The sea cucumber Stichopus japonicus, is widely distributed along all the coast of Japan. Commercially it is divided into three forms due to the body color, the green, red and black. Of these three, the former two are important, through they have a different market value.
As for the taxonomy of this species, there are Stichopus japonicus and S. armata described by E. SELENKA (1867); S. japonicus var. typicus by H. THEEL (1886); and S. armatus and S. roseus by E. AUGUSTIN (1908). K. MITSUKURI (1897) and H. OHSHIMA (1918) made it clear that these were all but synonyms of the S. Japonicus and this remains still accepted. The two forms of the Green and the Red, on the other hand, were first reported by K. MITSUKURI (1912) and were ascribed by the same author to a color variation due to the difference in habitat and thereafter his opinion has been widely accepted by the investigators.
Between these two forms, however, there are found some distinct differences, not only morphological but also ecological. The present authors have studied this problem and hereby propose to separate them into two independent communities, ecologically and taxonomically different from each other with the following grounds:
1) The predominant color of the Green is dark bluish green, which varies with individuals from light bluish green to yellowish brown or dark brown. The key tone for the Red is red or brownish red, and the ventrum being in red without exception and to be its characteristic. There are, though very low in their occurrence (no more than 1 to 2 per cent), some intermediate forms with the dorsum similar to the Green and the ventrum similar to the Red, and vice versa.
2) The Polian vesicles of the Green are usually of stumpy shape with dully round tip, while those of the Red are slender with projected tip.
3) The body contractibility of the Red is remarkable and under a mechanical stimulus, for example, it contracts into an almost round shape, while no such habit is observed with the Green.
4) The shape of the ossicles of tentacles is much more complicated in the Red than the Green, whereas the disk of the tables of the former is more round-shaped and the circum-ference of its holes is broader than the latter. As for the size of the disk of tables, when compared with individuals of the same body weight, the Green wins over the Red.
5) The Green has a preference to somewhat lower salinity and eventually finds its favorite habitat on sand and muddy-sand grounds, while the Red with fondness for higher salinity lives mainly in the rock, pebble and gravel grounds.
6) The matured egg of the Red has a gelatinous coating 23-26 microns thick, while the Green has without it. In the coasts of Mie and Aichi Pref., the spawning period of the Red is earlier and shorter than that of the Green.
7) In view of the modes of reaction to the environments, such as growth and adaptation for salinity, the Green and the Red are judged to belong to a different environment, respectively.
8) In summer season when the water temperature is high, almost all of the Red take aestivation, on the contrary 50 per cent of the Green of 6 to 30gr. (weight of body-wall) and 75 per cent of the grown-up are found in aestivation, and younglings of less than 5gr. totally dispense with it.