The magnitude of the worldwide invasions of Didemnum vexillum Kott, 2002 has taken a number of years to be comprehended. During the past 15 years, it has been identified as different species depending on its location - D. carnulentum on the U.S. west coast, D. lutarium or D. vestum in New England, D. lahillei or D. helgolandicum in France and the Netherlands, D. vexillum in New Zealand, D. pardum or D. moseleyi in Japan. A number of recent publications refer to it as Didemnum sp. or Didemnum sp. A. This paper presents a chronology of the steps in the development of our awareness and understanding of this species based on comparative morphology and genetics, and lists invaded regions and the approximate minimum length of time it has been known in each area. Evidence is presented that D. vexillum may have originated in Japan. The importation of vast quantities of Japanese oysters and spat into many countries prior to the 1960s is discussed but eliminated as a likely vector because there are no reports of a sudden didemnid ascidian appearance prior to the 1970s. Introductions (including to the type locality in New Zealand) are very likely due to shipping (either via hull or sea chest fouling), with subsequent local spreading by fouled recreational craft, barges, etc., drifting and reattachment of dislodged fragments, and movements of fouled aquaculture stock and gear. Based on morphological and genetic comparisons of hundreds of world-wide samples, museum type specimens, and anecdotal information on the presence of this species in various locations over several or many decades, the valid name is concluded to be Didemnum vexillum Kott, 2002 due to the lack of any pre-existing published description. D. vestum Kott, 2004 is synonymized under D. vexillum.

A colonial tunicate belonging to the genus Didemnum has recently been found in many temperate coastal regions throughout the world, as well as large areas of Georges Bank in the NW Atlantic. It continues to spread rapidly and compete aggressively with native, hard substrate species (e.g., mussels, barnacles, bryozoans, other ascidians). In addition, it can form dense mats on deep-water cobble-gravel substrates and influence the abundance and species composition of benthic epifauna and infauna. Thus, its ever-increasing presence is creating potentially severe detrimental economic and ecological impacts. This invasive species, referred to in recent publications as Didemnum sp. A, has been misidentified as five previously described species native to the regions where Didemnum sp. A has been discovered and has been described as two new species based solely on morphological characteristics. There are relatively few diagnostic characters and a great deal of variability in the relevant characters, making the task of identification very difficult. Adding to the confusion has been the widespread and apparently disjunct distribution of the species. Here, we present molecular data on both mitochondrial and nuclear genes from colonies sampled from Europe, east and west coasts of North America, Japan, and New Zealand. These data strongly indicate that Didemnum sp. A is a single species, possibly native to the northwestern Pacific Ocean, that has become established globally. Considering genetic and morphological evidence, the most appropriate name for this species is Didemnum vexillum Kott, 2002.

Styela plicata (Lesueur, 1823) is a solitary ascidian found in shallow, protected environments in tropical and warm-temperate oceans. Its origin is uncertain, given that it has already been identified in several oceans since it was first described, showing a very broad geographical distribution. Although S. plicata has been historically classified as a cosmopolitan species, in the past few decades it has been considered as an introduced or invasive species in some regions of the world. The present study investigated the genetic variation among populations of S. plicata. A total of 51 samples were obtained from locations on the Atlantic and Pacific coasts of the USA, Japan, and southern and southeastern Brazil. The amplification of a fragment of the cytochrome oxidase subunit I gene (COI) generated nine distinct haplotypes. There was considerable variation in the geographical distribution of haplotypes, yet the highest nucleotide and haplotypic diversities were clearly found in the Pacific samples. Interestingly, one of the haplotypes showed more than 3% divergence in relation to the remaining haplotypes, suggesting the possibility of a cryptic species. These results, together with historical records, indicate that commercial shipping could be the main cause for the global distribution of S. plicata. The northwestern Pacific region is hypothesized as the center of distribution of the species.

A revision of the non-indigenous ascidians in the Mediterranean Sea has been carried out, from published and unpublished records. The records considered include the last 50 years, which encompasses the period after the Pérès inventory (in 1958). Our aim is to analyze the ancient and recent records with comments about their validity and possible introduction vectors.

