This paper was written as part of the 2011 Alaska Oceans Sciences Bowl high school competition. The conclusions in this report are solely those of the student authors.

Invasive Tunicates and Their Impact on the Ecosystem Surrounding Petersburg Alaska


Sharece Randrup
Tyler Lantiegne
Daniel Cardenas
Shelton Brett
Christian Hernandez
Gabe Seaman

Team AquaTeen HungerForce

Petersburg High School
PO Box 289
Petersburg, AK 99833


The beautiful, diverse marine ecosystem surrounding our small community of Petersburg, Alaska is being threatened by potentially dangerous, stealthy invaders—the tunicates (also known as sea squirts). The economy of Petersburg, as well as the economies of many other southeast Alaska towns, relies heavily on salmon, crab, and shellfish. If the population and spread of the invasive sea squirts is not controlled and monitored, then the tunicates could devastate the marine ecosystem in Southeast Alaska. Tunicates, being filter feeders, consume planktonic organisms, which would not only create competition with native species, but it would also impact the food web. The loss of plankton would affect salmon populations, as salmon are dependent on native organisms that consume plankton. Tunicates are also capable of suffocating shellfish populations. The negative impacts to salmon and shellfish would be a detriment for Petersburg's economy. In this paper we explain why tunicates are successful invaders, the possible impacts they would have, and we develop a possible management plan for controlling and monitoring the spread of invasive tunicates, one that emphasizes education and early detection.


We are being invaded by aliens, but not the green men from Mars. The invader we are profiling would not be out of place in a science fiction movie. Much like the Blob, these alien invaders can spread everywhere. In one case, a species of invasive tunicate, Didemnum vexillum, spread approximately six square miles in one year off the coast of Massachusetts (Gordon, 2008). The same invader from Massachusetts has recently been found in Sitka, AK during a "BioBlitz" (Ronco, 2010). A town hall-style meeting later, plans are being made to control this invader, with ADF&G biologists surveying the full extent this fall. The invader we are talking about is one of four species of invasive tunicate that have been found in Alaskan waters. I'm sure many of you are asking, "What is an invasive species and why should we care?"

Invasive species are primarily determined by their origin and current location, their ability to colonize, and their ability to adapt to a new location. Invasive species are considered invasive when they have been introduced to an area that they were not originally found in. Once there, the invading organism's ability to reproduce quickly and abundantly and their lack of natural predators contribute to their staying power. All organisms have a niche, though some have a larger niche than others. Often, when a species is introduced to a new environment, it will not thrive immediately. In fact, if the organism is outside its fundamental niche it might very well fail to "take root" in the new environment. However, some organisms have a very wide niche and are able to adapt to a variety of environmental factors. The combination of these factors causes invasive species to be a problem because they will compete with native species.

Our Home

Here in Petersburg, bountiful waters surround the majestic perhumid temperate Tongass Rrainforest. The Tongass ranges from the narrow mountains and ice fields of the mainland to the over 1,000 islands scattered about the labyrinth we call Southeast Alaska. In this great rainforest, we have an average temperature of 4-12 degrees Celsius and an average precipitation of >1400 millimeters (USDA Forest Service, 2008). In these conditions, an incredibly rich ecosystem has sprung up.

The Alexander Archipelago sits on the Pacific Ocean side of southeast Alaska. These islands protect the inner waters from the rough waters and bad weather of the ocean. Although the Alexander Archipelago protects the Inside Passage from bad weather, it does nothing against the extreme tides found in southeast Alaska. The semi-diurnal tides in the Inside Passage can have a nine meter difference from high to low tide (Inside Passage). There are also a variety of currents found in the Inside Passage.

On the outside of the Alexander Archipelago, the Alaska current flows to the north along its coast. In the inside passage, however, the effects of this current are not too strongly felt. Instead, there are a number of smaller currents in various straits and passages. The predominant current of southeast Alaska is directed to the southeast, out of Clarence Strait, from where it flows north against the western coast of Prince of Wales Island (RaLonde, 2010). More currents in southeast include an east and west flowing current across the south tip of Kuiu Island and a north and south flowing current in Chatam Strait (RaLonde, 2010). These conditions are a major reason for Petersburg's abundance of marine resources.

