One bright morning in November 2018, state biologists in Louisiana loaded three dead and toothy alligators, each more than 5 feet long and weighing hundreds of pounds, into the back of a pickup. The biologists bumped along Louisiana State Route 56, driving the carcasses as far south as one can go before plunging into the marsh at the outer mouth of the Mississippi River.
Waiting at the end of the road was Craig McClain, a tall, bald, and bearded marine biologist who studies how living things get food at the bottom of the ocean. McClain leads a coastal research institute called the Louisiana Universities Marine Consortium, or LUMCON, that explores life at the interface of land and sea. Its main building hovers over the wetlands on big concrete pilings, driven deep into the muck.
Coastal Louisiana teems with alligators (Alligator mississippiensis)—more than a million of them. They sit, mouths agape, on riverbanks and roadsides; they’re crushed by cars; they swim in canals and bask in the sun on nearby barrier islands. McClain can look out the window at work and sometimes see them swim by. He wondered: What happens when they die? If they died at sea, or were washed out, might they provide food to the strange things that dwell in the depths of the Gulf of Mexico? Are they an important part of deep-sea energy cycles? He conjured up a plan to find out, but to carry it out, he would need dead alligators.
When big alligators threaten pets, livestock, or humans—charging, biting, eating—the Louisiana Department of Wildlife and Fisheries considers them “nuisance animals,” and sends out hunters to remove them. In October 2018, McClain had asked the department if they could provide him with a few dead gators for an unusual research project. That November morning, he’d gotten the call.
“They said, ‘hey, we have three alligators for you. Are you prepared for them?’” McClain says.
McClain’s group unloaded the alligators, wrapped them in thick plastic sheets, and stashed them in a large freezer. They stayed there until February 2019, when the biologists transferred the alligators into another freezer on board the R/V Pelican, LUMCON’s 116-foot research vessel, which houses four laboratories and can support 14 scientists for up to three weeks at sea. McClain’s plan was to motor out into the Gulf to sink the alligators at various ocean locations and depths, to study what happens to their carcasses—a kind of crocodilian burial at sea.
The idea driving the experiment was simple: If you feed them, they will come. In this case, “they” were the scavengers that float, swim, burrow, and crawl in the muck at the bottom of the Gulf of Mexico. These scavengers can’t live without food, but because plants and phytoplankton cannot grow in the deepest ocean where there is no light, biologists believe that the organisms found there largely subsist on “food falls” that drift down in the form of kelp or dead fish and other animals. McClain suspected that decomposing alligators might play a role in feeding the invertebrates that dwell at the bottom of the Gulf of Mexico. Understanding the fate of dead creatures—like alligators—on the seafloor would help to fill gaps in knowledge about both the food chain and the carbon cycle in the ocean depths.
McClain’s group was the first in the world, as far as he knows, to sink dead reptiles for research—but not the first to study sunken carrion in the sea. In recent years, biologists have sunk whale carcasses, turkeys, shark craniums, pig carcasses, fish bones, and cow bones—in waters deep and shallow, warm and cold, from California to the Mediterranean, Japan to Antarctica—in order to draw out the exotic scavengers that hide in the deepest ocean and can’t be studied in any other way. “It’s mind-boggling,” says McClain. “You can be two or three kilometers deep, and there’s this whole set of animals that are just made to eat whatever carbon comes down.”
It’s not just a quest for novelty that drives such research. Precise studies of the lives and genes of exotic worms, crustaceans, and other deep-sea denizens have expanded biologists’ ideas about the diversity of marine life and how life evolves in the depths. Recent research, for example, indicates that such scavengers emerged as far back as 100 million years ago, during the time of the dinosaurs, likely feasting on the remains of giant marine reptiles like plesiosaurs.
