Secrets of Southern Australia s Underwater World - Marine Life Revealed

The sheer size and scale of Australia is astonishing. Seemingly endless, stretching as far as the eye can see. While roads now link the vast open spaces of the southern Australian outback, this hasn't always been so. Before the arrival of the roads, wheat and wool, the staple cash crops of the region, could only be shipped by sea to the cities. And so, in the late 19th century, jetties sprang up along the stretch of inhospitable coastline. Disused now, many have fallen into disrepair. Others have been preserved. A small number are still in use. All have become landmark features of this coastline. For the creatures of the sea, they have provided a new living space. A place of safety from the open ocean. The pilings themselves have provided a surface for anemones, sponges and ascidians. Every inch being coated with their bright coloured coverings. And they, in their own turn, offer a sanctuary to fish that have evolved to match perfectly their colours. Old industrial structures of steel and wood have become festooned with colour. And other creatures that live under the jetties have their own coats for protection. Decorator crabs have evolved an individual and unique style of camouflage. Plucking parts of anemones and sponges, they attach them to their bodies, softening the contours and helping them to blend in with their surroundings. If in areas of weed, then that is plucked as their chosen camouflage attire. Frogfish go one better than decorator crabs. They actually grow filaments on their bodies and remaining motionless, merge into the background. It's only when they move that the frogfish becomes visible and is at risk from predators. Even the best of camouflage may not prove to be enough. A group of fish pick over the carcass of an unlucky crab. The slugs of the sea are called nudibranchs and they come in a rainbowed array of colours and shapes. Nudibranchs lay thousands of eggs, attaching them to a rock or some other solid object on the sea floor. These pygmy leather jackets are feasting on a freshly laid egg cluster. No matter how sharp and numerous your spines, no matter how seemingly impenetrable your defences may be, there is always someone who can break through them. Few would take on a spiny sea urchin, but the spines are no problem for the tough mouth of the blue grouper. Other strange creatures of the southern ocean shallows also make their homes below the man-made structures. Much of the ocean floor is just flat sand, offering little shelter, and few creatures live there. It's where the sea meets the land, where rocky reefs and islets occur, that there are the greatest concentrations of life. These outcrops and reefs offer shelter and protection, a surface on which weed and sponges can grow. Inevitably, such profusions of life attract predators and the whole food chain lives in a fragile balance. Man-made structures in the sea, whether accidental such as shipwrecks or deliberate such as jetties, provide welcome additional living space and rapidly become colonised, an artificial environment which quickly becomes a complete undersea community. This undersea battle lasted for two days, and each time we returned to this particular spot under a jetty to film, the fight was still continuing. To human eyes, there seemed no good reason for the crabs to slug it out for so long. Why didn't one simply retreat, or move on to a more peaceful spot on the seabed? But to soft-bodied crabs like the hermit crab, the empty shell of a sea snail is the secret to survival. As they outgrow their temporary homes, finding a larger one becomes essential. Maybe this was a contest to determine which crab would move into a larger shell and so gain its protection. Have you ever walked along the city street at night, looking over your shoulder, feeling that someone's watching you, following you, maybe going to mug you? Well, there are also villains in the nocturnal sea. Carrier crabs are creatures of the night. They cut lumps of living sponge to use as both a shield and as camouflage. So valuable is this protection that they will even resort to mugging other crabs for a choice piece. When this crab spots another with a coat of sponge that he fancies himself, he starts to pull it from the back of a smaller rival. When he separates the sponge from its previous owner, he takes it for himself to wear. Adjusting the fit of the sponge with his back legs, the victor leaves the scene of the crime with his spoils. A perfect fit. Seahorses are big business. Whether for souvenirs, or to meet the insatiable demand from the traditional Chinese medicine trade, the market is ever increasing. If taken from the wild, the species would become extinct in no time at all. So in Australia, in a proactive move to protect the population in the wild, seahorse farms are now in business to supply the seahorse trade. This factory specialises in breeding for the aquarium trade, and seahorses from here may be exported to collectors around the world. Pregnant seahorses give birth to huge numbers of perfectly formed live young, which are then separated from the parent, and raised here until large enough to either join the breeding stock, or to be sold. But in the