If lifespan is defined as the entire lifetime of an animal from birth until death, then the record for shortest average lifespan belongs to a group of microscopic worm-like animals called gastrotrichs.
The entire life cycle of a gastrotrich only lasts three or four days. There are many varieties of these aquatic microorganisms, but none live more than a few days under normal circumstances. This life is spent floating about, eating and reproducing. For some gastrotrichs, reproduction involves an exchange of genetic material with another individual, but many species reproduce by parthenogenesis, in which the offspring is a genetic copy of the adult animal.
The mayfly is often listed as the shortest-lived creature, as adult mayflies only live for a day or less after breaking free from the cocoon. However, the adult phase of the mayfly is only the last part of its lifespan. The naiad, or nymph, stage of a mayfly's life can last well over a year.