Due to the rich marine-terrestrial environment surrounding Petersburg, commercial fishing is a crucial part of the town's economy. According to the State of Alaska (2010), in 2009 Petersburg had 129 crabbing permits issued and 405 permits issued for salmon. There were also 66 permits issued for shellfish fisheries. For all the fisheries combined, Petersburg had a total of 1,100 permits issued. From those total permits fished, the estimated gross earnings were $43,297,571 (ADF&G, 2010).

Invasives in Alaska

Of the invasive species that occur in Alaska, we chose tunicates and eliminated other possible invasive species for a few reasons. The green crab could pose a threat to Alaskan waters over time, but it has not yet been found this far north. The mitten crab, which has been discovered on the west coast, could also be a threat but it has not come very far north. Last but not least, we excluded the northern pike from our paper because in order for pike to affect Alaska's marine life it would have to be through purposeful stocking of lakes.

Tunicates are a fouling species that are capable of spreading and reproducing quickly. They pose a threat to crab and shellfish populations because of their ability to suffocate out other species. Moreover, they could affect the food web, disrupting the food source for commercial and sport fish species. Tunicates have no known natural predators, which allows them to increase in population rapidly. In the following pages we will explain why species of tunicates are invasive and of concern, their possible ecological impacts, how their proliferation might affect us economically here at home, and finally, how we might manage for the occurrence of invasive species.

Tunicate Life History

In order to understand what makes these seeming "blobs" a strong invader, we need to first understand the biology of tunicates. As adults, benthic tunicates are sessile, or unmoving. They are most common in rock pools ranging from 1–400 meters in depth. Tunicates may be solitary or colonial. Many tunicates are translucent or whitish colored, although this feature may vary.


According to "Tunicates", although tunicates are hermaphroditic and their eggs are brooded with the body until they hatch, they are also capable of reproducing sexually. Gonads develop on either side of the zooid. The ovary is located behind the testis. When the tunicate is ready to reproduce, an egg is ovulated into a sac-like organ that forms as an outgrowth from the wall of the tunicate's body. The egg is fertilized inside of the brood pouch and develops until the larvae are mature enough to escape. Generally, gestation lasts more than one month. After escaping, the larvae are between 0.8 and 0.12 inches long and usually have bright coloration (Alton's Dive Center, 2006).

Metamorphosis begins shortly after the larva escapes (Alton's Dive Center, 2006). Two buds grow on the right of the oozoid and one bud grows on the left (Alton's Dive Center, 2006). "All of the tunicates of the same generation appear, grow, and die at the same rate" (Alton's Dive Center, 2006).

As also stated in "Tunicates," sexual reproduction in tunicates does not affect the process of asexual reproduction cycles. The asexual reproduction may occur through either self-division or outgrowth developing, also known as budding. Asexual reproduction produces colonies of genetically identical zooids (Alton's Dive Center, 2006).

Brain Eating, oh, and the Circulatory System

As an interesting fact, humans are more closely related to tunicates than to invertebrates. Tunicates are classified as Urochordates, and according to "Tunicates", they begin their lives as free-swimming tadpoles with primitive sensory organs and a primitive backbone called a notochord. This larval stage cannot last, however, because the tadpoles are incapable of feeding. The larval stage is primarily for dispersal (Alton's Dive Center, 2006). Once a larva finds a good place to settle, it cements itself to a rock and begins its transformation (Alton's Dive Center, 2006). The backbone dissolves and the tunicate absorbs, or eats, its cerebral ganglion, which was previously used to control movement (Alton's Dive Center, 2006). The tunicate develops a barrel-shaped body with a "tunic" that is supported by cellulose (Alton's Dive Center, 2006).

According to "Tunicates or Sea Squirts: A Wet Link," tunicates have a circulatory system that is largely without vessels. The heart is basically a simple tube that contracts to force blood through it. Interestingly, the heart of a tunicate is capable of reversing the direction of blood flow. That is, for about 100 beats the heart causes blood to flow one direction. Then the heart stops for a moment and pumps blood in the opposite direction (Shimek, 2008).

