"This is one of the strangest fish that I've seen in my whole career", said Ralf Britz, zoologist at the Natural History Museum. No wonder—it's the smallest fish ever discovered. Measuring 7.9mm long and a member of the Carp family, Paedocypris progenetica is the world's tiniest vertebrate.
Found living in a forest swamp on the Indonesian island of Sumatra, the minute creature lives in conditions previously thought to be uninhabitable due to high acidity levels of up to pH3. This is 100 times more acidic than rainwater. Remarkably, further research has indicated that many more species have made homes there.
Fish experts Maurice Kottelat of Switzerland and Tan Heok Hui from the Raffles Museum of Biodiversity Research in Singapore were working with colleagues from Indonesia and with Kai-Erik Witte from the Max Planck Institute in Germany when they stumbled upon the fish. Their paper states, "Paedocypris has many 'larval' features typically associated with paedomorphic fish (e.g. narrow frontals that leave the brain unprotected dorsally by bone and a precaudal larval-fin-fold) but, uniquely among fishes, males also possess highly modified pelvic fins with hypertrophied muscles and a keratinised pad in front of the pelvic girdle, which, we hypothesize, function together as a clasping or holding device, thereby suggesting an unusual reproductive mode."
Paedocypris progenetica has a reduced head skeleton leaving its brain unprotected by bone and has the appearance of larvae, as though it has not been fully matured. It was a fortunate find in 1997 amongst Indonesian peat swamps, which had been damaged by forest fires, increased logging, agriculture and urbanisation. It is possible that species similar to this tiny fish have already been lost and the future of its survival is still not definite due to their very survival now being dependant on highly acidic conditions. Although they can survive through extreme drought due to their small size, having sought refuge in the last puddles of the swamp, they are now threatened by continual and widespread forest destruction and drainage of the peat swamps for palm oil plantations.
Paedocypris sustain their small bodies by feeding on plankton at the bottom of pools. The fish have few adult characteristics in order to maintain a reduced size. The females have just enough room to carry a few eggs and the males have very little bony protection over the brain. The males have a tiny clasp underneath their bodies that may help them fertilise eggs individually.
The previous record for smallest vertebrate was held by an 8mm-species of Indo Pacific goby. The UK's smallest fish is the marine Guillet's goby, Lebetus guilleti , reaching 24 mm in length.
