The smallest known vertebrate is the Paedophryne amauensis, a species of frog found in Papua New Guinea. It measures only about 7.7 millimeters in length. ¹⁾

The world's smallest mammal is the bumblebee bat (Craseonycteris thonglongyai), also known as Kitti's hog-nosed bat. It weighs only about 2 grams and has a body length of around 30-40 millimeters. ²⁾

The smallest primate is the pygmy mouse lemur (Microcebus myoxinus), which is found in Madagascar. It weighs about 30 grams and has a body length of around 57-67 millimeters. ³⁾

The bee hummingbird (Mellisuga helenae) holds the title for the smallest bird in the world. It measures around 5-6 centimeters in length and weighs about 1.6-2.6 grams. ⁴⁾

The smallest fish is the Paedocypris progenetica, a species of carp found in swamps on the Indonesian island of Sumatra. It reaches a maximum length of about 10 millimeters. ⁵⁾

The world's smallest reptile is the Brookesia nana, also known as the nano-chameleon. It measures around 13.5 millimeters in length and is found in the rainforests of northern Madagascar. ⁶⁾

The tiniest known arachnid is the Patu digua, a type of mite found in Colombia. The adult females of this species are approximately 0.37 millimeters in length. ⁷⁾

The smallest butterfly is the Western Pygmy Blue (Brephidium exilis). It has a wingspan of about 0.5-0.7 inches and is found in North and Central America. ⁸⁾

The world's smallest crustacean is the Stygotantulus stocki, a species of water flea. It measures about 0.08 millimeters in length and is found in groundwater habitats in Western Australia. ⁹⁾

The Etruscan shrew (Suncus etruscus) holds the record for the smallest mammalian brain size. It weighs only around 1.2 grams and has a brain weighing about 0.003 grams. ¹⁰⁾

The world's smallest snake is the thread snake (Leptotyphlops carlae), which is found on the Caribbean island of Barbados. It measures around 10 centimeters in length. ¹¹⁾

The dwarf goby (Pandaka pygmaea) is the smallest known fish species in terms of body mass. It weighs only about 4 milligrams and is found in the Philippines. ¹²⁾

The Barbados threadsnake (Tetracheilostoma carlae) holds the title for the smallest known snake species in terms of length. It measures approximately 10 centimeters. ¹³⁾

The smallest known jellyfish is the Staurocladia dwarf jellyfish (Staurocladia capillata). It has a bell diameter of about 1.5 millimeters and is found in the Mediterranean Sea. ¹⁴⁾

The world's smallest invertebrate is the rotifer species called Rotaria magnacalcarata. It measures about 0.1 millimeters in length and can be found in freshwater habitats. ¹⁵⁾