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NOBANIS - European Network on Invasive Species
Introduction to Ascidiacea - Sea squirts
Ascidiacea is a class of the
subphylum Tunicata, formerly known as Urochordata, and phylum Chordata. This
means that sea squirts are more closely related to Vertebrates (including
Homo sapiens) than to any invertebrate phylum! The chordate relationship is
seen in the presence of a notochord in larval ascidians and in the formation of
the nervous system (Millar, 1970). There are two other classes in the Tunicata:
Larvacea and Thaliacea, both of which are pelagic, often colonial species.
Molecular studies indicate that these two classes cluster within the Ascidiacea,
which means that classification will need to be reorganized (Stach & Turbeville,
Ascidians are often part of the
fouling community and this means that they are also often transported and
introduced to new regions. Adult ascidians are sessile and many species are
colonial. Even the solitary species often form clusters of individuals. Fouling
communities may include several species attached to one another, which sometimes
makes identification confusing.
Generally, the ascidian body is
embedded in a more or less gelatinous tunic (hence the name Tunicata) or test,
which is composed of a cellulose-like material. There are two siphons extending
from the surface; one, the branchial (or oral) siphon, is connected to the most
prominent organ, the branchial sac (sometimes called branchial basket); the
other one, the atrial siphon, expels water from respiration/filter feeding,
faecal material from the intestine and genital products from the gonoduct. The
branchial sac is used for respiration as well as for filtration of food
particles. Thus ascidians are suspension feeders. The digestive system is
basically a U-shaped tube, but the actual shape and length differ among species,
which is sometimes used for species identification.
Drawing of schematic ascidian
to show important structures (Ciona intestinalis form Millar 1970)
Some ascidians are known to take
up and accumulate rare chemicals, like vanadium, in their bodies (Carlisle,
1968; Swinehart et al., 1974), some have highly acidic tunic fluids (Stoecker,
1980) and some species have symbiotic micro-organisms (Sybesma et al., 1981;
Saffo, 1982). Because ascidians filter food particles by a combination of cilia
and mucus, they can trap very fine particles (Lambert, 2005), and the large
surface area of the branchial sac makes them highly efficient filtrators
(Petersen & Svane, 2002).
Classification of the Ascidiacea
has been based on the structure of the branchial sac, but molecular studies have
shown that certain groups at family, genus or species level may need
reorganization (Stach & Turbeville, 2002). However, at the present time few
molecular studies are available and all existing identification keys use the
classification based on morphology. There are some discrepancies among the
experts about the rank of certain taxa, especially orders and suborders (Millar,
1970; Petersen & Svane, 2002).
Reproduction is usually by
shedding eggs and sperm directly into the water, and the fertilized eggs undergo
rapid development, usually forming a “tadpole-larva” within one day. The
swimming larva does not feed and settles within hours of hatching. Some species
are brooders, and the short-lived swimming larva escapes through the atrial
siphon. Colonial species (and some solitary ones as well) also have asexual
reproduction (Millar, 1970).
An overview of ascidian ecology
is found in Lambert (2005a).
There are a few species of
introduced ascidians in Nordic waters (see separate fact sheets). The number of
native species is too high, about 80 species total, to list here, but there are
a few introduced species in neighbouring waters that deserve mentioning because
they are likely to expand their ranges with increasing sea temperatures. It
should be noticed that the native NE Atlantic/ Mediterranean species
Botryllus schlosseri (Pallas, 1766) has been introduced to North America,
and is considered a nuissance species there (see e.g.,
Species found in neighbouring
Kott, 2002 is a colonial species, which was known as Didemnum sp. prior
to its formal description, or it was misidentified as one of the native species
(Lambert, 2009). It was found in the Netherlands in 1991 (Gittenberger, 2007),
in France in 1998, and in Ireland in 2005 (Minchin, 2007; Lambert, 2009). In
late 2008 it was found for the first time in Great Britain (Griffith et al.,
2009). The species listed as D. helgolandicum Michaelsen, 1921 in
Synopsis of British Ascidiacea (Millar, 1970) and Scandinavian Ascidiacea, with
records from Norway and the Faroe Islands (Millar, 1966), has been
misidentified, but it is not possible to identify it from Millar’s description
(Lambert, 2009). D. vexillum has also been introduced to North America
and New Zealand, and most likely is a native of Japan. It has caused
considerable trouble for aquaculture in New Zealand and North America (Bullard
et al., 2007; Denny, 2008), and is beginning to trouble mussel culture in the
Netherlands (Gittenberger, 2007). For more information see:
(excellent pictures for identification),
Arenas, F., Bishop, J.D.D., Carlton, J.T., Dyrynda, P.J.,
Farnham, W.F., Gonzalez, D.J., Jacobs, M.W., Lambert, C., Lambert, G., Nielsen,
S.E., Pederson, J.A., Porter, J.S., Ward, S. and Wood, C.A. 2006. Alien species
and other notable records from a rapid assessment survey of marinas on the south
coast of England. Journal of the Marine Biological Association of the United
Kingdom 86: 1329-1337.
