Abstract
Study on the cold-tolerance of Halocynthia roretzi has focused on the squirting movement of their oral and atrial siphons. These squirting movements have been classified into three patterns according to decreasing temperature. In particular, the oral and atrial siphons were closed in temperatures < -1 ℃, and only a movement that stretches and contracts the entire body was observed. With regard to the relationship between temperature and survival, no individuals died even when kept for 20 h at -1.0 to -1.5℃ if the cooling rate was -1.0 to -0.5℃/h. In the coast of Japan, marine organisms with a lower limit of habitation in the negative temperature range are only show in shellfish, such as clams and scallops, and certain fish, such as cod. Considering this, the resistance to cold temperatures exhibited by H. roretzi is probably in the high category among marine organisms.
