to slow their heart rate from 70 beats a minute to just 10, decreasing their need for oxygen. By employing this method, some can stay down for almost half an hour. To move efficiently on the surface, they've developed a cruising behaviour called porpoising. Like a dolphin, they can propel themselves forward while jumping out of the water, catching their breath while airborne. If extra speed is required to flee predators, penguins can release air trapped under their feathers. The bubbles reduce the density of water around them, allowing them to exit at three times the normal pace. Back to a world where breathing is easy but the body is slow. Penguins aren't the only marine animal that needs air. Sea turtles spend virtually all their life in the ocean, often in deep water far from shore. A strange habitat for an animal at risk of drowning. They need to come to the surface regularly to breathe, which is where they're most vulnerable to attack. Fortunately, these cold-blooded reptiles have a slow metabolism which helps them reduce their heart rate. Up to nine minutes can pass between beats. Depending on the size, species and stress level, turtles can remain submerged for hours at a time. They've also evolved to metabolise salt water, doing away with the need for fresh. Excess salt fluid is secreted through their eyes and falls like tears when ashore. For a nesting mother, it also serves to lubricate and protect her eyes from the sand. Simple strategies to survive a life all at sea. Marine mammals also face the challenge of a hostile habitat designed for those with gills. Unlike humans, dolphins lack a breathing reflex. They need to make a conscious decision to come up for air, at least once every 10 minutes. But these energetic cetaceans also need their rest. To sleep without drowning, they shut down only half their brain at a time. The other part remains awake, alerting them when it's time to breathe. They stay close enough to the surface where only a small flick of the tail will push them up for air. By maintaining a slow and steady swim speed, dolphins can conserve energy during these sleep cycles. Almost a third of their day is devoted to this semi-brained slumber, a sleeping solution that helps to sustain life. It's one of the largest creatures in the sea, with the biggest brain of any animal on the planet. Its huge head takes up a third of its body size and houses the most powerful sonar of any mammal. Sperm whales are an ocean oddity on a grand scale. 40 tonnes of predator that can dive down deeper than two kilometres, searching for sharks, fish and squid, holding their breath for more than an hour. Yet they sleep for only 10 to 15 minutes at a time, neither breathing nor moving. Often head down in a ghostly display called tail standing. If a warm-blooded marine mammal is inactive for too long, it risks lowering its body temperature, which could be deadly. The sperm whale stays on the safe side, sleeping motionless for less than two hours a day in total. Power naps obviously aren't a problem for this strange animal. They've been recorded living well into their 70s. The ocean's weird and wonderful are what happens when nature adapts to the challenging world around it. Subtle and sometimes substantial changes designed to give animals the edge to endure in this competitive environment. Many of these oddities are still evolving, continuing to develop strange traits, tricks and tools, all in the hope of sustaining themselves under the sea.