Crabs and other scavengers put nutrients back into the marine ecosystem
There is a recent and rather macabre addition to the marine biologist's toolkit. Scientists in Canada are using the bodies of dead pigs, diverted from the butcher's shop, in their undersea laboratory.
By ending up in ocean experiments (rather than on the dinner table), the pigs have provided scientists with some intriguing new data.
"The dead pigs were not my idea," says Dr Verena Tunnicliffe, professor of marine biology at the University of Victoria in Canada.
She explained that she and her colleagues were "piggybacking" on forensics research in order to study low-oxygen zones, sometimes known as "dead zones", in the ocean.
A lot of people [when they die] end up in the ocean - either accidentally or through foul play
Professor Vernea Tunnicliffe, University of Victoria
The pigs are used by forensics researchers because - strange as it might seem - we humans are remarkably pig-like in our anatomy. We share their relatively hairless skin, are similar in size, and our flesh has a similar make-up.
And these things are important when you are trying to find out what happens to a human body if it ends up in the ocean.
Because another unpleasant reality is that many bodies - particularly in a coastal area like British Columbia - are discovered there.
"The ocean is a big place and there are a lot of people who work at sea and a lot of people who live on the coast. As a result, a lot of people [when they die] end up in the ocean," says Professor Tunnicliffe. "Either accidentally or through foul play.
"If you need to know how long it's been since death, if you're looking at the remains and there are marks on them, you need to find out - how did those marks come about?"
Looking at what happens to the pigs enables forensics researchers to better understand those processes.
Deep sea pigs
This grisly research gave Professor Tunnicliffe the opportunity to study seafloor scavengers.
"This big hunk of meat on the seafloor represented a good food source for these marine creatures," she says.
"Scavengers are very important in the world. They're what allow things to restore."
So she and her team used the pigs to investigate the increasing problem of oxygen depletion in coastal areas.
Low-oxygen (hypoxic) zones are caused by the nutrient-rich run-off from agricultural land. This feeds algae in the ocean. When this algae dies, sinks and decomposes, it consumes most of the vital oxygen supply in the water.
At its worst, this can lead to a dead zone - an area of seafloor that is so low in oxygen that all of the marine animals either die or, if they can survive the move, are forced into habitats in shallower water.
The scavengers know something's there and they arrive almost immediately and set up residence
Professor Verena Tunnicliffe
One dramatic example is the dead zone in the Gulf of Mexico. Here, nutrient-laden water runs out into the Gulf from the Mississippi River causing one of the world's largest dead zones.
The zone, which is continually contracting and expanding, has been estimated to average approximately 16,000 sq km. Almost nothing, apart from bacteria, can live there.
So Professor Tunnicliffe and her team set out to find out "how low marine scavengers would go", in terms of oxygen, for a free lunch.
They placed three pigs into very oxygen-poor zones in the Saanich Inlet, which is off the coast British Columbia. Each pig was lowered with a camera and oxygen-measuring equipment.
Scientists could operate the cameras from shore thanks to an experimental undersea observatory called Venus (The Victoria Experimental Network Under the Sea).
Sea scavengers feast on dead pig
"We could put the pigs down and observe the scavengers as they arrived," explains Professor Tunnicliffe.
"On day one, we lowered the pig," she says. "By day two, we've had crabs and shrimp, then octopus. Then sea stars arrive.
"They've had to travel across the bottom. They know something's there and they arrive almost immediately and stay there."
At levels down to almost 7% oxygen in the water - well below what scientists predicted would be tolerable - the animals still coped.
"They know something's there"
"The oxygen levels are well into what the chemists tell us would cause severe hypoxia (oxygen shortage in the animals' bodies).
"Yet they stay there day after day - the same individuals. They take up residence."
All of these marine animals need oxygen in their blood. Ideally, they want 100% of their blood cells to be loaded with life-giving oxygen to feed their organs.
"When the outside water only has 7% oxygen, and you want 100% in your blood, you have to pump that oxygen against a tremendous gradient - that's pretty tough and that's what these crabs have to do," says Professor Tunnicliffe.
But there were limits to their tolerance.
"The third pig we put down there - for three months nothing touched it," said Professor Tunnicliffe.
The oxygen there was below that severe level, so even these hardy scavengers had reached their limit.
"That means nothing's getting recycled on the seafloor. It's just stuff lying there waiting for bacteria to do a very slow decomposition," says Professor Tunnicliffe.
"These scavengers get the carbon cycle back. We have to get the nutrients back in the system and start this restoration process."
She says that this provides insight into how marine ecosystems can recover from hypoxia - and that it gives scientists some hope.
"Think of a dead zone that's killed off a whole bunch of animals," the professor explains. "The dead zones occurring off the west cost of America are coming up from below - pushing the animals up [from the seafloor].
"If some of these animals can survive, with a little bit of oxygen coming in, perhaps they can get down there and start that clean-up. There's lots of food down there now."
Pig vs shark
This combination of the pigs along with the Venus observatory, which is operated via a fibreoptic cable on the seafloor, has allowed researchers to record footage of much more elusive creatures in the suffocating depths.
Dr Richard Dewey, also from the University of Victoria, and one of the researchers leading the Venus project explains: "We install a node - or an internet hub - into which we can plug in instruments," explains Dr Dewey. "This gives us high bandwidth access to all the instruments.
"We pan and tilt, zoom and focus on the subject - all from a remote point on the shore."
He says the pigs have been down "in relatively deep water for these coastal environments" - 100m to 300m.
Pig carcass tempts six-gill shark
"Even at those depths we've seen seals swimming across the film.
"And in the most recent experiment in February, we put down a pig at 300m in the Strait of Georgia; and we saw six-gill sharks having a feast on the carcass."
Dr Dewey explains that there is very little data about six-gill sharks, partly because they live in deeper waters than their more famous cousins, the white sharks.
"We now have footage of them working together," he says. "And they arrived at the carcass within 12 hours.
"We have evidence of four or five that are maybe eight feet (2.4m) in size."
Some of the footage may be gruesome, but it continues to unveil the lives of these ecosystems.
Hear more from the researchers on Science in Action on the BBC World Service on Friday 5 March.
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