Nurturing a Mystery of the Deep Sea
Donal Manahan, a developmental biologist at USC and father of two young sons, thought he knew the basics of good parenting.
“You keep babies warm,” he said. “Feed them a lot. Never crush them with pillows.”
Or so he believed, until he started rearing his newest charges, giant tubeworm babies.
Tubeworms are among the strangest creatures on Earth. Six feet long, glowing red and white, the animals make their way in the world with no mouth or digestive tract. The worms, which are now popular characters on scientific documentaries about deep sea life, went undiscovered until 1977 because they live at hydrothermal vents about 8,000 feet below the surface.
They are being studied intensively because they are widespread occupants of one of the Earth’s largest and least understood ecosystems: the ocean floor, an area scientists say is filled with at least as many surprises as other planets.
With their remarkable adaptability and bizarre biochemistry, the worms stretch the limits of known biological mechanisms. Their chemical makeup may offer useful new enzymes, ones that can endure intense heat.
How baby worms fared so well--and how they migrated across powerful ocean currents to settle new areas--mystified scientists until Manahan figured out the way to nurture them. His formula: Keep them cold, starved and crushed.
“They thrive on it,” Manahan said. “They love it.”
Tubeworms, after all, come from one of the harshest environments on Earth. They flourish around superheated undersea geysers that dot the otherwise frigid sea floor. They manage to grow tall under crushing deep-sea pressures and a near total absence of food.
Before Manahan began his work, a series of studies had revealed that the animals survive their barren environment because their bodies are filled with symbiotic bacteria. The bacteria process foul hydrogen sulfide gas from undersea vents and use it to convert the ocean’s carbon dioxide into sugars and other molecules that tubeworms use for nutrition.
But scientists still couldn’t figure out how young worms got from one place to the next.
Scientists are interested in these creatures because they offer clues to how life evolved on Earth and how it might exist today in the oxygen-starved environments of space.
The strange habitat of the worms is unstable, characterized by ridges, undersea volcanoes and 700-degree geysers that form when tectonic plates rip apart the sea floor. The geysers, or ocean hot spots where the worms thrive, come and go as the sea floor changes. How then do the worms move? How do microscopic baby tubeworms, tossed about by powerful undersea currents, get to and colonize new hot spots that might be 30, 60 or 100 miles away?
“It’s dark down there. It’s hard to find a mate and even harder to ensure the next generation lands on a vent,” said George Somero, an evolutionary physiologist who directs Stanford’s Hopkins Marine Station. “How do the larvae find their way?”
The question had nagged Manahan since the worms were discovered. Manahan, a 47-year-old native of Ireland who is a professor of biology and dean of research at USC, has spent a decade studying how larvae survive extreme environments like the Antarctic Ocean.
Tubeworms were not thought able to survive a long journey because their eggs are relatively small at 100 microns, about the diameter of a human hair.
Larger eggs, at 500 microns, are thought to survive longer in barren areas because they are packed with fats that can serve as food for developing larvae. Because tubeworm babies were obviously surviving their oceanic trek and colonizing new vents, Manahan knew there had to be more to the story.
“Overemphasis on the size of the egg is like studying the gas tank of the car,” he said. “The important thing is fuel efficiency.” In this case, fuel efficiency translates into metabolic rate: How efficiently were tubeworms using the picnic lunch they packed?
The only way to find out was to raise the babies in the lab and test them. During four dives in 1998 and 1999, Manahan and his team dove in the Woods Hole Oceanographic Institution’s deep sea submersible, Alvin, to a vent in the Pacific several hundred miles off the coast of Costa Rica.
On their research vessel, Manahan quickly dissected some of the worms, collected eggs and sperm and mixed them together.
The developing larvae sailed with the ship back to San Diego, then were driven up Interstate 5 to Los Angeles. Attempts by others to keep baby worms alive in the lab, although they had used cold and pressure, hadn’t worked well.
Back at USC, the machine shop had constructed custom nurseries: Teflon-coated, stainless steel tubes that applied two tons of pressure per square inch and could be flushed continually with fresh sea water to keep the babies healthy.
Manahan and his students, acting like nervous parents, continually checked the babies. They thrived, growing for 34 days before succumbing to their new, unnatural environment--weeks longer than in other experiments. Months later, Manahan is still showing off a picture of a mouthless, month-old larva taken by graduate student Douglas Pace.
Though it looks like a pregnant sock puppet, Manahan beams like a proud father. “Just look at it,” he said. “Isn’t it amazing?”
To determine how long the larvae survived on their own fat stores, Manahan needed to measure their respiration rates by seeing how much oxygen they used. The task involved measuring--at high pressures and frigid temperatures--barely perceptible changes in oxygen content. In an hour, a tubeworm larvae uses a trillion times less oxygen than a human uses in a single breath.
The team found that the animals could survive in the ocean for nearly 40 days, six days longer than they had survived in the lab. That’s long enough, according to models of sloshing ocean currents, for a small percentage to settle new vents up to 50 miles away. Life in the ocean is a numbers game; mature tubeworms spew out millions of eggs. Most drift away and eventually die. The lucky ones colonize--and keep the species from going extinct.
Manahan’s results explain, said evolutionary physiologist Somero, why tubeworms are able to colonize new vents in neighboring regions but aren’t able to spread across vast distances or into different ocean basins.
Manahan conducted the study with his postdoctoral researcher Adam Marsh, marine biologist Lauren Mullineaux of the Woods Hole Oceanographic Institution and Craig Young, a deep sea biologist at the Harbor Branch Oceanographic Institution in Fort Pierce, Fla. The study was published this month in the journal Nature.
“What I like about the study is it pulls together a lot of different information: how long they live, the oceanography. It’s a complete picture,” said James Childress, a biologist at UC Santa Barbara who was one of the first to conduct studies of the elusive tubeworms. “Usually with deep sea research, you end up with one or two pieces of information and you have to guess the rest.”
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A one-minute film about Manahan’s research can be seen at: https://www.usc.edu/manahanlab
