What happens after the largest animal on Earth dies
A blue whale dies in the North Pacific. Her body sinks more than a kilometre and a half, out of the light, to a floor of mud where almost no food arrives. What grows on her there can last for decades.
She was 27 metres long and weighed about 100 tonnes. Blue whales are thought to live 80 or 90 years, so she may have been born in the 1930s, when whaling ships were still hunting her kind. Blue whales have been protected since 1966.
This morning she breathed out at the surface and did not breathe in again.
A whale that dies of age or hunger is usually thin, and a thin whale is slightly heavier than seawater. As the air leaves her lungs she starts to go down.
Most great whales that die at sea are thought to end this way.
150 m
Water takes sunlight apart one colour at a time. Red and orange are mostly gone within ten metres, then yellow, then green.
At 150 metres only blue is left, and not much of it.
200–1,000 m
Below 200 metres there is rarely enough light for anything to grow.
Every evening the largest migration on Earth passes through this water. Small fish, shrimps and jellies rise to feed near the surface in the dark, and go back down before sunrise.
1,000 m
Below about 1,000 metres no sunlight can be detected. The light down here is made by animals. In 17 years of filmed dives off California, three out of every four animals seen in open water were kinds that can glow.
From here on the scene is lit by a lamp, as it would be from a submersible.
1,400 m
Nothing grows in the dark. Animals on the deep sea floor live on what sinks from the surface: dead plankton, droppings and dust, in pale flakes called marine snow.
It falls all year, and it is never much.
1,675 m
She comes to rest on soft mud 1,675 metres down, the depth at which one sunken whale off California was followed for years. The water is about 4 °C and the pressure is about 170 times that at the surface.
A whale of 40 tonnes holds as much carbon as a hectare of deep sea floor receives from the snow in 100 to 200 years. She weighs more than twice that. The mud beneath her gets about 2,000 years of food in one delivery.
Weeks
The scavengers come first. On a whale watched off California, 400 to 800 hagfish covered the body six weeks after it sank. Sleeper sharks up to three and a half metres long had eaten more of it than any other animal.
Amphipods, crustaceans half a centimetre long, came in the hundreds of thousands. Together the scavengers removed 40 to 60 kilograms of flesh a day.
Years
Even at that rate, a 35-tonne whale lasted a year and a half. For a blue whale of 100 tonnes, the feast may go on for five years.
Then the mud around the skeleton, soaked with fat and scraps, fills with worms and small crustaceans: up to 45,000 animals in a square metre, the densest crowd recorded on the sea floor below 1,000 metres.
Years
In 2002, on a whale skeleton 2,893 metres down in Monterey Canyon, researchers found the bones covered in red plumes. They were worms with no mouth and no gut. Each one grows roots into the bone, and bacteria living in the roots digest it for her.
Every worm you can see is a female. The males are microscopic and live inside her tube, sometimes a hundred to one female. The worms were named Osedax, the bone-eater. More than 30 kinds have been found since.
Decades
The bones are rich in fat. Bacteria deep inside them break it down without oxygen and give off hydrogen sulphide, the gas that smells of rotten eggs. Other bacteria feed on the sulphide and spread over the skeleton in white and yellow mats. Mussels and clams carry bacteria like these in their own bodies.
More than 30,000 animals can live on a single skeleton, of 185 species on average. The core of one whale vertebra was still full of fat after 50 years on the sea floor.
Decades
By 2003, researchers had counted 407 species on whale falls. Eleven of them also live at hot vents on the sea floor and 20 at cold seeps, places that are far apart and hard to reach. A whale skeleton may be where their young can land, grow and breed on the way from one to the next.
Other animals have been found only on whale bones. By 2015 the list of those stood at about a hundred.
Put a light on every whale skeleton that is still feeding animals, look down from far above, and the sea floor might look like this. Each light stands for about a thousand skeletons.
This is the sea before the whaling ships, by one calculation.
In the twentieth century, whaling fleets killed nearly 2.9 million great whales, 379,000 of them blue whales. A whale hauled onto a factory ship never reaches the sea floor.
The same calculation makes whale falls six times rarer now than before whaling. A 2003 estimate put the skeletons still feeding animals at about 690,000, twelve kilometres apart on average. Researchers think some animals that lived only on whale bones may have died out before anyone saw them.
Commercial whaling stopped in 1986. Blue whales are still endangered, but their numbers are rising in the North Atlantic and around Antarctica.
A blue whale can live for ninety years. Then she feeds the deep sea for decades more.
The whale in this story is a composite. No single blue whale has been followed from the surface to the bones; the depths, timings and counts come from carcasses that researchers have found or sunk off California since 1987, and each number is from the sources below. The animals are drawn to show who is there and roughly where, not to scale in every detail.