Types of tunicates
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Invasive species

  The topic of types of tunicates includes invasive species, considered here, and SOLITARY and COLONIAL forms considered in another section.
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Research study 1

From the beginning of the 1900’s there have been successful invasions onto the Californian coast by at least 14 ascidian species (10 solitary, 4 colonial), 11 of which occurred between 1917 and early 1960s.  The means of these colonisations are thought to be by transport of larvae and adults in ballast water of ships, or by transport of adults on boat hulls.  Since 2 of these species also occur in British Columbia, namely, Ciona intestinalis and Botryllus schlosseri, it is likely that they also arrived in similar ways and possibly around the same time.  Lambert & Lambert 1998 Mar Biol 130: 675. Photograph courtesy Angus Jackson and the Marine Biological Assoc UK; for additional surveys and data see Lambert & Lambert 2003 Mar Ecol Progr Ser photograph of tunicate Ciona intestinalis courtesy Angus Jackson, MarLIN, and the Marine Biological Assoc UK, Plymouth259: 145, Lambert 2007 Cah Biol Mar 48: 95, and Lambert et al 2010 Aquat Invas 5: 369.

NOTE  the dates span a time from a survey of California tunicates in 1917 (Ritter & Forsyth 1917 Univ Calif Publ Zool 16: 439) to collections in the early 1960s done by the authors of this Research Study

NOTE  based on extent of genetic divergence exhibited in California collections of Ciona intestinalis from Atlantic/European counterparts, the authors advocate that the species should be split into two.  Morphological and reproductive evidence also support such a split.  Nydam & Harrison 2007 Mar Biol 151: 1839.

Ciona intestinalis 1X

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Many marine species are thought to invade new habitats by trans-oceanic transport on or in ships, but what explains, in the case of these 14 species of ascidians, their colonisation success after settlement? Think about the answers given, then CLICK HERE for explanations.

Asexual budding allows quick colonisation of open space.

Ascidians compete strongly for space.

Brooding leads to greater settlement success of the larvae.

Ascidians are chemically distasteful to predators.

Ascidians are hermaphroditic and thus can reproduce sexually without a partner. 

NOTE  in brooding species the adult does not release eggs to the outside.  Instead, they are held in a special region of the body, either an expanded tip of the oviduct or in a part of the exhalent siphon (see section on tunicate REPRODUCTION), where they are fertilised and go through development to the larval form in relative safety.

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Research study 2

As noted above, the colonial tunicate Botryllus schlosseri is thought to have been introduced from the Atlantic coast to the Pacific coast sometime during the last Century, likely between 1917 and the early 1960s.  The question arises as to whether the 2 forms are, indeed, the same species.  This is tested in a study in Pacific Grove, California in which colony morphologies are compared and reproductive crosses attempted.  Results show that colony morphologies are similar and interpopulation crosses from the 2 coasts yield fertile F1 progeny.  The authors conclude that populations from Woods Hole, Massachusetts and Monterey Bay, California are the same species.  They do raise the question, however, as to whether the species name should remain schlosseriBoyd et al. 1990 Biol Bull 178: 239. 

NOTE   in fact, a recent genetics analysis by a research consortium centred at Hopkins Marine Station, California shows that the east coast of North America is not the source of California populations of B. schlosseri.  Rather, either Europe or Asia (specifically, Japan) is the more likely origin.  Stoner et al. 2002 Mar Ecol Progr Ser 243: 93

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Research study 3

photograph of skin spicules of colonial tunicate Didemnum vexillumThe most notorious ascidian invader is arguably the ubiquitous Didemnum vexillum, a species that adopts different identities depending upon its location in the world. A researcher at the Friday Harbor Laboratories has collected and examined specimens from hundreds of sites worldwide and, on the basis mainly of morphology of skin spicules (see photo on Left) has confirmed that most are D. vexillum.  The author provides evidence that D. vexillum likely originated in Japan, and spread from there via shipping, movements of fouled recreational boats, drifting, and so on. Lambert 2009 Aquatic Invasions 4: 5; for a map showing distribution of this species along the west coast see Bullard et al. 2007 J Exp Mar Biol Ecopl 342: 99; see also Switzer et al. 2011 Aquaculture 319: 145 for methods of controlling fouling by D. vexillum and Botryllus spp. in oyster culture.

photograph of colonial tunicate Didemnum, possibly vexillum with an anemone Urticina sp.NOTE  different species names used include carnulentum on the west coast of N.A., lutarium and vestum in New England, lahillei and helgolandicum in Europe, vexillum in New Zealand, and pardum and moseleyi in Japan


Colonial tunicate Didemnum, possibly
, competing for space with
a sea anemone Uricina sp.

