No one knows exactly how many fish live in the twilight zone—researchers keep revising their estimates upward—but we do know that it contains more fish biomass than all the other regions of the ocean combined. In particular, it teems with a toothy, glittery predator called the bristlemouth. If a lone bristlemouth passed by—they wriggle, rather than swim—you would likely miss it, because it’s half the size of a crayon. In fact, aside from marine scientists, few people have ever seen one, or have any idea what it looks like (think barracuda in minnow form), or even know that it exists, which is
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