Ever since Jodie Foster starred in Contact, the search for extraterrestrial life forms has been a popular preoccupation. But a few dozen marine biologists and oceanographers are excited by amazing discoveries of life forms right under our noses — under our noses by several hundred fathoms, that is, in the little-explored realm of the deep ocean.
Craig Smith, professor of oceanography at the University of Hawai`i, was one of several such scientists who presented findings of their recent work at the 2002 meeting of the American Association for the Advancement of Science held in Boston in February. Smith participated in a panel on biodiversity in the deep sea.
While the sea floor has generally been regarded as a wasteland of mud, Smith noted, on average, a square meter of that mud is home to 250 separate species of animals.
“On local scales, the deep sea is extremely species rich,” Smith told Environment Hawai`i, “but how that scales up to basins and beyond is a subject of debate and investigation.”
We’re not talking charismatic megafauna here. When Smith and others talk about species richness, they’re referring to organisms that are sieved out on a 300-micron screen, the rough equivalent of a metal coffee filter. It’s a world filled with needle-sized polychaete worms, microscopic amphipods, tubeworms, shrimp, and clams resembling fingernails more than anything you’d find on a plate of pasta.
In many respects, what Smith and his colleagues are finding is, to the lay person, as startling and alien as the discovery of little green men on Mars. Scientists are puzzled not so much by the life forms, but by the paradox of such abundance and diversity in a seemingly monotonous environment.
“In contrast to many highly diverse ecosystems (e.g., tropical rainforests and coral reefs), the muddy plains of the deep ocean have relatively little structural complexity,” Smith wrote in his AAAS presentation. Without light, overall productivity of the organisms is low and many of the species living in the same vicinity appear to be similar ecologically – that is, “in this nutrient poor environmentÉ dozens, and perhaps hundreds of species may be exploiting limited food resources in a similar manner.”
Smith and his colleagues have been studying what might explain such unexpected diversity and richness. Some of the most diverse areas occur where occasional “pulses” of food drop to the sea floor, providing a veritable banquet for the denizens of the deep. This might come in the form of decaying kelp or a bolus of bycatch from an ocean trawler. But the mother lode, the deep-sea equivalent of an all-you-can-eat smorgasbord, is a “whale fall” — the carcass of a dead cetacean. This can foster the growth of a sea-floor metropolis and keep it in robust health for half a century or more.
– Craig Smith
“There appears to be a suite of 25 or 30 species adapted to utilize whale falls,” says Smith, who has been studying whale falls in the Pacific for about a dozen years. “Clearly, whale falls are very important to them. Also, whale falls are interesting because of the communities that live on whale bones, which produce sulfides as they decompose.” The organisms that convert the energy in sulfides to organic matter form a sulfophilic community, and include small mussels that, Smith says, are an ancestral form of the mussels found on hydrothermal and cold seeps in the deep ocean.
– Craig Smith
Just as whales populations were devastated during the heyday of whaling, so, too, were these seafloor communities almost certainly harmed. “There’s a high likelihood that there were species extinctions in this community that has adapted to whale falls,” he said “In fact, it was probably the most heavily impacted of any deep-sea community thus far.”
The community is dynamic. Initially, there are the scavengers feeding on the flesh: hagfish, scavenging amphipods, and giant sleeper sharks (up to 6 meters long). After that, the “enrichment opportunists” move in to take advantage of the organic material that has “rained down on the sea floor, creating a massive enrichment.” These include polychaete worms and a variety of crustaceans. “The pattern looks much like the response to a sewage treatment plant outfall,” he says. “Some of the animals we find at this stage have never been collected anywhere else. One polychaete in particular – its closest relative lives in the Black Sea. It’s been found only on two whale falls off the coast of California.” (A polychaete worm found at a whale fall near Hawai`i might be the same species, Smith says, but it is much smaller. Genetic testing would need to be done to resolve the question.)
– Craig Smith
After the “enrichment opportunists” move out, the sulfophilic community moves in. This is the stage “when you get sulfide production from anaerobic bacteria degrading the rich lipids in the whale bones,” he says. And how long can this take?
A very long time indeed, he answers. Smith and colleagues at the University of Miami have come up with a technique to age the whale bones, using isotopes of radium and lead, and have dated the carcass of the first whale fall Smith came upon in 1987, in the Santa Catalina Basin at 1,240 meters depth. In 1999, the whale fall was about 50 years old, Smith says, and “it still had a robust sulfophilic community.”
— Patricia Tummons
Volume 12, Number 10 April 2002
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