Paedocypris progenetica

In life, the species is a translucent orange, with males displaying an iridescent orange spot between the eyes and on the nape. The species has a total length ranging from 6.63 to 12.50 mm with a standard length of 5.72-11.11 mm, making it one of the smallest known fish and vertebrate.^[6][3] P. progenetica is known as one of the most developmentally truncated vertebrates, with 61 physical characters affected by developmental truncation.^[7] The fish resembles in many ways that of the larval and juvenile stages of its relatives, retaining transparency, a long caudal peduncle, and a lack of frontals protecting the brain.^[7][3] Males have a more complex state, notably exhibiting one of the most extreme instances of skeletal sexual dimorphism.^[7] The genus contains a number of unique physical attributes. These include a complex novel structure to the pelvic girdle, hypertrophied (meaning enlarged due to increased cell size) abductor and ventral arrector muscles in males, a pad of keratinized (meaning cells fill with keratin) skin preceding the pelvic fin in males, and a highly modified outermost pelvic-fin ray with the ventral hemitrich significantly expanded and flattened.^[3] Traits distinguishing P. progenetica from its genus include the projection of the keratinized pad, the straight arrangement of breeding tubercles on the dentary, vestigial or often missing pelvic fins in females, and tiny black vermiculations on the belly from throat to anus, among others.^[3]

Upon first discovery of two species of the genus in 2006, Paedocypris was placed in the formally recognized family Cyprinidae. As of 2010, Paedocypris has been placed in a new superfamily, Paedocypridoidea, and family, Paedocyprididae.^[8] This family includes the three species of Paedocypris: P. progenetica, P. micromegethes, and P. carbunculus.^[8] A study in 2007 hypothesized the minimum age of divergence for the genus Paedocypris estimated at ca. 24 MYBP (million years before present).^[9] An additional study investigated the evolution of the genus using morphological and molecular analyses to estimate divergence times between species. The study concluded that P. carbunculus diverged first compared to P. micromegethes, then later followed by P. progenetica. It was hypothesized that P. progenetica may have split from its relatives 24.5 MYBP during the late Oligocene.^[6]

The genome, like the morphology of Paedocypris, is notably characterized by reduction. The species demonstrates an overall loss in DNA content, with a lower number of chromosomes compared to that of close relatives.^[10] Along with a considerable loss of intron sequences, the genome lacks multiple homeobox (HOX) genes and other genes that play critical roles in development.^[10]

P. progenetica is found in tropical Southeast Asia within the Peninsular Malaysia region and on Sumatra and Bintan Island in Indonesia.^[4][3] It is predicted that additional populations of the species will be found with future inventory efforts.^[8] They live in blackwater peat swamps, wetlands with waterlogged soils that prevent plant decay forming layers of acidic peat, with highly acidic (pH 3-6), tannin-stained waters. This acidity is due to large amounts of humic acids and other phenolic acids.^[4] The water ranges in color from a light tea brown to deep red brown, gradually reaching black, with depths ranging from 10 cm to 3 m.^[4] Blackwater peat swamps in Malaysia are some of the richest ichthyofaunal regions in the world.^[4] During the dry season, P. progenetica can survive in small animal burrows, shallow pools, and even in the soil, in part thanks to its small size. This survival skill provides considerable advantage when water levels fall during times of drought. In dry periods, the peat acts as a buffer to retain isolated pools of water, and in some cases can sustain permanent creeks.^[7]