Two unique bays, Great Salt Pond system of Block Island, Rhode Island and Lagoon Pond and adjacent portions of Vineyard Haven Harbor, Martha’s Vineyard, Massachusetts, undergoing bay scallop Argopecten irradians irradians restoration had similar tunicate faunas. We found that Great Salt Pond and Lagoon Pond contained similar tunicate fauna dominated by invasive species, Ascidiella aspersa, Botrylloides violaceus, Botryllus schlosseri, Didemnum vexillum and Styela clava, along with native species Aplidium constellatum and Molgula manhattensis and cryptogenic species Ciona intestinalis. In both regions, most tunicate fouling was on artificial surfaces. Tunicate fouling occurred but at lower biomass on natural benthic surfaces including marine plants and algae Ulva lactuca, Sargassum filipendula, Fucus spp., Zostera marina and Codium fragile tomentosoides especially near docks. Tunicates were absent on rocks, free-living scallops, the sedentary snail Crepidula fornicata and open meadows of marine plants.

The conditions for a successful invasion involve the intersection of a species, its vector, and an appropriate receiving environment. Species distribution (biogeography) and availability of a shipping vector were used as filters to reduce a list of 57 tunicates with a history of invasion in marine or estuarine waters worldwide, to a more manageable basis for a “watch list” or “trigger list” for non-indigenous invasive tunicates in Atlantic Canada. Seven species from the worldwide invasives list were already present in Atlantic Canada: the non-indigenous Styela clava, Ciona intestinalis (cryptogenic in southern Nova Scotia but non-indigenous in northern Atlantic Canada), Botryllus schlosseri, Botrylloides violaceus and Molgula manhattensis, and the native Aplidium glabrum and Didemnum candidum. Nine species, not currently present in Atlantic Canada, were removed from the analysis due to insufficient distribution data. All of the remaining 41 species co-occurred in one or more bioregions with species presently found in Atlantic Canada. Examination of distributions relative to shipping patterns eliminated eight species not present in the areas with the most shipping traffic to Atlantic Canada: the eastern seaboard of the USA, the Caribbean Sea, northern Europe and the Mediterranean Sea. A climate zone filter to remove species found only in subtropical or tropical waters eliminated 21 species. Applying both the shipping and climate zone filters resulted in a “watch list” of 17 tunicate species considered the most likely to successfully invade Atlantic Canada: Ascidia sydneiensis, Ascidiella aspersa, Botrylloides leachi, Clavelina lepadiformis, Cnemidocarpa irene, Corella eumyota, Cystodytes dellechiajei, Didemnum vexillum, Diplosoma listerianum, Perophora japonica, Perophora multiclathrata, Phallusia mammillata, Polyandrocarpa zorritensis, Polyclinum constellatum, Styela canopus, Styela plicata, and Symplegma brakenhielmi.

Many ports and marinas contain exotic ascidians, ostensibly because they offer adequate physical habitat and harbour many boats or ships, which are considered important vectors of exotic ascidians. Standardized quantitative sampling of these habitats would be useful to assess exotic ascidian presence and abundance, as well as for more detailed ecological studies. In this paper two methods for sampling exotic ascidians are compared using data from nine marinas along the Olympic Peninsula and Upper Puget Sound, Washington, U.S.A. One method is considerably less expensive, drier and easier: it consists of laying a simple grid over the side of the floating dock from the surface, with species identity and abundance being measured on the spot. The other method involves swimming underneath marina floating docks, taking standardized digital photographs and analyzing the photographs later. Differences between these two methods might be expected because the environment sampled, the underneath versus the sides of floating docks, is slightly different, especially with respect to light. However, this study finds that both methods observed almost identical presence/absence patterns for exotic ascidians and, at least for the two most common exotics Botrylloides violaceus and Diplosoma listerianum, observed similar abundance trends across sites. Further analyses show that, while there are differences in the overall communities observed by the two methods, community patterns are correlated. Overall, this study finds that the easier surveys of the sides of floating docks are as effective for rapid assessment of exotic species presence/absence and relative abundance as those of the undersides of floating docks.

