THE alligator lay motionless on a flat expanse of mud and stared into the camera with a toothy grin. Two metres long, its skin covered in dark green scales, it wasn’t in the usual location for a dead reptile.
A day earlier, the carcass had been loaded into a wire cage and lowered over the side of the ship I was aboard. For an hour, it had travelled down to the sea floor, 2 kilometres below. There it was met by a deep-diving submersible with live-feed cameras. Controlled by pilots on the ship, a robotic arm had reached into the cage, picked up the reptile and placed it on the seabed. This was to be the alligator’s final resting place, at the bottom of the Gulf of Mexico.
The next day, I joined the scientists and crew of the ship around the monitor screens to watch a grisly scene unfolding. Despite its tough hide, the alligator was already being eaten by a horde of scavenging giant isopods – think pink woodlice the size of rugby balls.
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This was February 2019 and the first time an alligator had been left by scientists in the deep sea, so we didn’t know what to expect. But no one aboard the ship had anticipated just how quickly it would be found and eaten. This reptile’s demise would show how entire ecosystems spring up on the carcasses that fall from the surface in this strangest of . It would also shed light on a mystery dating back to the era of the dinosaurs.
More than a century ago, naturalists dispelled the myth that the deep oceans are a lifeless void. Even today, exploration of the largest habitat on the planet is redefining the possibilities of life on Earth with discoveries of bizarre new species and even whole new ecosystems in this vast realm that covers 71 per cent of the planet’s surface. There is a snailfish that lives 8 kilometres down in the Mariana trench, which in 2014 broke the record for the deepest living fish known, and a hydrothermal vent system in the Gulf of California with surrounded by beds of red-tipped tubeworms. “Even after 10 years working in this field, I’m still amazed there are organisms down here, under these extreme conditions,” says MacKenzie Gerringer, a deep-sea biologist at the State University of New York at Geneseo.
Below 200 metres, the only sunlight that pierces through the water is dim and blue, not strong enough to power photosynthesis. Below 1000 metres, there is no sunlight at all. It means that in open, deep water, no new food is made and animals rely on a shower of organic particles made from clumps of dead plankton and their faeces, known as marine snow, sinking from the surface.
A lot of that marine snow gets eaten on the way down. Shrimp sift the water with combs on their legs. Swimming snails, called pteropods, build snow-catching mucous webs. Vampire squid reel out a long filament to collect falling flakes, then pack them into snowballs to swallow. At most, about 2 per cent of the food produced at the surface reaches 2000 metres. With such meagre supplies, any larger consignments of organic matter that arrive in the deep, like a dead alligator, are likely to be welcomed with open jaws.
The was carried out by the Louisiana Universities Marine Consortium (LUMCON). The experiment was a first, but it probably wasn’t the first alligator to be eaten by giant isopods. Conservation efforts mean there are now close to a million American alligators in the southern US – the specimens LUMCON used for this and other drops were obtained from a culling programme. Storms can sweep dead reptiles offshore. Elsewhere, saltwater crocodiles, caimans and other large reptiles live along coastlines and could also end up sinking, without the help of scientists.
“We gathered around the monitor screens to watch a grisly scene unfolding”
The aim of the alligator study, led by Craig McClain, executive director at LUMCON, was to trace the effects of this reptilian carbon in the abyss. Alligators, crocodiles and caimans are the closest living analogues of the enormous marine reptiles such as ichthyosaurs and plesiosaurs that dominated the oceans in the Jurassic and Triassic eras between 250 and 66 million years ago. Their bodies would have made a significant contribution to the deep-sea carbon budget. McClain wanted to find out which scavengers are lying in wait in the deep ocean for a modern-day reptilian meal.
By the time the submersible returned to the scene, the dead alligator had lain in the abyss for 50 days. It had been stripped to bare bones. No one had been there to watch the feast take place but, based on similar studies of dead whales, we can guess how events unfolded.
“What scavengers are lying in wait in the deep ocean for a reptilian meal?”
Food oasis
Sunken whales form food-rich islands in the deep that last for decades. First, mobile scavengers, such as the eel-shaped, slime-producing hagfish and bristle-covered polychaete worms, strip away the soft tissues. Opportunists like crabs and snails congregate around the carcass to feed off scraps dropped by the scavengers. If a dead whale is in the deep for long enough, bacteria begin to break down its bones anaerobically, releasing sulphur and methane. These “chemosynthetic” microbes grow in thick mats, turning the whale bones white, yellow and pink, attracting shrimp, crabs and snails to graze on them.
