The natural world is full of spectacular extremes, and the question of the tiniest animal highlights the incredible diversity of life on Earth. While large mammals like the blue whale capture the imagination, the true record-holders for smallness exist in a microscopic universe hidden from the naked eye. Determining the absolute smallest animal is complex, as the title depends entirely on the criteria used, such as body length, mass, or volume. The contenders for this title span a vast range of animal groups.
Defining the Limits of Smallness
Scientists classify all animal life, or Metazoa, as multicellular organisms, a distinction that immediately excludes bacteria and single-celled protists. This classification is fundamental when seeking the smallest animal, as it sets the lower boundary for complexity. The two primary measurements used to compare extreme smallness are linear size, such as length or height, and mass, which represents the overall volume and weight of the organism.
Different species hold the record depending on whether length or mass is prioritized, creating a nuanced set of “smallest” titles. For instance, an animal that is extremely thin but long might be larger by volume than a slightly shorter, spherical one. This framework is necessary to understand why the smallest animals overall are invertebrates, while the smallest vertebrates are still comparatively large.
The Smallest of the Small: Microscopic Multicellular Life
The absolute smallest animals by volume and mass are found among the Myxozoa, a subphylum of highly specialized, obligate parasites derived from cnidarians (the group that includes jellyfish). Myxozoan spores are often composed of just a few cells and typically measure between 10 to 20 micrometers (µm) in length. One of the smallest species, Myxobolus shekel, measures no more than 8.5 µm when fully grown, making it a strong contender for the smallest known animal overall.
Myxozoans achieved this extreme miniaturization through parasitic evolution, losing many genes and complex structures like a nervous system or gut. Their tiny spore stage is the infective form that alternates between two hosts, typically a fish and an annelid worm. The body of the myxozoan is so reduced that for many years, they were incorrectly classified as single-celled protists rather than true animals.
Moving up slightly in size are the microscopic invertebrates that still maintain a more conventional body plan. Rotifers, often called wheel animals, are common in freshwater environments and can be as small as 50 µm (0.05 mm) long, possessing a complete digestive system and a small brain. Another group of minute invertebrates are the fairyflies, a family of parasitoid wasps that includes the smallest known insects. The male Dicopomorpha echmepterygis averages only about 186 µm (0.186 mm) in length, making it smaller than some single-celled organisms.
Miniature Vertebrate Marvels
When attention shifts to vertebrates (animals with a backbone), the smallest examples are significantly larger than their invertebrate counterparts, yet still represent a remarkable degree of miniaturization.
Fish and Amphibians
The category of fish contains some of the strongest contenders for the smallest vertebrates. The males of the deep-sea anglerfish Photocorynus spiniceps measure a mere 6.2 millimeters long, but this size is an adaptation for sexual parasitism where the male fuses to the much larger female. The stout infantfish, Schindleria brevipinguis, holds a strong claim for the lightest vertebrate, weighing less than one milligram, with males maturing at about 6.5 millimeters in length. The former record holder, Paedocypris progenetica, is a carp-relative from the acidic peat swamps of Sumatra, with females maturing at 7.9 millimeters. The current record for the smallest vertebrate by length is the Brazilian flea toad, Brachycephalus pulex, a recently described amphibian whose adult males average just 7.1 millimeters from snout to vent.
Reptiles and Mammals
The smallest reptile is the male nano-chameleon, Brookesia nana, found in Madagascar, which has a body length of only 13.5 millimeters, excluding the tail. This minute size is believed to be a product of island dwarfism. Among the mammals, the title is split between two species: the Etruscan shrew (Suncus etruscus) and the bumblebee bat (Craseonycteris thonglongyai). The Etruscan shrew holds the record for the smallest mass, weighing only about 1.8 grams, while the bumblebee bat is slightly smaller in length, measuring 29 to 33 millimeters.