Dad (bignose) at bottom, mum (whalefish) middle, and baby (tapetail) at top: so different they look like separate species (Image: G.David Johnson/Donal Hughes/Bruce Robinson)
IT IS the very opposite of family resemblance: three groups of strikingly different-looking fish that turn out to be males, females and young of the same family.
Tapetails (pictured, top) live in shallow waters and are named for the long streamers that trail behind them. Whalefish and bignoses are both deep-sea fish, but while whalefish (middle) lack scales and have huge jaws, bignoses (bottom) have long nasal organs and immobile jaws, and live off energy stored in their gigantic livers.
Nobody thought these groups were related. "The differences were so extreme," says marine biologist David Johnson of the Smithsonian Institution in Washington DC.
Then a study found that whalefish and tapetail mitochondrial DNA is virtually identical, prompting Johnson to re-examine museum specimens. This revealed one in the process of changing from a tapetail into a whalefish. Specimens intermediate between tapetails and bignoses were collected in 2007, and together with more DNA analysis this proved that the three families are really one (Biology Letters, DOI: 10.1098/rsbl.2008.0722).
A "tapetail" larva grows up to be a "whalefish" female or "bignose" male. Johnson claims this is the most extreme metamorphosis ever seen in a vertebrate.