The potential for boating to disperse the clubbed tunicate Styela clava Herdman, 1881 and green crab Carcinus maenas (Linnaeus, 1758) in the southern Gulf of St. Lawrence was investigated using interviews with recreational and commercial boaters in eastern Prince Edward Island (PEI). Boaters were asked how long their boat had been at the present location; the primary use of the boat; if anchors, sounding equipment or fishing gear were used; whether any organisms were attached to these items when retrieved; and the fate of those organisms. Bilge water and hull scrapings from the vessels contained 31 and 47 taxa, respectively. Recreational boats, nearly half of which were docked outside their home estuary, were a more likely vector of dispersal than commercial fishing boats that tended to return to the same port each night. Northeastern Nova Scotia and the southern coast of PEI were most at risk for the spread of clubbed tunicate, while green crab could be transported to PEI and eastern New Brunswick. The Magdalen Islands, Quebec, were also predicted as a site to which green crabs could spread, and the first green crabs were detected there two years after our study.

The invasive clubbed tunicate (Styela clava) was first identified in Georgetown harbor, Prince Edward Island (PEI), Canada, in 1998 and has since spread to other estuaries in PEI. Worldwide, the dispersal of this species is attributed to hull fouling and contamination of bivalve aquaculture products. We tested how clubbed tunicate settlement patterns differed among the most common boat hull surfaces and colors as well as the ability of these tunicates to survive extended atmospheric exposure similar to that of boats being transported on trailers during summer months. Untreated hulls made of fiberglass, painted wood, and bare aluminum were quickly colonized by larval tunicates with hulls painted black attracting significantly more colonists compared to white-painted hulls. Use of anti-fouling paint kept colonization to a minimum, even after 12 weeks in the water. Bare aluminum hull material attracted the highest numbers of tunicates, which is a problem because boats operating around bivalve aquaculture sites are mainly constructed of aluminum and cannot be painted with anti-fouling paint according to industry codes of practice. Aluminum and fiberglass hull material that had one-year-old tunicates growing on them were exposed to open air for 48 h during September (mean daytime high temperature 29.7˚C, night-time low 8.5˚C). Nearly all tunicates were alive after 8 hours with only 10 to 11 % mortality after 48 h. Based on a 48 h survival time, viable tunicates on boats removed from infested waters of PEI could be spread on boats transported on trailers to any waters within 1600-2000 km of PEI – a distance encompassing the entire Atlantic seaboard of Canada.

Following anecdotal reports of tunicates on the carapaces of rock crab (Cancer irroratus) and American lobster (Homarus americanus), we evaluated the role of these species and northern lady crab Ovalipes ocellatus as natural vectors for the spread of invasive tunicates in the southern Gulf of St. Lawrence. Several hundred adult specimens of crabs and lobster from two tunicate-infested estuaries and Northumberland Strait were examined for epibionts. Small patches of Botrylloides violaceus were found on rock crabs examined from Savage Harbour and a small colony of Botryllus schlosseri was found on one lobster from St. Peters Bay. Lobster and lady crab collected in Northumberland Strait had no attached colonial tunicates but small sea grapes (Molgula sp.) were found attached on the underside of 5.5% of the rock crab and on 2.5% of lobster collected in Northumberland Strait in August 2006. Lobster and rock crab clearly represent a vector for the spread of invasive tunicates regionally and wherever living crustaceans are shipped globally.

Botryllid tunicates have become increasingly important as model experimental organisms in a variety of biological and ecological fields, including invasion ecology. This paper summarizes existing botryllid culture methods and offers new ones for culturing Botryllus schlosseri (golden star tunicate) and Botrylloides violaceus (violet tunicate) under laboratory conditions. Techniques for maintaining adult colonies, obtaining larvae, and establishing juvenile cultures are presented. Considerations for designing laboratory experiments using botryllid tunicates are discussed.

