Paedocypris

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Paedocypris is a genus of fish in the family Cyprinidae (carps, minnows). It includes two species, Paedocypris progenetica and Paedocypris micromegethes.

Paedocypris progenetica has been claimed to be the smallest known species of fish and vertebrate in the world. The smallest mature female measured 7.9mm and the largest known individual 10.3mmMaurice Kottelat, Ralf Britz, Tan Heok Hui, Kai-Erik Witte, 2005. "Paedocypris, a new genus of Southeast Asian cyprinid fish with a remarkable sexual dimorphism, comprises the world's smallest vertebrate." ''Proceedings of the Royal Society B 10.1098/rspb.2005.3419. [Abstract].

Male individuals of the species Photocorynus spiniceps have been documented to be 6.2-7.3mm at maturity, and thus claimed to be a smaller species. However, these survive only by sexual parasitismSandra Hines, 2006. "[Flap over fishes: Who's the smallest of them all?]" University of Washington News. and the female individuals reach the significantly larger size of 50.5mmRaffles Museum news, 2006. "[RMBR: So which is the smallest species of fish in the world? (Part II)]".

Paedocypris progenetica lives in the peat swamps of the Indonesian island of Sumatra . Paedocypris micromegethes lives in the peat swamps of Sarawak (Borneo). Their small size helps them to survive through extreme droughts, as they can live in small remaining puddles of acid water (pH as low as 2.9).

Their miniature 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, with modified male pelvic fins, with the first ray terminating by a hook-like projection of keratinized skin and with hypertrophied pelvic musculature. Males also have a pad of keratinized skin in front of the pelvic fins.

Both species were discovered and identified by ichthyologists Maurice Kottelat and Tan Heok Hui of the National University of Singapore and their osteology was studied by Ralf Britz at London's Natural History Museum.

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