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Miniature relations of world's smallest fish revealed

18 April 2007

Scientists have worked out the relations of the world's smallest fish, the 7.9mm-long animals discovered living in acidic peat swamp forests in Southeast Asia, in 2006.

The miniature fish genus, or group, called Paedocypris attracted worldwide media attention last year as it comprises the world's smallest vertebrate .

Scientists were intrigued to find out more about this little fish and Dr Lukas Rüber and Dr Ralph Britz, ichthyologists (fish experts) at the Natural History Museum , and colleagues from Singapore and Switzerland have worked out the position of Paedocypris in the fish family tree.

Tiny relatives
World's smallest vertebrate is a member of the carp family of fish.© Kottelat, Cornol/Raffles Museum

World's smallest vertebrate is a member of the carp family of fish. © Maurice Kottelat, Cornol /Raffles Museum

Finding the relatives of such a tiny fish isn't easy. The minute see-through Paedocypris have the appearance of fish larvae. Although clearly a member of the carp family Cyprinidae , the simplified anatomical structure of Paedocypris , combined with a number of highly unusual characters, make it difficult to explain where it fits among members of this largest family of fishes.

'We were curious to find out where Paedocypris belongs and whether it is closely related to other tiny carp-like fishes or not,' said Rüber and Britz.

To resolve this puzzle Rüber and Britz employed a dual approach, using DNA sequence data and morphological data (looking at the body structures). The results of the DNA based study are published in the journal BMC Evolutionary Biology .

Relations revealed

By comparing the data from Paedocypris with other similar fish, Paedocypris was found to be most closely related to Sundadanio , another miniature cyprinid from peat swamp forests in Southeast Asia.

Both tiny fishes belong to a larger group of fishes in the carp family called the Rasborinae . Rasborines include many popular aquarium fishes such as the harlequin rasbora and the pearl danio. They also include the zebrafish Danio rerio , the most important vertebrate model organism scientists use to study development. Interestingly, rasborines comprise an unusually high number of minute species compared to other groups of the carp family.

Miniaturisation

The research also found that other closely related species had become miniature independently. Miniaturisation is an evolutionary process that leads to dwarfed sexually mature organisms. It is widespread among vertebrates, especially in fishes.

'We were surprised to see that the tiny species are not necessarily closely related and that miniaturisation evolved several times independently in rasborines' commented Rüber and Britz.

Two ways to miniaturisation

The scientists found two types of miniaturisation among rasborines, proportioned dwarfism and developmental truncation .

In the first, the miniaturised species is a dwarfed but otherwise identical image of its larger ancestor.

In the second, the miniaturised species closely resembles an early developmental stage of the larger ancestor. Fishes of the latter type tend to look like larvae and this is the type of miniaturisation found in the Paedocypris species. However, others like the harlequin rasbora are proportioned dwarfs.

Threatened acidic swamps

Many of these fish species live in acidic peat swamp habitats that are threatened by forest fires, logging, urbanisation and agriculture. Several populations of Paedocypris have already been lost.