To understand factors contributing to sessile marine species mortality, we assessed the role of two species of small predaceous neogastropods (Mitrella lunata and Anachis lafresnayi) on mortality patterns of different species of ascidian recruits. By using the functional response approach (i.e., predator response to variations in prey density) we obtained comparative information on the survival and potential local persistence patterns of several species of ascidians common to southern New England, USA, coastal waters. In addition, we were able to compare the influence of the predators on ascidian species which were relatively recent invaders (e.g., last ~30 yrs or less) into southern New England from those species which have been resident in the region for more than a century. Ascidian recruits examined included both solitary (Ciona intestinalis, Styela clava, Molgula manhattensis, Ascidiella aspersa) and colonial (Botryllus schlosseri, Botrylloides violaceus and Diplosoma listerianum) forms. Collectively, our studies indicated that the predators displayed fairly generalized predation patterns and neither species readily consumed Botrylloides recruits. When ascidian recruit densities were manipulated over a 1 to 3 order of magnitude range, predator consumption rates differed between prey species. Proportional mortality was prey density independent (Type I functional response) when Anachis was preying on Styela, Ciona, Botrylloides and Diplosoma and when Mitrella was foraging on Diplosoma. Mortality was inversely prey density dependent (Type II functional response) when Anachis was feeding on Molgula and Mitrella was feeding on Styela, Ciona and Botryllus recruits. Low prey density mortality rates, suggestive of a Type III functional response, were observed when Anachis was feeding on Ascidiella and Botryllus and when Mitrella was feeding on Molgula recruits. Collectively, results indicate that, for the most part, the predators are capable of effectively controlling the prey regardless of naturally occurring variations in ascidian recruitment densities.

The colonial ascidian, Didemnum vexillum, is a relatively recent invader to the east and west coasts of North America and since its appearance it has often become a dominant member of a variety of coastal communities in these regions. The species has the unique ability to colonize cobble-pebble substrates and form extensive mats in this habitat type. The mats essentially ‘glue’ the small pebbles and cobbles together and alter habitat complexity of the seafloor from a more three-dimensional system to a more two-dimensional one. We examined the potential impacts of the mat-forming activities on benthic macro-invertebrate population and community structure by comparing sets of samples collected inside and outside of the mats. Sampling was conducted at a site located in eastern Long Island Sound, USA, over a period of approximately one year using a corer and a suction sampler. Contrary to our prediction that the presence of the ascidian mats would reduce benthic species richness and abundance we found that these parameters either were not different or were significantly higher in samples taken inside Didemnum mats compared to samples collected immediately outside the mats. The presence of the mats did result in subtle shifts in benthic community structure and functional group dominance with greater numbers of infauna and deposit-feeders residing inside the mats compared to samples collected adjacent to the mats.

Since the discovery of the invasive colonial tunicate Didemnum vexillum Kott, 2002 on Georges Bank in 2002, research has focused on investigating the spread of the tunicate invasion, evaluating its potential impact on the benthic community, identifying it to species level, and determining its region of origin. The percent cover of Didemnum vexillum, measured from bottom photographs, ranges from 0-100% in individual photos and between 0-79% when averaged within photo transects. Individual photos represent an area of the seabed measuring ~ 0.39 m² while photo transects range from ~ 700-1000 meters in length. Hydroids are the second most abundant epifaunal taxon. The macrofauna identified in bottom photo analysis comprises 21 different taxa, of which burrowing and non-burrowing anemones are the most numerous. Our detailed analysis of bottom photographs suggests that Didemnum vexillum is able to out-compete other epifaunal and macrofaunal taxa. An Analysis of Similarity (ANOSIM) test on macrofauna abundance data collected with a Naturalist dredge from 1994 to 2006, indicates that Didemnum vexillum has had a significant impact on the species composition of the benthic community. The abundance of two polychaete species, Nereis zonata Malmgren, 1867 and Harmothoe extenuata Grube, 1840, increased significantly in infested areas compared with uninfested areas, according to two-way Analysis of Variance (ANOVA). We found four distinct nucleotide sequences of the 18s rDNA gene among 17 samples of Didemnum species, three from Georges Bank and one from New Zealand. Two of the Georges Bank sequences were identified as Didemnum albidum Verrill, 1871, a species native to the northeast United States. The third sequence represents the invasive Didemnum vexillum from Georges Bank, and the fourth sequence an undescribed species from New Zealand (not D. vexillum).