Digestive and Excretory Systems

As explained by "Tunicates or Sea Squirts: A Wet Link," the excretory system of tunicates is relatively simple—tunicates have only two siphons through which water enters and exits the branchial basket (Figure 1). Water may enter the basket through the modified gill slits, going into the atrium and exiting through the atrial siphon. The atrium is also where gonads and feces gather in the tunicate. These materials can be blown out through the atrial siphon as well (Shimek, 2008).

As stated in "Tunicates or Sea Squirts: A Wet Link," "Cilia lining the branchial basket pumps water though the gill slits into the branchial basket." Within the branchial basket is a ventral groove, called the endostyle, that produces mucus. The mucus forms a sheet around the inside of the branchial basket so that when particulate organic material is taken in by the branchial siphon, it becomes lodged in the mucus. Cilia inside the branchial basket move the mucus, with the food inside of it, "to a food groove located on the dorsal midline of the branchial basket." The food then travels from the food groove to the tunicate's mouth. The food filled mucus travels down the esophagus into the stomach where it is digested (Shimek, 2008). From there, waste moves into the intestine and out the anus. Fecal matter is "deposited in the atrium and flushed out with the extracurrent water passing out the extracurrent siphon" (Shimek, 2008). In other words, these biological attributes contribute to tunicate invasiveness.

Species of Concern

Of the possible aliens to be on the look-out for, there are four main species of invasive tunicate currently found in Alaska—Ciona sp., Botrylloides sp., Didemnum vexillum, and Styela clava. These tunicates can become the cause of much grief to Alaska's ecologic and environmental prosperity. These species of tunicates have many similarities, first and foremost being that they can reproduce copiously both sexually and asexually. The four tunicate species of concern are originally from the eastern coast of Asia (Pagad, 2007). They eat a variety of plankton. All of these tunicates are considered fouling species, and as a result, all of them can be a big nuisance to the human species. The first of these grief-causing tunicates I will be profiling is Didemnum vexillum.

Didemnum vexillum

Didemnum vexillum (Figure 2) is a tunicate that was originally found in Japan (Puget Sound Partnership, 2009). This tunicate is considered invasive in Africa, Asia, Australasia-Pacific, Europe, North America and South America (Pagad, 2008). The most commonly accepted explanation of the global distribution of this species of tunicate is transfer from shellfish sales, larvae detaching from the bottom of boats, or in ballast water (New South Wales Fishing and Aquaculture). In order to control this species, one must first understand the morphology and habits of Didemnum vexillum.

Didemnum vexillum is a tunicate that can be found in a variety of forms. It is found in bays, harbors and coastal water as either a beardlike colony or a low undulating mat with appendages. Didemnum vexillum reach sexual maturity in a few weeks after attaching to a surface and have long breeding seasons. They can also breed in a wide range of temperatures and salinities. Recently, Didemnum vexillum has been found in Sitka (McCann 2010).

Ciona sp.

Ciona sp. is considered to be a cryptogenic species; it has been identified on both sides of the North Atlantic, west coast of North America, South America, Australia, New Zealand, and Africa (Therriault and Herborg, 2008). Ciona sp. is a tunicate that was considered invasive in the Puget Sound area in the late 1990's. It has since moved into Alaska where it is also considered invasive. The Ciona sp. rests at the bottom of the ocean, where it filter feeds and eats up sources of food for benthic organisms. Ciona sp. is a strange tunicate that is very different from the others we will be profiling.

Ciona sp. is a translucent, gelatinous tunicate that traps food in mucus net in its oral cavity (Figure 3). This species competes with native and aquaculture species for both space and food (Ciona, 2008). This species has also been found near Prince Edward Island in Canada.

Styela clava

Styela clava is one of the worst invasive tunicate species not only in Alaska but down the west coast of North America. Styela clava was originally found on the eastern coast of Asia, but has spread to the Pacific Islands, Europe and coasts surrounding North America. Styela clava is spread just like Didemnum vexillum, detaching from boats or hitching a ride in ballast water. Styela clava is most commonly found on coasts in low wave energy environments or sheltered embayments at depths of up to 25m (Pagad, 2007). Because of its wide tolerance, Styela clava is one of the most dangerous invasive tunicates.

