Volcanic spires thatspew superheated toxic chemicals deep in the Indian Ocean are home to some of the earth's strangest creatures. Steve Farrar reports.
A swarm of ghostlike shrimps billowed out of the inky darkness. The highly animated crustaceans flickered into the circle of light cast by one of the robotic probe's floodlamps while two-and-a-half miles above, a group of scientists watched the silent drama on a shipboard monitor onboard the research vessel Knorr . Although this was the first time this international team of experts had come face-to-face with these Indian Ocean denizens, many recognised the tiny creatures from previous expeditions thousands of miles away on the other side of Africa. Yet Cindy Lee Van Dover, the expedition's chief scientist, whooped with delight - she had been hoping for just such an encounter.
Since the first mid-ocean hydrothermal vents were discovered in 1979, Van Dover, an ecologist at the College of William and Mary in the United States, has been exploring the global biodiversity of a remarkable ocean-bottom fauna. Given the vast expense and effort involved in such expeditions - the current 40-day central Indian Ocean trip was five years in the planning - it is a painstaking process. The habitats Van Dover frequents are among the most remote on earth, the meeting places of our planet's oceanic plates. As these gradually move apart, ridges of volcanic vents sometimes can form along the seams. Tall, thin spires of rock, as high as a house, thrust up from the ocean bottom, pouring out dense clouds of superheated water rich in metals, acids and other products of the unhappy marriage of molten basalt and seawater.
In complete darkness, what billows forth from these black smoker chimneys can provide a source of life - chemosynthesis as opposed to photosynthesis. The microbes capable of performing such biochemical feats form the bedrock of a food web in which 90 per cent of species occur nowhere else in the world. Van Dover's search has taken her from the Pacific to the North Atlantic, sometimes at the controls of a robot but also at the helm of the submersible Alvin . She has probably spent more time in this realm than anyone else. She loves it. "It changes people's lives," Van Dover says. "Once you have seen the sea floor, you can't look at the ocean in the same way again - you just drain the water away in your mind."
Despite being separated by the Americas, the Mid-Atlantic Ridge and East Pacific Rise, the two vent regions that have been relatively well explored, have much in common. However, their differences are as interesting to ecologists such as Van Dover. Some are obvious, such as the mysterious tubeworms found only in the Pacific. These creatures, a distant relative of the earthworm, grow up to 2m in length inside hard, chitin tubes. Without mouth or stomach, they use red, gill-like plumes to collect chemicals from the water, possess uniquely adapted haemoglobin and host billions of symbiotic bacteria that secrete the sugars the tubeworm needs to survive. Of the 500 or so new organisms discovered living around hydrothermal vents, they are undoubtedly the most famous. They are not necessarily, however, the most unusual.
Van Dover led 33 colleagues - biologists, geneticists, geologists and chemists from a dozen institutions including the Southampton Oceanography Centre - to the Indian Ocean to explore the middle ground between the Atlantic and Pacific. Japanese scientists discovered the site last year and expectations of new insights into the dispersal of vent species were high. When, three weeks ago, the one-tonne, remotely operated vehicle Jason , caught a glimpse of swarming white shrimps in one of its six cameras, those hopes were being realised. "At least one Indian Ocean vent is dominated by swarming shrimp that appear to us to be morphologically identical to the ones we know from many of the Mid-Atlantic Ridge vents," Van Dover says.
The Atlantic shrimp, already studied in some detail, has baffled the experts. They do not have the eyestalks of their shallow-water cousins. In fact, it appears that their eyes have migrated to their backs, where they lack the lenses to form visual images but possess high concentrations of the light-sensitive pigment rhodopsin and a reflective layer that maximises the chances of detecting photons.
Although she admits it is akin to reconstructing the lifestyle of a bat by examining a few dead specimens, Van Dover guesses the shrimp may be able to detect the faint light gradient that warns of the proximity of a black smoker. She believes it may be an adaptation to the extremely hazardous environment in which the crustaceans live. Feeding on microbes close to superheated columns of water, a matter of a few millimetres can be the difference between a cool 2C or a shrimp-barbecuing 300C.
Preliminary findings from the Indian Ocean suggest connections with the mid-Atlantic Ridge communities where such creatures commonly gather on the sides of smokers at up to 30,000 per square metre. Nevertheless, Van Dover observes: "Other elements of the fauna are more familiar from Pacific vent sites." The first Indian Ocean tube worms were brought to the surface this week.
As the data about the shrimp and other creatures such as mussels and snails accumulate, a picture of global distribution will start to emerge. In these linear habitats, unaffected by climate, the physical influences on biodiversity are limited in number to the likes of volcanic activity levels and depth of the overlying water column. Links between biology and physical phenomena can be revealed while variation between ridges separated for tens of millions of years, yet perhaps connected by deep ocean currents, can be plotted. Taxonomic and genetic analysis will further clarify the story.
With work in progress in the Indian Ocean, there are still many locations to be explored, such as the Southern Atlantic, the Cayman Rise in the Caribbean and the Arctic. "Each time we go to different ocean basins, we know we'll find different faunas," Van Dover says. And each will help reveal the intimate workings of one of the earth's most alien networks of life."
Daily updates on the mission, which ends on May 5, can be viewed at www.divediscover.whoi.edu </a>