Science correspondent, April Holladay, said that the answer is probably gastrotrichs (a minute aquatic animals) - they live as short as a three days.
Gastrotrich, any of about 500 species of the phylum Gastrotricha, is a group of microscopic aquatic invertebrates that live in the spaces between sand grains and soil particles and on the outer coverings of aquatic plants and animals. They occur in salt water and freshwater and also on sandy seashores.
Others claim that the title belongs to mayflies. Adult mayflies live only a few hours because they do not or cannot eat. Their whole existence is devoted to finding a mate and reproducing. They accomplish this task in no more than eighteen hours.
However, although short-lived as adults, these insects may spend one to two years in the larval or nymphal stage before changing into adults.
How long do animals live? Man has been trying to answer this question for years, but until recently most of the life-span information resulted from keeping age records on captive animals. Although these records showed how long animals could live, the information was misleading.
See Texas.gov's Animal Life Spans article to learn more.