Ciona robusta is a species of marine invertebrate in the genus Ciona of the family Cionidae. The holotype was collected on the northeastern coast of Honshu Island, Japan. Populations of Ciona intestinalis known as Ciona intestinalis type A found in the Mediterranean Sea, the Pacific Ocean, east coast of North America, and the Atlantic coasts of South Africa have been shown to be Ciona robusta. Ciona robusta is a delineation of C. Intestinalis. In the early 2000s, molecular research concluded that C. intestinalis was composed of four distinct lineages named type A, B, C and D. Conia intestinalis type A was later reclassified as C. robusta. It was not until 2015 that the four different classifications of C. intestinalis were accepted into the scientific community, and thus reported on. A vigorous and highly invasive species, the Ciona robusta is a solitary, marine invertebrate attached at its base to a substrate, with a water-filled, sac-like body structure. It is a recently identified tunicate appearing to be native to the Northwest Pacific, though more research needs to be done on its true native range. Ciona robusta has a translucent tunic and white or off-white body, with orange to red dots on the scalloped edges of the siphons. The majority of the tunic is soft, flexible, and gelatinous, with the exception being the posterior end, which is tough and mostly white or yellowish white. Muscle bands and organs can usually be seen beneath the tunic, as well as tubercles, which are most visible near siphons and scattered throughout the surface of the tunic. The tubercles are very small, and histological sectioning or 3-D imaging may be required to see all of them. The body of Ciona robusta is elongated, cylindrical, and vase-shaped, with five to seven longitudinal bands on each side, extending nearly the entire length of the body. The body is easily torn, though other species such as Ciona savignyi are more fragile. The siphons of Ciona robusta are short and directed forward, with the oral siphon, containing eight lobes, larger than the atrial siphon, containing six lobes. Both siphons have a pale white or yellowish tinge visible, if any color at all. Ciona robusta are marine filter feeders (suspension feeders), and to feed, they draw water in through the oral siphon, which contains gills that filter phytoplankton, bacteria, and detritus, which are their main food sources. Water is then drawn into the stomach and intestines using the mucus strings, then finally expelled using the atrial siphon. Size wise, they can reach 210 mm, though they usually do not grow past 100– 120 mm. They tend to be found in both protected harbors and natural substrates, with areas such as docks, boat hulls, buoys, ropes, pilings, rocks, shells, aquaculture gear, and boulders being common habitats for them. They are subtidal, though they sometimes can be found in low intertidal, and prefer salinity of 11 to 50 ppt, though they are highly tolerant and can reproduce up to 40 ppt. They survive temperatures ranging from ten to thirty degree Celsius, and can tolerate temperatures as low as -1 C for months at a time. This tolerance for varied conditions leads them to be invasive, and they are widespread, having invaded the West coast of North America, Hawaii, Australia, New Zealand, South America (on both coasts), South Africa and Europe. Being a fouling organism, they can have negative economic impact on aquaculture operations through fouling equipment and reducing the growth of culture shellfish. Their consumers include fish and crabs, however this predation seems to happen more on rocky habitats, which leads to them becoming rarer or absent in rocky habitats, instead populating in areas that they have fewer predators. Ciona robusta are similar to several other tunicate species, such as C. intestinalis, C. savignyi, and Ciona Species C and D. They used to be considered C. intestinalis, but morphological, genomic, and ecological data has indicated that C. intestinalis and C. robusta are separate species, with genetic analysis indicating that they have been largely isolated for three to five million years. Ciona intestinalis has different genetics, pigmentation, tubercles presence on tunic, and larval morphology than Ciona robusta. Ciona savignyi is similar in appearance to Ciona robusta, but it always has white pigmented flecks or spots in the body walls, while Ciona robusta never has these. Ciona savignyi also has a more fragile tunic with much brighter yellow markings on the siphon edges rather than orange and a white not red dot on the vas deferens, and tends to have fewer tentacles around the oral siphon than Ciona robusta, though this tends to be variable. In Ciona robusta, the pharyngeal-epicardial opening pair is usually very small and located near its base, while in Ciona savignyi these openings are located close to the esophageal opening. The final tunicate species commonly misidentified with C. robusta are Ciona Species C and D, which are undescribed species identified by molecular means from the Mediterranean and Black Sea respectively.

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