Styela clava is a large club-shaped solitary ascidian with bumps on it, growing up to 8-12 cm in length (Pasag, 2007) (Figure 4). Styela clava is a leathery tunicate that has dark brown or yellowish wrinkled skin. This species of tunicate can breed in temperatures as high as 15 degrees Celsius.

Botrylloides sp.

The final species of invasive tunicate found in Alaskan waters is the Botrylloides sp. Botrylloides sp. is a tunicate that was originally found on the east coast of Asia from Japan to southern China. It has since spread to the west coast of the United States from Prince William Sound and Ketchikan in Alaska south to Baja, California. This tunicate can be found in temperatures of 8-25 degrees Celsius and salinities of 26-34 ppt. (Cohen, 2008). Botrylloides sp. is also known as the "chain tunicate."

Botrylloides sp. common name came about because this tunicate is really made up of many small individuals, called zooids (Figure 5). The zooid is an oval or teardrop shaped organism that can be up to 1-2 mm long. These zooids can be a variety of colors, usually orange, yellow, red or purple, all zooids in a colony are the same color. In Botrylloides sp., each individual zooid filters its own food and oxygen and removes its own waste. Eggs are hatched in pouches that protrude from the zooid. The larvae are about 1mm long and are tadpole-shaped. The larvae spend a day free swimming, then fasten themselves to a hard surface and form a new colony.

With the combined quadruple threat of these four tunicates, Botrylloides sp., Didemnum vexillum, Styela clava, and Ciona sp., something must be done to halt their progress before our ecosystem is negatively impacted in a big way.

Ecological and Economic Impacts

Biogeochemical Impacts

Most of the information in this section regarding the filtering capabilities comes from salps—a pelagic tunicate. Salps are efficient filter feeders, and their waste contributes to the re-mineralization processes in deeper parts of the ocean (Sutherland et al., 2010). Given that salps are in the water column, they play a significant role in food webs through biogeochemical cycling of nutrients (Sutherland et al., 2010). Salps are not a benthic species like the four non-native tunicates that we are worried about. Many parallels can be drawn between these two categories of tunicates, with respect to filtering rates, diet, and production of fecal material. However, they differ in that the invasive species of concern to our community are benthic, pulling nutrients from the water column and placing them on the ocean floor. The three main nutrients that influence ocean productivity are carbon, nitrogen and phosphorus.

Carbon is an essential nutrient as the building block of all living matter. The carbon is given basically two options—it can contribute to the carbon sink at the bottom of the ocean and stay there for a long time, or it can go into the water column and become available for microbes. The carbon that is taken up by microbes forms the base of the food chain. Since tunicates are benthic and attached to many submerged surfaces, then the most plausible scenario would be for the majority of the carbon to sink and become part of a reservoir. Some would still be available as a food source in the water column.

In the nitrogen cycle, bacteria fix nitrogen out of the atmosphere making it available for plants or phytoplankton. This introduces nitrogen, typically a limiting resource, into the food chain. Many bacteria are able to fix nitrogen gas; however, much of the nitrogen available to living organisms comes from mineralization/decomposition of other living organisms. Tunicates pull the nitrogen out of the water column as they filter feed. Due to the number of invasive tunicates, this could limit the amount of nitrogen available for other organisms that are not benthic.

Phosphorous enters into the ocean through erosion of rock and human causes. This phosphorous is then taken in by plants and turned into organic phosphorous available to other organisms. Thus as tunicates die or eliminate waste the phosphorus is released onto the substrate where the organisms occur, typically the ocean floor. Again, this removes a limiting nutrient from the water column, possibly limiting productivity. Much of the nitrogen, phosphous, and carbon are pulled into the tunicates and are not recycled into the ocean, but the ocean floor.

Food web

If this tunicate problem continues, our ecosystem could be significantly impacted. Tunicates are master filter feeders, which means they would remove plankton that power the entire food chain. The nano and picoplankton are the main contributors to marine productivity and biomass. They are a major base of the food web. If the tunicate population grows then the nano and picoplankton will not be as available for other organisms such as mussels, clams, and fish.