Findings from food-fall experiments have also supported new hypotheses about how species that live in resource poor environments, like the deep ocean, evolve as their food sources change. Such research, McClain says, may ultimately help scientists better predict how deep marine life will respond to changes in carbon levels due to climate change. “If we give a little food, what happens? And then we give a little more, what happens then?” he says. “Hopefully, we can predict what the losers and winners will be in future oceans.”
“It’s mind-boggling. You can be two or three kilometers deep, and there’s this whole set of animals that are just made to eat whatever carbon comes down.”
— Craig McClain, Louisiana Universities Marine Consortium
A Waste of Utter Darkness
The deep ocean is the largest ecosystem on the planet, with more than half of Earth’s surface submerged beneath at least two kilometers of water. It’s also among the least hospitable to life.
Indeed, until the mid-19th century, scientists believed that no life could survive at all below a certain depth. There was, they maintained, a “bathymetrical limit to life,” below which “the conditions became so peculiar… as to preclude any other idea than that of a waste of utter darkness, subjected to such stupendous pressure as to make life of any kind impossible,” as Scottish zoologist and explorer Charles Wyvile Thomson explained in his 1873 book The Depths of the Sea. That limit, according to 19th-century biologist Edward Forbes, resided at precisely 550 meters (1,804 feet) beneath the surface.
But even back then, there were clues to the contrary. In 1810, for instance, French naturalist Antoine Risso identified and named a number of new fish species pulled from the bottom of the Mediterranean, including a boa dragonfish (Stomias boa boa) caught on a line dropped 1,500 meters (nearly 5,000 feet) down. Based on this and other new finds, Risso introduced the idea of underwater zones, defined by depth, where different creatures flourished. In 1818, explorer John Ross, while scouting the Northwest Passage, found a brittle star wrapped around a kilometer-long sounding line—the first evidence of deep-sea life in the colder North Atlantic Ocean. In the 1860s, engineers retrieved a broken North Atlantic telegraph line from a depth of 2 kilometers (6,562 feet) and found colorful corals, brittle stars, and other unusual invertebrates attached, further refuting the idea that life stopped at some set distance below the surface.
It was Thomson, however, who is generally credited with disproving the notion of a sterile, lifeless ocean bottom. From 1872 to 1876, he led an expedition on the HMS Challenger, a British warship whose guns had been replaced with laboratories and workrooms. From the boat’s deck, Thomson and his colleagues sank small traps attached to a rope 4.8 kilometers (3 miles) long and pulled up creatures that were, he wrote, “exquisitely beautiful in their soft shades of colouring and in the rainbow-tints of their wonderful phosphorescence”— every bit as diverse and exquisite as fauna found in shallower waters. He dared future scientists to enhance our understanding of these creatures: “Their mode of life, and their relations to other organisms,” he wrote, “and the phenomena and law of their geographical distribution, must be worked out.”
The Secret Taxonomy of the Deep
In the subsequent 150 years, oceanographers have endeavored to answer Thomson’s challenge. Using increasingly sophisticated technologies, they have unearthed a vast diversity of species and begun to learn how energy moves in the deepest seas. They have learned that sunlight can usually penetrate only a few hundred meters below the ocean surface, which means that nothing living a mile underwater can use photosynthesis to turn sunlight into life. They have come to understand that while some surface-dwelling fish, kelp, and zooplankton do die and sink to the bottom, such “marine showers” provide insufficient nourishment to support the wild diversity of life there. This suggests there must be other pathways for food to travel from the surface to the bottom, which is what experiments based on artificial food falls—like McClain’s alligators—are designed to explore.
Artificial food falls owe their origin, in large part, to another marine biologist named Craig—Craig Smith, now at the University of Hawaii. In the 1980s, when Smith was a graduate student at the Scripps Institution of Oceanography in San Diego, he found himself frustrated with how little scientists knew about life in the deep sea. They had discovered a growing catalogue of wild species there, but had generated few ideas about how life could subsist in such an inhospitable environment. “We already recognized that [the ocean bottom] has high diversity, but ecological theory couldn’t explain why,” he says.
