Nature guards the Southern Ocean well. Frozen winds howl around the continent, building the biggest waves on the planet. Beneath lurks life unlike anywhere else on Earth, unseen to early seafarers except for terrifying glimpses of squid as big as their ships. Fifty years ago the world formed the Antarctic Treaty to protect the polar continent. Now Britain is leading an international push to protect the unseen depths and the life — both known and unknown — that the Southern Ocean contains.

The protected area of the Southern Ocean starts at 60 degrees south latitude, but in reality the border of its frigid waters are defined by a wandering cold front. North of this latitude, protection for the region’s extraordinary life must come from individual nations. Yesterday South Africa announced its intention to establish the world’s fifth-largest marine reserve around the Prince Edward Islands at 48 degrees south, about 2,000 kilometres (1,250 miles) south of South Africa. At 180,000 sq kilometres, the reserve will encompass Prince Edward Island and Marion Island, benefiting huge populations of seals, seabirds and killer whales as well more mysterious Antarctic species such as the 14-metre long Colossal Squid, famed for its battles with sperm whales.

The frigid waters of the Southern Ocean store less oxygen than usual and so encourage the growth of invertebrates such as sea spiders the size of dinner plates, outsized crabs and huge worms. Chemosynthetic communities have been discovered recently clustered around hydrothermal vents in the area, living independently of sunlight. Many secrets still remain, hidden from scientists.

Threats are building. As fish become scarcer, trawlers are turning to Antarctic species that have been ignored until now. Krill — tiny shrimp-like creatures whose swarms are the biggest on Earth — form the glue that binds together all Antarctic food webs but are under threat to feed the increase in farmed fish.

The final designation of the Prince Edward Islands protected area will come after a 60-day gazetting period. “In many respects the declaration of this marine protected area is a test case to pilot international co-operation on areas such as this, and as such we look forward to sharing experience and expertise with our international friends,” Marthinus van Schalkwyk, the South African Environment Minister, told The Times.

Marine Protected Areas benefit wildlife and fishermen. The provision of an undisturbed haven allows a strong, diverse and resilient ecology, which is especially important as climate change drives the acidification of seawater. The size and populations of fish have been shown to increase both within and surrounding their borders.

British-administered South Georgia, in the sub-Antarctic, enforces a ban on fishing within 12 nautical miles of its shores, and hosts the world’s only Marine Stewardship Council certified fishery for the toothfish — also known as the Chilean sea bass.

In the internationally managed Southern Ocean, where reserves cannot be designated by unilateral action, such reserves are rarer. Currently less than 0.2 per cent is set aside as protected areas, compared with the 40 per cent recommended by campaign groups. “There’s a lot of work being done on where the best places to protect are,” says Susie Grant, of the British Antarctic Survey, who adds that Britain is leading the way in pushing for marine protected areas in the Southern Ocean. “Momentum is good. Such areas need to be based on scientific information, so it’s a slow process, but in the next few years we will start seeing large offshore areas being developed.”

The Foreign and Commonwealth Office congratulated the South African Government on its intention to establish a marine reserve. “The UK is committed to the designation of a network of marine protected areas in the Southern Ocean and welcomes all steps taken in pursuit of this goal,” a spokesman said.

Priority areas include the Ross Sea, a wide-mouthed bay on the Pacific side of Antarctica, whose environment is thought to be among the most pristine of all the oceans. In most of the oceans 90 per cent of predatory fish have gone, but the populations of the Ross Sea remains intact. The Scotia Sea and the western Antarctic Peninsula are also being considered, and Britain’s proposal for the South Orkney Islands will be presented to the Convention on the Conservation of Antarctic Marine Living Resources this autumn.

“What’s missing is speed and scale of thinking,” said Richard Page, ocean campaigner for Greenpeace. “We have no time frame. Everything is moving at glacial pace and is incredibly small. If it continues, we’ll end up with only a patchwork of postage stamp-sized reserves. If we’re going to make a difference, we have to think big.”

Some of the most intriguing areas in the southern seas have yet to be investigated. The South Sandwich Trench — which passes close to the Britishadministered islands of the same name — is a chasm more than 7 kilometres deep and is all but unexplored.

Antarctic drama: The cast

Antarctic krill (Euphausia superba): The biomass of the Antarctic krill is estimated to be over 500 million tonnes, twice that of all the human beings on Earth. They are at the very heart of the Antarctic ecosystem. They graze on zooplankton beneath the ice pack, making their response to climate change uncertain.

Colossal squid (Mesonychoteuthis hamiltoni): The stuff of legend, below right. Never yet caught alive, the monstrous invertebrates with the largest eyes in the animal kingdom grow up to 14 metres (45ft) long. Their suckers are the size of dinner plates and are equipped with swivelling hooks. Their outline matches scars found on the skin of sperm whales — thought to be the only animal brave enough to predate them in the depths.

Giant sea spider (Pycnogonida): Thought to feed on hydroids and bryzoans (coral-like animals) by sucking them up through an extended proboscis, these animals are far bigger and more common in the Southern Ocean than anywhere else on the planet. Vital organs such as gonads are located in their long legs. Not true arachnids, they may be the last remaining members of an ancient and long-gone group of arthropods that lived in Cambrian-era oceans.

Wandering albatross (Diomedeidae): The largest flying bird, with a wingspan of up to 3.5 metres (11ft). Though efficient travellers that use the rolling Southern Ocean swells to power their flight, they nest on land, making them vulnerable. They feed on squid (though not Mesonychoteuthis), fish and krill, but unfortunately many mistake plastic debris in the ocean for food, and so poison themselves and their young.

Leopard seal (Hydrurga leptonyx): The awesome predator of penguins punches above its weight — its jaw can open almost 180 degrees to allow it to catch large prey, such as other seals. Inflatable boats are also targeted, and in 2003 a snorkelling biologist was killed.

Sperm whale (Physeter macrocephalus or Physeter catodon): Perhaps the most impressive of the 20 or more species of whale in the Southern Ocean. Bulls grow 20 metres long, and can dive more than three kilometres. The sperm whale’s brain is the largest in the animal kingdom, its sonar click the loudest. Loud sonic bursts are thought to be used to stun the colossal squid into submission.

Great white shark (Carcharodon carcharias): Some scientists fear that if ocean circulation patterns change as a result of the changing climate, sharks will enter the Southern Ocean with disastrous results.

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