The invasive colonial ascidian Didemnum vexillum occurs in Japan, North America, northern Europe, and New Zealand. It forms adhering mats on living and non-living hard substrates and alters seabed habitats. We examined the relationship of the first and last occurrences of D. vexillum recruits to water temperature and suggest it is possible to identify coastal and offshore locations that are at risk of colonization by the species based on seasonal water temperature trends. Temperature loggers and settlement plates for recruits were deployed and monitored at three shallow coastal settings in New England, including an open harbor, a marine estuary, and a nearshore island. In addition, the distribution of D. vexillum at sites on Georges Bank, an offshore fishing ground (where settlement plates could not be deployed), was compared to long-term bottom temperature data. Recruits of D. vexillum are small (0.2 to 1.0 mm) but easy to identify, and photographs illustrating the developmental stages of oozooids are presented. Recruits of D. vexillum did not appear on settlement plates at the same water temperature at the three coastal sites. Recruitment occurs in the range of 14 to 20°C and apparently is dependent on local climatic conditions. At coastal sites where we have complete records, recruitment persisted for 3.5 to 5 months; and recruits continued to appear as waters cooled to below the temperature of first occurrence. Recruitment ceased in the range of 9 to 11°C. On Georges Bank, the yearly temperature range (4 to 16-17°C) is the same in areas where D. vexillum is present and in a nearby area where D. vexillum is absent. However, in the reproductive season on the bank, temperature variability is but a few degrees in areas where the species is present; whereas it is high (11°C) where the species is absent. It is known from previous studies that in extremely cold conditions in shallow water D. vexillum colonies degenerate and all but disappear, only to regenerate as waters warm. We suggest that: 1) the degree to which colonies degrade in the cool season influences the length of time they require to regenerate, reproduce sexually, and brood larvae; 2) larval recruits will be released at the end of a developmental period as water temperatures warm, not necessarily when a particular water temperature is reached; 3) larvae likely will appear at different temperatures at climatically different sites, and at approximately the same time and temperature at climatically similar sites; 4) highly variable temperatures during the warm season likely inhibit the reproductive process; 5) the time required for a colony to develop and release larvae and the length of the warm season probably affects the length of the recruiting period at a site; 6) as waters cool, larval release by healthy colonies and recruitment might be regulated chiefly by declining temperature and therefore could end at approximately the same temperature at all sites. At deeper water sites, where minimum temperatures are warmer than at shallow sites, it is possible that D. vexillum colonies are not as affected in the cool season (i.e., do not degenerate) and thus could have a longer recruiting season. This may explain the species’ successful colonization of several large areas of gravel habitat on Georges Bank.

In 2004, an exotic tunicate, Ciona intestinalis, was detected in Montague River, Prince Edward Island. Since it was first detected, this exotic species has become an invasive nuisance species creating production problems in bivalve aquaculture industries including handling difficulties and resource competition with the cultured blue mussel, Mytilus edulis. Ciona intestinalis has become a challenging species to manage because of its long reproductive period and rapid biomass accumulation. Population development of C. intestinalis in a single season was investigated to inform the aquaculture management of this fouling species. The study focused on seasonal recruitment patterns and the subsequent development of the C. intestinalis population after settlement. Recruitment on experimental collectors occurred from mid-June until late November, with a peak in late August. A rapid increase in biomass was documented in late July, six weeks after the initial recruitment. No substantial increase in C. intestinalis biomass was observed in new recruits after mid-August.

This study was undertaken to quantify the ecological interactions between blue mussels (Mytilus edulis Linnaeus, 1758) and vase tunicates (Ciona intestinalis Linnaeus, 1767) in the context of mussel farming. To quantify the extent of competition for food resources at varying temperatures, clearance rates for both species were calculated using Tetraselmis striata (Butcher, 1959). Between 4-13°C, mussel clearance rates were at least three times higher than those of tunicates. At 16°C and 19°C, the mussel clearance rates fell to the same level as the tunicates. Clearance rates were also examined using different sized algal species, and a substantial overlap in mussel and tunicate particle size utilization was observed. To determine the effects of tunicate density on mussel productivity, size and condition indices were measured in the field under varying tunicate density. Mussel size and condition decreased with increasing tunicate densities. In addition, up to 50% mussel mortality was observed under heavy tunicate fouling (~2 kg•m^-1). Mussels and tunicates have the potential for substantial food resource competition, and tunicates were found to have a negative effect on mussel productivity. Heavy tunicate fouling was associated with higher mussel mortality, lower overall size and condition. The cost effectiveness of removing the vase tunicate from mussel lines is also discussed.

