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Tide Bites: Searching for Sea Squirts by Megan Powers

  • Written by Megan Powers

INTRODUCTION by Dr. Megan Dethier, FHL Director

Most people are aware of the concept of invasive species – plants or animals that don’t “belong” in a given area and end up disrupting the local ecology in many ways. Eastern grey squirrels, tansy ragwort, and spurge laurel are all problems on various San Juan County islands, and of course there are infamous invasives in other parts of the U.S. such as kudzu and even certain earthworms.

The marine realm’s invaders are perhaps less visible but no less disruptive to local ecosystems. For example in the Salish Sea, invasive Sargassum or “wireweed” may displace native kelp forests. More subtle are the “sea squirts” that Megan Powers writes about in this essay. They can be hard to distinguish from native species but because they spread by mobile larvae and can rapidly overgrow many other marine organisms, they are of considerable concern.

Megan’s work will help us understand the degree and nature of the threat they pose, and all her time in public marinas leads to opportunities for good outreach!

- Best,

Dr. Megan Dethier FHL Director

Searching for Sea Squirts

Invasive species often pose great ecological and economic challenges. In the Swalla Lab, we study a group of animals called ascidians or “sea squirts.” The Salish Sea is home to dozens of native ascidian species. We are interested in the non-native species, and their ecological effects on other marine animals. A few ascidian species are successful invaders all over the world as they are moved from place to place on the hulls of boats, in ballast water or on aquaculture equipment. Ascidians reach sexual maturity in just a few weeks, each individual can produce large quantities of larvae at a time, and some species can also reproduce asexually. Therefore, large populations of ascidians can be started in a new location with only a few individuals. Invasive ascidians are especially a headache for aquaculture. They live on cages, bags, and ropes that hold shellfish and sometimes on the shellfish themselves, competing for food. They also can smother the shellfish, so invasive ascidians must be removed (often through power washing) to maintain the flow of water through shellfish cages.

There are currently seven documented non-native species in the southern Salish Sea (south of Blaine, WA) based on a 1998 rapid assessment of invasive species in the Puget Sound (Cohen et al. 1998) and a survey by Gretchen Lambert and the Washington Department of Fish and Wildlife in 2006-2007 (WDFW 2008). Three of these species are considered invasive, meaning that in addition to being non-native, they displace or threaten native species, pose a threat to natural resources, can interrupt commercial or recreational activities causing economic harm, or threaten human health (WDFW 2008). The other four non-native ascidians are considered “potentially invasive” in Washington waters. Four of these non-native species, including Ciona savignyi (Figure 3B) are solitary ascidians, meaning each individual lives on its own and can perform all the functions it needs to survive by itself. The other three non-native species of interest are colonial species. Colonial ascidians like Botrylloides violaceus (Figure 2B) are made of many connected individuals that depend on each other to carry out various functions.

Since 2007, there have been no major surveys aimed at assessing non-native ascidian species. My research is to revitalize monitoring invasive ascidians in the Salish Sea. I want to know whether invasive species have been spread to new locations in the past 15 years by determining where each species is found now, and why non-native ascidians are incredibly abundant in some Washington marinas but not found in other marinas nearby.

I am currently conducting surveys for non-native ascidians at marinas throughout Washington in two ways: settling plates and floating dock surveys. The first of these methods uses settling plates and is possible because of the ascidian life cycle. Ascidians have a biphasic life cycle which means they spend part of their lives as mobile larvae, then those larvae settle and undergo metamorphosis to become sessile adults. Larval ascidians look like tiny tadpoles and can swim around for a few hours up to a few days before they stick themselves down to one spot and stay there for the rest of their lives, filter feeding food from the water surrounding them. I take advantage of this by providing a place for ascidian tadpoles to settle on plastic tiles known as “settling plates.”

I hang settling plates from ropes off the side of docks, then check on the plates after a few months to see which species are present, and how much of the settling plate they cover compared to other species. This year I left settling plates at five locations: Squalicum Harbor Marina (Bellingham), Shilshole Bay Marina (Seattle), Roche Harbor Marina (San Juan Island), Friday Harbor Labs docks (San Juan Island), and Port Angeles Boat Haven (Port Angeles). At each marina I left 8 settling plates submerged from May until October in 2022.

I used these settling plates as a way to test my methods for next year by hanging the plates at four depths (0, 2, 4, and 6 feet below the surface of the water, Figure 1). I found that ascidians are consistently the most abundant on settling plates positioned 4 feet below the surface, so next year I can expand my survey and hang all my settling plates at that optimal depth in more locations.

