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Key to BC Tunicates Background, glossary, species list

BACKGROUND

Tunicates, also known as ascidians, belong to the phylum Chordata and subphylum Urochordata. In the larval stage urochordates possess three distinctive chordate traits: a notochord, a dorsal tubular nerve cord, and pharyngeal clefts or pouches. These chordate traits are not all retained in the adult stage. However, by possessing these chordate traits at one stage in their development, tunicates are the invertebrates most closely related to humans!

Adult tunicates are primarily sessile, benthic, filter-feeding organisms. They can be found at all ocean depths worldwide attached to any substrate. Tunicates prefer hard substrates including natural surfaces, such as rocky outcrops and invertebrate shells, but may also be found on artificial structures such as pilings, docks, and boat hulls in marinas.

Tunicates are encased in a protective covering, called the tunic, which has two apertures that are usually borne on tubular extension of the tunic called siphons (Figure 1). Water, carrying food and oxygen, is drawn into the body through the oral/incurrent siphon into a large feeding structure called the pharynx. Similar to a sieve, the pharynx is perforated, with openings called stigmata (=pharyngeal gill slits). As the cilia in the stigmata draw water through the pharynx, food particles are trapped in a mucous sheet inside the pharynx. The mucous-food bundles pass from the pharynx and enter the digestive tract (esophagus, stomach, and intestine). Waste products and gametes are transported by the water as it exits the tunicate body through the atrial/excurrent siphon. Adult tunicates have one of three body forms upon which this basic body plan is slightly modified. Tunicates are categorised as either solitary, social, or colonial tunicates.

Solitary tunicate species are usually larger in size (up to 60 cm long). Though solitary tunicates may be gregarious in nature, individuals are housed separately in their own tunics. In social tunicates individuals are joined together by creeping stolons or by sheets of tunic. Colonial tunicates have multiple individuals embedded in a common tunic. The individuals, called zooids, are very small (a few mm) but the colonies can be up to several meters large. The zooid body maybe organised into one, two (thorax and abdomen), or three (thorax, abdomen, and postabdomen) regions. In some colonial species the zooids are arranged into systems where they share common cloacal chambers and atrial aperture openings.

Tunicates are primarily hermaphroditic species; meaning that they have both female and male reproductive organs. Though there is a great deal of variation in reproductive strategies, most solitary tunicates release their gametes into the surrounding water, resulting in external fertilization and embryonic development. In many colonial and some solitary tunicates, fertilization and development occur in the cloacal chambers (=brooding). The swimming tadpole larvae of all tunicates are short-lived, and within a few days settle and metamorphose into the adult tunicate form, often not far from the parents. Colonial and social species can also reproduce asexually by budding.

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Figure 1. Schematic of the generalised anatomy of a solitary tunicate.

NOTES ABOUT THE KEY

This key was designed to be easy-to-use and accessible to a diverse audience. As such, I have focused on describing gross morphology and external features as much as possible. Biologists searching for more detailed information on internal anatomy should consult the references listed at the bottom of each species description page.

In some cases it is necessary to observe internal anatomy to make an identification. Following proper narcotisation and preservation some organisms will need to be dissected. For

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Royal BC Museum Heidi Gartner solitary and social tunicates, begin by cutting from the oral siphon towards the base until the tunicate opens like a book. Cut through the tunic, the body wall, and the pharynx, but carefully avoid the digestive and reproductive tracts. Remove the pharynx from the tunic (i.e., sever the pharynx from the digestive tract). Zooids and internal anatomy of colonial species can be observed by dislodging zooids from the tunic, either by prodding the colony surface or by cutting the colony parallel to the long axis of the zooids.

GLOSSARY OF TERMS

Abdomen – The body region containing the loop of the gut (esophagus, stomach, intestine) posterior to the pharynx; in colonial tunicates with two or three body regions.

Atrial aperture – Also known as the exhalent aperture. The opening in the tunic through which waste water exits the tunicate body.

Atrial siphon – Also known as the exhalent siphon. The tubular extension of tunic through which wastewater exits the tunicate body.