The first confirmed report of the invasive solitary ascidian Styela clava (clubbed tunicate) in the blue mussel (Mytilus edulis) production area of March Water in Malpeque Bay, Prince Edward Island (PEI), occurred in September of 2002. Mussel farms in eastern PEI waters have been challenged with heavy infestations of this tunicate since 2000, causing both production and processing problems for the industry. A multi-year study was initiated in June of 2003 to document the spread of S. clava within the mussel producing areas of Malpeque Bay. The study design consisted of the establishment of a series of 4 transect lines extending outwards from the initial area of detection and the subsequent deployment of PVC collector plates at predetermined distances. Collection plates were retrieved in late fall of each of 4 study years, and S. clava specimens were quantified for abundance and body length. Mean recruitment levels of S. clava per collector increased from 0.4 individuals in 2003 to 370.8 in 2006. By November 2006, the geographical spread within Malpeque Bay reached approximately 12 km from the initial area of detection. This study demonstrated that within 3 years of detection, a few individual S. clava expanded to a population at nuisance levels for the mussel industry of PEI.

Commercial shellfish farming is increasing in Brazil to provide for the growing local and international markets. Shellfish (mussel and oyster) production in Brazil is greatest in the state of Santa Catarina where three sea ports with national and international commerce are sources of non-indigenous species (NIS) that have the potential to find suitable habitat in the shellfish farms. Here we describe the tunicate community associated with shellfish farms during the past 10 years. Survey results identified 17 species of which only one was native. Two were clearly introduced. Of the 14 species classified as cryptogenic, 10 were probably introduced. Tunicates become very abundant in farms and as a consequence farms require periodic cleaning of the shells as well as associated equipment. Styela plicata and Didemnum perlucidum are the most abundant and may become a threat to natural communities, but they are currently not found on natural substrates. Ciona intestinalis and Styela clava were not found in shellfish farms, even though C. intestinalis, at least, has been introduced many times in Brazil. We recommend that a public awareness program be developed to prevent colonization by C. intestinalis in the bivalve farms. Additionally, we emphasize that the diversity of possibly introduced species also requires close monitoring for rapidly expanding populations, since a diversity oflife cycles and biological requirements also increases the probability of the appearance of a new pest.

The invasive colonial tunicate Didemnum vexillum has become widespread in New England waters, colonizing large areas of shell-gravel bottom on Georges Bank including commercial sea scallop (Placopecten magellanicus) grounds. Didemnum vexillum colonies are also fouling coastal shellfish aquaculture gear which increases maintenance costs and may affect shellfish growth rates. We hypothesized that D. vexillum will continue to spread and may affect shellfish larval settlement and survival. We conducted a laboratory experiment to assess interactions between larval bay scallops (Argopectin irradians irradians) and D. vexillum. We found that larval bay scallops avoid settling on D. vexillum colonies, possibly deterred by the low pH of the tunicate’s surface tissue. The results of this study suggest that widespread colonization of substrata by D. vexillum could affect scallop recruitment by reducing the area of quality habitats available for settlement. We propose that the bay scallop can serve as a surrogate for the sea scallop in estimating the negative impact D. vexillum could have on the recruitment of sea scallops on Georges Bank.

Blue mussel Mytilus edulis aquaculture lines and associated gear provide habitat for sessile and mobile epifaunal fouling organisms. Due to food limitations and substrate space, these species are likely to interact among themselves and with mussels. In some areas of Prince Edward Island and elsewhere in Atlantic Canada, invasive sea squirts such as the vase tunicate, Ciona intestinalis, colonize mussel socks and rapidly become the dominant species in terms of abundance and biomass. A relevant question for these systems is which native epifauna are most affected by the growth of these invasive tunicates. Our study documents local variations in the abundance of vase tunicates and relates this information to the abundance patterns of three groups of native epifauna with distinctive levels of mobility: sessile tunicates of the genus Molgula, sedentary polychaetes and errant polychaetes. Following recruitment, large abundances of vase tunicates created a significant among-site variation pattern that was consistent over time irrespective of the season when the mussel socks were deployed. In contrast, native tunicates of the genus Molgula colonized the mussel socks in lower numbers and in a spatial pattern opposite to that of invasive tunicates. With the exception of one sampling period, sedentary polychaete colonization also displayed a negative relationship with invasive tunicates. Errant polychaetes displayed erratic patterns that were apparently unrelated to the other species studied. Overall, these results suggest that invasive tunicates have negative effects on many epifaunal species, particularly those that are sessile or have limited mobility.

