Researchers believe they have solved the puzzle of three seemingly different fish, one all males, one all females and one all juveniles. They're the same fish, and undergo remarkable changes as they mature.
"You can imagine it was a pretty exciting discovery," said G. David Johnson, an ichthyologist at the Smithsonian's National Museum of Natural History. "The pieces kept falling into place."
"And it tells you how little we know about the deep sea, Johnson said in a telephone interview.
The fish live in the sparsely populated deep water thousands of feet below the surface, though as youngsters they rise to shallower levels where there is more to eat.
Cetomimidae, a type of whalefish, had been known since the 19th century, but only females had been found.
Seemingly related species called Mirapinnidae, or tapetails, and Megalomycteridae, or bignose fish, were identified in the 1950s and 1960s. Tapetails were only found as juveniles and bignoses only as males.
Although their skeletons indicated the three were related, there were so many differences no one could believe they were the same fish at different sexes or stages in life, Johnson said.
But it turns out that is the case, Johnson and colleagues report this week in Biology Letters, a journal of Britain's Royal Society.
All three will now be classified as Cetomimidae, he said.
Johnson said the researchers were able to link the fish through comparative anatomical study and, once they obtained fresh samples, by their DNA.
The larvae are called tapetails because they grow long streamers, he said. The purpose of the streamer remains unknown, but several fish larvae develop similar appendages, so it must have some value, he said. They reside within 600 feet of the surface, a region well stocked with food.
As adults, however, these fish descend thousands of feet down into the dark ocean.
There is scarce food there and the females cope by developing a large mouth — a common trait among fish living in the deepest waters — and they even develop teeth in their gill area that can serve as an additional mouth.
Even stranger, males who reach adulthood don't eat at all. Having gorged as larvae, their jaw fuses and they develop a vestigial gut that only stores shells from previous meals. That's an advantage, Johnson said, because in the deep ocean "there's not a lot of food, you're better off taking your lunch with you." The males gorge as larvae and grow a giant liver, storing energy there to live on.
"This thing was basically a set of testes looking for the female," Johnson said.
The males also develop a large nose to sense smells in the dark water.
Meanwhile, researchers had noted that females have some unusual tissue, separate from the skin, on their body. It's not luminous, so Johnson speculated that this tissue may produce a pheromone that the big-nosed male can home in on.
Co-authors of the paper were John R. Paxton of the Australian Museum, Sydney; Tracey T. Sutton of the Virginia Institute of Marine Science, Takashi P. Satoh and Mutsumi Nishida of the University of Tokyo and Tetsuya Sado and Masaki Miya of the Natural History Museum, Chiba, Japan.