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Research study 4

photographs showing predation by green sea urchins Strongylocentrotus droebachiensis on the ascidians Botrylloides violaceus and Styela clava courtesy Epelbaum et al. 2009 Mar Biol 156: 1311What is the susceptibility of invading ascidian species to invertebrate predators?  This is assessed by researchers at the Pacific Biological Station, Nanaimo, British Columbia using 4 non-indigenous species of ascidians and 12 species of potential predators, including various sea urchins, sea stars, crabs, amphipods, nudibranchs, and snails.  Results from single- and multiple-choice tests in the laboratory show that the majority of these potential predators do, indeed, consume one or more of the invasive ascidian species, but only where there is just a single choice (see photos showing grazing patterns of green sea-urchins Strongylocentrotus droebachiensis).  In multiple-choice tests all predators choose their normal preferred foods over ascidians.  The authors conclude that predation is unlikely to curtail large-scale recruitment and spread of alien ascidian species in British Columbia.  However, in that green sea-urchins S. droebachiensis are the most effective grazers of these ascidian species, the authors suggest that they may be useful in reducing ascidian fouling in shellfish aquaculture.  Epelbaum et al. 2009 Mar Biol 156: 1311.

NOTE  these are Styela clava, Botryllus schlosseri, Botrylloides violaceus, and Dedemnum vexillum

NOTE  the only shelled gastropod used in the study, Fusitriton oregonensis, is known to prey upon the solitary tunicate Styela gibbsii, a close relative of S. clava

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Research study 5

map showing present and possible future distribution of colonial tunicate Didemnum vexillumIs it possible, knowing something about the habitat requirements of a certain invasive species, to predict1 its future movements and forecast its potential distribution?  This is attempted by researchers at the Pacific Biological Station, Nanaimo for the colonial form Didemnum vexillum.  The species is an aggressive global invader and has spread to both coasts of North America.  Although present for some time along the entire coastline of the western U.S., it is known in southern British Columbia only from 2007.  The species has a limited natural dispersion, owing to a short (approx. 1d) planktonic larval stage, and human-mediated transport is thought to be the most important vector for long-distance movements.  To identify new potentially suitable habitats along the coastline southern B.C. the authors combine in a model 5 possible means of transport2 with 9-10 environmental variables3 known from already established colonies of D. vexillum on the coasts of the 3 western U.S. states. As a test of the predictive ability of their model, the researchers compare predictive results with actual distributions of D. vexillum and find good correspondence.  The overall results, shown in the accompanying map4, identify potential hotspots for future establishment of the species along parts of the central coast, west photographs of colonial tunicate Didemnum vexillumcoast of Vancouver Island, Strait of Georgia (including, as a “bonus”, parts of Puget Sound), and to some extent along the east
coast of Haida Gwai.  Herborg et al. 2009 J Appl Ecol 46: 64. Photographs
courtesy Linda Shaw and National Marine Fisheries Service, Juneau, Alaska.

NOTE1  this is known as “ecological niche-modelling”, where suitable environments are identified based on occurrence data and a knowledge of relevant environmental variables.  Over the past decade or so the method has been used to forecast potential distributions of invasive species such as beetles, mussels, and fishes

NOTE2  these include hull fouling of slow-moving, large commercial, and small vessels, commercial fishing activities, and transfer during aquaculture activities

NOTE3  the data are obtained from 46 locations along the 3 western states.  The 9 variables include temperature, salinity, dissolved oxygen, and chlorophyll (several other factors are not identified by the authors) in surface waters along the B.C. coast

NOTE4  it is unfortunate that environmental conditions in southeast Alaska are not included in the model, as D. vexillum has recently been discovered at Whiting Harbor near Sitka.  Identification is confirmed through DNA-sequence comparisons.  This represents a jump of about 1000km in the species distribution.  Cohen et al. 2011 Aquat Invas 6 (3): 263.

Colonial tunicate
Didemnum vexillum

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Research study 6

map showing areas of coastal British Columbia that should favour growth and survival of tunicate species Botryllus schlosseriA companion article done by some of the same Canadian government researchers cited in Research Study 5 above applies similar ecological niche-modelling techniques to distributions of 2 non-indigenous ascidian species in British Columbia Botryllus schlosseri and Botrylloides violaceus.  The authors culture larvae and resulting colonies (1-3 zooids) of each species in the laboratory in a 2-factorial design with different levels of temperature and salinity treatments, and determine growth, survival, and reproduction.  The idea is to develop a model that will predict the most suitable locations along the B.C. coast for colonisation of each species.  Results show that, based upon these 2 environmental factors, both species have broad tolerances and no areas are totally unsuitable for growth and survival (see maps showing colour codings for varying expectations of colonisation success).  The findings may be useful to the shellfish-aquaculture industry, for these colonial ascidians are a nuisance because of their overgrowth propensities.  The authors discuss their results in the context of climate change.  Epelbaum et al. 2009 J Exp Mar Biol Ecol 369: 43.