wild, not only is this southern ocean a haven for seahorses, but also for their close relatives, the weedy and leafy sea dragons. The most treasured secret of these seas. The leafy sea dragon is perhaps the most spectacularly camouflaged fish in the ocean. Its leafy appendages are there for just one purpose, to make them invisible against their environment. And both leafy and weedy sea dragons are only found in one area of the ocean, in the cool waters of the southern Australian coast. This is one of the most ancient creatures in the world's oceans. It's also one of the sea's most voracious predators. They hunt by day, and little is safe from their lethal strike. And of all the cuttlefish found in the world's seas, growing to over a metre in length, the giant Australian cuttlefish is the biggest. Once a year, they congregate in their thousands in a very small area of the southern Australian coast. Moving in from deep water to shallow reefs only a couple of metres deep, where they engage in a frenzy of mating. It's an ancient and stylised ritual. The pulsating patterns of the larger male have two purposes. They serve both to attract females, and to warn off potential rivals of this mating free-for-all. To woo the female, the male shows his finest display. If she's receptive, then they couple face-to-face. Locked together, the male passes sperm-filled packets to a pouch under the female's mouth. She then starts to lay her eggs, concealing them individually and carefully in crevices and under rocks on the seabed, passing them each in turn over the male's sperm package in order to fertilise them. But it's at this stage that there's trouble. Rival males attempt to replace the sperm with their own, so the male must stand guard over the female while she lays her eggs. It's an example of the principle of natural selection and survival of the species. The strongest will fertilise the eggs, passing on his genes to the next generation. In the frenzy, there can be many rivals attempting to get to the female as she lays. Competition's fierce, and in the free-for-all, competing males can mob a single available female. Flaring his arms out to their full length to make himself appear at his largest, with colours pulsating their electronic signals, the male squares up to his rival, both rolling onto their sides to maximise the impact of their display. The speed and intensity of the signals vary with the message and emotion that the cuttlefish is transmitting. Often the posturing alone is enough to chase off a weaker or smaller rival. But if two evenly matched males square up, a fight can develop, and many bear breeding scars from the bites and suckers of their rivals. While the male's attentions are distracted by one rival, others seize the opportunity to sneak in to the momentarily unguarded female. Once the eggs are laid, the interest of both male and female ends, and the eggs are left alone to hatch. Mating is the culmination of the life of the cuttlefish, and after the mating aggregation, the cuttlefish return to the open ocean. The lifespan of the giant cuttlefish is just a couple of years, and they see just one mating aggregation in their short lives. Wherever fishermen anchor up to clean their catch, there is always the chance of a free meal. Stingrays have learnt this lesson over time, and they thrive around the fishing harbours scattered along the coast. In this southern Australian bay, smooth rays spend their days cruising the harbour shallows in search of scraps. It's very unusual to see eagle rays in shallow water. They're usually fish of the open ocean. But they have also learnt of the chance of free food, and are from time to time also spotted here in the shallow waters of harbours. Rays and shark are all part of the same family of fishes, known to scientists as elasmobranchs, and the seafloor of southern Australia is home to many unusual rays. There are about 50 different species of rays in Australian waters. Much of their lives are spent hidden under the sand, only their eyes protruding. Their mouths are on the underside of their flattened bodies, and they feed on crabs and fish buried in the sand. But there are other rays that have a far more sinister hunting technique. The numbray has a very unique method of taking its prey. 220 volts. A severe electrical shock. The same as the jolts you get from your home electrical system. If you know what that feels like to a fully grown human being, then try and imagine the effect on a fish. A shock is more than enough to stun, and probably kill a fish, giving the ray all the time it needs to eat it. And the numbray is capable of repeating that charge as many as 50 times in just 10 minutes. The speed of the strike is faster than the eye can see, as the ray spits out a fish stunned and juddering from the voltage coursing through its body. But why let the fish escape? Let's rewind the tape and watch it again. It's only when the shot is replayed in slow motion that the answer becomes apparent. In a lightning fast movement, the ray envelops not one, but two fish. And even this creature of nightmares can only consume one fish at a time. There are many surprising, often beautiful, and sometimes shocking secrets hidden beneath the seas of southern Australia.