A study on the impact of a non-native tunicate called Didemnum vexillium—a Western Pacific native that has spread to the U.S., Europe and New Zealand within the past two decades—found that it hampered the growth of eelgrass in a study area on the west shore of Tomales Bay. Ted Grosholz, one of the study’s authors and a professor at the University of California, Davis, who does research at the Bodega Bay Marine Lab, said he and fellow researcher Holly Long undertook the study because “we knew it was there and potentially an important invader.” Seagrasses play a critical role in marine and estuary ecosystems; fish and other sea creatures hunt and hide in the grasses and use them for nursery grounds. Grasses also prevent erosion and have other benefits. Ms. Long said at a public presentation in January at the Red Barn that the East Coast is known to have huge areas covered with the tunicate, though it’s patchily distributed on the West Coast. She noted there are few studies on the impacts to eelgrass, but there are concerns that it could block the light necessary for the grass to grow and interfere with seeding. Mr. Grosholz added that he also pursued the study because Didemnum can impact oyster culture since it can grow on both eelgrass and hard substrate like oyster racks. The study, published late last year in the Journal of Experimental Marine Biology and Ecology, measured the length and biomass in grass plots on the west shore in an area where some patches of grass had up to 4.3 percent of shoots covered—high rates compared to other areas that were surveyed. Over the span of a month in 2012, the researchers found that eelgrass with the tunicate grew less than unaffected grass, an impact characterized in their findings as a “modest effect.” That finding was in line with expectations. But contrary to one of the researchers’ other hypotheses–that fewer invertebrates would colonize near Dvex–they found a higher density of invertebrates near grasses with the tunicate. The researchers wrote that perhaps some invertebrates use the tunicate to hide from predators. Ms. Long said the tunicate had been present since around 2001, and Mr. Grosholz said the fact that it has not spread widely may be good news—with an emphasis on maybe. “We haven’t seen it take over the bay. We haven’t seen coverage like in New England,” he said. “I think we would have seen dramatic takeover by now if we were going to see that… But species can also hang out and then explode.” He said he had no plans, or funding, for another study on the issue, but that it was on his “watch list.” He also said that his own discussions with oyster growers have not revealed problems with the tunicate. “One of the things, from my perspective, is that [the tunicate] seems to do well on the far side of the bay where there isn’t much aquaculture.”