When plankton are removed from the environment, competition for food will increase. If plankton are being removed, then meroplankton such as shrimp, crab, and other larvae are also being removed; affecting animals that eat the adult versions of these larvae. If tunicates increase in number too greatly, then the plankton might not be able withstand the native organisms that prey on it as well as the invasive tunicates. When tunicates deposit waste, the waste remains on the ground, which then removes organic carbon from the water column, which is a food resource for many plankton. This, along with the filter feeding of tunicates, would kill off many plankton and endanger our way of life and out ecosystem.

Tunicates also foul shellfish farms. Tunicates suffocate shellfish by blocking off their filter feeding, cutting them off from nutrients and oxygen (BCSGA, 2007). This threat of both invasive shellfish and tunicate would most likely leave southeast Alaska in a state of severely impacted economy and in huge ecological trouble.

Economic impact

In 2009 shellfish farms in Southeast were worth $200,000, with a 10% growth rate each year (Figure 6) (ADF&G, 2010). Shellfish farms boost economic growth and also create a handful of jobs. Seventy-two percent of money generated from shellfish farming stays in the local area (Welch, 2010). Tunicates consume many of the same plankton that shellfish eat, decreasing the availability of plankton will affect shellfish farms in Southeast communities, such as Petersburg where there are other forms of income for example, fishing, logging, and tourism.

The absence of $150,000 does not impact the economy greatly in communities such as Petersburg, In smaller communities such as Kake and Coffman Cove, however, the impact will be large as there are not many options for economic income (Figure 7). The absence of shellfish farms would impact these communities. In addition to economic impacts on the shellfish industry, officials in Puget Sound have spent $750,000 to eradicate the spread of tunicates from their harbors (Gordon, 2008). So if tunicates were exposed to the harbor of Petersburg, the cost of maintaining a tunicate free harbor is less than, the price compared to the price of exterminating a tunicate infestation.

Management Plan

There are many ways to deal with the threat of invasive tunicates in the diverse marine ecosystem surrounding the Petersburg area. This species can be devastating to this area, either through genetic contamination or through fouling and smothering area's of ocean bottom. To deal with the threat of invasive tunicates, we propose we take the following management actions.

Coordinate all aquatic invasive programs at the regional, national and international levels. If we are to deal with the invasive tunicate problem, we will have to get as many organizations together as possible so that we can utilize the combined resources of these organizations. Some of these organizations include the National Oceanic and Atmospheric Administration, Alaska Department of Fish and Game, the United States Coast Guard, local canneries, community members and harbormasters. Although many of these organizations have little to no advancement in policy and program development, and have almost no coordination of efforts, we would have to instigate cooperation between these organization.

Prevent more tunicates from being introduced into Alaskan Waters

In order for tunicates to be controlled and exterminated, we have to begin to prevent their introduction. One of the ways to prevent tunicates from arriving in Alaskan waters is to regulate ballast water. According to Zemach, approximately 100,000 ships arrive in Alaska each year, half of them from overseas. Fifty million metric tons of ballast water are released by foreign ships in United States coastal waters, while U.S ships release 130 million metric tons in our own waters (Zemach, 2010). Ballast water can harbor tunicate larvae and when the ballast water is distributed invasive tunicates may be dispelled and establish a population of these parasites. Ballast water would have to be properly filtered or amounts of ballast water would have to be regulated.

Another way for tunicates to be controlled is for boat owners to thoroughly clean the bottoms of their watercraft. Adult tunicates may be able to attach and release their spawn from their position, spreading the species to new locations. Yet another way for tunicates to be controlled is for shellfish farmers to treat their crop with a white vinegar solution to kill off the adult tunicates before they can spawn in profuse amounts (Zemach, 2010). Many treatments are in experimental form.

Detect, monitor, control, reduce, or exterminate the tunicates

Tunicates can be monitored and detected at the objects they foul. SETL[Security Environment Threat List] has already set up monitoring locations throughout southeast Alaska and the world, and because of this they would be the forefront of invasive tunicate detection in Alaska (Figure 8). SETL is a network of volunteers monitoring for invasives, of which Petersburg High School is a member (Figure 9). Problem areas for tunicates would need to have specialized squads of tunicate checkers to watch areas of fouling and areas of potential fouling. If tunicate populations were found, the affected area would have to be treated to destroy the populations—early detection. To monitor problem areas, a mapping system would have to be developed that would track sightings and areas where these tunicates were cleared.

