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Your Questions, Answered: Why are we seeing so many sea squirts?

MLCA Staff

It can be like a bad science-fiction movie. Thick mats of blob-like creatures enveloping lobster traps, swallowing aquaculture nets, engulfing even buoy ropes. Tunicates, also known as sea squirts, have proliferated in the Gulf of Maine in the past two decades as the Gulf waters have warmed.


This tunicate-covered trap reportedly weighed more than 200 pounds. D. Closson photo.


The marine invertebrates are filter feeders, drawing in and expelling water through siphons. Tunicates occur either as single individuals or as colonies. After a brief period floating freely, a tunicate will attach itself to a solid surface — any solid surface. A lobster trap, dock piling, a rock, you name it, a tunicate will attach to it.


There are roughly a dozen different species of tunicates in the Gulf of Maine. More than half are believed to be invasive species brought to the Gulf from other regions.


Solitary tunicates, like the European sea squirt, are distinct individuals and can grow together in large groups. They reproduce quickly and can easily spread into new areas by attaching themselves to boats or gear.


Colonial tunicates, such as the pancake batter or sea vomit tunicate, form incredibly dense colonies. “Pancake batter” got its name because the colony forms a mat with dripping tendrils. These tunicates are individual organisms that live in colonies within a shared protective tunic.


Warming Gulf of Maine waters have allowed tunicates to grow quickly in many parts of the Maine coast.


They also are less susceptible than many invertebrates to the impacts of ocean acidification and low oxygen conditions created by algal blooms. Tunicate populations vary with the seasons, peaking at the beginning of fall. They are more likely found in shallow water.


As Brian Beal, a University of Maine at Machias marine scientist and director of the Downeast Institute, commented in a recent Bangor Daily News article, “Sea squirts have exploded in the Gulf of Maine over the past decade or so. They’re just a huge nuisance.”


A colony of tunicates can easily double the weight of a lobster trap, making it difficult and dangerous to haul.


They can cover the trap escape vent and the wire mesh itself, limiting the amount of water that can pass through the trap. Typically, however, tunicates won’t thrive in deeper water, where sunlight is limited.


Like the bad guys in a science fiction movie, tunicates are hard to kill. If scraped off a trap or hull at sea, the little bits hosed into the water will grow into new tunicates. Getting them off gear is best done on land, after they have dried out and died.


Researchers have found that during a severe winter when the Gulf temperatures drop and stay cold, tunicate populations will be reduced. Rain and a heavy influx of freshwater into coastal areas will also knock the populations back.


For now, however, with the Gulf of Maine continuing to warm, the invasive tunicates will remain irritating pests.

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