As the beach-bound swimmers gaze longingly at the sparkling but forbidden Mediterranean, they should take comfort that the Spanish environment ministry had been preparing for the arrival of hordes of “mauve stingers” for some weeks, as part of Plan Medusa. Like the hapless authorities in a Fifties’ monster movie, they had produced posters and advertising campaigns warning swimmers to brace themselves for an onslaught of the purple jellies (Pelagia noctiluca), just one of the species posing a hazard to bathers this year, as well as the potentially deadly Portuguese Man o’ War with its 100-foot tentacles and the box jellyfish, which leaves welts that burn for three weeks.

Some of those barred from the water may be gladdened by the news that the Mediterranean has been identified in the Census of Marine Life as one of the world’s top five areas for marine biodiversity. The others are the oceans off Australia, Japan, China and the Gulf of Mexico, each containing as many as 33,000 individual forms of life that can be scientifically classified as species. In total, the census now estimates that there are more than 230,000 known marine species – but that this is probably less than a quarter of what lives in the sea.

The Census of Marine Life is being co-ordinated in Washington DC by the Consortium for Ocean Leadership and has involved scientists in more than 80 countries working over the past decade. They hope that by creating the first catalogue of the world’s oceans we can begin to understand the great ecological questions about habitat loss, pollution, overfishing and all the other man-made plagues that are being visited on the sea. The truth is that at present, much of what pass for scientific “facts” about the sea and what lives in it are still based on guesswork.

So far, the census tells us that fish account for about 12 per cent of sea life, and that other easily recognisable vertebrates – whales, turtles, seals, and so on – are just 2 per cent of what lives beneath the waves. It is the creepy-crawlies that are out there in really big numbers; almost 40 per cent of identified marine species are crustaceans and molluscs – things like crabs, shrimp, squid and sea-snails. Jellyfish are part of the Cnidaria group, along with anemones and corals – about 5 per cent of the total.

The census continues to gather images and data relating to a myriad range of creatures that could have slithered from the pages of science fiction. Neither Jules Verne nor Isaac Asimov could do justice to the shape and form of Chiasmodon niger – “the great swallower” – with its cadaverous skull, metallic pink flesh and needlelike teeth, accompanied by an enormous ballooning stomach that allows it to swallow animals bigger than itself. And surely there is something enchanting about the “Yeti crab” (Kiwa hirsuta), another new discovery from the Pacific, with a delicate, porcelain-smooth carapace and arms longer than its body, encased in “sleeves” of what look like ginger fur.

Chiasmodon Niger - definitely not a beer belly!

In shallower waters, the iridescent pink fronds of Platoma algae from Australia resemble the sheen of a pair of pink stockings caught in the glow of a nightclub stage. Juvenile Antarctic octopuses, speckled brown, mauve and orange, look like exquisitely carved netsuke ornaments, perfectly proportioned and endearing for their donnish domed “heads”.

For its bizarre variety and for its enduring mystery, we must learn to treasure the sea. It is easy to be captivated by intelligent, seemingly friendly sea creatures such as dolphins or even by the hunting prowess of the more sinister sharks. The Marine Census helps us understand that it is the less glamorous, less appealing and less dramatic creatures that are the great bedrock of life on which the oceans depend. As Nancy Knowlton, one of the census scientists, observes, “most ocean organisms still remain nameless and unknown” – and how would we begin to start naming the 20,000 types of bacteria found in just one litre of seawater trawled from around a Pacific seamount?

In the clear blue waters of the Maldives I once found myself ignoring a pair of giant manta rays just five metres from where I was kneeling on the sand staring at a coral outcrop. I had spotted a small black shape that looked like a sponge. Sponges hold a particular fascination for me, because they are so varied in colour, shape and size and they have an almost miraculous life cycle. But this sponge was moving, and it was in fact a Maldivian sponge-snail – a mollusc that survives by mimicking a sponge.

Afterwards, back on the surface, I was ridiculed by the other divers for taking photographs of the small black snail instead of watching the manta rays. But it is the mystery of the sea and its life forms that holds me, and makes me want to fill my mind with the images I gather when I dive, knowing that they exceed anything I can ever conjure from my imagination.

The final Marine Census results will be formally presented at the Royal Society in October, but hidden within the already released information is a dark message. Maps showing the density of large fish populations in tropical waters reveal that numbers of many of the biggest open ocean species have declined by more than 50 per cent since the Sixties and specific species, including many of the sharks, by as much as 90 per cent.

The Census of Marine Life also points to the effect of so-called “alien species” being found in many of the world’s marine ecosystems. The Mediterranean has the largest number of invasive species – most of them having migrated through the Suez Canal from the Red Sea. So far, more than 600 invasive species have been counted, almost 5 per cent of the total marine creatures in the Mediterranean.

The well named "Yeti Crab"

Those annoying jellyfish on the Spanish holiday beaches may be sending us a message, or at least a warning. In recent years there have been other jellyfish “invasions”. In 2007, 100,000 fish at Northern Ireland’s only salmon farm were killed by the same “mauve stingers” that are affecting the Spanish beaches. The swarming jellies covered 10 square miles of water.

In 2005, and again last year, Japanese fishermen battled swarms of giant Nomura jellyfish, each measuring six feet across and weighing 200kg. Once seen infrequently, they now regularly swarm across the Yellow Sea, making it impossible for Japanese boats to deploy their nets. One fishing boat capsized after the jellyfish became entangled in its nets.

There is evidence that the global jellyfish invasion is gathering pace. As Mediterranean turtles lose their nesting sites to beach developments, or die in fishing nets, and the vanishing population of other large predators such as bluefin tuna are fished out, their prey is doing what nature does best: filling a void. Smaller, more numerous species like the jellyfish are flourishing and plugging the gap left by animals higher up the food chain. According to the Spanish environment ministry: “Jellyfish blooms have been increasing in recent years, and one of the suggested causes is the decline in natural predators – as well as climate change and pollution from land-based sources.”

Beware - large enough to capsize a fishing boat!

In the meantime, the warning posters are pasted along the beachfront, spotter boats cruise the bays looking for the “invaders”, and patrols along the sand teach holidaymakers what to do if they are stung.

This is not quite Invasion of the Body Snatchers, and clearly it is unwise to talk as if the jellyfish had some kind of plan. But many marine experts have been saying for several years that we need to start loving jellyfish – because in the not too distant future, they may be the most plentiful marine species around. With typical culinary inventiveness, Japanese chefs have already begun experimenting with recipes for jellyfish ice cream and jellyfish tofu. It may be the future of seafood.

Article reproduced with the kind permission of Tim Ecott – author of Neutral Buoyancy/Stealing Water/Vanilla:Travels in Search of the Luscious Substance (timecott@hotmail.com)