Mesenchytraeus solifugus, commonly called an ice worm, is a species of oligochaete worm that inhabits coastal glaciers in northwestern North America. M. solifugus is dark brown and grows to about 15 mm long and 0.5 mm wide. It has a high population density and is common in suitable habitat. It can only survive at temperatures of approximately –7 °C to 5 °C. The ice worm is studied for its ability to survive in low temperatures. It has heavy pigment, absorbing most colors; however, it burrows into the glacier to avoid strong sunlight. History and taxonomy This species was first reported from Muir Glacier in Alaska. Carlo Emery named it solifugus in 1898, meaning "fleeing from the sun". Phylogeny suggests that it evolved from an aquatic ancestor. Description Ice worms are the only known psychrophilic annelids. Adult ice worms are approximately long. They live their entire lives in snow or ice. The ice worm’s dark pigmentation helps protect it from solar ultraviolet radiation. Their fluid and flexible body are able to squeeze through air holes and tiny crevasses in ice. Their lifespan is believed to be five to ten years. Ice worms have an elongated head pore, with sensing organs located on their prostomium and ventral surface. These organs are responsible for chemo-, thermo, and photorecepetion. However, the worms lack eyes or eyespots. They have setae that curve at the tips, unlike those of related organisms. Little is known about the life cycle of the ice worm; it is thought to be unusually slow. It requires temperatures of 0 °C for a successful reproduction. Cocoons are dispersed by birds. Not much is known about the ice worm's diet, but it is thought to eat snow algae, as well as some bacteria. Habitat Ice worms inhabit glaciers and can be found in snowfields, avalanche cones, and glacier rivers and pools. These environments have temperatures near the freezing point and are nutrient poor. The organisms require permanent glacial environments for survival and reproduction. As well as the northwestern United States, ice worms have been found in glaciers of southern Alaska and certain parts of British Columbia. They can be abundant, with typical densities of about 100 per square meter and peak recorded densities of up to 6,000 per square meter. Though they are found in snowfields, rivers and pools, they are not found on dirty snow or rock. Ecology and behavior M. solifugus