Home on the Bone

Tuesday, October 01, 1996
RELATED TAGS: UNUSUAL ORGANISMS
Some of Earth’s strangest creatures live around hydrothermal vents--hot springs that bubble from volcanic fissures in the ocean depths. These remarkable animals--tube worms, giant clams, and others--form a food chain that depends ultimately not on photosynthetic plants but on bacteria that live on and inside their bodies. The bacteria break down hydrogen sulfide in the vents, in the process releasing body-building energy for their hosts.

Marine biologists have long wondered how these animals manage to colonize different vents, which may be hundreds of miles apart. Deep currents are too slow to carry vent larvae far before they starve. Some researchers have suggested that whale bones on the sea bottom may act as stepping stones for the larvae by supporting the bacteria on which these communities depend. Larvae landing on the bones would give rise to tube worms and other creatures that eventually spawn their own larvae. Those larvae would in time colonize either another whale-bone oasis, a thermal vent , or perhaps a cold seep, an area where mineral-laden cold water, squeezed from ocean-floor sediments by the motion of tectonic plates, gushes forth.

To test the bony oasis idea, Yoshihisa Shirayama of the University of Tokyo and Hiroshi Kitazato at Shizuoka University used a small research submarine to deposit a cow’s thighbone on the bottom of Sagami Bay, Japan, midway between two cold seeps 19 miles apart. Shirayama, a marine meiobenthologist--one who studies tiny ocean-floor-dwelling animals--and Kitazato, a paleontologist, thought that marrow and fat in the bone would provide food for oxygen-burning bacteria in the sediments around the bone and pave the way for hydrogen-sulfide-loving bacteria.

Once the oxygen in the sediments was used up, the researchers reasoned, sulfur-eating bacteria floating dormant could move in to feed on the fat and marrow and release hydrogen sulfide as waste. This in turn would provide an energy source for sediment bacteria that feed on hydrogen sulfide, and for all the tube worms and clams that rely on these bacteria for energy.

And that’s what happened. After a year Shirayama and Kitazato returned to examine the bone. Sediments they collected from around the bone smelled strongly of hydrogen sulfide--immediately indicating that colonization by bacteria had occurred. The bone itself was patchily coated with a white mat of hydrogen-sulfide-eating bacteria, and a galatheid crab- -a species typically found in vents--was grazing on the mat. Unfortunately, a navigational mishap prevented further study, as Shirayama explains. We erroneously operated the submersible and it ran over the bone, so the bone was buried. But we had no option to pull it up, because a storm was approaching.

Luckily, the two were later able to lay down new bones, and Shirayama is optimistic that a full-blown community of vent animals will arise in a few years.
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