ship hulls, and farmed shellfish cause significant economic problems, and sea squirt invasions have disrupted the ecosystem of several natural sub-tidal areas by smothering native animal species. Sea squirts are the natural prey of many animals, including nudibranchs, flatworms, molluscs, rock crabs, sea stars, fish, birds, and sea otters. They are also eaten by humans in many parts of the world, including Japan, Korea, Chile, and Europe (where they are sold under the name "sea violet"). As chemical defenses, many sea squirts intake and maintain an extremely high concentration of vanadium in the blood, have a very low pH of the tunic due to acid in easily ruptured bladder cells, and (or) produce secondary metabolites harmful to predators and invaders. Some of these metabolites are toxic to cells and are of potential use in pharmaceuticals. Evolution Fossil record Ascidians are soft-bodied animals, and for this reason, their fossil record is almost entirely lacking. The earliest reliable ascidians is Shankouclava shankouense from the Lower Cambrian Maotianshan Shale (South China). There are also two enigmatic species from the Ediacaran period with some affinity to the ascidians – Ausia from the Nama Group of Namibia and Burykhia from the Onega Peninsula, White Sea of northern Russia. They are also recorded from Lower Jurassic (Bonet and Benveniste-Velasquez, 1971; Buge and Monniot, 1972) and the Tertiary from France (Deflandre-Riguard, 1949, 1956; Durand, 1952; Deflandre and Deflandre-Rigaud, 1956; Bouche, 1962; Lezaud, 1966; Monniot and Buge, 1971; Varol and Houghton, 1996). Older (Triassic) records are ambiguous. The representatives of the genus Cystodytes (family Polycitoridae) have been described from the Pliocene of France by Monniot (1970, 1971) and Deflandre-Rigaud (1956), and from Eocene of France by Monniot and Buge (1971), and lately from the Late Eocene of S Australia by Łukowiak (2012). Phylogeny The ascidians were on morphological evidence treated as sister to the Thaliacea and Appendicularia, but molecular evidence shows that ascidians are polyphyletic within the Tunicata, as shown in the cladogram. Uses Culinary Various ascidians are used as food. Sea pineapple (Halocynthia roretzi) is cultivated in Japan (hoya, maboya) and Korea (meongge). When served raw, they have a chewy texture and peculiar flavor likened to "rubber dipped in ammonia" which has been attributed to a naturally occurring chemical known as cynthiaol. Styela clava is farmed in parts of Korea where it is known as mideoduk and is added to various seafood dishes such as agujjim. Tunicate