World's Smallest Freshwater Fish

Paedocypris is a genus of cyprinid fish found in Southeast Asia where it occurs in Malaysia and Indonesia. Paideios (Greek), children; Cypris (Greek), Venus, common suffix for cyprinid genera; gender feminine. Three species are known. Paedocypris progenetica has been claimed to be the smallest known species of fish in the world. The smallest mature female measured 7.9 mm and the largest known individual 10.3 mm.

Paedocypris carbunculus

Paedocypris micromegethes

Paedocypris progenetica 

 

        Two species were discovered and identified by ichthyologists Maurice Kottelat from Switzerland and Tan Heok Hui from the Raffles Museum of Biodiversity Research and the National University of Singapore in 1996. Their osteology was studied by Ralf Britz at London's Natural History Museum. In 1994, the same ichthyologists had already discovered in Sarawak (Malaysian part of Borneo) another miniature species of the same genus Paedocypris. Like P. progenetica, P. micromegethes was found in the slow-flowing blackwater streams and peat swamps. The genus, Paedocyrpis, and two species, Paedocypris progenetica and Paedocypris micromegethes, were first described in 2006. Paideios is Greek for children and Cypris is Greek for Venus and is a common suffix for cyprinid genera; the gender is feminine. Progenetica (from the word progenetic) is used as an adjective. Micromegethes is Greek for small in size, and is used as a noun in apposition. 

       Paedocypris progenetica has been claimed to be the smallest known species of fish and vertebrate in the world, particularly before the description of the frog Paedophryne amauensis in 2012. The smallest mature P. progenetica female is only 7.9 millimetres (.31 in) standard length, smaller than the female of any other vertebrate species, including those of P. amauensis. The largest known individual is 10.3 mm (.41 in). Their miniature transparent body lacks typical features characteristic of adult fish, for instance a bony skull structure around its brain, and it retains the post-anal larval-fin-fold along the ventral edge of the caudal peduncle, characteristic of fish larvae. They have a unique sexual dimorphism: The males have highly modified pelvic fins, with the first ray terminating with a hook-like projection of keratinized skin, supported by hypertrophied pelvic musculature. Males also have a pad of keratinized skin in front of the pelvic fins. It is hypothesized that these modified fins are used to grasp the female during mating, or to keep position over a spawning surface. 

     Fishes are also the smallest known freshwater vertebrates, the current record being held by the Burmese cyprinid Danionella translucida (12.0 mm, size at maturity unknown), followed by the Southeast Asian cobitid Kottelatlimia katik (mature at 13.0 mm) and cyprinid Boraras micros (13.3 mm, size at maturity unknown) (Kottelat & Lim 1992; Kottelat & Vidthayanon 1993) and the South American characid Xenurobrycon polyancistrus (13.1 mm) (Weitzman & Vari 1988). The discovery of P. progenetica, with a mature female of just 7.9 mm and a maximum size of 10.3 mm, makes it the smallest recorded vertebrate species, slightly smaller than the marine goby T. nanus. Paedocypris micromegethes, the females of which mature at 8.8 mm (maximum 11.6 mm), comes a close second as the smallest freshwater vertebrate. 

Source: http://en.wikipedia.org/wiki/Paedocypris ; http://rspb.royalsocietypublishing.org/content/273/1589/895.f