NOTE  the first, B. schlosseri is native to Europe, while the second, B. violaceus is native to Asia

NOTE  five levels of temperature from 5-25oC and five levels of salinity from 14-38‰ are used, over a growing period of 8wk

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Research study 7

histogram comparing temperature lethalities in native and introduced speciesMarine fouling communities in Bodega Bay, California comprise several common species of tunicates, bryozoans, and hydroids, and other less common species.  Interestingly, of 11 species studied by researchers that occur on a floating dock near the Bodaga Marine Laboratory, only 4 are native to Bodega Harbor; the other 7 are introduced (invasive).  In 2 sets of laboratory experiments the researchers investigate the potential for climate change, specifically, ocean warming, to facilitate new invasions of fouling species and to change community composition.  First, they find that invasive species tolerate significantly higher water temperatures than do native species (see histogram; “LT50” represents the temperature at which 50% of individuals die after 24h exposure). 

Second, in mesocosm experiments they test effects of 3 and 4.5oC warming scenarios on survival and growth, and find, as expected, variable species-specific responses but with generally greater survival and photograph of fouling community in Bodega Harbor, California courtesy C.J.B. Sortegrowth in non-native species than in native ones (data not shown here).  Overall, the authors conclude that as oceans warm, invasive species will likely increase in abundance while native species will decrease, with resultant shifts in community composition.  Sorte et al. 2010 Ecology 91: 2198; see also Sorte et al. 2010 Oikos 119: 1909 for similar studies in the Bodega Marine Laboratory using mesocosms and simulated heat waves. Photograph courtesy C.J.B. Sorte, Bodega Marine Laboratory, UC Davis, California.

NOTE  6 of the 11 species are tunicates and, of these, only Distaplia occidentalis and Ascidia ceratodes are native.  The 4 introduced species are Botrylloides violaceus, Botryllus sp., Didemnum sp., and Diplosoma listerianum.  The 11 species selected for study account for 80% of the occupied space

NOTE  the “mesocosms” are 2.5-L plastic aquaria containing test specimens settled in the field onto 10 x 10cm PVC plastic plates.  For survival and community-structure experiments, all specimens settled are left intact.  For growth experiments, all individuals but those of a single selected species are weeded out.  The survival/community structure experiments last for 5wk; the growth experiment, for 6d

Fouling community after 6wk
development in Bodega Harbor

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Research study 8

Is the success of recruiting larvae of invasive compound species like Botrylloides violaceus more, or less, affected by large grazers/predators like chitons, snails, sea urchins, flatworms, fishes, and so on, than those of native species?  A researcher from the University of Chicago employs exclusion cages in fouling communities on docks at 4 sites in northern Washington and shows that absence of these large grazers, as predicted, does not affect recruitment or abundance of B. violaceus, but does lead to increases in these parameters of 2 native fouling species Corella inflata and Obelia sp. (a hydroid).  However, despite these and other supportive results, lack of  an overall correlation of grazer/predator abundance with exotic-species abundance across 4 study sites, leads the author actually to conclude that release from grazing/predation does photograph of colonial tunicate Botrylloides violaceus courtesy Judith Oakley, UKnot fully explain the success of the exotic species in these particular fouling communities.  However, notwithstanding this conclusion, one wonders if the miscellaneous group of grazers/predators selected was really a “true” selection of predators of recruiting and/or juvenile colonial tunicates. Perhaps some “inclusion”-type studies where known, rather than supposed, native predators are selectively caged, would be a useful follow-up study.  Grey 2010 Mar Ecol Progr Ser 411: 89. Photograph courtesy MARLIN and Judith Oakley, UK.