The proliferation of invasive tunicates in Prince Edward Island (PEI) estuaries has necessitated the development of approaches for managing tunicates that foul aquaculture structures, especially Styela clava and Ciona intestinalis. Spraying or immersion with a saturated solution of hydrated lime (calcium hydroxide) or 5% acetic acid are effective against these tunicates, but are also biocidal to a variety of non-target organisms as demonstrated by bioassays with the bacterium Vibrio fischeri, sand shrimp Crangon septemspinosa and threespine stickleback Gasterosteus aculeatus. Both chemicals have the potential to alter estuarine pH, which should remain within the limits 7.0-8.7 mandated by federal water quality guidelines. Acetic acid is no longer used as a commercial treatment in PEI and currently poses no risk to the environment. The pH of saturated hydrated lime solutions used for immersing mussel socks reached 12.6, but the “pH footprint” in the estuarine water column was limited to a radius of <1 m around the treatment site and rapidly returned to ambient pH. In studies elsewhere, heavy applications (>5 tonnes/ha) of quicklime (calcium oxide) pellets mainly converted to hydrated lime within minutes, but chemical conversion of hydrated lime to harmless calcium carbonate took up to 18 days depending on temperature, pellet size, and the amount of quicklime applied. Conversion of hydrated lime to calcium carbonate should be more rapid at the much lower daily application rates (<0.007 tonnes/ha) of hydrated lime in powdered form, which represented the maximum amount likely to be used in mussel aquaculture for tunicate management in PEI. In most PEI estuaries, dilution by tidal mixing alone is probably sufficient to return the pH to normal values within a tidal cycle at these relatively low inputs of hydrated lime, even without taking into account the chemical conversion which would be occurring simultaneously. Decisions about whether to use chemical treatments must balance economic and potential environmental costs of treatment against the known economic consequences of unmanaged tunicate biofouling. A recent shift to use of pressure washing will reduce the potential for impacts associated with chemical treatments, but may not be feasible in estuaries infested with Styela clava.

This paper is the first in a three-part series that addresses rapid response as a management approach to the recurrent problem of colonization by non-indigenous species, invasive tunicates in particular. “Rapid response” refers to the steps taken, starting before detection of the invasion of a non-indigenous species, through a decision process that may culminate in an attempt to eradicate the species before it becomes established in the new habitat. Rapid response is the second line of defence against non-indigenous species, when prevention measures have failed. We review the goals of rapid response, and its history in the marine environment, to place rapid response into context for the subsequent papers which will review the history of non-indigenous tunicate management in Prince Edward Island, Canada, and propose a framework for rapid response.

The term “rapid response” refers to the steps taken upon the detection of a non-indigenous species and encompasses a range of possible actions ranging from eradication, management of population abundance or dispersal, to a decision against an active response. Rapid response to non-indigenous species in Canadian waters is in its infancy and has been carried out on ad hoc basis in the absence of formal rapid response plans. Even so, many of the essential elements of a rapid response plan were in fact implemented in the management of recent invasions of the non-indigenous tunicates Styela clava, Botryllus schlosseri, Botrylloides violaceus, and Ciona intestinalis in estuaries of Prince Edward Island (PEI), Canada. In this second of a three-part series, we examine case studies of the PEI tunicate invasions, as a step in formulating a rapid response framework that can be used in Canadian waters to manage future invasions by nuisance or noxious species.