A few years earlier, a Navy bathyscaphe—a bulky, self-propelled submersible built to withstand tremendous pressures—had stumbled across a well-preserved whale skeleton off the coast of California, near Santa Catalina Island. Smith speculated that such whale falls might serve as living laboratories that could unlock the secret taxonomy of the deep, and in 1987, he had the good fortune to find one. He was exploring a 1,200-meter- (3,937-feet-) deep undersea canyon with an ROV—a remotely-operated vehicle—when he encountered a 20-meter- (65-foot-) long whale carcass. Over five dives the following November, Smith and his colleagues identified an assemblage of bacteria, snails, clams, and other fauna, many of which hadn’t been found in the area before or had never been known to science at all. His analysis, published in Nature in 1989, jumpstarted a new effort among marine biologists to document and analyze the ecosystems around fallen sea creatures.
Smith knew he was fortunate to have stumbled upon the whale. While hundreds of thousands of whales and other marine food falls likely litter the world’s seafloors, “it’s hard to find them by luck,” he says. So when it was time to find another fallen whale to study, he decided to sink his own. In 1992, he set up an experimental agreement with the National Oceanic and Atmospheric Administration’s Marine Mammal Stranding Network: When a whale washed up on a California shore, NOAA would call Smith, who would bring boats and scientists to tow the carcass back into the ocean and sink it using thousands of pounds of retired railroad cogs and wheels as ballast. (On occasion, his team had to resort to poking holes in the carcasses—with arrows, harpoons, or bullets—to release the gases that can keep them afloat.)
Smith’s group sank and studied three gray whales (Eschrichtius robustus) in 1992, 1996, and 1998, at depths of between 1,200 and 2,000 meters (3,937 and 6,562 feet), tracking their decay over a decade. Whales, they learned, decompose in several stages in the deepest oceans. First on the scene are necrophages that feed on soft tissue—hagfish, ratfish, sharks, amphipods, even octopuses. Crustaceans and snails arrive next, nibbling on leftover blubber and picking the bones clean. Then come even smaller creatures—an intermingling of worms, mollusks, bacteria, and crabs—that turn the skeleton to dust. The entire process can take between a year and a decade, depending on the depth of the carcass. The deeper the fall, the fewer the scavengers, the longer the decay.
In 2002, around the same time Smith was studying his artificial whale falls, a marine biologist named Robert Vrijenhoek was using an ROV to explore another whale carcass—one his team at the Monterey Bay Aquarium Research Institute (MBARI) had stumbled upon a month earlier. Vrijenhoek’s group was looking for a particular species of clam he believed might be found on whale remains. Instead, they spied big patches of red on the whale bone. From a distance, it looked like a rash. A closer inspection of specimens brought up from the site revealed a forest of tiny tubes extruding from the bones. “It was like a red shag carpet,” says Vrijenhoek.
They were tubular worms that scientists had never before identified, and they were bizarre. The females are about as long as a finger and half a millimeter in diameter—narrower than most splinters. They have enormous ovaries in relation to their body size, and spawn hundreds of eggs per day. The males are typically microscopic, living inside the females’ oviducts. Lacking both mouths and guts, the worms burrow into bone by excreting an acid and sending root-like tendrils within, relying on symbiotic bacteria to digest the lipids and collagens from the center of the bones and produce nutrients. The team named them Osedax—Latin for “bone-eaters”—though they go by other names as well: snotworms, boneworms, zombie worms.
The worms’ novelty and bizarre appearance made Osedax a celebrity among biologists. Their efficient means of absorbing nutrition from whale bones suggested that they had evolved with decomposing whales. What wasn’t clear was their role in the carbon pathways that connected air, land, and ocean surface to the invertebrates that live far below. Did Osedax thrive only on the bones of certain whales, or only in some oceans? Was there a depth at which they could no longer survive? Or a temperature? It was hard to know, without first knowing how ubiquitous they might be in the oceans.