NOTE  the idea that exotic species are less regulated by resident predators than native species is hardly new.  After all, this is usually the most obvious explanation of the success of an invading species.  The author is attempting to test the “enemy-release hypothesis”, a rather fancifully named idea that originated with Darwin’s 1859 “origin of species” and later applied in tests of herbivore- and pathogen-effects on terrestrial plants (with mixed results)


Colonial tunicate Botrylloides violaceus 1.5X

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Research study 9

photograph of clubbed tunicate Styela clava courtesy Janna Nichols & Washington Department of Fish & Wildlifemap of world haplotypes of clubbed tunicates Styela clava courtesy Janna Nichols & Washington Department of Fish & Wildlife

The solitary ascidian Styela clava originates in eastern Asia but has spread widely over the past 8 decades to America, Europe, Australia, and New Zealand.  It is a dominant invader of fouling communities, especially within ports, marinas, and it occurs on many types of submerged structures. It also grows on intertidal rocky shores and on subtidal mudflats. The first west-coast records date from the 1920s.  Through investigative effors of a world-wide consortium of researchers, the global phylogeography of the species has been determined.  Results show haplotype diversities ranging from 1-13 in each population, with greatest diversity in New Zealand and Japan,  and least diversity on the west coast of North America (see map).  Two of the New Zealand populations and both Japanese populations are siginificantly different from most other world populations.  In general, northern-hemisphere populations are less diverse than southern-hemisphere ones.  Shared haplotypes between one of the Japanese populations and most of the northern-hemisphere populations suggests that this may have been the source population for the incursions.  In general, high diversity in some populations, notably in New Zealand, suggests multiple founders from different sources, most likely via shipping.  On the west coast, the marked difference between the Los Angeles site (8 haplotypes) and other nearby west-coast sites such as Santa Barbara and Mission Bay (1-3 haplotypes) likely reflects site-specific selective processes.  The authors note that greater genetic diversity in an area is likely associated with several or many incursions of S. clava, while reduced genetic diversity is likely associated with rapid expansion from small initial population bases.  Goldstein et al. 2011 PLoS ONE 6 (2): e16755.  Photograph courtesy Janna Nichols & Washington Department of Fish & Wildlife.

NOTE  sequencing of a fragment of the cytochrome oxidase subunit I mitochondrial gene (COI) from 20-30 individuals sampled at 20 world locations

NOTE  map codings for the 5 west-coast populations are PS=Puget Sound, SF=San Francisco, SB=Santa Barbara, LA=Los Angeles, and MB=Mission Bay

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Research study 10

Is it possible to determine genetic attributes that contribute to invasive success of a tunicate species?  This is attempted for a colonial species Botryllus schlosseri by a research group from the National Institute of Oceanography in Haifa, Israel who collects specimens from a Santa Cruz, California boat harbor and measure genetic drift and mutation rate over a 13yr period.  During this time the researchers estimate that the population would have undergone more than 150 generations.  Results from microsatellite typings at 5 alleles each January over 9 selected years (out of the 13yr total) reveal limited gene flow and relative genetic isolation.  However, mutation rate is found to be high and the authors conclude that potential adaptability to new habitats will be correspondingly high.  Thus, even though a single invasive event by a small, intitial population risks serious loss of genetic variability, a high inherent mutation rate leading to high genetic diversity, combined with relatively high fecundity, will contribute to invasive success.  The study is valuable in that it evaluates genetic change in a single population across a prolonged period of time.  Reem et al. 2013 Biol Invas 15: 225.

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Research study 10.1

Just how concerned should we be about negative ecological effects of invasive tunicates on the west coast of North America? This large question is addressed by researchers at the University of Washington using several sites in Puget Sound, Washington. Target species at the sites include epibenthic and infaunal copepods, polychaetes, and amphipods that represent food of juvenile salmonids, and cultured mussels Mytilus sp. and oysters Crassostrea gigas that represent commercially important species. The research methodology for the first part consists mainly in comparing abundances of target species in areas with and without tunicates and, for the second part, in comparing growth and condition indices in shellfish with and without fouling growths of tunicates. Results overall for this ambitious project are generally inconsistent and statistically insignificant, not unlike those of several other past similar ecological studies. Cordell et al. 2013 Biol Invas 15: 1303.

NOTE of particular interest in the study are the colonial species Didemnum vexillum and Botrylloides violaceus, and the solitary species Ciona savignyi and Styela clava

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Research study 11

map showing sampling sites for tunicate community study in Mission Bay, Californiaschematic showing levels of statistical similarity of 4 tunicate communities in Mission Bay, California in relation to environmental conditions over a 14mo period








