The Department is keeping an eye on a number of marine invasives that are spreading toward Alaska, or have already made it here. Green crab and Atlantic salmon probably get the most headlines.
Didemnum has yet to draw much attention, but that is probably about to change.

Marnie Chapman is an associate professor of Natural Science with the University of Alaska. She described the life-cycle of Didemnum to the audience of mostly commercial fishermen. The organism is already well-established for something that most people have never heard of.

“This is actually a pretty dramatic photo, once you realize what you’re looking at. This is a bunch of rockweed. Someone’s standing in the intertidal area looking down and taking a picture of what it looks like under the water at Whiting. All that white stuff there is Didemnum. This is the kind of picture that there’s the potential, if we don’t check the spread of this beast and get rid of it, this is the kind of coverage you can get. It really is a habitat-altering species.”

The largest known Didemnum infestation is on the famed Georges Bank on the Atlantic coast. Approximately 143 square miles of sea floor have been smothered by the organism, causing local exctinctions of some benthic fauna like clams, scallops, and mussels.

Didemnum is commonly called a colonial sea squirt. They’re also called tunicates, because of a shared outer membrane that resembles a tunic.

Some other invasive tunicates have been found in Alaskan waters, usually growing on boat hulls, docks or mooring lines. For example, there are several species of invasive colonial Botryllids, also found in Sitka. Tunicates may be described as “solitary” or “colonial”, depending on the species. Colonial tunicates may fuse or grow together tightly to form a thick mat on the sea floor or on underwater structures like net pens used in aquatic farming.

Part of what makes the invasive Didemnum so menacing is its resemblance to a native species that does not form these thick mats, however.

When it was discovered during the June 2010, Bioblitz in Sitka, Chapman says even the scientists gathering the samples required laboratory confirmation.

When it was discovered during last summer’s Bioblitz in Sitka, Chapman says even the scientists gathering the samples required laboratory confirmation.

“Think about the fact that there were experts out there on the dock looking at this stuff in June, and they took until August to decide what it was, through genetic analysis. It’s really tricky.”

Chapman, and her co-presenters at the meeting have been organized into ADF&G’s rapid response team. Tammy Davis is the Department’s invasive species program project leader. They were joined over the phone by Sarah Cohen, a geneticist with the Romberg Tiburon Marine Research Center, and Linda McCann, with the Marine Invasions Research Lab at the Smithsonian’s Environmental Research Center, also in Tiburon, California.

Davis called the origins of Didemnum “cryptic.” Although it’s been found in Japan, Europe, New Zealand, and both coasts of North America, no one really knows what the organism’s native habitat is. It wasn’t formally classified by taxonomists until 2002.

She said that the owner of the defunct Sitka Sea Farm, which now lies on the bottom of Whiting Harbor, corresponded with ADF&G about a colonial tunicate, and sent samples to a lab. It’s possible that the tunicate fouling the farm’s nets was Didemnum, even in the late 1990s.

“It goes back to the idea that we didn’t know a lot about Didemnum vexillum prior to 2007, when everybody started to realize that even though the Didemnum in Japan, New Zealand, and Wales had been given separate names, actually they were the same organism.”

ADF&G’s “rapid response” did not seem fast enough for some in the Sitka audience. Some asked why ADF&G had not begun eradication efforts soon after Didemnum was discovered. Some offered to help. But part of what makes Didemnum such a major concern is its ability to reproduce asexually, just from pieces of itself.

Sue Aspelund, the deputy director of the Division of Commercial Fisheries, stressed that well-intentioned efforts to remove Didemnum could have disastrous results.

“This is just a really tricky critter. Just pulling the lantern nets out of the water like they did at the Bioblitz, you risk fragmentation. So it’s not simple. But I think you can be assured that we’re taking a careful and measured and rapid response as we possibly can to try to address the situation.”

Aspelund and the panel also calmed worries that the past and present oyster farming operations in Whiting Harbor were the source of the infestation. Hull fouling on boats is the more likely culprit; aquafarms, on the other hand, create ideal habitat.

ADF&G shellfish dive crews conducted transect studies in Whiting Harbor late last summer following the discovery of Didemnum. The divers will return again in January to try and map the exact extent of the invasive, possibly assisted by remotely operated vehicles to cover those areas too deep for human divers.

After that, the eradication plan itself is a bit vague. The panel suggested that whatever strategy emerges in Sitka might be used as a model for eradication elsewhere. For the time being, they are encouraging a voluntary quarantine of the area, to prevent Didemnum from spreading to Sitka’s other harbors.
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