The first step would be to study whether Osedax lived only on whale bones, or if they could also be found on other decaying carcasses, like seals, fish, and seabirds—and perhaps McClain’s alligators, too—which were even more difficult to find on the ocean’s bottom, because smaller creatures typically degrade faster.
Thanks to Smith’s pioneering whale work, however, scientists had a method for studying deep-water food falls: dropping lots of different bones in lots of different places. In 2005, Vrijenhoek’s group sunk a series of slivered cow femurs, hung on a PVC “bone tree” and anchored in concrete-filled buckets, near whale falls off the California coast. When they returned to retrieve the tree just two months later, they found a shag carpet of Osedax on the bones. In 2010, Vrijenhoek and his team sank assemblages of cow bones and calcified shark cartilage in Monterey Bay. Five months later, they not only confirmed colonies of known Osedax species, but also identified a few new ones.
In 2012, they even sunk a holiday turkey. “It was Thanksgiving, and we were at sea,” says Vrijenhoek’s colleague and frequent collaborator, Scripps Institution of Oceanography biologist Greg Rouse. The worms colonized the bird bones, too. “The bone worms didn’t care,” Vrijenhoek says. Osedax, it seems, does not discriminate.
Since then, artificial food-fall experiments around the world have continued to fill gaps in the Osedax story. “Many people were skeptical they could live in shallow waters,” says marine biologist Sergi Taboada, now at the Complutense University of Madrid , but in 2012, he and his colleagues sank the de-fleshed bones of a beached minke whale (Balaenoptera acutorostrata) along with a collection of pig and cow bones in the Mediterranean off the coast of northern Spain. They found no evidence of Osedax worms in warm water at a depth of 20 meters (65 feet), suggesting that the creatures might indeed be limited to colder environs. Two years later, though, Taboada’s group dropped a similar cache of bones at a lower depth, in a nearby undersea canyon 53 meters (174 feet) down, and identified a new species which they named Osedax mediterranea.
Osedax evolved to eat whatever bones drifted down into the dark, including, perhaps, the bones of prehistoric ichthyosaurs.
More recent studies have also explored not only where the scavengers feed, but also how long they’ve been around. In 2010, researchers found telltale boreholes in whale fossils from the Pacific Ocean dating back about 30 million years, around the time whales evolved—leading scientists to speculate that Osedax likely emerged with whales. But in 2015, researchers at Plymouth University in the United Kingdom identified Osedax boreholes in fossil remains of plesiosaurs and sea turtles stored at the University of Cambridge. Those fossils dated back 100 million years—meaning that the worms have been around since at least the late Cretaceous period. Osedax evolved to eat whatever bones drifted down into the dark, including, perhaps, the bones of prehistoric ichthyosaurs.
By sinking carcasses (and studying some natural food falls as well), scientists have now identified dozens of new species of Osedax in the Pacific, Atlantic, and Southern Oceans, in the Arctic Sea, and in the Mediterranean. They’ve found other new critters, as well: In May 2020, scientists at the University of California, San Diego even reported finding a new kind of worm, covered in dazzling scales that look “like sequins from an Elvis costume,” says Rouse, a coauthor on the ZooKeys paper that reported the discovery. The researchers named the species Peinaleopolynoe elvesi, after the singer.
Alligator Number Three
Thanks to McClain’s alligators, we now know that Osedax worms reside in the Gulf of Mexico, as well. The experiment marked McClain’s first foray into sunken animal carcasses, though not his first effort at submerging items in the name of science. Before arriving in Louisiana to lead LUMCON in 2016, he had worked with Monterey biologist Jim Barry on a 2006 experiment to study underwater snails and slugs on sunken logs. “I’ve always been interested in how biological systems are tied to the energy that’s available to them,” he says.
