What is the 1 smallest animal?

The smallest animal ever discovered is Myxozoa, a parasitic organism measuring just 20 micrometers (0.02 millimeters)—smaller than many single-celled organisms and barely visible even under a microscope. These microscopic parasites, which primarily infect fish and invertebrates, are so simplified that scientists initially classified them as single-celled protozoans before DNA analysis revealed they're actually highly degenerate animals that lost most typical animal features through extreme evolutionary simplification.

Among more recognizable animals, the Paedocypris fish from Indonesia holds the record for the smallest vertebrate at just 7.9 millimeters long when fully grown, while the Etruscan shrew claims the title of smallest mammal by mass, weighing around 1.8 grams. For terrestrial invertebrates, certain species of fairyflies (parasitic wasps) measure only 0.139 millimeters in length. These extreme size adaptations demonstrate remarkable evolutionary solutions to specific ecological niches—smaller body size can mean lower energy requirements, ability to exploit food sources unavailable to larger animals, and access to microhabitats that provide protection from predators. The discovery of increasingly smaller animals continues to challenge our understanding of the minimum biological requirements for complex animal life.