A leech capable of surviving exposure to extremely low temperatures

Abstract

It is widely considered that most organisms cannot survive prolonged exposure to temperatures below 0°C, primarily because of the damage caused by the water in cells as it freezes. However, some organisms are capable of surviving extreme variations in environmental conditions. In the case of temperature, the ability to survive subzero temperatures is referred to as cryobiosis. We show that the ozobranchid leech, Ozobranchus jantseanus, a parasite of freshwater turtles, has a surprisingly high tolerance to freezing and thawing. This finding is particularly interesting because the leach can survive these temperatures without any acclimation period or pretreatment. Specifically, the leech survived exposure to super-low temperatures by storage in liquid nitrogen (-196°C) for 24 hours, as well as long-term storage at temperatures as low as -90°C for up to 32 months. The leech was also capable of enduring repeated freeze-thaw cycles in the temperature range 20°C to -100°C and then back to 20°C. The results demonstrated that the novel cryotolerance mechanisms employed by O. jantseanus enable the leech to withstand a wider range of temperatures than those reported previously for cryobiotic organisms. We anticipate that the mechanism for the observed tolerance to freezing and thawing in O. jantseanus will prove useful for future studies of cryopreservation.

Figure 1. Stereoscopic micrograph of Ozobranchus jantseanus (dorsal view).

Scale bar = 1 mm.

Figure 2. Survival rates of Ozobranchus jantseanus after long-term storage at −90°C.

Values below data points refer to the number of individuals subjected to storage. Leeches were considered to be alive if they moved within five hours of thawing.

Figure 3. Survival rates of Ozobranchus jantseanus after repeated freeze-thaw cycles.

Freeze-thaw cycles determined by differential scanning colorimetry consisted of cooling from 20°C to −100°C and then thawing to 20°C at 10°C/min.

Figure 4. Thermal analysis of Ozobranchus jantseanus by differential scanning colorimetry for temperatures 20°C to −120°C and then back to 20°C at ±10°C/min.