The seagrass originated in the Indian Ocean and made its way west through the Suez Canal to the Mediterranean Sea. Now, as ocean temperatures rise, a fast-growing species of seagrass known as Halophila stipulacea is spreading through the Caribbean, displacing important native seagrasses — the very definition of an invasive alien species.
But just how dangerous is it to these planet-trotting creatures? Healthy seagrass is so important to marine life – and to us – that scientists are trying to understand the consequences of its spread.
David Patriquin, a former professor of marine biology at Dalhousie University in Nova Scotia, Canada, lists a number of reasons why we need healthy seagrass: It supports a wide variety of species, from turtles to sharks to birds; it provides essential nursery grounds for fish; it protects coasts during storms, with each blade of seagrass absorbing and dispersing the impact of waves; and it fights climate change by absorbing carbon dioxide from the atmosphere.
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“The carbon gets stored in the sediments and in the roots and stored for a long time,” Patriquin says, “and that's key for carbon sequestration.”
In fact, seagrass is one of the world's most efficient natural carbon sinks, sequestering up to 18 percent of the ocean's total carbon.
But not all seaweeds are equally useful, and new arrivals can cause problems – or bring unexpected benefits.
In fact, Halophila stipulacea does both.
Hello, Halophila
Imagine a vast expanse of grassland the size of New York City's 850-acre Central Park submerged under the ocean and sprouting up from the ocean floor, and multiply that by 88,000 to get a conservative estimate of the current extent of the world's seagrass ecosystems, stretching from the tropics to the Arctic.
Overall, seagrass habitats are shrinking due to climate change, deteriorating water quality and other hazards.
But not Halophylla stipulacea. This plant has been growing since the opening of the Suez Canal in Egypt in 1869. In fact, climate change has helped it grow.
“Previously, there was a barrier to how far west this seaweed could move because of the cold Atlantic waters, but that's now changing,” said Demian Willett, a professor at Loyola Marymount University and one of the world's leading experts on Halophila stipulacea.
By 2002, it had reached Grenada, where two scientists, Hector Ruiz of the University of Puerto Rico and David Ballantyne of the Smithsonian Institution, “discovered a swarm the size of a tennis court offshore near a medical school,” Willett said.
Willett made the discovery in 2007 while working on his doctorate in Dominica, a small Caribbean island nation southeast of the Dominican Republic.
“I was swimming along the coast, looking at native seagrasses and learning about them,” Willett says, “and photographed something odd. I came back a month later, read Lewis and Ballantine's paper and realized what it was.”
Over the next few years, Willett watched as the species continued to spread across Dominica: by 2016, the species had spread to 19 Caribbean islands and even reached South America.
Fish traps used by Demian Willett in his research on Halophila stipulacea in the Dominican Republic. Photo by Demian Willett.
Willett estimates that the plant spreads about 22 miles per year, an incredibly fast rate for a marine plant.
“How do they get there? They probably travel from the Mediterranean to the Caribbean on recreational boats… they probably share a boat,” Willett said. “And once they get there? They just move between island boats, getting caught on anchors or fishing traps.”
This is what makes this bacterium such an excellent mobile organism: “It can live in tiny fragments, about four centimetres in size, and can float in the water and grow to be very large,” Willett says. “It doesn't have to be in the soil. It can fragment, photosynthesize, and eventually settle and land in a new location.”
Very versatile!
“Most seagrasses are limited in how deep they can grow by light,” Willett said. He noted that H. stipulacea may be able to grow in deeper, darker waters because it can consolidate the seagrass' photosynthetic structures, chloroplasts, into parts of its body. “I've seen this seagrass at depths of 60 feet, or 18 meters, but in the Mediterranean it's been measured at depths of more than 100 meters,” he said. “Most seagrasses can't do that. By chance, or for some reason, this seagrass is in a good position to spread very quickly.”
“Right now, the only thing we're wondering is if and when it's going to get to Florida,” said Lauren Olinger, a quantitative coral reef ecologist and postdoctoral researcher at the University of the Virgin Islands.
Scientists have tracked the plant, Halophylla stipulacea, for decades as it displaces native species, outdoing Thalassia testodinum, known as turtle grass in the Caribbean, and Syringodium filiforme, known as manatee grass.
