A scientist at the College of William and Mary’s Virginia Institute of Marine Science has helped solve a long-standing deep-sea mystery: Three separate families of fish — the tapetails, the bignose fishes and the whalefishes — have been found to be the juvenile, male and female forms of the same fish, respectively.
A study revealing the true genetic nature of the whalefish was published last month in the scientific journal “Biology Letters.”
It’s a tale that begins years ago and thousands of feet below sea level. VIMS professor Tracey Sutton said his colleagues, the other authors on the paper, had been exploring the oddity for some years.
“You’ll have a problem and you’ll work on it for a long time, and often it’s these kind of random occurrences where, all of a sudden, you get just that little piece of evidence you’re looking for and it kind of cracks the case, just like solving a crime,” Sutton said. “In this particular instance, we had a couple of key pieces of evidence that popped up at just the right time that allowed us to put it all together.”
Sutton’s co-authors, David Johnson of the Smithsonian Institution’s National Museum of Natural History and John Paxton of the Australian Museum in Sydney, had been working on putting together the pieces of the puzzle that separated the three fish families: the tapetails, the bignose fishes and the whalefishes.
Sutton said Johnson and Paxton had long suspected the three fish to be more closely related than previous scientists held.
A recent effort by Japanese scientists to identify and classify all of the world’s fish found that the three families’ DNA indicated they were in the same family.
“That was kind of a eureka moment, thinking, well, okay, these are related; they don’t look anything alike,” Sutton said.
So Johnson, an expert in skeletal morphology, set about examining the specimens’ bone structures.
In a process called staining, Johnson turned the muscle clear, the bones red and tendons blue to easily examine a specimen’s structure. He determined that the three fish were morphologically related.
Despite the genetic and structural evidence, the scientists needed a living fish to bridge the gap between the two families. Few specimens have been discovered due to the difficulty in finding and capturing them.
Of the three former fish families, the whalefish is the most prominent, with several hundred examples in museums around the world. The fishes formerly known as tapetails and bignose fish, however, number no more than a few dozen specimens.
“They’re so rare that a lot of ichthyologists would look at them and have no idea what they were,” Sutton said.
After finding a specimen, the scientists then face the problem of actually collecting it.
“These real deep-living fishes are extremely fragile. They’re built to live in an environment [in which] there’s nothing solid except the other animals. There’s no bottom, no surface or whatever, there’s just midwater animals and they’re really fragile, so when you tow a net and bring them up often they’re broken, bent or in some way mangled,” Sutton said. “But if you use the right kind of gear or if you just get lucky — if you catch a bunch of jellyfish in your tow — then it kind of cushions the sample.”
Despite these difficulties, Sutton happened upon the very evidence they needed off the African coast during a research expedition in 2007.
“We got a female whalefish who was metamorphosing from one form to the other,” he said. “We got kind of a missing link specimen.”
Sutton explained that the new specimen — formerly known as a tapetail — was growing into an adult female whalefish.
The specimen, he said, still had remnants of the pelvic fins that mark the juvenile form but are lost in the transition to adulthood.
“It shows that they have pelvic fins when they’re young and then lose them when they get older,” Sutton said.
“And that was the piece of evidence we needed.”
From that specimen Sutton, Johnson and Paxton were able to conclude that the tapetail is a juvenile whalefish, the bignose fish is the male whalefish, and the family previously known as whalefish is actually just the female form.
“I was very excited for my co-authors to … get a nice conclusion out of something they’d worked on a long time,” Sutton said.
This study will have a strong impact on marine science and biology, Sutton said. New fish textbooks, for example, will have to be edited to reflect the discovery.
It also reminds Sutton of how little about the ocean is known.
“What it does is kind of highlight our lack of knowledge of most of the living space on Earth,” he said. “The deep sea, that’s by far our largest living space on Earth.”
The study also holds lessons on the process of scientific discovery.
“At the end of the day, if you have DNA data and careful study of something, that’s the key, rather than relying on what’s been said before is written in stone,” he said. “Science in general can still proceed by looking at something from a different angle.”
Sutton said the project also proves scientists from across continents can work together for a common purpose.
“There’s this kind of excitement that you can use a team of people from different countries all over the world to team up on one study,” he said. “It kind of shows how the communication era now can facilitate projects that are worldwide; it doesn’t have to just be a little group of people.”