The ability to conduct an effective rapid response to the detection of a novel non-indigenous species is greatly enhanced by the development of appropriate protocols and action plans before the arrival of species for which rapid response may be required. Worldwide, much effort has been expended on the development of such plans. Rapid response in Canadian waters is in its infancy and in Atlantic Canada, at least, rapid response against invasive tunicates has been carried out on an ad hoc basis in the absence of formal rapid response plans. Even so, many of the essential elements of a rapid response plan were implemented in the management of recent colonization of estuaries of Prince Edward Island (PEI) by non-indigenous tunicates. This paper proposes a framework that can be used to develop rapid response plans against future colonization of Canadian waters by non-indigenous species. The framework builds on principles derived from the preceding two papers that reviewed case histories of marine rapid response internationally and of four non-indigenous tunicates that have recently become pests in PEI estuaries. The status of non-indigenous tunicate management in PEI will be discussed with reference to the proposed framework in order to highlight areas where rapid response planning in Canada may benefit from further development.

We assessed the relative growth of Didemnum vexillum under different environmental conditions. Pouches containing pre-weighed D. vexillum colonies were deployed at three different depths and in three different salinity regimes for two weeks. Upon recovery, colonies were reweighed to determine their change in biomass. D. vexillum grew fastest in shallow water (1.0 m) and in high salinity areas (26-30 ppt).

In the Dutch province of Zeeland, fouling species on a mussel line form a diverse assemblage of mainly exotic evertebrate species. This differs strikingly from the situation in the estuaries of eastern Prince Edward Island, Canada. Despite the fact that about the same species are present, monocultures of exotic tunicate species dominate there. The import of mussels has been strictly regulated in The Netherlands, aiming at a reduction of the introduction of invasive species among which tunicates.

The solitary ascidian Styela clava Herdman, 1882 has recently been found in the Bassin de Thau, France, an area of intensive oyster and mussel farming. The shellfish are grown on ropes suspended in the water column, similar to the technique employed in Prince Edward Island (PEI), Canada. S. clava is considered a major threat to the mussel industry in PEI but, at present, it is not considered a threat to oyster production in the Bassin de Thau. Anoxia or the combined effect of high water temperature and high salinity may be constraining the growth of the S. clava population in the Bassin de Thau. Identification of the factors restricting the population growth may provide clues to potential control methods.

Didemnum vexillum is an invasive tunicate that aggressively grows on and fouls all manner of substrates in coastal New England habitats. Most alarmingly, D. vexillum acts as a shellfish pest and is capable of completely encapsulating and smothering bivalves, causing them to have reduced growth or be killed. Fouling by D. vexillum on aquaculture gear and product requires remediation. While there are numerous manual eradication methods, they are labor intensive and expensive. We investigated whether the common periwinkle snail Littorina littorea can be utilized as a biological control for D. vexillum. The only known predator of senescing D. vexillum is the snail L. littorea and there are no known predators of healthy D. vexillum. Field observations indicated that L. littorea may be consuming, scouring, or otherwise removing stressed D. vexillum from rocks in intertidal pools at Sandwich, Massachusetts, during all seasons. We used two methods to investigate whether L. littorea could be used as a biological control by either consuming or scouring D. vexillum off shellfish. We examined L. littorea’s fecal pellets and conducted a laboratory experiment to determine if the snails would “clean” unhealthy D. vexillum from aquaculture product. Fecal pellets from L. littorea collected on unhealthy D. vexillum contained the characteristic spicules of this tunicate, thus confirming that L. littorea consumes D. vexillum under field conditions. The laboratory experiment indicated that L. littorea did not notably consume or scour D. vexillum from shellfish under the conditions we provided. At this time, we recommend that manual eradication methods be considered the primary defense for shellfish aquaculturists and others interested in controlling D. vexillum and that L. littorea should merely be considered as a supplement to these more reliable control methods.

Styela clava is a solitary tunicate that has spread rapidly from its native waters in the Philippines and is now a dominant resident in marine habitats of much of the world. It was first reported on the east coast of the U.S. at Beverly, Massachusetts in 1970 and is now commonly found in marine environments throughout the state. The alien sea squirt is a pest to the shellfish aquaculture industry that causes nuisance fouling on aquaculture gear. In southern Korea, Styela clava is considered to be a seafood delicacy and has acquired a cultural distinction as an aphrodisiac. A consumer market already exists for the fouling tunicate in Korean markets in the U.S. that sell imported frozen Styela clava at a retail price of at least $8 per pound ($3.63/kg). It is suspected that local fresh tunicate product, if available, could sell for an even higher price. A market study to turn this alien pest into a profitable seafood product is proposed.