One might guess from their very presence that invasive species of tunicates are more tolerant of differences in environmental conditions than native ones.  This is one topic examined by researchers from University of San Diego, California at 4 locations in Mission Bay characterised by seasonal and spatial differences in temperature, salinity, dissolved oxygen, currents, and seawater clarity.  The researchers deploy replicate PVC settling plates at each location and monitor them weekly over a 14mo study period commencing in autumn.  A total of 15 ascidian species settle on the plates, 11 of them invasive.  Results show that patterns in species composition, density, and growth change in time correlative with differences in temperature, salinity, and water visibility at the 4 locations, but the only notable difference between invasive and native species appears to be a greater abundance of the former during winter.  Location differences are significant, with the “back-bay” Hilton site being most different from the others (see figure above Right) and photographs below.  Note in the figure that 4 temporal episodes of rain at the “back-bay” Hilton site produce statistically significant differences in community structure. In one instance for the Hilton site (datum point on the far Right) and presumably based on effects of one of these rain-storms, community structure is completely different from the others. The authors comment on successional patterns, the main one being the early colonisation of plates by  a handful of colonial species, with solitary species arriving only later (some, however, being competitively dominant for space).  The study is a fine one, and the authors are to be congratulated on their use of such interesting statistical treatments.  Tracy & Reynes 2014 Aquat Invas 9 (4): 441.

NOTE  the Bay is characterised by being more oceanic at its mouth than in the back.  In fact, during summer the surface waters in the back-bay regions may become warm and hypersaline, while in winter they become cooler, dilute, and more turbid owing to inputs from creeks that bear variable nutrient and sediment loads


These photographs show representative views of settling plates at each site at the end of the study period in October, 2012: photograph of tunicate community at the Seaforth Landing study site in Mission Bay, California at the end of a 14mo investigation on the effects of environmental factors on community dynamics photograph of tunicate community at the South Shores study site in Mission Bay, California at the end of a 14mo investigation on the effects of environmental factors on community dynamics photograph of tunicate community at the Yacht Club study site in Mission Bay, California at the end of a 14mo investigation on the effects of environmental factors on community dynamics photograph of tunicate community at the Hilton study site in Mission Bay, California at the end of a 14mo investigation on the effects of environmental factors on community dynamics
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Research study 12

graph showing percentage cover of 2 species of tunicates Ascidia ceratodes and Ciona intestinalis growing in competition with one anotherThus far in this account the advantage seems consistently on the side of the invader, but what do the other members of the community have to say about it?  This question of biotic resistance, that is, the ability of a community to minimise the success of new colonisers,  is examined for solitary tunicates, the invasive Ciona intestinalis1 and the native Ascidia ceratodes2 by a consortium of European, American, and Australian researchers at Bodega Marine Laboratory, California.  Instead of limiting their approach to a single life-history stage (usually adult), as is so often done in such studies, the researchers examine community responses to the presence of egg, larvae, and post-settlement juvenile and adult stages of Ciona, focusing on processes of competition and predation.  The method involves settling each species separately onto plastic discs, then growing each in the field under the protection of fine-mesh cages that screen out predators but not other sessile community members (some predation experiments use settling plates with half the plate protected, the other half left open).  Sizes of each tunicate species are recorded at intervals over a 14wk period.  Another separate and unmanipulated 35wk experiment3 starting in August follows successional dynamics of the community.  Results over a several-week period are sometimes inconsistent with the notion of what it takes to be a successful invader.  For example, although the invasive Ciona would be predicted to grow faster than Ascidia, this happens only in mid-successional stages.  In other respects, such as resistance to predation and competitive ability, Ciona actually seems more vulnerable and weaker, respectively, than the native Ascidia (see graph on Right).  Note in the graph that Ascidia is actually competitively dominant over Ciona in one-on-one confrontation.  As for biotic resistance to Ciona’s early life stages, these appear to be absent or with only modest effect.  The study is a good one, not necessarily for its results, but for its holistic approach, and we hope to see more of this type of study in the future.  Rius et al. 2014 J Anim Ecol 83: 296.

NOTE1  this  is a well-known invasive species that lives in so many different areas of the world that its place of origin is unknown, otherwise termed cryptogenic (“hidden origin” G.)

NOTE2  in the Bodega Bay area of the study, this native species is often the dominant late-successional occupier of space.  It is a larger species than C. intestinalis, but has similar resource needs

NOTE3  other laboratory experiments determine potential sperm interference between the 2 species; results not considered here

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Research study 13

photograph of conpound tunicate Didemnum vexillosum lethally covering an eelgrass blade Few marine organisms seem immune to being overgrown by the invasive colonial tunicate Didemnum vexillum. A recent addition to a long list is eelgrass Zostera marina. Field observations and outdoor mesocosm experiments in estuaries in central California and at the Bodega Marine Laboratory show that overgrowth cover of shoots as little as 20% is enough to reduce their growth significantly. Long & Grosholz 2015 J Exp Mar Biol Ecol 473: 188. Photograph courtesy Dann Blackman (USGS)with thanks also to Mary Carman and OCEANUS Magazine.



Colonial tunicate Didemnum vexillosum
lethally overgrows an eelgrass blade Zostera
sp. in Martha's Vineyard, Massachusetts

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