Plenty of energy could be found in the alligators that roamed all along the Louisiana coast: “I’ve seen more alligators in the state than I have squirrels,” he says. One day at LUMCON, while leading a discussion of a recent paper Rouse had written about artificial food falls, one of his students asked, “What if we sank an alligator?” McClain warmed to the idea: Perhaps the reptiles filled a similar niche along the Gulf coastline as sunken whales off the coast of California. Which is how, one crisp morning in February 2019, he found himself on a research boat with a freezer full of dead alligators, motoring out to sea.
McClain and his group dropped the first alligator about 24 kilometers (15 miles) from shore, securing the animal in a sturdy basket attached to 45-pound weights and using a system of pullies—like a deep-sea dumbwaiter—to lower it 2,181 meters (7,156 feet) beneath the water’s surface. The next day, they lowered a 1,585-kilogram (3,500-pound) ROV to examine the gator. The goal was to study what happens in the very early stages of a food fall.
McClain suspected that the reptiles’ tough hide might deter would-be scavengers, but video footage revealed eight giant isopods—Bathynomus giganteus, which look like pillbugs but are pink and as large as armadillos—gnawing on the creature’s leathery flesh. A day later, the rover recorded even more of the creatures; some had already burrowed inside the carcass.
The crew then motored 80 kilometers (50 miles) east to sink the second alligator at a depth of 2,034 meters (6,673). They planned to give this one more time to decompose—and headed back to shore. Six weeks later, they motored back to the site with the rover and observed that the animal’s flesh was nearly gone. Small crustaceans called amphipods darted in and out of the remains. All manner of octopuses, crabs, snails, and others gnawed on the scraps and bones. And a reddish carpet of Osedax now covered the bones—the first ever identified in the Gulf of Mexico. These worms belonged to the same genus as the ones found on whales, turkey bones, and all the rest, but they were a brand-new, as-yet unnamed species.
McClain’s team sank the third and final alligator at around the same depth but about 105 kilometers (65 miles) northeast of the second. Eight days later, they returned, expecting to find another partially devoured carcass. Instead, they discovered that the beast had vanished. “We were like, are we in the right place? Did we mess this up?” says LUMCON graduate student River Dixon, who watched the rover feed from a cabin on the Pelican. Video images showed an alligator-shaped depression in the sediment—but, says Dixon, “it was completely gone.”
The rover scoured the area for clues, and about 6 meters (20 feet) away, came across the discarded weight and a severed rope. “Something was big enough to haul it away,” says McClain. Even at such great depths, fallen carcasses feed other, larger creatures as well.
McClain’s experiment showed that dead alligators and other large reptiles such as crocodiles or pythons, when they die at sea or are washed out by the tides, likely serve as meaningful food sources in the Gulf, and further expanded scientists’ knowledge of the diversity and reach of Osedax. In only 16 years, scientists have found 26 species—suggesting that they’ve only just begun to chart the biodiversity of the weird little worms. We now know that they live in most of the world’s oceans and have been there for many millions of years, feasting on the dead as an integral part of the complex energy web of the deepest seas.
Continuing to study them may help answer one of the most important questions driving deep-sea research today: How will the recycling of carbon from land to water respond to a rapidly-warming, acidifying, ocean? Models suggest that deep-sea ecosystems are particularly sensitive to change. Studies of the diversity and evolution Osedax may show how, in the past, organisms have adapted and thrived—or not—as new pressures bring changes to the food web.
The missing alligator, meanwhile, illustrates just how much we still have to learn about deepwater ecosystems and the way energy moves through them. McClain still wonders what hungry creature might have been able, at a depth of 2,000 meters, to dispatch with his alligator so quickly. “Maybe it was a big shark. Or maybe a bunch of sharks, working together,” McClain says.
But he doesn’t know—and in this uncertainty, he is in good company. Scientists have only just begun to understand the complex intricacies of what happens in the ocean dark, and in the deep.
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