At the bottom of the ocean, several kilometres down, is the abyssal seafloor.

The pressure is crushing, the temperature is two to three degrees Celsius.

The darkness is absolute: no light means no nutrients, and thus almost no life.
Except

when

a

whale

falls.
When whales die, they fall to the bottom of the ocean. And death brings life.

Whale falls are the dark inverse of coral reefs.
A coral reef is composed of microscopic, living organisms. In the sun-dappled shallows, it hosts a thriving ecosystem. Many reefs are readily accessible to divers and snorkelers, and the species hosted by  coral reefs are familiar and even beloved, like the clownfish.
A whale fall is the dead body of a single, giant organism. In the darkest depths, it too hosts a thriving ecosystem. But most whale falls are only accessible to submersible vehicles, and the species hosted by whale falls are largely unfamiliar to us.
Some we do know, like the famously disgusting hagfish, with its habit of burrowing face-first into dead flesh and its defense mechanism of exuding slime.
Along with sleeper sharks, hagfish eat the soft tissues of dead whales. To feed, jawless hagfish extend their dental plates out of their mouths, unfolding four rows of horizontally-moving teeth which grab onto and pull off bites of food as they are retracted.
When hagfish are disturbed, they exude molecules that instantly turn the seawater around them into slime. Would-be predators (including sharks) find themselves gagging and suffocating as the slime fills their mouth and coats their gills. The hagfish feeds placidly on, unmolested.
It takes about two years for the soft tissues of a whale fall to be consumed, after which  only the skeleton is left. Bone consists of two components: a hard, brittle mineral called hydroxyapatite, and the protein collagen.
Whalebone is about sixty five percent mineral and thirty percent protein. What’s more, sequestered inside whale bones are energy-rich fats. Like a frugal cook who makes gelatin-rich stock from bones, nature doesn’t let the nutrition locked up in a whale skeleton go to waste.

Enter the Osedax.
Discovered in 2002, Osedax are a family of annelid worms that can consume bone. They have no heads, mouth or guts.

Instead, the treelike worms grow branches that collect oxygen dissolved in the water, and ‘roots’ that contain symbiotic bacteria. The bacteria pump out acid and enzymes to dissolve the bone, drilling through the bone and anchoring the worm, and they pass on the nutritious protein and fats to their host.
When scientists began to look at the Osedax specimens they had collected, they were puzzled: all the specimens were female. Where were the males? Then they looked more closely. It turns out that Osedax males are dwarfs; they consist of fully mature sperm-producing testes and not much else. And they live inside the bodies of the females, up to hundreds of dwarf males inside a single female.

Zombie bone-eating all-female harem-keeping worms at the bottom of the ocean.
Since that first species of Osedax was found, a score or so more have been discovered, all around the world. Processing a whale fall can take decades, and the best estimate is that upwards of 700,000 whale falls are on the seafloor at any given time. Since many of these would be on migratory routes, the distance between whale falls is estimated to be as little as five kilometres.
It is thought that Osedax larvae are free-floating in the ocean; when a suitable whale skeleton presents itself, they settle on it. Environmental cues likely ensure that the females develop first, and then the dwarf males.
But Osedax had yet another surprise in store. A new species was discovered recently which, to the shock of the scientists, featured free-living males. The genetic evidence suggests that in this species, for unknown reasons, the dwarfism (paedomorphism, to be precise) of the males was reversed. In this species, the males can extend their trunks to many times their contracted length, and they use them to transfer sperm to the females. This behaviour gave the species its name: Osedax priapus.
Osedax – with their recent discovery, unusual habitat, and bizarre behaviour – are a reminder of how much of our home planet remains alien to us.

A world of surprises remains to be found.