Educating the public on tunicates

The public would have to be educated about these fouling species to ensure their control and halting of population growth. First and foremost, we must regulate the keeping of these invasive tunicates in aquariums. Just one accidental release could jeopardize all of the prior efforts we have taken to control this problem. We would also have to educate the boat owners and fisherman about measures they can take to ensure that they keep their vessel tunicate free.

Research, Research, Research!!!

In order to effectively prevent a future infestation of tunicates in our waters, then we must conduct research into the dispersal methods, effects on human health, and the speed of habitation, and effects on the ecosystems and economy of the area. We could also benefit from studying other locations and how they dealt with invasive tunicates, such as Prince Edward Island and their problems with Stylea clava.


Regulations will be critical in the control of invasive tunicates. As previously mentioned, we would have to place new regulations on ballast water and boat cleaning. The Native Invasive Species Act of 1996 says that any ships operating outside of the EEZ should have a ballast water exchange, retain their ballast water or have a United States Coast Guard approved treatment method. These guidelines were not mandated until 2004, if these guidelines are not met consequences include a class C felony for all on board and a daily fine of $27,500. These regulations would have to also encompass vessels inside the EEZ, if one area is infected with tunicates then these vessels could spread the tunicate larvae to new uninfected locations. We would also have to regulate shellfish farmers, if tunicates infested their crop and they just dumped the ruined clams into the ocean, yet another infestation would occur.

The management of these tunicates will be a long and hard process. The methods and steps outlined in this paper will be crucial in defending our waters from these fouling organisms.


Invasive tunicates are one of the most grievous threats to our Alaskan ecosystem. The four main species of concern are Didemnum vexillum, Ciona sp., Botrylloides sp., and Stylea clava. These tunicates are a major threat to our ecosystem because of their fast reproduction rate and the absence of predators. For Alaska, the most important step in our management plan is the prevention and detection/monitoring, followed by the extermination and controlling stages.

Alaska has a low distribution and abundance of marine invasive species. This allows the state to work hard to identify and prevent before the invasive species have had a chance to establish themselves. Climate change adds an additional layer of complexity; however, the changes that may increase habitat suitability for invasive species do not necessarily change our assessment of ecological and economic impacts or our proposed management plan. The predicted temperature shows an average monthly increase of 6 degrees Celcius by the year 2041 and 10 degrees Celcius by the year 2091 (SNRAS, 2009). As anecdotal observations by students involved with the SETL monitoring program indicate here in Petersburg, warmer summers have had higher growth in comparison to cooler summers (Figure 10). Many of the invasive tunicates profiled prosper in warmer environments, spelling bad news to our usually cooler area. With increased climate change, tunicate growth could skyrocket and leave us all in an altered marine ecosystem.

Luckily for Alaska, the threat of invasive tunicates has just arrived and is still at a point where we can prevent, and more or less control the organism. If left unchecked the tunicate problem could create an extreme deficit in Petersburg's economy and a very bleak outlook for our ecosystem.


anatomy of a tunicate

Figure 1. Tunicate anatomy (Shimek, 2008).

picture of Didemnum vexillum

Figure 2. D. vexillum (USGS).

picture of Ciona savignyi

Figure 3. Ciona savignyi (PNWScuba, 2008).

picture of Styela clava

Figure 4. Styela clava (PNWScuba, 2008).

picture of Botrylloidies sp.

Figure 5. Botrylloidies sp. (Cohen, 2005).

graph of the dollar value and number of permitted shellfish farms in southeastern Alaska between 1990 and 2009

Figure 6. Value and number of permitted shellfish farms in southeastern Alaska, 1990-2009 (ADF&G 2010).

a map illustrating where the shellfish permits are located around Petersurg and Wrangell, Alaska

Figure 7. Location of shellfish permits in the Petersburg-Wrangell vicinity (ADF&G 2010).

a map of the world showing where the SETL monitoring locations

Figure 8. SETL monitoring locations around the world (Nakurangi 2008).

a student sampling a SETL plate

Figure 9. PHS student sampling SETL plate.

Corella inflata growth on a SETL monitoring plate

Figure 10. Heavy Corella inflata growth on SETL monitoring plate, Petersburg, Alaska.