Tinier than a pinhead, tardigrades are some of the toughest creatures on the planet. Some are so tough that they have even survived being sent into space. While Halobiotus crispae isn't at the top of the list of tardigrade tough guys, it's still pretty robust. Enclosing itself in a cyst-like second skin while hibernating during winter in the icy waters around Greenland, the marine tardigrade eventually sheds its protective outer layer in the spring before becoming active during the short northern summer. Curious to find out more about the tardigrade's strategy for survival, Nadja Møbjerg, Reinhardt Møbjerg Kristensen and their colleagues from the Universities of Copenhagen and Roskilde decided to test the resilience of a local population of H. crispae from the Danish coast. However, the Danish tardigrades prefer to hibernate during the relatively warm Danish summer, becoming active in late winter and early spring. Would the two populations use the same strategies to survive the effects of ice and salt water, or had they come up with alternative approaches to weather their local conditions (p. 2803)?

Collecting tardigrades from the sediments at the bottom of the Danish Isefjord, Møbjerg and her colleagues, Kenneth Agerlin Halberg and Dennis Persson, tested the tiny animals' resistance to freezing. Teaming up with Hans Ramløv and Peter Westh, they gently cooled the active and hibernating animals and measured the exact moment when the tiny creatures froze. Amazingly, the tardigrades remained unfrozen until the temperature dropped to –20°C, when both active and hibernating animals froze. Somehow the animals are able to prevent their body fluids from freezing,remaining supercooled until the temperature plummeted to –20°C. And when the team thawed the animals, the hibernating animals were absolutely fine, having survived the icy experience.

Next the team tested the Danish tardigrades' responses to saline conditions by immersing the animals in seawater ranging from a salinity of 2 ppt to 40 ppt and found that the active animals seemed able to cope better with the drastic changes. And when they measured the volume of the animals' bodies, it was clear that the animals reacted very differently to high and low salt concentrations. The animals' bodies initially swelled by 60% when transferred to dilute seawater, before shrinking back down to their normal size 2 days later, while the animals that were transferred to more concentrated seawater initially shrunk by 40% before slowly reinflating over the next day to a new smaller, but stable, body size.

Wondering what effect the tardigrades' dramatic size change had on their body fluids, Halberg carefully extracted nanolitre samples of body fluid from the microscopic beasts and measured the number of particles dissolved in the fluid. No matter what salt concentration the tardigrades were exposed to,their body fluids were always slightly more concentrated than the environment surrounding them. And when the team tested the effects of salt and ice on the Greenland H. crispae population, they found that the animals use the same strategies for survival as the Danish tardigrades.

Having found how different H. crispae populations protect themselves from the damaging effects of salt and ice, Møbjerg suspects that all H. crispae use the same approaches, and is keen to find out more about the ways the tiny creatures regulate their body fluid composition to survive in some of the planet's most inhospitable environments.

Halberg, K. A., Persson, D., Ramløv, H., Westh, P.,Kristensen, R. M. and Møbjerg, N. (
). Cyclomorphosis in Tardigrada: adaptation to environmental constraints.
J. Exp. Biol.