When McClain and the team returned to the fallen alligator, it was probably too soon for sulphurous, chemosynthetic stages to have developed. But the carcass did harbour something else commonly known from whale falls. The alligator bones had a “furry, red shag carpet-look to them”, says McClain. This red fuzz turned out to be a type of worm called ). Sometimes known as zombie worms, Osedax look more like plants than worms, with roots and pink, flowery gills. They have no mouth and no stomach, instead absorbing and digesting collagen from the bones through their roots, with the help of symbiotic microbes living inside them.
First discovered on a whale fall in 2002, dozens more such worms have since been found on a variety of sunken bones. But two of the types found on the alligator were previously unknown species.
Some researchers initially thought Osedax may specialise in eating whale bones, but it was soon established that they will colonise any bones they find. They do, however, need a large vertebrate, like a whale, whose skeleton will remain on the deep seabed for a long time, not chewed up and eaten whole by a predator. This raises the question of which came first, the whales or bone-eating worms.
To find out, we can look at the , comparing them with other types of worm to find out when they first evolved. However, the results of such genetic studies can give conflicting results depending on how they are processed. One of the genetic clocks used to trace the evolutionary timescale indicated that the zombie worms originated 45 million years ago, shortly after the ancestors of whales took to the oceans. But another genetic clock put the origin of Osedax back in the Cretaceous era, which began 145 million years ago. If that clock tells the correct evolutionary time, whose bones were those ancient worms eating?
An answer came in 2015, when a team at the University of Plymouth took a fossilised bone from a plesiosaur that swam in the oceans 100 million years ago and put it into a CT scanner. The bone was perforated with characteristic worm holes, supporting the idea that Osedax evolved long before whales. It is possible that the skeleton of the alligator dropped by McClain and his colleagues could have been devoured by zombie worms descended from those that consumed the bones of these ancient reptilian sea monsters.
Reptiles aren’t the only unlikely food source sustaining life in the deep ocean. As well as dropping an alligator, the LUMON team sank chunks of wood in the Gulf of Mexico. Remarkably, there are animals living on the deep sea floor that seem to specialise in eating trees.
Perhaps this shouldn’t be surprising, given rivers carry lots of uprooted trees and woody debris to the sea, where they become sodden and sink. Previous studies have shown that most species colonising rotting logs on the sea floor and have come to rely entirely on dead trees and branches.
The LUMCON team left logs on the sea floor for between 12 and 18 months and is analysing which animals made them their home. The results haven’t been published yet, but there seem to have been as many as 60 species per log. As in previous wood falls McClain has carried out off the Californian coast, the Gulf of Mexico logs are riddled with clams called Xylophaga (in Latin, this means wood-eaters) that dig their way in with the sharp edges of their shells, creating boreholes that other species could then inhabit, including sea cucumbers, squat lobsters and sea stars.
“Reptiles aren’t the only unlikely food source sustaining life in the deep ocean”
For the latest wood-fall study, McClain collaborated with Clifton Nunnally, a specialist at LUMCON on the Gulf’s deep ecosystems, to test a concept called the theory of island biogeography – but in the deep. This ecological “law” was proposed in the 1960s to explain the diversity of animals on islands. The smaller and more isolated an island is, the fewer species live on it. McClain and Nunnally want to know if the same goes for isolated oases of wood in the abyss. As well as understanding how these ecosystems assemble, the study could help predict how changes on land – deforestation or increased hurricanes because of climate change, for example – may be felt in the remote reaches of the deep.
The alligator and wood drop studies do show a strong link between the land and the sea floor, with a host of organisms waiting in the deep for food parcels from the shore. “There’s this highway that connects the land to the oceans,” says McClain.
But not all the items dropped to the deep meet a slow fate as they are eaten by a variety of organisms. Another of the alligators sunk by the LUMCON team met a very different demise.
Eight days after it was dropped, the alligator had disappeared. All the team could find was the weight that had once been holding the carcass in place. The rope fixed to it had been bitten clean through. “You could see where the weight had been drug through the sediment,” says McClain.
It is probable that the alligator was snatched away by a shark. Six-gill and Greenland sharks, which can reach 7 metres long, are known in these depths in the Gulf of Mexico. “They have the bite strength and the sharpness to be able to chew through a half-inch polypropylene line,” says McClain.
Nobody was there to witness the event, but the missing alligator is another piece of evidence showing the multiple pathways that land-based carbon can take when it enters the deep-sea food web. And it goes to show that there is more than one way to eat an alligator in the abyss.
