Identifying organisms is key to understanding the ecosystems in which they live and their role within those ecosystems. Scientists have discovered and identified many different organisms, some big and some very small.

The Guinness Book of World Records lists the smallest mammal as the Kitti’s hog-nosed bat, which has a wingspan of only 5.5 cm (2.2 in) and weighs 2–3 grams (0.071–0.106 oz). The smallest bird is the bee hummingbird, which measures 5 cm (2 in) in length and weighs 2 g (0.07 oz).

The smallest reptiles are two species of chameleons, Brookesia micra and B. minima, which measure 29 mm (1.14 in) and 27 mm (1.06 in), respectively. The smallest amphibian is Paedophryne amauensis, a frog from Papua New Guinea that measures 7.7 mm (0.30 in) in length.

The smallest fish is the contended freshwater goby, Trimmatom nanus; males of this species measure 10.6 mm (0.42 in), and females 11.5 mm (0.45 in). The smallest marine fish is Amblygobius minima, which measures 8 mm (0.31 in) in length.

The smallest mollusc is the bivalve mollusc Idiomyzon pygmaeus, which has a shell length of only 0.8 mm (0.031 in). The smallest gastropod is Cyclostrema minutum, which has a shell length of only 0.3 mm (0.012 in).

The smallest arthropod is the species Nanosesarma denhami, a tiny crab which measures 1.5 mm (0.059 in) in length. The smallest insect is also a tiny crab louse, Philopterus minutus, which measures only 0.4 mm (0.016 in) in length.

The smallest free-living organism is the bacterium Mycoplasma genitalium, which measures only 0.3 μm in length. The smallest protozoan is the parasitic Nanomonas espinae, measuring only 0.2 μm in length. There are also reports of smaller viruses; for example, the cowpea mosaic virus particle measures only 18 nm in length.

As of June 2016, the smallest mammal is the Etruscan shrew (Suncus etruscus), with a body mass of only 1.8 grams (0.063 oz). The smallest bird is the bee hummingbird (Mellisuga helenae), measuring 5 cm (2 in) in length and weighing 2 g (0.07 oz). The smallest reptile is the gecko species Sphaerodactylus ariasae, measuring 14 mm (0.55 in) in snout-vent length. The smallest amphibian is the frog Paedophryne amauensis, measuring 7.7 mm (0.30 in) in length. The smallest known fish is the goby species Trimmatom nanus, measuring 10.6 mm (0.42 in) in standard length. The smallest mollusc is the bivalve mollusc Idiomyzon pygmaeus, with a shell length of only 0.8 mm (0.031 in). The smallest gastropod is Cyclostrema minutum, with a shell length of only 0.3 mm (0.012 in).

At 2.5 μm in diameter, the marine alga Ostreococcus tauri is thought to be the smallest free-living eukaryote. The smallest parasitic eukaryote is Spirostomum minus, measuring only 0.5 μm in length. The smallest virus is the marine virus MS2, which measures only 18 nm in diameter.

## 9 Smallest Organisms Ever Discovered

The debate over the definition of life, and what entities qualify as organisms, continues. What is certain, however, is that there are many extremely small organisms in the world.

Here are 9 of the smallest organisms ever discovered.

*Mycoplasma genitalium*

This bacterium is thought to be one of the simplest and smallest organisms capable of self-replication. It measures just 0.3 micrometers (µm) in length, and has a genome of only 580,000 base pairs of DNA.

*Ostreococcus tauri*

This marine alga is just 2.5 µm in diameter, making it the smallest free-living eukaryote known.

*Nanomonas espinae*

This parasitic protozoan was discovered in 2009, and measures just 0.2 µm in length.

*Spirostomum minus*

This tiny parasitic eukaryote was discovered in 2011, and is even smaller than *Nanomonas espinae*, measuring just 0.5 µm in length.

### MS2 virus

This marine virus is the smallest known virus, measuring just 18 nanometers (nm) in diameter.

*Paedophryne amauensis*

This frog from Papua New Guinea is the smallest known amphibian, measuring just 7.7 mm (0.30 in) in length from snout to vent.

*Trimmatom nanus*

This goby fish is the smallest known fish, measuring just 10.6 mm (0.42 in) in standard length.

*Sphaerodactylus ariasae*

This gecko species from the Dominican Republic is the smallest known reptile, measuring just 14 mm (0.55 in) in snout-vent length.

*Mellisuga helenae*

This hummingbird from Cuba is the smallest known bird, measuring just 5 cm (2 in) in length and weighing 2 g (0.07 oz).

Though these are some of the smallest organisms known, it is certain that there are many more waiting to be discovered. With new technology and techniques, scientists are constantly finding smaller and smaller organisms. It is amazing what they may find next!