A stray observation helped one researcher to uncover the strange connection between the seashells and lobsters of his childhood.
Born in the Bahamas to a family of lobster fishermen, Nicholas Higgs spent much of his childhood diving in Caribbean waters, working on boats, and collecting shells on the beach. That connection to the sea stayed with him. He moved to the UK and became a marine biologist. He studied whales and marine worms. And on his wedding day, he asked his parents to bring some shells from the Bahamas to decorate the dining tables. Those shells, which symbolized his past, would also define his future.
At the wedding, his former boss picked one up and identified it as a lucinid clam—a group that feeds in a strange way. While most clams filter food from the surrounding water, lucinids get almost all their nourishment from bacteria that live in their gills. And the bacteria create their own food—just like plants, but with one critical difference. Plants make nutrients by harnessing the sun’s energy, in a process called photosynthesis. But the clam bacteria get their energy by processing minerals in their surroundings. That’s chemosynthesis—making nutrients with chemical power instead of solar power.
And Higgs studied chemosynthesis. He had worked on other animals that also form partnerships with chemosynthetic bacteria, including deep-sea worms that colonize the carcasses of whales. Those clams on his wedding table were his bread and butter. And he realized that he had seen plenty of them before.
The fishermen whom he grew up with would catch spiny lobsters by dropping artificial shelters onto the seafloor. And around these shelters, Higgs remembered seeing lots of empty, chipped lucinid shells. “The lobsters were obviously feeding on them,” he says.
If his hunch was right, that would be a big discovery. Scientists used to see chemosynthesis as an obscure way of life, confined to environments like the deep oceans, and practiced by creatures that most people never see. By contrast, photosynthesis underpins all the food webs we depend upon: the sun’s energy feeds plants, which feed animals, which feed more animals.
Over time, this distinction has eroded. We know that millions of birds dig up chemosynthetic clams and gorge upon them to fuel their migrations. We know that indigenous people in Brazil and the Caribbean harvest and eat lucinid clams.
But if Higgs was right about the lobsters, then chemosynthesis is important on an even greater scale—a commercial scale. Spiny lobsters are a delicacy, and their fisheries are some of the most lucrative in Central America. They keep some 50,000 people in jobs, and create more than US $450 million every year. If the lobsters depend on the lucinids, for even just part of their diet, then chemosynthesis isn’t so niche after all. It puts food on our plates and money in our bank accounts.
As a lifestyle, chemosynthesis is billions of years old—perhaps as old as life on earth itself. But as a concept, it’s roughly as old as Higgs, who is 31. Its story begins in 1977, when a submersible called Alvin descended into the waters just north of the Galapagos Islands.
At a depth of 2,400 meters, the scientists who were crammed inside saw something startling: huge chimneys of rock, belching hellish plumes of superheated water. These were hydrothermal vents—the first ever discovered. And while the researchers had expected to find those vents, they weren’t expecting to find them teeming with life.
The Hadean landscape was covered in clams and shellfish, and patrolled by crabs and fish. The belching chimneys were encrusted white tubes containing giant worms, whose red gills resembled extended tubes of lipstick. As I wrote in my book, I Contain Multitudes, “the team were so unprepared to find life that there wasn’t a single biologist among them—they were all geologists. When they collected specimens and brought them back to the surface, the only preservative they had was vodka.”
One of those giant tube worms made its way to the Smithsonian Museum of Natural History, where zoologists noted that it had no mouth, gut, or anus. How could it possibly eat? A young student named Colleen Cavanaugh solved the mystery. She showed that an unusual yellow organ, which filled up half the worm’s length, was loaded with bacteria. These microbes oxidize the sulphide compounds that belch out of the vents, and use that energy to make nutrients. In other words: chemosynthesis.
Cavanaugh published her discovery in 1981, four years before Higgs was born. It explained the simple mystery of the mouthless, gutless worms—their bacteria give them all the food they need. But It was also revolutionary. It described an entirely new way of making food, and one that turned out to be surprisingly common. Cavanaugh and others soon showed that many deep-sea animals carry chemosynthetic bacteria, including the clams and shrimps of hydrothermal vents, and bone-devouring worms that feast on dead whales.
But chemosynthesis isn’t restricted to the deep. It’s actually common in shallow habitats, from mudflats to coral reefs. For example, in 1983, Cavanaugh showed that clams, living in the mud off the coast of New England, also contain chemosynthetic bacteria. They get their sulphides not from volcanic vents, but from rotting seagrasses. Clams all over the world do the same, including the lucinids that decorated Higgs’s wedding tables.
Higgs confirmed his hunch that spiny lobsters eat these clams by returning to the Bahamas and catching them in the act. He even found a few individuals with clams clamped around their legs. “I don’t know exactly how that comes about,” he says. “I suspect they prod around to find the clams in the sediment, and then dig them out.”
He also analyzed all the lobsters’ food—clams, snails, starfish, and more. Each of these prey items contains a signature blend of chemical isotopes in its flesh—heavy and light versions of carbon, nitrogen, oxygen, and sulphur. By measuring these, and then doing the same for the lobsters themselves, Higgs could reconstruct what these animals were eating, and in what proportions. And he concluded that, on average, the clams made up 20 percent of the lobsters’ diet. In some populations, that figure is doubled.
Given the value of the lobsters, this suggests that the clams are contributing U.S. $17 million to the Bahamas fishery alone—and more elsewhere in the world. “It’s the best and only demonstration of a strong direct tie between chemosynthesis and commercially important animals,” says Chuck Fisher from Pennsylvania State University.
Higgs’s dad, and the other lobster fishermen he knew, owed part of their livelihoods to the clams and their chemosynthetic bacteria. And this partnership might even sustain the very seagrass meadows that Higgs snorkeled over as a kid.
The hydrogen sulphide released by decaying seagrasses is toxic; by soaking it up, and using it for chemosynthesis, the clams and their microbes allow new grasses to flourish. That’s why, as Dutch scientists showed in 2012, the clams and the grasses grow best in each other’s company. And the grasses in turn provide lush underwater meadows that support spiny lobsters, turtles, seahorses, manatees, and more.
“It’s a wonderful example of how the discovery of chemosynthetic symbioses at the bottom of the sea has led to so many new discoveries that were in our backyards the whole time,” adds Jill Petersen from the University of Vienna.