Thanks to the Monterey Bay Aquarium and their research and technology partner MBARI for partnering with us on this episode of SciShow.

They are super excited for folks to learn about the deep sea. [♪ INTRO] Even though they live in Earth’s largest habitat, we just don’t know a whole lot about the extraordinary creatures found in the deep sea. So we typically have to base our research on animals that were caught in trawl nets, or more recently, by combing through thousands of hours of deep-sea video recording.

This means we’re often left with more questions than answers. That was certainly the case for a few different taxonomic families of deep-sea fish that had long been puzzling scientists, some for over a hundred years. Despite collecting many individual fish with shared traits that helped us place them into three distinct groups, some key specimens from each family were weirdly absent, leaving huge gaps in our understanding of their lives.

So we thought we knew very little about these three different families of fish, but as it turned out, we actually knew a lot about one. The fish family Cetomimidae has been known to scientists since as early as 1895. They’re called whalefish because, well look at them, and they’re about as unusual-looking as you’d expect a fish found in the deep sea to be.

They have tiny, poorly developed eyes, but seem to get along just fine without sophisticated eyesight. After all, there’s not much detail to see way down in the midnight zone, the midwater area below 1,000 meters and the ocean floor. Instead they’re equipped with large sensory pores that detect the movement of predators and prey, allowing them to navigate their surroundings even in complete darkness.

But it’s not just their unexpected appearance that made scientists puzzle over the whalefish. Even though researchers have collected more than 600 specimens of several different species from within this family over a hundred-year span, all of them were female, and they were always sexually mature. Without any known males or juveniles, this family remained a mystery.

Up until the early 2000’s, we were also puzzling over the Megalomycteridae, a family known as the bignose fishes, also found below 1000 meters. These fish are long and fairly small, with their namesake large schnozz. It houses large nasal organs, presumably to sniff out a mate.

Adding to the curiosities about these unusual fish, adult males don’t bother to eat. They have immobile jaws, and no esophagus or stomach, just a big liver storing energy. They also have a pair of enlarged gonads, so it would appear their strategy is to survive just long enough to reproduce.

And they represented a parallel head-scratcher to the whalefish. All bignose specimens were male. Enter puzzle number three, the Mirapinnidae family, which are generally found above 200 meters.

These astonishing-looking fish have large mouths with jaws pointed dramatically upward, like an intense under bite. The family is mostly composed of species of tapetails, which have a long streamer on their tail fin that can be up to nine times their body length. That is, this is what they look like as larvae.

Despite having collected over one hundred individuals since the 1950s, only sexually immature tapetail specimens were ever found. Well, put together one fish family of only females with one family of only males, add in another of only juveniles, and you get one true family of fish! Sure, congratulate yourself for calling the twist, but nobody laid it out that way for the scientists who had to piece it together!

Thankfully, while looks can be deceiving, DNA doesn’t lie. In 2003, researchers compared small segments of the genome called mitochondrial DNA between a juvenile tapetail and female whalefish. And their mitochondrial genomes were almost identical.

And not long after, new bignose male specimens were found together with whalefish females at depths below 1500 meters, which sparked a connection between these two families. In 2009, scientists also determined that some of the bignose males were actually juvenile tapetails, changing shape as they matured! They performed further DNA analyses to close the book on both the bignose and tapetail families, confirming that they were in fact all one family, now collectively known as Cetomimidae!

And now that we’ve finally put their story together, we can better appreciate their unique adaptations through different stages of their life cycle, and between sexes. As juveniles, fish in this family gorge on small crustaceans, thanks to those big vertically-positioned mouths. But when they mature, they not only travel a thousand meters or more down into the ocean depths, but their bodies become radically different depending on their sex.

Species within this fish family showcase an extreme example of sexual dimorphism, where the males and females can look dramatically different from one another. As the males develop into adults, they lose the ability to eat. Instead, they store the energy from their last meals in a giant liver, which they rely on as their sole source of nutrients for the rest of their lives.

Presumably, this gets them through their search for a mate and not much longer. Meanwhile, females continue eating and grow to be much larger. Their jaws get longer rather than smaller, which allows them to gulp up very large prey.

While there’s still a lot to learn about the life strategies of these fish, these two opposing adaptations would help both males and females survive in the ocean depths where food might be hard to come by. While we’ve cracked the case that puts these three families together into one, we still haven’t fully matched together all the different life stages. We now know there are over 20 individual species of Cetomimidae, but we won’t fully understand who’s who when it comes to matching larvae with males and with females until more genetic analysis is done.

So although we’ve solved one big part of the puzzle, we’re still far from having a full understanding of these extraordinary deep-sea fish. It just goes to show we still have much to learn about deep-sea inhabitants and the extreme lives they live. Thanks to the Monterey Bay Aquarium and their research and technology partner MBARI for partnering with us on this episode of SciShow.

You can make like a whalefish and swim over to their websites and social media. MBARI’s YouTube channel has awesome videos about the deep sea, and on the Aquarium’s website, you can make a donation to support their exhibits, education programs, and ocean conservation work. Oh, and the Aquarium’s YouTube channel posts these amazing lofi mixes set to relaxing Aquarium videos that are perfect for getting work done, if you need somewhere to go next! [♪ OUTRO]