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Compared to other creepy deepwater critters like gulper eels or angler fish, Chiasmodon niger looks pretty unassuming—kind of like an ugly anchovy.
It grows to about 25 centimeters long, and is found all around the world at about 700 to 2800 meters deep in what scientists call the twilight and midnight zones of the ocean because there’s so little light.
And while Chiasmodon is not, you know, a beautiful fish, with its rows of long, needle-like teeth and a mostly scaleless body, it’s still pretty normal looking… as long as you catch it before it eats.
It’s common name is the black swallower because it can eat meals several times larger than itself,
and make competitive eaters worldwide jealous of its stretchy stomach.
In 2007, a fisherman off the Cayman Islands found a dead 19 centimeter-long black swallower floating on the surface of the ocean.
Inside its guts was an 86 centimeter-long snake mackerel—
that’s four times as long as the swallower!
Other reports suggest they can eat meals twelve times their mass!
If humans could eat that much in one sitting, the world record for competitive hot dog eating would be closer to 8,500 hot dogs, rather than just over 70.
Swallowers manage this incredible feat of gastronomy thanks to their large mouths and very elastic stomachs, which can expand so far that the skin becomes thin enough to see through.
Fish in the swallower family also have pelvic fins that aren’t fused, much like the lower jaw bones in a snake, so they can stretch their chest to make room when their belly expands.
Ichthyologists think that when swallowers hunt, they first seize their victim by the tail,
and then bit by bit scoot the jaws up like a boa constrictor until the luckless prey is completely engulfed.
And it makes sense that they’ve evolved the ability to eat pretty much anything they can get their big mouths around.
Down in the deep ocean, where food can be pretty scarce, having that kind of flexibility would be a great adaptation for survival.
But no one knows yet exactly how they pull it off.
When snakes eat a big meal, they have to ramp up their metabolism and even enlarge organs like their liver and heart to fully digest their food.
It’s likely these little fish have evolved similar ways to cope with such large meals.
But their big appetites can sometimes get the better of them.
Digestive juices can only work so fast,
and if a meal is just too big—like a nearly 90 centimeter-long snake mackerel—the prey can start to rot and decompose inside the stomach.
Like all rotting food, a decomposing meal releases gases.
And these can inflate the swallower’s stomach like a balloon, lifting the creature to the surface of the ocean and killing it.
That’s what happened to the one from the Cayman Islands,
and it’s actually how we first discovered this species back in the 1860s.
In fact, for over a century their habit of fatally overeating was the main way scientists collected specimens, other than a few that were dragged up by deepwater fishing.
Now, thanks to better deep-sea vehicles, we’re able to spy on them in their natural habitat.
In 2017, for instance, a NOAA rover spotted one in the Gulf of Mexico with a bit more of a dignified profile.
And, unsurprisingly, a very full belly.
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