Invasive tunicates have shellfish farmers crying “foul”
As shellfish farmers struggle with invasive tunicate invasions, scientists are trying to gain insight into the thermal tolerances for these strange critters and determine where they might show up next
Estimated reading time: 10 minutes
It looked like someone spray foamed the farm.
When Bill Doyle arrived at his four-acre oyster farming site off the coast of Plymouth, Massachusetts for the morning tide, he felt a pit in his stomach. Rows and rows of oyster cages, normally coated in black mesh, were plastered with a mysterious brown-and-orange foam-like film.
“Every piece of equipment—we had more than 3,000 pieces out—was completely covered,” says Doyle, owner of Plymouth Rock Oyster Growers. “We didn’t even know where to start.”
Doyle and his crew began hauling in the equipment to power-wash the leathery film off, which had been weaved together by millions of invasive tunicates known as Didemnum vexillum, or “sea vomit” as the species is commonly referred to. The stealthy invasion took three staff members months to deal with. The vomit-like slime also reduced water and nutrient flow around the oysters, suffocating thousands of them.
“The monetary loss was huge,” says Doyle, who has witnessed a spike in invasive tunicates since he began his farming operation in 2016. “Before then, if you had a piece of equipment sitting out there for a long period of time, you might run into some issues with tunicates every so often. Now, all the sudden we’re blanketed with it.”
Doyle is just one of many shellfish farmers grappling with invasive biofouling tunicates, which have spread along the North American east coast since the 1980s. Also known as sea squirts, owing to a pair of siphons that draw seawater in and squirt it out, these spineless filter feeders seek out and bond themselves to hard surfaces ranging from aquatic animals and plants to floating docks and buoys.
Originally, invasive tunicates likely made it to the U.S. east coast by hitching rides on ship hulls, anchors, and chains. They’ve been found from Florida all the way up to Maine. This extensive biogeographic range has captured the attention of scientists, causing some to wonder, are there any limits to where these weird critters can survive? And more importantly, where might they show up next?
Zac Tobias is lying face down on a dock clawing his right arm over the side. It’s early June, 2022, and the MIT-WHOI Joint Program marine biology student has taken up temporary residence at the Rutgers University Marine Field Station in Tuckerton, New Jersey for two weeks of field work. With his right arm in the water, he reaches for a layer of multi-colored slime lining the dock’s slippery underside and plucks a small piece off. After inspecting it for a moment, he confirms that it’s a golden star (Botryllus schlosseri) —the daisy-shaped tunicate species he’s studying. It has a sac-shaped body and uses a glue-like substance to attach to surfaces. While it has long been considered an invasive species, it’s cryptogenic and there’s debate within the scientific community that it may be native to the U.S. east coast. Regardless, it’s widely considered a nuisance species—that’s something everyone seems to agree on.
Tobias drops the gelatinous creature into a small saltwater-filled Tupperware container, crawls back onto his stomach, and peels another one off the dock. The golden star happens to be much prettier than any of the marine vomit tunicates that snuck up on Bill Doyle’s oyster farm. But just as pesky.
“These things have been spreading northward like crazy over the past few decades,” says Tobias. “As the ocean continues to warm, we want to try to understand how golden stars may be adapting to various temperatures at different latitudes—something there hasn’t been much research on.”
Tobias says that investigating the golden star’s potential for local adaptation—evolutionary-biology-speak for when a species evolves to suit its immediate environment—could help shed light on how they have been so successful across a wide range of temperatures. And that information, in turn, could help predict and possibly control their spread to certain areas as ocean temperatures continue to creep up across the North Atlantic.
“If you ignore the potential for the species to evolve, and the potential for the species to tolerate different temperatures, your predictions for where it may spread may be off,” says Tobias. “This has the most important management connection for designing early monitoring efforts to detect nascent invasions.”
Inside the field station, Tobias sets the container of a dozen freshly picked golden star samples onto a lab workbench. The ocean sampling took all but five minutes, and was as simple and basic as it gets—no research vessels, fancy sensor arrays, or underwater robots required. But he has what he needs to start figuring out what kind of water temperature limits these weird animals can live within, and where they might pop up next.
Tobias inches over to a 12-compartment plastic case that looks like a tackle box and places a single adult golden star colony onto a rectangular glass plate at the bottom of each water-filled compartment. The tunicates run a spectrum of colors. The lighter, peachy ones show off their tiny flower-like arrangement of zooids—the asexually-produced individuals that make up the larger tunicate colony. The darker golden stars look like submerged patches of moss.
The tunicates bathe in the organizer case overnight. At some point, by design, the water temperature hits 53° F (12° C), causing some of the golden stars to spawn and unleash dozens of nearly-microscopic tadpoles into their respective compartments. The larvae swim around frenetically, but not aimlessly: like their parents, they just need a hard surface to cling to. Eventually, some forge themselves to the glass plates, allowing Tobias to move to the next phase of the experiment.