Branchial folds – Folds that develop in the pharyngeal wall.

Colonial tunicate – A colony composed of multiple individuals, called zooids, embedded within a common tunic.

Dorsal lamina – A longitudinal curved ridge on the dorsal surface of the pharynx that rolls food- laden mucous sheets into cords that are then passed into the digestive tract.

Endostyle – A grooved band of ciliated and glandular tissue that produces mucous for capturing food particles in the pharynx. The endostyle runs longitudinally along the ventral axis of the pharynx.

Gregarious – Tunicates that grow in close association or proximity to each other but that are not connected by tunic.

Kidney – A bean- or sausage-shaped organ found in the Mogulidae family that functions in osmoregulation.

Oral aperture – Also known as the inhalant aperture. The opening in the tunic through which water enters the body of the tunicate.

Oral siphon – Also known as the inhalant siphon. The tubular extension of the tunic through which water enters the body of the tunicate.

Pharynx – The large internal organ that is used for filtering food particles from the passing water. Often described simply as the perforated internal filtering sac or the branchial basket.

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Postabdomen – The region of the body that contains the gonads and the heart; in colonial tunicates with three body regions.

Social tunicates – Individuals that appear as discrete individuals but that are joined basally by stolons or sheets of tunic.

Solitary tunicate – An individual that may be in close association or proximity to others but that is always separated within its own tunic.

Stellate – Shaped like a star; having rays arranged around a common center.

Stigmata – The perforations of the pharynx through which the water passes.

Stolon – A ‘runner’, or tubular structure, that creeps over the substratum and connects individual vascularly. New individuals arise by budding from the stolon.

Thorax – The region of the body that contains the oral aperture, pharynx, and atrial aperture in colonial tunicates with two or three body regions.

Tunic – The protective outer covering of solitary and social tunicates. Also the matrix in which zooids are embedded in colonial tunicates.

Zooid – In colonial tunicates, the individuals that are embedded within the common tunic.

SPECIES LIST

To date, 66 species of tunicates have been recorded from British Columbia. There are 34 solitary, 5 social, and 27 colonial species. Some of these species (highlighted in bold below) are not included in the online key because they are relatively rare in British Columbia. However, if you find a tunicate that does not fit with the description of any of these species, do not despair; new species are always being discovered and described, species ranges are shifting in response to climate change, and species are being transported and introduced to new regions by human activities. Keep exploring and researching.

SOLITARY

FAMILY ASCIDIIDAE Ascidia callosa Stimpson, 1852 Ascidia columbiana Huntsman, 1912 Ascidia ceratodes (Huntsman, 1912) Ascidia paratropa Huntsman, 1912 Ascidia prunum Müller, 1776 – An Arctic and northern Pacific species that has been found in northern British Columbia.

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FAMILY CIONIDAE Ciona savignyi Herdman, 1882 Ciona intestinalis (Linnaeus, 1767)

FAMILY CORELLIDAE Chelyosoma columbianum Huntsman, 1912 Chelyosoma productum Stimpson, 1864 Corella inflata Huntsman, 1912 Corella willmeriana Herdman, 1898

FAMILY MOLGULIDAE Molgula cooperi (Huntsman, 1912) – A species that is likely conspecific with Molgula regularis, a Californian species. M. cooperi was reported once in British Columbia waters and was separated from M. regularis based on its oviviparous character. Molgula manhattensis (DeKay, 1843) Molgula pacifica Huntsman, 1912 Molgula pugetiensis Herdman, 1898

FAMILY PYURIDAE Bathypera feminalba Young and Vazquez, 1995 Boltenia echinata (Linnaeus, 1767) Boltenia villosa (Stimpson, 1864) Pyura haustor (Stimpson, 1864) Pyura mirabilis (Von Drasche, 1884)