Native turtle grass (Thalassia testudinum) is being slowly eaten up by the invasive Halophylla stipulacea in the U.S. Virgin Islands. Photo by Dan Melle Seagrass is the primary food source for green sea turtles. Green sea turtles have been observed eating Halophylla, but preliminary research suggests the turtles prefer the native species, finding and eating single blades of turtle grass amid the vast seabeds of invasive species. Photo by Dan Melle
There's just one problem: H. stipulacea doesn't seem to be all that good for some fish. In June, Willett and six other researchers published a paper showing that juvenile yellowtail seem to thrive better in native seagrass beds than in H. stipulacea beds.
“Seagrass is known to be a nursery for fish, but with this invasive species it doesn't work that way – the grass is too short,” Willett says, “so the fish can't live there. They don't multiply. So the whole fish community changes.”
Another problem is that it offers little protection against storms.
“When a storm comes, the tall seagrass slows down the waves,” Willett said. “It's a small piece of seagrass, so (the storm) just passes over it.”
Halophila lends a helping hand
But it's not all bad news. For example, some other fish prefer it. A 2017 study by Olinger on St. Thomas in the U.S. Virgin Islands found that its seagrass meadows supported more nocturnal carnivores, like snappers, but fewer diurnal carnivores and herbivores than native seagrass meadows. In another study, Willett and colleagues found that Halophylla and native seagrass meadows supported roughly the same amount of fish.
In 2023, Olinger returned to St. Thomas and noticed a thriving pink sponge called Desmapsama in the Halophylla beds of Brewers Bay.
“At the same time that we saw the pink sponge, we also saw a tremendous number of juvenile angelfish,” Olinger said. “Angelfish are sponges, so we suspect that the Halophila provides a surface for the pink sponge to grow on and also serves as food for the juvenile angelfish.”
An Atlantic giant octopus navigates a H. stipulacea meadow at sunset, just one of many species that call the H. stipulacea colony home in the U.S. Virgin Islands. Photo by Dan Melle Southern stingrays are commonly seen gliding over H. stipulacea colonies in the U.S. Virgin Islands, but it is unclear whether they prefer to feed in the native seagrass beds. Photo by Dan Melle
Dan Mele, a coral biologist and conservation photographer based in the U.S. Virgin Islands, has been taking videos to see the differences in seagrass up close.
“When we were filming in native seagrass beds, like the long Thalassia beds, we actually had a hard time finding a variety of life,” Mele says. “When we were exploring the Halophila seagrass, which are short, thick leaves about the size of your pinky finger, we found lots of little sea slugs and sea slug-like creatures.”
But that's not all.
“What's interesting is that in Caribbean waters like Dominica, there's more seagrass than there was 20 years ago,” Willett says, “because we have invasive species in places that were previously just sand.”
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Seagrass cover around Dominica doubled in just five years, from 2008 to 2013.
Halophila deserves credit, Willett said: “It's spreading everywhere.”
More seagrass in what was previously sandy areas means more carbon sequestration, and Halophila stipulacea is so good at that, it doesn't just outcompete other seagrass species, it may actually help them.
“It keeps the soil down, it releases carbon, it secretes nutrients into the soil,” Willett says, “and then hopefully native species will repopulate those areas.”
Halophila stipulacea floating in the water column in Dominica in 2009. Photo by Demian Willett.
This is an ecological concept called “facilitation”: one species establishing itself in an environment can set off a positive chain reaction.
Scientists say protecting native seagrasses is still a good idea: If they're healthy and dense, there's no room for Halophila to dominate and disrupt ecosystem functions.
But Willett thinks differently about invasive species than he did at the start of his career.
“These creatures aren't inherently mean or evil,” he says. “They're just animals. They're just life. They don't know they're invasive. They're just trying to complete their life cycle.”
And the study highlights an increasingly nuanced view of these species: some may indeed wreak havoc in their new locations, while others may not.
“With Halophylla, it's not that simple,” says Mele, the coral biologist.
Not necessarily for the better, but not necessarily for the worse either.
“Over time, this invasive species will become integrated into the local ecosystem,” Willett says, “and that's my hope.”
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thank you,
Bin Lin
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Bing is a conservation scientist and fifth-year PhD student at the Princeton School of Public and International Affairs. Trained as a marine ecologist, Bing's research sits at the intersection of conservation ecology, environmental policy, data analytics, and behavioral science. Outside of research, Bing is a conservation photographer, a member of the International Union of Conservation Photographers' Emerging League, and an avid science writer and communicator. His photographs and writing have appeared in National Geographic, Scientific American, BBC Wildlife, Nature, and other outlets. He is a 2024 AAAS Mass Media Fellow for Inside Climate News.