The next day, Tobias begins removing the glass plates with baby tunicates stuck to them. He puts each plate under a microscope to count the number of larvae living on them and then places the plates in a series of five experimental water tanks, one plate per tank. The water tanks are essentially shoebox-sized plastic storage bins equipped with miniature water heaters that heat the water at designated temperatures, from 85° F (29° C) in the first tank to 90° F (32° C) in the last.
The increments across this temperature gradient may seem small, but perhaps not for the baby tunicates. They will be trying to survive these miniature hot tubs over the next 20 hours.
In the wild, the spread of invasive tunicates has increased as the ocean continues to warm. A 2018 study suggests that a species known as Botryloides violaceus can double their production with a water temperature increase of just 3.6° F (2° C). The idea is that warmer sea temperatures can extend the length of reproductive seasons and shorten the time for these invaders to reach sexual maturity.
As such, the ocean warming trend has the potential to create more headaches for shellfish farmers like Doyle at Plymouth Rock Oyster Growers.
“Tunicate invasions here have spiked in the last three or four years, and our water temperature has increased a lot during the same time period,” says Doyle. “And we now see some of the oysters starting to spawn which means consistent water temperatures above 70° F (21° C). We never had oysters spawn before. Water temperature is what I’m blaming it on.”
It could be to blame for invasions at aquaculture farms farther north as well. According to a study last year, water temperatures in the Gulf of Maine hit their warmest five-year period between 2015-2020 in recorded history, and is warming faster than 96% of the world’s oceans. Carolyn Tepolt, an associate scientist at WHOI and Tobias’ advisor, says one of the big concerns with warming is the northward spread of fouling species that are established further south.
“Many of these species have been limited by cold, and as those limits move north, so do several high-profile tunicate species,” she says. “In some areas, like Alaska, we suspect that this process will be accelerated by increased traffic as shipping lanes through the Arctic are opened up and more biofouling vectors like boats and structures are moved through these waters.”
Alex de Koning, a shellfish farmer at Acadia Aqua Farms in Bar Harbor, Maine, says his staff has felt the pain of the northward tunicate spread—literally—after their oyster growing cages became caked with heavy mats of golden star and other tunicates in the summer of 2021.
“The cages became really heavy and hauling them out of the water caused muscular-skeletal issues for our workers,” says de Koning. “They were developing problems like tennis elbow, bad shoulders, and bad backs.”
Back in the lab, the 20 hours of tank time is up. Tobias removes the baby tunicate plates from each of the five water tanks and begins examining them under the microscope to see how many survived and at what temperatures. To his surprise, a number of the larvae survived tank #5 – the hottest one at 90° F (32° C). The information, in and of itself, doesn’t tell us much about when and where the tunicates might spread in the future. But Tobias is just getting started. Over the next several months, he’ll run the same series of experiments at field sites in Massachusetts and Maine in order to compare how golden star populations fare across wider ocean temperature gradients.
“I expect that the tunicates south of New England should be more resistant to heat than ones in the north, potentially reflecting local adaptation to ocean temperatures” he says.
Jennifer Dijkstra, a research professor and marine biologist at the University of New Hampshire who is not involved in the study, says the work Tobias is doing is important because once you know an invasive species is likely to spread to a certain location—particularly one with strong aquaculture potential—you can start planning mitigation strategies.
“When you start combining predictive models with ocean warming predictions for certain areas, you can start to think about ways to locally control these invasive species,” she says. “You could choose aquaculture sites that are likely to have fewer tunicates,” she says.
Tepolt points out that knowing where invaders are coming from, another aspect of Tobias’ work that he’s addressing with cutting-edge genomics, can also help. Coastal managers could direct rapid detection and mitigation resources to vectors like the ships most likely to be spreading them. “While the goal is to prevent invasions, even slowing them down can have big economic benefits by providing more years of no- or low-impact on aquaculture operations.”
In the meantime, shellfish farmers like Doyle and de Koning are more or less stuck with their own unwelcome invaders. They can be difficult and costly to manage, but de Koning says it all comes down to keeping a watchful eye over the farm and taking action once any visible signs of tunicate growth appear.
“The story with biofouling from a farmer’s perspective is you’ve got to deal with it early,” he says. “It’s like keeping up with the weeds in your little vegetable garden at home. You pull weeds a little bit here and there. But if you go on vacation for a month, suddenly you have an overgrown mess to deal with.”
This research is funded by the National Defense Science and Engineering Graduate Fellowship Program and the American Philosophical Society’s Lewis and Clark Fund for Exploration and Field Research.