Bullard, S.G., Lambert, G., Carman, M.R., Byrnes, J.,
Whitlatch, R.B., Ruiz, G., Miller, R.J., Harris, L., Valentine, P.C., Collie,
J.S., Pederson, J., McNaught, D.C., Cohen, A.N., Asch, R.G., Dijkstra, J. and
Heinonen, K. 2007. The colonial ascidian Didemnum sp. A: Current
distribution, basic biology and potential threat to marine communities of the
northeast and west coasts of North America. Journal of Experimental Marine
Biology and Ecology 342: 99-108.
Carlisle, D.B. 1968. Vanadium and other metals in
ascidians. Proceedings of the Royal Society B 171: 31-42.
Denny, C.M. 2008. Development of a method to reduce the
spread of the ascidian Didemnum vexillum with aquaculture transfers. ICES
Journal of Marine Science 65: 805-810.
Dupont, L., Viard, F., David, P. and Bishop, J.D.D.
2007. Combined effects of bottlenecks and selfing in populations of Corella
eumyota, a recently introduced sea squirt in the English Channel. Diversity
and Distributions 13: 808-817.
Gittenberger, A. 2007. Recent population expansions of
non-native ascidians in The Netherlands. Journal of Experimental Marine Biology
and Ecology 342: 122-126.
Griffith, K., Mowat, S., Holt, R.H.F., Ramsay, K.,
Bishop, J.D.D., Lambert, G. and Jenkins, S.R. First records in Great Britain of
the invasive colonial ascidian Didemnum vexillum Kott, 2002. Aquatic
Invasions 4(4): 581-590.
Lambert, G. 2004. The south temperate and Antarctic
ascidian Corella eumyota reported in two harbours in north-western
France. Journal of the Marine Biological Association of the United Kingdom 84:
Lambert, G. 2005a. Ecology and natural history of the
protochordates. Canadian Journal of Zoology 83: 34-50.
Lambert, G. 2005b. First North American record of the
ascidian Perophora japonica. Journal of the Marine Biological Association
of the United Kingdom 85: 1011-1012.
Lambert, G. 2009. Adventures of a sea squirt sleuth:
unravelling the identity of Didemnum vexillum, a global ascidian invader.
Aquatic Invasions 4(1): 5-28.
Lützen, J. 1967. Sækdyr. Danmarks Fauna 75, Dansk Naturhistorisk Forening,
Millar, R.H. 1970. A synopsis of the British ascidians.
Synopsis of the British Fauna No. 1. The Linnean Society, Academic Press,
Minchin, D. 2007. Rapid coastal survey for targeted
alien species associated with floating pontoons in Ireland. Aquatic Invasions
Nishikawa, T., Bishop, J.D.D. and Sommerfeldt, A.D.
2000. Occurrence of the alien ascidian Perophora japonica at Plymouth.
Journal of the Marine Biological Association of the United Kingdom 80: 955-956.
Petersen, J.K. and Svane, I. 2002. Filtration rate in
seven Scandinavian ascidians: implications of the morphology of the gill sac.
Marine Biology 140: 397-402.
Saffo, M.B. 1982. Distribution of the endosymbiont
Nephromyces Giard within the ascidian family Molgulidae. Biological Bulletin
Stach, T. and Turbeville, J.M. 2002. Phylogeny of
Tunicata inferred from molecular and morphological characters. Molecular
Phylogenetics and Evolution 25: 408-428.
Stoecker, D. 1980. Relationships between chemical
defense and ecology in benthic ascidians. Marine Ecology Progress Series 3:
Swinehart, J.H., Biggs, W.R., Halko, D.J. and
Schroeder, N.C. 1974. The vanadium and selected metal contents of some
ascidians. Biological Bulletin 146: 302-312.
Sybesma, J., van Duyl, F.C. and Bak, R.P.M. 1981. The
ecology of the tropical compound ascidian Trididemnum solidum. III.
Symbiotic association with unicellular algae. Marine Ecology Progress Series 6:
Varela, M.M., de Matos-Pita, S.S. and Ramos-Esplá, A.A.
2008. New report of the Antarctic ascidian Corella eumyota (Tunicata:
Ascidiacea) on the Galician coast (north-west Spain). JMBA2 – Biodiversity
Records 1: e59 (http://www.mba.ac.uk/jmba/pdf/5657.pdf).