Freezing-tolerance of Macrobiotus harmsworthi (Tardigrada) and Plectus antarcticus (Nematoda) in the Antarctic region

Abstract

In early January of 1988,we collected mosses in the ice-free Yukidori Valley area of Langhovde, near Syowa Station in East Antarctica. The mosses were stored at -20℃ and shipped to Japan. After ten months of storage, Macrobiotus harmsworthi J. MURRAY (Tardigrada) and Plectus antarcticus DE MAN (Nematoda) were extracted from these mosses to test their freezing-tolerance. These extracted samples were cooled from -10℃ to -40℃ at -2℃/day, as slow as cooling conditions similar to the natural habitats, then down to -80℃ rapidly at -20℃/hr. At -10℃, -18℃, -32℃, -40℃ and -80℃ in the course of cooling, some individuals were taken out of the freezer and cultured at 0℃. The ratio of active individuals (RA) was assessed for the periods of two hours, four hours, two weeks and five weeks after thawing. Most individuals of M. harmsworthi were dormant at low temperatures. RA of the individuals cooled to temperatures below -32℃ ranged from 80 to 100% in two hours after thawing. The values remained as high as 75–86% even after five weeks. The individuals cooled to -10℃ or -18℃ showed a smaller RA which was about 70% two hours later and only about 50% five weeks later. In P. antarcticus the RA values of four hours after thawing ranged from 46% (-32℃) to 100% (-10℃). Unlike M. harmsworthi, most of them quickly died during the incubation at 0℃; RA of two weeks after thawing was 0% in the individuals cooled to -10℃ or -18℃ and 3–11% in the individuals cooled to -32℃ or below. In five weeks after thawing, all of them had died. Overall, M. harmsworthi and P. antarcticus were freezing-tolerant; moreover, they seemed more tolerant when cooled to lower temperatures.