Misplaced visitors
By Phil Colarusso
Cryptic hitchhikers on boats
Food webs are altered
While doing eelgrass restoration work in Gloucester, I became aware of a bluish-gray growth appearing on a large number of shoots. Shortly thereafter, I read in the Woods Hole journal Oceanus about a researcher who was seeing a new species of invasive tunicates (Diplosoma listeria) appearing on scallops, boat hulls, mooring lines and eelgrass on Martha’s Vineyard. The photo was of exactly the same thing I was seeing in Gloucester.
Tunicates are filter feeding organisms that can grow as small zooids in extensive colonies or as large solitary individuals. The colonial forms tend to be prolific breeders and filter enormous quantities of water. They can grow quickly and will cover just about any surface that is bare, including pilings, clam shells, algae and eelgrass. Recent research has shown that literally miles of the seafloor can be covered by one of these colonial species, smothering other sessile life and altering the availability of the habitat.
My team decided to conduct a study in a salt pond on Martha’s Vineyard, where these organisms had appeared to be particularly abundant. We initially had focused on the impact of these animals to the eelgrass in the pond, but quickly realized their prolific filter feeding may pose an additional risk to the food web of this small coastal pond.
Using stable isotopes, we determined the tunicates were feeding on the same resources as several commercially important shellfish species. Based on their high abundance, their prolific feeding rates and the small volume of the pond, our modelling suggested the tunicates could potentially filter a volume of water equivalent to the entire pond in somewhere between 1 and 17 hours. This represents a significant challenge for commercial shellfish stocks in these waters. You can see a video on this project here.
It is not always clear where and how these invaders arrive, but shipping is believed to be a major vector. Planktonic life forms and small creatures are carried in ballast water and along the hulls or larger vessels. Globalization has significantly increased shipping all over the planet and as a result the unintentional transportation of organisms as well. Early detection may allow for some level of control, but often once a new species is detected in the ocean, control options are untenable. Persistent monitoring is the most prudent tool in identifying and controlling the spread of non-native species.
Original research:
Colarusso, P. et al. (2016) Quantifying the ecological impact of invasive tunicates to shallow coastal water systems. http://dx.doi.org/10.3391/mbi.2016.7.1.05
Valentine, P.C. et al. (2007) The occurrence of the colonial ascidian Didemnum sp. on Georges Bank gravel habitat – Ecological observations and potential effects on groundfish and scallop fisheries. https://doi.org/10.1016/j.jembe.2006.10.038
Dr. Phil Colarusso is a marine biologist with US EPA Region I. He has been working on eelgrass restoration, conservation and research for 31 years. He and his team just recently had a paper on carbon sequestration rates in eelgrass in New England accepted for publication.
Enjoyed Phil’s sciku? Check out his other of his sciku Blue Carbon and Diving for Science.