Through a glass, darkly

This immortal jellyfish danced "like a tornado", says photographer Jialing Cai
(Credit: Jialing Cai/ Ocean Photographer of the Year)

Something wondrous and most mysterious happens every night on planet earth,
the largest migration of the tiny, largely invisible, until now.

Dance of the immortal jellyfish

This mature immortal jellyfish "looked like a lightbulb in the night of the ocean", says Cai. The immortal jellyfish gets its name from its ability to "revert itself back into its baby stage, its polyp stage" on sensing danger, such as pathogens in the water, she explains. "It can restart its lifecycle again and again." In theory, immortal jellyfish can live forever. 

A juvenile fish has captured a jellyfish in its mouth and uses its tentacles as a weapon
(Credit: Jialing Cai/ Ocean Photographer of the Year)

The diel vertical migration crew is a crowd jostling with both predators and prey. "These little critters form the basis of the food chain," says Hobbs, "so they're really important for everything from fish to whales." To survive being eaten by larger predators – or each other – many of these animals have developed ingenious defence strategies to protect themselves.

These tiny animals face a gauntlet of predators as they migrate to the surface
(Credit: Jialing Cai/ Ocean Photographer of the Year)

"Here you can see a juvenile trevally, a kind of jack fish, sitting on the bell of a jellyfish," says Cai. Inside the jellyfish, you can see a baby octopus is being digested. "Its soft, blood-tinged eyes are visible within the gelatinous stomach. For the baby octopus, this jellyfish is brutal." For many small zooplankton, said Cai, the jellyfish's gut becomes their final resting place. For others, like the jack fish, the jellyfish offers refuge.

Cai's images, says Hobbs, offer a fascinating insight into the diversity of life at this scale in the ocean. "These huge migrations are happening as part of a complex three-dimensional environment, where tiny zooplankton move towards the surface to feed whilst simultaneously running the gauntlet past a variety of predators. It's amazing to think that each of these species is following their own strategy, shaped by its preferred prey, potential predators, and reproductive needs."

As larvae, crabs drift in the water column undergoing multiple stages of metamorphosis (
Credit: Jialing Cai/ Ocean Photographer of the Year)

Backgrounder

When Cai was studying marine biology at the University of Virginia in the US she thought exploring the deep sea would be as ambitious going to outer space. Then, one day, her lecturer explained the phenomenon of diel vertical migration (DVM). 

Every single night, he explained, trillions of tiny zooplankton rise from the deep sea to feed in the relative safety of the cover of darkness. They swim for hundreds of metres to shallow waters, some even making it to the surface. During this nightly migration, the ocean hums with an "evening chorus". This is the chatter of countless fish, shrimp, jellies and squid making their way to the surface. 

Then, just before sunrise, they return to the abyss.