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Mussel family’s
missing link may
have been found

UH scientists and others
make the discovery studying
dead whales on the sea floor

By Helen Altonn
Star-Bulletin

University of Hawaii oceanographers and mainland colleagues studying rotting whales on the sea floor have discovered what they believe is a missing link in the mussel family.

Craig Smith, UH oceanography professor, said their findings are exciting because they show a relationship between giant mussels at hydrothermal vents and small mussels found on whale bones and wood on the ocean floor.

Mussel specialists didn't believe there was a connection, he said.

"It looks like very small mussels that live on whales and wood falls in the deep sea are likely to be evolutionary stepping stones from shallow water, more traditional, mussels people might see on a plate in a restaurant," he said.

Results of the study by Smith, UH graduate assistant Amy Baco and University of Maine and Harvard University researchers appeared in the Feb. 17 issue of the British journal Nature: "Do Mussels Take Wooden Steps to Deep Sea Vents?"

Eleven years ago, in a 1989 paper in Nature, the researchers had proposed that whale falls may be dispersal stepping stones for animals to colonize hydrothermal vents.

"We were pretty heavily criticized for that over the years," Smith said. "It's nice to see some vindication of the data, that these ideas are not so crazy. They do make sense, for mussels at least, particularly in an evolutionary sense."

He said other groups of animals might also move down the continental slope, following similar habitats in the deep sea and going from those to the hydrothermal vents.

'Reasonable stepping stones'

Whale and wood falls occur all over the ocean at various depths, he said.

"So they provide very reasonable stepping stones for the kinds of animals that live in shallow water and gradually move down slope and colonize rich sulfide hydrothermal vents."

Smith's team studied five whale skeletons on the seafloor off Southern California, going back to them over the years in a submersible to study animal communities on the bones and bacteria decomposing the whale fat and proteins.

The researchers collected at least 11 new species of animals, including some that seem to specialize on dead whales.

The detergent industry has been interested in Smith's research because enzymes from bacteria feeding on whale carcasses could dissolve greasy stains in a cold-water detergent.

Smith has been collecting bacteria samples from whale bones for Diversa, a San Diego-based biotechnology firm, which is searching for enzymes that break down fats and proteins under cold conditions to save energy.

Expanding on his work, Baco said one species of mussel found on whale bones and wood fragments appears to be an intermediate relative to the shallow water mussels and those living at hot deep sea vents.

May answer questions

"The question is how they evolved," she said.

"This gives you a look at what may have happened along the way."

Mussels may have evolved on whale bones or wood because they have lower levels of sulfide, which is toxic to organisms.

"It would be hard for organisms to go straight to an environment where there is no sulfide (in shallow water) to one with heavy concentrations (at the hot vents)," Baco said.

Lower sulfide levels at rotting whale falls, wood and plant material would allow the mussels to gradually increase sulfide tolerance, she said.

Smith said edible mussels in shallow salt marshes and rocky interdial zones get their nutrition by picking particles out of the water column -- not from sulfide oxide bacteria.

Giant mussels at hydrothermal vents appear to get all their nutrition from sulfide-eating bacteria in their gills, he said.

Mussels at whale carcasses are very small, with somewhat modified gills to get nutrition out of bacteria.

"It looks like, based on adaptions, they may be an intermediate between shallow water mussels and hydrothermal vents."

Smith said the findings are supported by genetic data.

"Genes suggest the same thing: that whale fall and wood and hydrothermal vent mussels are all closely related and they are divergent from the shallow water cousins -- the edible ones," he said.

As far as he knows, Smith said this is the first evidence "of what we might call missing links -- animals that might serve as evolutionary precursors, direct ancestors ... to animals at hydrothermal vents.

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University of Hawaii
Ka Leo O Hawaii