In the summer of 2020, Alicia Gaiero began to realize that sea squirts were putting the success of her new oyster farm in jeopardy. She and her two sisters, Amy and Chelsea, were working together to fulfill their dream of a family aquaculture business, Nauti Sisters Sea Farm, in Yarmouth, Maine.
But the critical work of flipping oyster cages—turning them over to combat biofouling and introduce sunlight and air to the bottom—had become impossible. They couldn’t even lift some of them, even though they were made of hollow plastic mesh.
In large numbers, sea squirts—some as large as a pair of tube socks, some smaller than a pinky ring—are surprisingly heavy, and are impacting lobster, oyster, and other fisheries.
The cages were covered and also filled with the globby bodies of sea squirts, invasive marine invertebrates that thrive in the warming waters of the Gulf of Maine and along the coasts of Alaska and the western United States. In large numbers, sea squirts—some as large as a pair of tube socks, some smaller than a pinky ring—are surprisingly heavy, and are impacting lobster, oyster, and other fisheries.
Gaiero had heard that sea squirts could be challenging, but this was out of control. “I’d never seen anything like it,” she says.
By the next season, she felt overwhelmed. “This was ruining my life.”
Blob Invasions Impact Maine’s Fisheries—and Beyond
There are now over 150 independently owned oyster farms in Maine, in part thanks to investment by the state. Under the glistening, still surface of the water, nearly every line and buoy marking a trap or cage is encased with gooey sea squirts—formally known as tunicates, for the tunic-like sheath of fleshy cellulose that covers their siphons, which suck in and filter sea water. The nickname “sea squirts” comes from the fact that they often squirt water when they’re disturbed.
Tunicates, commonly known as sea squirts, are a problem for commercial shellfish farmers, as they glom onto cages and the shellfish themselves. Here, tunicates cover an oyster cage in Casco Bay in Maine. (Photo credit: Alicia Gaiero/Nauti Sisters Sea Farm)
For more than 500 million years, tunicates have existed as simple creatures clinging to underwater substrates and filter feeding on plankton and bacteria. There are hundreds of subspecies. Some have inhabited the Gulf of Maine since the 1800s, arriving in the ballast waters of ships from distant seas; new subspecies have come from Europe and Asia in oyster seed and on cruise ships.
As tunicates spread across oyster cages, mooring lines, and buoys, they add incredible weight, turning a 5-pound oyster cage into an unmanageable 100-pound obstacle. As they proliferate, they compete with bivalves—oysters, mussels, and scallops—for resources and can eventually choke them out entirely. A bivalve covered in globby tunicates can no longer open its shell to feed, and will eventually starve to death.
They were only a mild nuisance to Maine’s working waterfronts until the past decade, when their populations started to soar.
“The biggest thing driving this invasion,” explains Jeremy Miller, research associate and coordinator of the System Wide Monitoring Program at Wells Reserve, “is the warming Gulf of Maine. The Gulf of Maine is getting warmer and warmer every year. Ever since about 2012, we have been going in one direction, and we haven’t had an anomalously cool year since 2007.” According to the Gulf of Maine Research Institute, sea surface temperatures in the gulf have been steadily rising at an average of 0.84° F annually, roughly three times that of the world’s oceans.
The Wells Reserve team researches and tracks changes to the environment along the Wells Estuarine Research Reserve, monitoring changes year on year and sharing their findings with the broader scientific community. (Although they are concerned about potential federal cuts to their overall funding, their studies on invasive species receives private foundation money.) The warming waters have had profound impacts on Maine’s fisheries and waterfronts, from the disappearance of Northern shrimp to more frequent flooding events, including so-called “blue sky flooding” in the coastal city of Portland. And those rising temperatures are now driving a sea squirt population boom.
Internal anatomy of a tunicate (Urochordata). Adapted, with permission, from an outline drawing available on BIODIDAC.
Tunicates thrive and spread faster with warmer ocean temperatures. And the rising number of aquaculture farms are providing plentiful structures to which sea squirts can attach themselves and grow.
And there are other factors as well. “The Gulf of Maine, compared to a lot of other parts of the world, is actually fairly low in diversity,” says Larry Harris, Professor Emeritus of Biology Sciences at the University of New Hampshire and co-author of UNH studies on tunicates and warming ocean temperatures. Harris explains that development along the coast of Maine has created the perfect ecosystem for tunicates, with new docks and moorings offering an abundance of substrates for them to attach to in addition to aquaculture farms. Also, tunicates have few true predators in the Gulf of Maine, and overfishing has reduced the number.
Because tunicates are effective filter feeders that grow extremely quickly, they can reproduce alarmingly fast; certain species can double their populations in as little as 8 hours. Some species are considered “colonial,” growing in a super-organism, like coral. Others are called “solitary,” but often appear in clusters and groups because their offspring do not travel far.
