Aquatic science has identified 16,764 species of fish living on Earth. Scientists continue to discover new varieties of aquatic vertebrates each year. 41% of them live in freshwater lakes, rivers, and streams, while the rest live in saltwater oceans and seas. They range from the smallest, the Paedocypris progenetica, to the largest, the whale shark.

The Paedocypris progenetica is the smallest fish in the world. Discovered only a decade ago on the Indonesian island of Sumatra, some mature female members reach only 7.9 mm. The largest Paedocypris progenetica found was a 10.3 mm mature female. Male Paedocypris progentica average around 9.8 mm.

When the Paedocypris progenetica was first discovered, it was believed to be the smallest vertebrate on Earth. This title was lost, however, when scientists found a miniscule species of frog in the New Guinea rain forest, the Paedophryne amauensis. While the males from the anglerfish species Photocorynus spiniceps are smaller, reaching only 6.2–7.3 mm, the females can be as large as 50.5 mm.

The Paedocypris progenetica is a member of the carp family and lives in the dark peat swamps and backwaters of Sumatra. The peat swamps and streams where they live are highly acidic and brown colored, with a pH of 3, which is 100 times more acidic than common drinking water. The swamps of Sumatra have been threatened by forest fires, logging, and plantations in recent years, leading to the extinction of some Paedocypris species.

Resembling a larva more than it does an adult fish, the Paedocypris progenetica has a translucent body and small head with a skeleton that leaves the brain unprotected. Interestingly, the males have pelvic fins that can grasp. It’s closest known relatives are the Paedocypris micromegethes and the Paedocypris carbunculus. The Paedocypris progenetica’s name derives from the Greek words ‘Paideios’, which means children, ‘Cypris’ meaning Venus, and the Latin ‘Progenesis’ which means precocious reproduction.

In contrast, the whale shark is the biggest non-mammal on Earth, reaching lengths as high as 12 meters, or 41 feet and weights as heavy as 30 metric tons. Considered the gentle giants of the seas, their size rivals that of some dinosaurs. They have flat heads with large mouths, sometimes as big as 4.5 feet wide. Each has grey skin with a white belly, and individually unique markings.

Whale sharks are believed to have originated around 60 million years ago, however Whale sharks were officially discovered by Andrew Smith in 1828 near Table Bay, South Africa and were given the scientific name Rhinocodon. They are known by a huge variety of names in languages throughout the world and are even considered a deity in Vietnam. Their picture has appeared on money in the Philippines. A small number have been kept in captivity in aquariums in Japan, China, the United States and other countries. It is illegal to hunt, sell, import or export Rhinocodon for commercial purposes in the Philippines, India, and Taiwan, but they continue to be hunted in parts of Asia.

Rhinocodon are filter feeders, eating plankton and small aquatic life. They have five sets of gills, and around 300 rows of tiny teeth. They swim with their mouths open, allowing water into their mouths. This water is filtered through the teeth and filter pads before being expelled from the gills.

The Rhinocodon gets its nickname from its large size, which is the same as some whales, and its method of filter feeding which is like the baleen. Rhinocodon are found in tropical and warm oceans all over the world, except for the Mediterranean. They swim in open waters and have even been known to let humans ride on their backs!
Scientists believe that Rhinocodon begin mating at around 30 years of age and give birth to live young. They aren’t certain, though, because they have never observed mating or pupping. They are estimated to live to between 70 to 100 years old.

From the smallest to the biggest, fish come in a wide variety of shapes, colors, habitats, and diets. Learning about them is important so that we can protect their habitats and ensure that they are still around for years to come.