FAMILY STYELIDAE Cnemidocarpa finmarkiensis (Kiaer, 1893) Dendrodoa abbotti Newberry, 1984 Dendrodoa grossularia (van Beneden, 1846) – A Northeast Atlantic species with a single report in British Columbia. Halocynthia aurantium (Pallas, 1787) Halocynthia igaboja Oka, 1906 Pelonaia corrugata Goodsir and Forbes, 1841 – A Japanese species with only one report in British Columbia. Styela clava Herdman, 1881 Styela coriacea (Adler and Hancock, 1848) Styela clavata (Pallas, 1774) – A Bering Sea species with limited reports in British Columbia. Styela gibbsii (Stimpson, 1864) Styela milleri Ritter, 1907 – A deep sea species with only one report in British Columbia. Styela montereyensis (Dall, 1872) Styela truncata Ritter, 1901 – A species that is listed as extending from southern Alaska to California (Abbott and Newberry), but with no confirmed reports of its presence in British Columbia. Styela yakutatensis Ritter, 1901 – An Alaskan species with only two reports in British Columbia.

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SOCIAL

FAMILY CLAVELINIDAE Clavelina huntsmani Van Name, 1931 Pycnoclavella stanleyi Berrill and Abbott, 1949

FAMILY PEROPHORIDAE Perophora annectens Ritter, 1893

FAMILY STYELIDAE Metandrocarpa dura (Ritter, 1896) Metandrocarpa taylori Huntsman, 1912

COLONIAL

FAMILY AGNEZIDAE Agnezia septentrionalis (Hunstman, 1912) – This is a deeper (30-86 m) subtidal species that has not been reported in the marine waters of British Columbia since the first half of the twentieth century.

FAMILY CLAVELINIDAE Cystodytes lobatus (Ritter, 1900) Distaplia occidentalis Bancroft, 1899 Distaplia smithi Abbot and Trason, 1968 Eudistoma molle (Ritter, 1900) Eudistoma psammion Ritter and Forsyth, 1917 Eudistoma purpuropunctatum Lambert, 1989 Eudistoma ritteri (Van Name, 1945) – A Californian species with two reports in British Columbia.

FAMILY DIDEMNIDAE Didemnum albidum (Verrill, 1871) Didemnum carnulentum Ritter and Forsyth, 1917 Didemnum vexillum Kott, 2002 Diplosoma listerianum (Milne-Edwards, 1841) Trididemnum alexi Lambert, 2003 Trididemnum opacum (Ritter, 1907) Trididemnum strangulatum (Ritter, 1901) – An Alaskan species that is likely in the marine waters of BC (but no published reports).

FAMILY POLYCLINIDAE Aplidiopsis pannosum (Ritter, 1899)(=Polyclinum pannosum Ritter, 1899) – Though there are no published reports of its presence in British Columbia, this Alaskan species

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has recently been found in northern Washington, and is easily confused with Distaplia occidentalis. Aplidium arenatum (Van Name, 1945) Aplidium californicum (Ritter and Forsyth, 1917) Aplidium propinquum (Van Name, 1945) Aplidium solidum (Ritter and Forsyth, 1917) Ritterella aequalisiphonis (Ritter and Forsyth, 1919) Ritterella pulchra (Ritter, 1901) Ritterella rubra Abbott and Trason, 1968 Synoicum parfustis (Ritter and Forsyth, 1917) Synoicum spp. – This genus includes a number of morphologically similar species, and is currently under taxonomic revision. Several northern species of Synoicum may inhabit the marine waters of British Columbia.

FAMILY STYELIDAE Botrylloides violaceus Oka, 1927 Botryllus schlosseri (Pallas, 1766)

REFERENCES FOR BACKGROUND AND GLOSSARY

Brusca, Richard C. and Gary J. Brusca. 2003. Invertebrates (2nd edition). Sunderland: Sinauer Associates, Inc. Carlton, James T., editor. 2007. The Light and Smith Manual: Intertidal Invertebrates from Central California to Oregon (4th edition). Berkeley: University of California Press. Morris, Robert H., Donald P. Abbott, and Eugene C. Haderlie. 1980. Intertidal Invertebrates of California. Stanford: Stanford University Press.

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