The Crystal Crawlers are crystalline organisms native to Pluto, in the outer solar system.
As their name indicates, they appear as crawling masses of crystals and exist in numerous varieties, each specialized to feed on a different element. For instance, aluminium-feeders consume deposits of frozen clay-like aluminum, which is somewhat abundant on Pluto. Other kinds are known to feed on iron, silicon, sulfur and carbon. There is none which feeds on copper, though.
According to Peri Maclane, Crystal Crawlers are extremely simple creature which have no eyes and no sense organs of any kind, and just crawl in any direction hoping to come across a food source. She describes them as being "on the borderline" between living and non-living things. They seem to be fragile creatures, as they're shattered easily when one steps on them.
Most kinds of Crystal Crawlers are harmless to Humans, but the carbon-feeders are incredibly dangerous. It's been speculated that if one such beasts could find its way to Earth they would keep feeding and multiplying until the entire biosphere was consumed.
- "The Red Peri", by Stanley G. Weinbaum (1935)
- At the time Weinbaum wrote the story, Pluto's actual size and features where not known yet. Weinbaum imagined it as a very large and dark planet, larger than the Earth and with a very low surface albedo.