The smallest animal with a backbone known to science, a fish from the carp family, has been discovered in the peat swamps of Indonesia.

　　Mature females of the fish species Paedocypris progenetica reach just 7.9mm (3/10 in) in length, making them the smallest vertebrates yet identified by a tenth of a millimetre.

　　The previous size record for a vertebrate was held by the Indo-Pacific goby, another fish, at 8mm. Britain's smallest fish, and vertebrate, is the marine Guillet's goby, Lebetus guilleti, which measures 24mm.

　　The species was discovered in the highly acidic peat swamps of the Indonesian island of Sumatra by a team led by Ralf Britz, a zoologist at the Natural History Museum in London.

　　"This is one of the strangest fish that I've seen in my whole career," Dr Britz said.

　　"It's tiny, it lives in acid and it has these bizarre grasping fins. I hope that we'll have time to find out more about them before their habitat disappears completely."

　　The species is transparent and lives in dark tea-coloured swamp waters, which at pH3 are 100 times more acidic than rainwater. Although these swamps were once thought to harbour very few animals, recent research has shown that they are home to a highly diverse range of species that occur nowhere else.

　　The peat swamps were damaged by forest fires in 1997, and are also threatened by logging, urbanisation and agriculture. The scientists behind the discovery said that several populations of P. progenetica had already been lost.

　　"Many of the peat swamps we surveyed throughout South-East Asia no longer exist and their fauna is eradicated," Dr Britz said. "Populations of all the highly endemic miniature fishes of peat swamps have decreased or collapsed."

　　Details of the discovery are published today in the journal Proceedings of the Royal Society of London, Series B. The male fish grow to 8.6mm, and boast enlarged pelvic fins with exceptionally large muscles relative to the size of the rest of their bodies. The researchers believe that these may be used for grasping females during sex. The females are smaller still, reaching 7.9mm.

　　The smallest known mammal is Kitti's hog-nosed bat, Craseonycteris thonglongyai, from western Thailand, which measures between 2.9cm and 3.3cm long.