NEXT time a whale becomes stranded on a beach in New Zealand, Craig Smith will be secretly hoping that any rescue attempts fail, so that he can tow the dead whale out to sea, strap a couple of tonnes of scrap metal to it, and dispatch it to a watery grave.
Smith, a marine biologist at the University of Hawaii at Manoa, has been sinking dead whales in this way since 1992. It’s smelly and expensive – each sinking costs about $8000 – which is why he’s only done it seven times. But with each whale sunk, Smith learns a little bit more about one of the oddest and most fascinating habitats on the ocean floor.
“Whale falls” host bizarre communities of creatures that feed off the whale’s huge carcass. They can last for decades, perhaps as long as a century, and support a succession of weird and wonderful organisms, some of which only live on dead whales. In terms of species richness, researchers are coming to realise that whale falls are right up there with hydrothermal vents and cold seeps (see “Springtime in the abyss”). “They are the biodiversity hotspots of the deep ocean floor,” says Smith.
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Biologists had long wondered what happens to whales that die in the open sea. As long ago as 1934, the Danish zoologist August Krogh speculated that dead whales sink to the bottom and are eaten by whatever lives down there. But it was more than half a century before researchers saw what becomes of a dead whale.
In 1987, Bruce Bennett of the University of Hawaii was mapping the Santa Catalina basin off the coast of California in a submersible when he spotted something peculiar on the seabed, 1240 metres down. At first he thought it was a shipwreck or a set of dinosaur bones. A closer look revealed it was an 18-metre whale skeleton. “It’s very eerie, there’s a real poignancy to it,” says Cindy Van Dover, a marine biologist at the College of William and Mary in Williamsburg, Virginia, who has visited the skeleton. “It’s laid out so perfectly on the sea floor, like it is in a museum.”
Drenched in blubber
The next year a team that included Smith went for a closer look. They estimated that the skeleton belonged to a 60-tonne blue or fin whale and had been there for several years (Smith now believes it has been there since the 1940s). The bones were teeming with life: the team saw about 12,000 animals from more than 43 species. Most intriguing of all, the life forms were uncannily similar to those found on vents, whose discovery a decade earlier had surprised the world.
Since then, Smith and other marine biologists have discovered and studied eight more whale falls and have created many more by towing dead beached whales out to sea. As a result, they are learning a good deal about the strange afterlife of a whale.
The biggest natural whale killers are starvation and disease, often during migration. Whales that die in the open ocean quickly sink to the seabed as they are usually malnourished. And once below about 1000 metres, they stay down: water pressure at that depth crushes the decomposition gases that make it impossible to sink a beached whale carcass without ballast.
For the nutrient-poor ocean floor, a dead whale is an absolute bonanza, delivering a package of nutrients that would take up to 4000 years to accumulate at background rates. “It’s a massive pulse of food in a food-poor environment,” says Smith. The average mass of a great whale is about 40 tonnes, about 90 per cent of which is soft tissue.
Within days of a whale touching down, the carcass is crawling with mobile scavengers such as sleeper sharks, hagfish and king crabs, which between them guzzle about 60 kilograms of soft tissue a day. Even at this rate it takes a long time to eat a whale. A 35-tonne grey that Smith sank in deep water in the Santa Cruz basin in 1998 took a year and a half to be stripped to the bone, and he estimates that a large, 160-tonne blue would take up to 11 years.
There’s nothing hugely unusual about the animals that feast on the soft tissues: all are opportunistic scavengers that eat pretty much any kind of dead animal that reaches the ocean floor. But what follows is unique. As the large scavengers finish up and move out, another community of creatures moves in. They start on the blubber-drenched sediments around the carcass, but their real target is the bones.
A whale skeleton weighs up to 20 tonnes and, uniquely among mammals, is 60 per cent fat by weight. Exposed whale bones quickly become covered in a writhing mass of lipid-slurping worms, snails, clams and limpets at population densities higher than anything else seen in the deep sea. Many of these species have only ever been found on whale carcasses, and appear to be whale-fall specialists.
Perhaps the weirdest are the bone-eating zombie worms. First described only last year (Science, vol 305, p 668), these appear to be about 5 centimetres long, but most of their body consists of “roots” that bore into whale bones and extract lipids with the help of the symbiotic bacteria living within the worm. Like the hydrothermal vent worms to which they are related, they have no gut. “They are an example of extreme evolutionary novelty,” says Smith. “A new feeding strategy that scientists never dreamed of.”
“It’s very eerie, there’s a real poignancy to it. The skeleton is laid out so perfectly, like it is in a museum”
This second stage can last a long time – four years in the case of the 35-tonne grey off Santa Cruz – but this is a mere blip compared with what comes next. As the easy pickings subside, yet another community moves in. These are the “sulphide lovers”, led by anaerobic bacteria that gradually invade the interior of the bones and break down the remaining lipids, all the while leaching out sulphides that sustain a complex community of clams, limpets, crustaceans, mussels and worms. A typical whale skeleton in this stage harbours 185 species. Again, some species are whale-fall specialists, though many are also found at sulphide-rich vents or cold seeps, or both.
This third stage goes on for a remarkably long time. The whale skeleton discovered in 1987 is still going strong after 60-odd years, and Smith estimates that a large whale could keep going for 100 years or more. “Whale bones are large – a blue whale’s skull is 4 metres long and 2 metres in diameter,” he says. “They have huge stores of lipids in the mineral matrix, with limited access by bacteria, so you get slow release for decades.” And it’s this that sets whale falls apart from other organic masses that reach the sea floor. Nothing else has large enough or fatty enough bones to sustain an entire ecosystem for decades.
There may be a lot of them, too. In Monterey Bay off California, researchers have found thousands of cold seeps, but just four whale falls. But seeps and vents occur in clusters – vents along the mid-ocean ridge and cold seeps along the continental margins – whereas whale falls can occur just about anywhere.
Even with today’s depleted populations, Smith and Amy Baco-Taylor of Woods Hole Oceanographic Institution in Massachusetts have estimated that 69,000 great whales die every year. The vast majority sink into the abyss. At any one time, they estimate, there are more than 850,000 active whale falls, on average 12 kilometres apart (Oceanography and Marine Biology: An annual review, vol 41, p 311).
That means whale falls could vastly outnumber vents, of which there are estimated to be 80,000. They are just as diverse, too, with 407 species known from whale falls and 469 from vents. And whale falls are still relatively unexplored territory – which is why Smith wants to drop one off New Zealand. “I’m absolutely sure we’ll find a lot more species,” he says. “It’s a big ocean!”
