Dead Whales Make for an Underwater Feast
When whales decay the seafloor, their enormous carcasses give life to mysterious worlds inhabited by an assortment of bizarre creatures.
Once the whale is stripped bare, a dense community of anaerobic bacteria dines among the decaying bones. These "sulfate-reducing" bacteria release sulfides up to eight feet into the surrounding sediment, supporting a rich mix of bacteria, mussels, clams, and worms. Just as some animals derive energy from the hydrogen sulfide and methane leaking out of hydrothermal vents and cold seeps on the seabed, similar "sulfur-loving" organisms found at whale falls get their metabolic fix from the chemicals released by whalebones. Finally, the carcass acts as a sort of reef, providing habitat for filter feeders and worms such as Osedax, which excavate the lipid-laden bones. Bacteria either live inside Osedax and other organisms as symbionts, or they provide a feast for worms and snails. Fish, octopuses, and crabs move in to munch on the degraded whalebones, consuming worms and mussels along the way.
Since natural whale falls are difficult to find, researchers must sink beached dead whales. To date scientists have sunk more than 25 whale falls and monitored about six natural ones. "Pebbles," the one Johnson is studying today, washed up in April 2007--smelly and decaying--amid the stunning vista of the 18th green at Pebble Beach in sight of $2,000-a-night hotel rooms. Johnson received an urgent call from a whale-stranding network and mobilized her team. "Every time a dead animal washes up on the beach, I get like five phone calls," she says. Donning hip waders, rubber gloves, and sweatshirts destined for the trash, the researchers often gut the whales, using machetes to remove the blubber, before they tow them to a drop spot. But towing and sinking a five-ton juvenile, let alone a 35-ton adult, is no small undertaking. When whales die, they quickly bloat up with decompositional gases, becoming buoyant, so it may take several tons of scrap metal, mostly from old train wheels, to weigh them down. (In nature, whales often sink immediately before gases build up. But those that do bloat float for days to weeks before washing ashore or being ripped apart by scavengers at sea and descending.)
Ashley Rowden, a marine ecologist at New Zealand's National Institute of Water and Atmospheric Research, understands the inherent rigors of the science. On one occasion, Rowden's team first had to acquire support from indigenous Maori, who prize the whales' enormous jawbones. When a 40-ton sperm whale washed up in 2008, they raced against a thunderstorm in the notoriously turbulent Cook Strait. Wearing masks to avoid breathing in dangerous particles from the decaying animal, the crew endured several hours of frantic rope work before finally securing it to their vessel. "It was a bit like the old days," says Rowden. "Every hand was needed on deck, and everyone was slipping all over the place because of the amount of blubber." It then took more than 24 hours to tow the whale to sea. "We were all very glad when it finally made its way to the bottom."
In 2007 and 2008 Glover, Thomas Dahlgren, a marine biologist at Sweden's University of Gothenburg, and Craig Smith, a University of Hawaii at Manoa biological oceanographer, sank whalebones in frigid Antarctic waters, where whale fall fauna haven't previously been studied. This past March they recovered some of the bones. To their delight, they found a new species of Osedax. Next they'll deploy bones off the Antarctic Peninsula to gauge the fauna's diversity and geographical distribution. "The Antarctic is one really fascinating piece of the puzzle," says Smith. "It's a big habitat with a long history of whales, and it is somewhat isolated from other regions. So I think we may find really unusual diversity there."
So far the most exotic find has been the nearly 20 species of Osedax worms. In 2002 Johnson's colleagues turned up the first two species on an unknown whale carcass in Monterey Canyon. Osedax have evolved a nightmarish feeding strategy. They have no eyes, mouths, or stomachs but possess elaborately branching root systems that tunnel into whale bones, whittling them into something that resembles Swiss cheese. From the marrow, the worms' roots extract fats and oils, which bacteria living inside the roots break down. Never before had scientists found this sort of microbe living in a symbiotic relationship with another animal. It isn't clear where the bacteria come from: Osedax don't have them as larvae or seem to inherit them from their parents. "They're somehow getting them from the environment, but we haven't found them in the environment yet," says Johnson. "They're out there somewhere."
Osedax evolved roughly 40 million years ago, about the same time scientists believe many whale species arose. To learn more about how specialized the worms are for life on whalebones, Johnson, led by Monterey Bay Aquarium evolutionary biologist Bob Vrijenhoek, planted cow bones near a study site. To their surprise, they found six of the eightOsedax species from nearby whale falls on the cow bones--though there were only a handful of worms; thousands can inhabit whalebones. So the giant, fatty whalebones may be the meal of choice, though preliminary data aren't conclusive. The team also recently detected some 100 worms on an elephant seal carcass they sank, but it's not yet clear how much biodiversity such bones can sustain. Whereas smaller cetaceans' bones may decompose totally in three months, some whalebones may support fauna for 80 years.