The Snailfish

A specimen jar arrived on deck just after noon. The scientist who brought it up the ladder from the wet lab held it carefully in both hands and walked to a folding table in the shade. Polly hopped over, the small round lenses of her glasses bobbing slightly with each step.

In the jar, suspended in clear preservative, was a fish about the length of her own wing. It looked, at first glance, like a soft-bodied tadpole that had been left too long in water. The body was gelatinous and translucent, a faint pinkish-white. The head was disproportionate, with two small dark eyes and a faint, almost worried-looking mouth. The tail tapered to a fine point.

This was Pseudoliparis swirei, the Mariana snailfish. It had been hauled up from 7,966 metres below the surface. As far as anyone yet knew, no fish lived deeper.

"At that pressure," said the scientist, more to the jar than to Polly, "an ordinary fish would simply crumple. The proteins in its cells would refuse to fold properly. The membranes of its cells would not work."

Polly cocked her green-feathered head. The scientist explained, slowly, what made the snailfish able to do what no other fish could.

Inside every cell, in every animal, there is a class of compound called osmolytes that helps the cell hold its shape against the surrounding fluid. The snailfish carries an unusually high concentration of trimethylamine N-oxide, or TMAO. The TMAO acts as a chemical brace against the pressure, stopping the proteins from folding incorrectly.

"There is, of course, a limit," the scientist said. "Past about 8,400 metres, even TMAO is not enough. That is, we believe, why no fish has ever been found below that depth. The cells simply cannot be held together any more."

Polly's red head tilted. Eight thousand four hundred metres. A natural boundary, marked not by water pressure alone but by the molecule that holds a fish together. Below that line, the trench was empty of true fish, only invertebrates: amphipods, sea cucumbers, strange gelatinous things.

The snailfish was small. Small enough to slip down past most predators. Its skin was thin, almost gel-like, with no scales. It had no swim bladder; at this depth, a gas-filled organ would be impossible. It moved, when it moved, by undulating its long body.

"It is the most successful vertebrate at the bottom of the world," the scientist said. He held the jar up to the light. The body of the snailfish glowed faintly. "And until 2014, we did not even know what it looked like."

Polly thought about that. A whole creature, with a clever protein trick of its own invention, and the world had simply not seen it until twelve years before this conversation.

The scientist screwed the lid back onto the jar and turned away. The snailfish hung in the preservative, unmoving, its small face still set in that worried expression. The breeze on the deck was warm. Eight kilometres below, in the dark, others of its kind were quietly carrying on.