The Whalefish Mystery

A red deep-sea fish specimen.
This deep sea creature has a whale-like body and gaping mouth. It grows up to 400 mm (15.7 in) long. (Kunio Amaoka)
Meet the Suspects

More than a century ago, in 1895, two Smithsonian scientists described a new kind of deep sea creature living at least 1000 m (3,280 ft) below the ocean’s surface—a part of the ocean that we still know very little about.

The scientists named their find the whalefish because of its whale-like appearance. Little did they know that this fish would become one of the prime suspects in a mystery that took scientists from around the world decades to solve.

Preview Tapetail fish with long tail.
This fish’s tail looks like a long streamer. It grows only up to 45 mm (1.7 in) long. (Chris Kenaley)
The Mystery Develops

Flash forward to 1956, when scientists described another new kind of fish. It was named the tapetail because of its long, streamer-like tail. It also had a large upturned mouth.

Unlike the whalefish, the tapetail was found living near the ocean’s surface. And there was something very curious about this sea creature: Every single one of the 120 tapetail specimens scientists studied was a larva or juvenile.

Where were all the adults?

The Plot Thickens

In 1966, based on 11 specimens, scientists added another deep sea creature to the list of mystery suspects: the bignose fish, found living deep in the sea like the whalefish. It has an unusual nose-like bulge on its snout with large organs for smelling. Its upper jaw can’t move. And something else proved odd about the bignose fish: Of the 65 specimens now collected, everyone is a male. Where were the females?

Then, in 1989, the whalefish also became a suspect. An Australian scientist studied all the whalefish specimens collected so far—a total of over 500 from all over the world. Every adult was a female. Where were the males?

Examine the Clues
Preview John Paxton, a scientist who studies marine life, looks into a microscope
Ichthyologist John R. Paxton of the Australian Museum identifies freshly caught lanternfishes. Paxton was on the team that solved the whalefish mystery. (R. Cornejo)

In 2003, a team of Japanese scientists analyzed the DNA of tapetails and whalefish. The results suggested that these two very different looking fishes were almost identical in one specific gene. But more clues were needed. An international team of marine biologists took a closer look at specimens of tapetails, bignose fish, and whalefish in museum collections. The team included Dave Johnson, an ichthyologist at the Smithsonian. Here’s what the team found:

  • Bignose fish have no throats or stomachs, but they do have enormous livers.
  • The immovable jaws of bignose fish could conceivably develop from the moveable jaws of tapetails.
  • Some tapetail specimens appear to be in the process of changing into bignose fish. With these and other clues from museum collections, scientists were able to crack the case.
Aha! They’re All in the Family

It may be hard to believe because they look so different, but tapetails, bignose fish, and whalefish are actually all members of the same family (Cetomimidae).

  • Tapetails are the young, or larvae. They use their upturned mouths to gorge on small shellfish.
  • Bignose fish are the males. They feed off of their huge livers and use their large nasal organs to sniff for females.
  • Whalefish are the females. They use their gaping mouths to capture large prey.

There are other examples of males and females with very different shapes (sexual dimorphism) and of animals changing from one shape to another as they grow older (metamorphosis). But this is one of the most amazing examples of sexual dimorphism combined with metamorphosis ever found among vertebrates.

Preview The tapetail is the larva of the family. It transforms into either a male (bignose) or female whalefish.
The tapetail is the larva of the family. It transforms into either a male (bignose) or female whalefish. (Composite by Dave Johnson/Smithsonian Institution)
Museum Collections Hold the Clues

“This is an incredibly exciting finding,” says Smithsonian ichthyologist Dave Johnson. “The answer to the puzzle was right under our noses all along—in the specimens. We just needed to study them more carefully.”

This scientific mystery clearly demonstrates the importance of museum collections. Many years after a specimen was collected, it may provide biologists with the answer to a new question raised by science. 

“The study also shows the need for continued exploration and collection in the open ocean—from the surface to the deep sea,” says Johnson. “Who knows what other mysteries remain to be solved there?”

Preview An image of collections storage at the Smithsonian National Museum of Natural History.
There are about 4 million specimens in the fish collection housed at the National Museum of Natural History. (Chip Clark/Smithsonian Institution)
October 2010