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What is the definition of ‘smallest’? This seemingly simple question is behind a friendly rivalry that has developed between three groups of scientists.

Mine's smaller than yours!

If the definition of the smallest species, means the shortest, then the newly described Paedocypris progenetica may take the title, at least for females. This tiny carp-relative that lives in peat swamp forests of Sumatra was described by Kottelat, Britz, Tan and Witte (see ‘Further Reading’) in a paper published in 2005. The species reaches a maximum size of 10.3 mm with females reaching maturity at 7.9 mm.

Ted Pietsch of the University of Washington, however, believes that the title of the world’s smallest fish belongs to Photocorynus spiniceps. Males of this deepsea anglerfish are parasitic on the much larger female. A 6.2 mm mature male attached to a 46 mm female was found by Pietsch in the Scripps Institution of Oceanography Fish Collection (Pietsch, 2005).

The claim of scientists from the Australian Museum, the Scripps Institution of Oceanography and the Southwest Fisheries Science Center is that the title of smallest fish rests with the Stout Floater, Schindleria brevipinguis. Females of this species, a relative of gobies, which is only known from the Great Barrier Reef, reach sexual maturity by 7 mm to 8 mm in length and males mature by 6.5 mm to 7 mm. The largest specimen known is only 8.4 mm long. Watson and Walker base their claim on the fact that the Stout Infantfish is far more slender, and thus lighter than either of the other species. It is noteworthy that all gamefishing records are based on weight not length. Most people would agree that a 4 m long elephant is a bigger animal than a 5 m long snake. The Stout Infantfish is slightly longer, but at less than one-millionth of a kilogram, is almost certainly much lighter than either P. progenetica or the male P. spiniceps and is thus the smallest species. Both males and females of the Stout Infantfish are small, unlike the anglerfish species championed by Dr Pietsch.

Clearly, which species is smallest depends upon your definition of ‘small’. In the long run, it probably matters little which species is the smallest. All three species - P. progenetica, P. spiniceps and S. brevipinguis are remarkable fishes and highlight the fact that there is still much to learn about fishes.

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Further reading

  1. Kottelat, M., Britz, R., Tan, H.H.& K-E. Witte. 2006. 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: 1-5.
  2. Pietsch, T.W. 2005. Dimorphism, parasitism, and sex revisited: modes of reproduction among deep-sea ceratioid anglerfishes (Teleostei: Lophiiformes). Ichthyological Research. 52: 207-236.
  3. Watson, W. & H.J. Walker Jr. 2004. The world's smallest vertebrate, Schindleria brevipinguis, a new paedomorphic species in the family Schindleriidae (Perciformes: Gobioidei). Records of the Australian Museum. 56(2): 139-142.