On this page...

Adult male cetomimid whalefish, Ataxolepis apus
An adult male (5.8 cm) cetomimid whalefish, Ataxolepis apus, from Gulf of Mexico. Image: G. David Johnson

International collaboration between scientists in Australia, the USA and Japan has resulted in one of the most amazing biological discoveries in decades.

The seven scientists have shown that what were previously recognized as three distinct families of deep-sea fishes are in fact only one. The whalefishes (family Cetomimidae) are all female, the smaller bignose fishes (family Megalomycteridae) are all male and the bizarre tapetails (family Mirapinnidae) are larvae.

The remarkable combination of developmental transformations and sexual dimorphism (differences between the sexes) the authors have documented are unparalleled within vertebrates.

The road to this discovery was long and winding. The scientists had studied the families individually over many years. Detailed analyses of changes in the anatomy of specimens as they transform from larvae to females and males were made on some recently captured fishes after DNA indicated one larva was similar to a female. Closer study of older museum specimens identified two specimens transforming from larvae to males.The recently captured specimens also allowed new DNA analyses of all three life stages. In a wonderful piece of good luck towards the completion of the research, a specimen which was transforming from a juvenile (tapetail) to a female (whalefish) was caught in the southeast Atlantic in late 2007.

A final twist to the story is that the small males do not eat after maturing. Reproduction and the search for a female are fueled entirely by a huge liver.

The deep-sea midwaters of the ocean between sunlit surface waters and the bottom constitute the largest and most poorly known and sampled ecological area on earth. There are many more discoveries to be made in this little-sampled zone.

Publications From This Research:
Johnson, G.D., Paxton, J.R., Sutton, T.T., Satoh, T.P., Sado, T., Nishida, M. & M. Miya. 2008. Deep-sea mystery solved: astonishing larval transformations and extreme sexual dimorphism unite three fish families. Biology Letters. http://dx.doi.org/10.1098/rsbl.2008.0722.