The world beneath the waves - 20,000 Worlds Under the Sea - includes related article on coral reefs - deep-sea ecology

A multitude of strange creatures inhabit parts of the sea floor where, some scientists believe, life itself may have originated.

'The deepest parts of the ocean are unknown to us. What goes on in these remote abysses? What creatures live beneath the surface of the waters? What is the constitution of these beings? We can hardly imagine. ... if Nature still keeps secrets ... nothing is more admissible than to suppose the existence ... of new species or even new genera."

These prophetic words were uttered by Professor Aronnax, the earnest marine biologist who is the narrator of Jules Verne's novel 20,000 Leagues Under the Sea, published in 1870.

More than a century after Jules Verne penned these words, one of Nature's deepest secrets was revealed to latter-day submariners. In the valley of a mountain range in the eastern Pacific Ocean, where the heat of volcanism inexorably drives the separation of two oceanic plates, geologists discovered animals worthy of nothing less than science fiction. I myself have seen the thickets of giant tubeworms with haemoglobin-red plumes emerging from white tubes, yellow mussels huddled in great piles over cracks in basalt, clams with thick chalky shells lying exposed on the sea floor. All of this and more, thriving in the noxious chemical soup of what has come to be known as one of the most productive habitats on our planet and the environment where some scientists think life itself may have originated: deep-sea hot springs.

Life in the abyss

New species and new genera are the least of the discoveries at deep-sea hot springs, though they abound and already make up a lengthy list of more than 400 novel Latin binomials. A whole new style of living was revealed as biologists rushed to understand how a giant tubeworm, taller than me and as thick around as my wrist, could be sustained in an environment as scarce in food as the deep sea. Bacteria, both free-living and symbiotic within host animal tissues, are the key. Animals at deep-sea hot springs are sustained by microscopic bacteria that grow on simple chemicals dissolved in the venting fluids.

Discovery of seafloor hydrothermal vents and their attendant communities stands out as a spectacular example of how poorly we have known our ocean. Scientists working in the soft-sediments of the bathyal deep sea tell another cautionary tale worthy of recollection should we ever become too sanguine about our knowledge of marine biodiversity. Working with sieves, microscopes and tremendous patience, biologists have identified nearly a thousand species of worms, snails, and crustaceans in a small volume of cold mud from deep waters off the east coast of North America. Our deep seas are repositories of biodiversity, with some estimates of the total number of global deep-sea species reaching into the millions, rivaling the celebrated diversity of tropical rain forests. Virtually nothing is known about their biology, their role in the ecosystem or their potential useful attributes.

Dwindling diversity

Deep-sea work requires high-tech tools for access, but anyone with an old pair of canvas shoes can explore the intertidal zone of coastal wetlands and beaches. Marine plants thrive in these sunlit waters: red, brown and green algae create three-dimensional forests within which live incredible numbers of molluscs, crustaceans and worms. There are rich and diverse littoral habitats: rocky shores and sandy beaches, mangrove swamps and salt marshes, coral reefs and mud fiats, shallow lagoons and deeper bays. These are the habitats patently at risk from a plethora of anthropogenic effects including pollution, development, over-fishing, trawling, and global warming. As coastal habitats are compromised or disappear altogether, whole suites of species disappear as well.

Not long ago I read about the dwindling populations of horseshoe crabs that gather ritually on beaches at full moon on a spring tide in June to mate and lay eggs. I used to haunt the beaches where horseshoe crabs landed, always astounded by their fantastic numbers. These animals are relicts of the Mesozoic, a time when dinosaurs ruled the earth, but their long history never prepared them for the demanding economics of catfood and fertilizer production in this modern world. Two decades ago they were uncountable. Are they to go the way of the passenger pigeon, slaughtered to extinction in the nineteenth century?

For a variety of reasons, healthy marine habitats do not necessarily support high numbers of species, and biodiversity per se may not always be paramount. Rather, as in terrestrial habitats, it is loss of diversity that generates concern. Because some marine habitats may naturally have relatively low diversity but a high abundance of animals, loss of a single species can greatly modify the ecology of a system. A case in point is Chesapeake Bay, the largest tidal estuary on the east coast of the United States. Oyster reefs once dominated the Chesapeake's shallow waters. These animals feed by filtering seawater, and not so long ago the equivalent of the entire volume of water in the Bay was filtered within a week. Oyster populations have declined due to exploitation and disease and what took one week to filter now takes one year. The effect cascades through the water column as the particulate load increases, light penetration decreases, and a host of other consequences transpire.

In terrestrial ecosystems, loss of charismatic mammals or the last remnants of virgin forest results in a dramatic visual sensation as well as a numerical debit on the tally of species or habitats. Loss of marine diversity takes place out-of-sight, invisible beneath the waves, yet the toll on the planetary legacy we leave to our children is the same. Worse, remediation of habitat damage or restoration where habitat has been lost is a difficult task at best in shallow waters and all but inconceivable in deep waters of bathyal and abyssal environments.

RELATED ARTICLE: Endangered coral reefs

Snorkel in sunny, warm tropical waters over the vibrant rainbows of coral heads and reef fishes and your head goes dizzy with the magic of diversity. Over 90,000 species living in coral reefs have been described, and the total number is at least one million. Why is the figure so high? The microtopography of a growing reef is complex, creating abundant surfaces for attachment and hiding places to accommodate small invertebrates and habitats for fish. Reefs are typically insular oceanic habitats. One can envision an always expanding species pool as a reef is successfully invaded by the chance arrival of drifting larvae of a species from some remote location. The invader, isolated from its parent population, gradually adjusts to its new environment and develops into a daughter species. Where there was once one species, now there are two. Each one has some likelihood of invading other reefs and in such a way diversity of coral reefs blossoms exponentially over geologic time.

This diversity is now threatened by the flash of dynamite of the myopic fisherman who destroys the habitat that sustains the resource he seeks to gather. Coral reefs are victims of many other unsound practices, with the result that as much as 10 per cent of them have already been degraded beyond recovery. If unchecked, loss of coral reef habitat will progress at a rate of 10 per cent per decade. Global warming is a particularly insidious danger. Corals lose their algal symbionts when the seawater temperature rises just a degree or two above normal, a phenomenon known as bleaching, which often results in death of the coral. Disease hits harder when corals are stressed by warmer than usual temperatures. A sick coral reef is dizzying only in the staggering loss of biodiversity.

C.L.V.D.

COPYRIGHT 1998 UNESCO
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