Tunicates Brooksville HW - Ascidiella aspersa, called the 'dirty ball' tunicate. PHOTO COURTESY OF HANNAH WEBBER

Sea squirts spread along the coast



BAR HARBOR — An alien invader is popping up more frequently along the local coastline. 

Very few tunicates, the small marine animals more commonly known as sea squirts, are indigenous to the Northeastern United States and Eastern Canada. Yet, there have been plenty of sightings by beachcombers, fishermen, sea farmers and others whose work is focused in and on the sea.

Tunicates come in many varieties. Some look like gelatinous ping pong balls or a clear bubble. Another, called the orange sheath tunicate, is bright orange or red in color with no defined shape. 

There is one called star tunicate or golden star tunicate. “It can be lots of colors; mostly I see the black and white version,” said Chris Petersen, who works for College of the Atlantic as the graduate program director and a professor of marine biology, in an email. “To me it is the prettiest one, the zooids are arranged in rosettes … They grow by asexual production, but they also produce small tadpole like larvae (but very tiny) sexually.”

Another looks like spilled pancake batter. 

“There is a relative newcomer — it is commonly called the pancake batter tunicate,” said Petersen. “The species name is Didemnum vexillum. I have only seen it over the last couple of years, although aquaculture folks and lobster fishermen have probably seen it longer on their gear.

“It is a tan color and can grow quickly and extensively. It is a major fouler of gear, especially for something like oyster aquaculture. I have seen it on gear in lines, but this year also saw quite a bit at Bartlett’s Landing on the dock.”

Some have dubbed it sea vomit.

These species are invasive over a lot of areas, they aren’t just invasive here,” said Petersen. “Most of them are from the Pacific — we don’t know for sure how they got here, but the most likely ways they arrived are from shipping (boat hulls), or aquaculture. They started in the south and moved north over time. Most have been here for at least a decade.”

One of the varieties of tunicates seen in recent years along the Maine coast. They will attach to a hard surface, even seaweed as in this photo, in order to reproduce. Sea Squirt Botryllus – Botryllus scholosseri, star tunicate or golden star tunicate. Tunicates come in many colors, but the more common variety in the area is black and white. Zooids, the small units that make up the colony, are arranged in rosettes. PHOTO COURTESY OF CHRIS PETERSEN

There are three classes of tunicates, according to the Britannica web page about them. They include Ascidiacea (ascidians, or sea squirts), Appendicularia (Larvacea) and Thaliacea. Most that are seen in and along the waters of Frenchman Bay, Blue Hill Bay and other areas of the Maine coast are those known as sea squirts. Although they take many different forms, this name was given to them because they can exhale, “squirt,” water when touched.

“They can grow really quickly, and they can grow over everything,” said Hannah Webber, the Marine Ecology Program director for the Schoodic Institute at Acadia National Park. “They are voracious eaters … They like hard surfaces.”

For those who maintain equipment used in the water, tunicates “foul things up,” said Webber, noting that the app iNaturalist lists 10 different tunicate species in the area. “Fishers or farmers have to spend a lot more time tending to gear than they would otherwise.” 

When they land on a solid surface, such as a mooring line, rocks, shells, traps, pilings, boat hulls, “really anything that’s not sand,” according to Webber, the tunicates can proliferate. They often form colonies made up of a few to many individuals, called zooids, which reach up to nearly 6 feet in length. 

“A lot of them are sort of a leathery texture,” said Alex de Koning, who farms mussels and scallops for his family’s business, Acadia Aqua Farms, on Mount Desert Island. “A lot of people mistake them for eggs … You won’t dislodge them once they are settled.”

In some areas of Canada where scallop farming is common there is also a big problem with tunicates. Once the sea squirt settles onto bags or crates underwater, it can grow to the point of covering it, preventing nutrients and water from reaching the shellfish inside, according to de Koning. 

“We use water depth to control for that,” he explained. “Deeper than 20 feet, they drop off quite rapidly.”

Although he did not state with certainty the reason, de Koning figures both the colder temperatures and lack of light contribute to the exodus. Over the last few years, he has noticed more of the marine invertebrates as he cultivates shellfish. 

“It hasn’t been to the level of being a problem for farming,” said de Koning. “It’s becoming more common.”

On Swan’s Island, Jason Joyce is a multi-generation lobster fisherman who recently added raising oysters to his resume. Tunicates are something he has gotten used to as part of the job.

“We are not seeing more than usual, in my opinion,” Joyce said in an email. “But, they are around and are thicker in shallow areas. I rarely see them attached to (lobster) traps that are deeper than 15 fathoms.”

Colder water temperatures seem to be a natural management tool for tunicates. 

“Sometimes you’ll see a vanguard population,” said Webber, noting that can be the case in warmer water temperatures. “A cold winter will knock them completely down. A warmer winter will help them settle in.”

Warming waters in the Gulf of Maine could contribute to their spread. According to NASA, a heatwave in the Gulf recorded in August 2018 showed the Gulf is one of the fastest-warming parts of global ocean waters. In the past three decades, the Gulf of Maine has warmed by 0.06 degrees Celsius (0.11 degrees Fahrenheit) per year, three times faster than the global average, according to NASA, and faster than 99 percent of the global ocean.

“Tunicates will change the coast of Maine,” said Webber. “It’s important for all of us to be paying attention to these changes.”

Sarah Hinckley

Sarah Hinckley

Sarah Hinckley covers the towns of Southwest Harbor, Tremont and neighboring islands. Send story ideas and information to [email protected]

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