Toolmaker (Xeelee Sequence)

The Toolmakers, also called tree stumps, are an alien species in Stephen Baxter's Xeelee Sequence. They are an ice-based, helium-blooded species native to Baked Alaska, a small and distant Kuiper Belt object. They are not very important to the plot, but are notable as the first sentient extraterrestrial species discovered by humanity.

Anatomy & Physiology
Toolmakers are composed not of organic matter but of water ice and rock. Their blood - and presumably other internal fluids - consists of superfluid helium. Because of this, they will die if exposed to temperatures more than a few degrees above absolute zero. Their bodies are shaped like tree stumps, with roots spreading out around the base in a regular pattern. In the immature, sentient phase of their life cycle, the roots serve as legs.

Diet
Toolmakers derive energy from thermoelectricity, created by their tops facing the sun and their roots facing Baked Alaska's surface. The temperature difference is very small, but it is enough to set up a current that moves superfluid helium around the body.

Reproduction & Life Cycle
Toolmakers undergo a highly unusual form of metamorphosis, similar to that of sea squirts. Young Toolmakers are sentient and mobile, but live short lives, only about a (local) day or so. Afterward, they root down, and the nervous system dissolves into the body, having outlived its use. Toolmakers reproduce sexually, but the specifics are not known.

Technology
Toolmaker technology is quite primitive, roughly on par with Earth's Stone Age. Technology is limited to the construction of simple tools. Due to their homeworld's extreme cold and isolation, it is unlikely they could ever have achieved more advanced technology.