I also wanted to see whether predation would play a role in which species were found on the plates, so I caged half of the plates at each site so that no ascidian-eating fish or invertebrates larger than ¼ inch could get inside to eat the sea squirts (Figure 1).

The cages surrounded the underside of the settling plates because that is where sea squirts settle and survive most frequently. They hang upside down from settling plates just like they do from docks so that the siphons they use to filter feed don’t get clogged by falling organic matter and sediment. Based on this preliminary data, there was not a significant difference between caged and uncaged settling plates.


Fig. 2: San Juan Island settling plates. (A) FHL docks settling plate covered primarily in barnacles and worm tubes. (B) Corresponding settling plate from Roche Harbor covered in many different ascidians including non-native Botrylloides violaceus (orange colonies) and native Corella inflata (transparent solitary ascidians).

Fig. 2: San Juan Island settling plates. (A) FHL docks settling plate covered primarily in barnacles and worm tubes. (B) Corresponding settling plate from Roche Harbor covered in many different ascidians including non-native Botrylloides violaceus (orange colonies) and native Corella inflata (transparent solitary ascidians).

The animals that I found on the settling plates varied widely from site to site, even across short distances. FHL docks had no invasive ascidians (Figure 2A), while settling plates at Roche Harbor Marina on the north end of San Juan Island included multiple large colonies of the potentially invasive species Botrylloides violaceus (Figure 2B). Studying differences between Friday Harbor and Roche Harbor may provide a great study system for understanding why invasive species thrive in some marinas, but remain absent from others.

The settling plates in Port Angeles were covered with Corella inflata, a native ascidian, when I checked on them in July. In November, however, there were far fewer Corella inflata, and many more colonies of the invasive Botrylloides violaceus.

On the other hand, the settling plates at Shilshole Bay and Squalicum Harbor marinas were almost or entirely empty in July (Figure 3A). There were very few ascidians. In October, however, the native species Corella inflata was present at both sites. Shilshole plates were also covered in potentially invasive Botryllus schlosseri in October (Figure 3B). No invasive species were found on the settling plates in Squalicum Harbor, however, an invasive colonial ascidian was present on the cage surrounding the settling plates and on the docks (Figure 4).

This brings me to my second method of surveying ascidians. There is a remarkably diverse community of marine invertebrates that already live on the sides of floating docks. Ascidians are often prominent members of these dock communities and I take advantage of this convenient sampling location by surveying the sides of docks for native and invasive species both manually and with photographs. I will compare my results from these dock surveys to the settling plate results to determine whether both methods are necessary or if one method alone can give us a clear picture of which invasive and potentially invasive species are present.


This preliminary work will guide my future experiments, both in the field and in the lab. I will use the knowledge gained from this ascidian survey to improve my methods and expand this survey to a greater number of marinas next summer.

Throughout my time working in marinas this summer, I had lots of great interactions with passersby on the docks. Sea squirts are a bit less popular than sea stars and crabs, so I had the opportunity to introduce ascidians to many new people and gauge the public’s interest in invasive species.

From discussions of my specific research questions to more general conversations about marine biology, climate change, or dock-dwelling animals, I thoroughly enjoyed meeting and learning about folks’ environmental interests and concerns.

I am incredibly grateful for the support of the Charles Lambert Fellowship and the University of Washington Biology Department Iuvo Award, which have helped to fund my research in the Swalla Lab. I would also like to thank the marinas that I worked with for allowing me a place to set up my experiment.

Megan Powers is a PhD candidate in UW Biology, working in the Swalla Lab at Friday Harbor Labs. She grew up in Iowa and received her bachelor’s degrees in biology and environmental science from the University of Iowa where she researched plant interactions with water pollutants. Megan was introduced to marine biology research as an undergraduate through summer internships at the Oregon Institute of Marine Biology and the Sanibel-Captiva Conservation Foundation Marine Lab and has been fascinated by marine invertebrate research ever since.


Cohen AN, Klinger T, Cordell JR, Mills CE, Li K, Kohn A, Bookheim B, Secord D, Wonham M, Berry H, et al. 1999. The 1998 Puget Sound Expedition: a rapid assessment survey for nonindigenous species in the shallow waters of Puget Sound. First National Conference on Marine Bioinvasions. https://www.sfei.org/documents/1998-puget-sound-expedition-rapid-assessment-survey-nonindigenous-species-shallow-waters

WDFW. 2009. Annual Report to the Puget Sound Partnership for the INVASIVE SPECIES TUNICATE RESPONSE. Washington Department of Fish and Wildlife. https://wdfw.wa.gov/sites/default/files/publications/00804/wdfw00804.pdf