Life in the deep ocean is tough. It’s dark, cold and the pressure of all that water crushing down is immense.
To make things even more challenging for the denizens of the deep, there’s also not a lot of food down there. What little light does manage to trickle down beyond the first few hundred metres is too dim to power photosynthesis.
So, in most places in the deep there’s no new food being made and animals either have to catch scraps of organic matter sinking from the surface, or prey on each other.
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As such, if you live down here, it pays not to be too fussy about what you eat – a rule that the black swallower fish, Chiasmodon niger, takes to an extreme.
Compared to other deep-sea fish, like anglerfish with their long fangs and pelican eels with their gigantic jaws, black swallowers don’t look all that outlandish. They’re relatively small, usually around 15–20cm (6–8in) long and never get any bigger than a school ruler.
And if they haven’t recently eaten, a black swallower has a slender, sardine-shaped body. Encounter one after it’s eaten, however, and you can see where they get their name.
Black swallowers can swing their jaws wide open and have incredibly stretchy stomachs, which can expand to such an extent that the skin around them becomes transparent.
Their pelvic fins aren’t fused, so their stomachs can inflate to huge sizes. It means the black swallower’s eyes are definitely not bigger than their bellies, and they can gulp down pretty much anything they find, without chewing.
They can swallow fish twice their size and more than 10 times their body weight. If humans had the same ability, we would be able to put away 300–400 burgers in a single sitting.
Another name for black swallowers is the snaketooth fish, which hints at another adaptation they have for feasting on relatively enormous prey. They likely grab their prey by the tail, then inch their teeth over its body, like a boa constrictor, until they’ve swallowed the whole thing.
This feeding habit is undoubtedly a useful adaptation for surviving between 700 and 3,000m down (approx 2,290–9,850ft), but it comes with a drawback. After swallowing an enormous meal, it takes time to digest it.
For a snake this isn’t a problem; they can just lie somewhere hidden away and get on with the job. But for a fish, their digestive enzymes may not be able to break down their food before it begins to rot.
If that happens, decompositional gases can inflate a black swallower’s belly like a balloon and make the fish pop up to the surface, which is a lethal, one-way journey.
This is how the species was discovered in the 1860s, when scientists found one floating on the surface with a huge, distended belly.
More recently, in 2007, a fisherman in the Cayman Islands, found a 19cm (7.5in) black swallower floating with the remains of an 86cm-long (34in) snake mackerel inside it.
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