AN UNUSUALLY ROBUST CIONA FROM THE NORTHEASTERN COAST OF HONSYU ISLAND, JAPAN

Abstract

While the first author had been engaged for years in the studies of the comparative anatomy of the circulatory system and in the cytological researches of the heart of Ciona, he became aware of a very strange fact. Specimens of Ciona for his works were first collected from the rafts in the oyster farm of the Oyster Institute in Mone Inlet ofKarakuwa-tyo, Miyagi Prefecture in the northeastern district ofHonsyu Island. And he found two types of Ciona in his material from this locality. The one has the test which is very soft as usually known for cosmopolitan Ciona intestinalis (L.) and quite free from any foreign materials on the surface, and its mantle is more or less sprinkled with orange pigment spots, especially densely near the tip of siphons. The test of the other type is, however, much thicker and harder and sometimes even harbours other organisms on the surface; its mantle is scarcely pigmented. Moreover, it was found that the two types differ from each other markedly not only in the feature of the test but also in the formation of the endostylar appendage and in the arrangement of the epicardia and pharyngo-epicardiac openings as shown later. For the operation to remove the test and body wall on the right side to expose or take out the live heart, the soft Ciona is much more suitable than the robust Ciona, while in the robust Ciona the body contraction caused by external stimuli is much less than in the soft Ciona, because the contraction is much checked by the existence of the thick and hard test regardless the sensitivity and contractility of the mantle itself. Thus, the robust Ciona is much better for the pigment injection into the blood vessels under anesthetization by menthol. To secure these two types of Ciona for respective operations, Ciona in another locality, the fish rearing tanks of the Onagawa Fisheries Laboratory ofTohoku University, was examined, and it was found that there most of Ciona were of the robust type.