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As the sun sets over the waves, vast numbers of animals begin to rise from the depths of
the ocean towards the surface. Creatures that had sought refuge from predators during the
day. But now, under the cover of darkness, they gather near the surface to feed on the
abundant food. Driven by a number of processes, these deep sea migrations unite planktonic
organisms and their predators in a constant cycle of movement. Let’s take a look at
some of the deep sea’s unique migrating animals, and uncover their incredible adaptations
to a wandering lifestyle.
It is a necessity for creatures of the Pelagic zone to become nomadic. Spending their entire
lives in the expanse of the open ocean, they must learn to use the currents and cycles
to their advantage. Thus, many have become migratory, adopting certain routes and behaviours
in order to follow prey, find partners, or seek out secluded shallows to raise their
young. But most animals of the midnight zone living below 1,000 metres spend their lives
within a narrow depth range, relying on the meagre resources available. This, however,
is not true of mesopelagic animals from the twilight zone, between 200 and 1,000 metres
deep. By living at this intermediate depth, they have an advantage. A very minimal amount
of light makes its way down here, but it is enough that animals can use their incredible
light sensitivity to distinguish night from day. Using this ability, a huge diversity
of organisms migrate up towards the surface as night begins to fall, and migrate back
down again come the dawn. A mass movement so dense that it shows up on echo sounders
as a false bottom.
This is a phenomenon known as diel vertical migration, and it is the largest mass migration
on the planet. As a gathering of creatures from vastly different groups, it is a vertical
feast that fuels the ocean’s food web. And it all relies on the smallest of marine organisms.
Zooplankton, which migrate vertically towards the surface at night to escape deep sea predators,
and to feed on the microscopic algae that flourish near the surface. During the day,
they return to deeper waters to be sheltered from surface predators that rely more heavily
on eyesight. But zooplankton and their predators are engaged in a never-ending evolutionary
race, with each adapting to the behaviour of the other in order to better their chances
of survival.
The prey live their lives on the edge. Existence is a careful balancing act to ensure they
are neither eaten, nor starve to death. Heading back to the secluded deeper waters before
dawn may mean going hungry but staying alive. A necessary compromise. For zooplankton and
other prey like lanternfish, being unfed is better than being dead. And indeed, the depths
are somewhat safer. Here, their adaptations of transparent or dark-coloured bodies, large
eyes, and bioluminescence help them disappear into the inky darkness. At the surface, these
features become a liability.
But the migrations are necessary. Without them, the steady supply of marine snow - the
organic material that sinks ever-downwards from the shallows - would be barely enough
to sustain the creatures of the mesopelagic. There is a great deal more food available
in surface layers of the ocean, largely due to the presence of algae and microscopic plants
that cannot exist in deeper waters where there is no light. Phytoplankton need high light
levels for efficient photosynthesis. And the zooplankton must gather at the surface to
prey on them. Consequently, the entire food web of the deep sea follows, and up from the
depths come the innumerable shoals of sharks, fish, squid, and all manner of peculiar creatures.
Comb jellies lie in wait of passing plankton, undulating their hair-like cilia to attract
prey. With teeth too big to fit in their mouths, and armed with alluring bioluminescence, toothy
viperfish also rise towards the surface.
But among the key prey animals that adopt this daily migratory behaviour are krill - tiny
crustaceans that are a vital source of food for a wide range of predators - from fish
to gigantic whales. During their migrations towards the surface, krill gather in large,
dense swarms that provide a concentrated source of nutrients.
Siphonophores, like the small
but abundant Nanomia bijuga, follow the prey in their migration. Another creature, the
giant larvacean builds a large mucous net around itself that can be over a metre wide.
A deadly trap used for capturing krill. Thus, the larvacean represents another strategy
used to hunt migrators. They, like many creatures, hang motionless in the water column in wait
of the migrating shoals to pass through.
The arms race between predator and prey has lead to deep sea predators evolving various
strategies for hunting migrators. Some active hunters chase them towards the surface. This
is demonstrated by the behaviour of many aggressive
predators squids like the Humboldt squid,
which hunts one of the world’s most abundant deep-sea prey species. Lanternfish. Aglow
with bioluminescent light, these fish are abundant enough to outweigh the entire planet’s
annual fisheries catch, and pursued by the gigantic predatory squid, they join the 5
billion metric tons of animals that migrate towards the surface each night.
But even these predators must watch their backs.
Overall, the 24-hour cycle of diel vertical migration is one of the great forces that
shapes the diversity and productivity of the oceans. Life in the deep sea would be nowhere
near as diverse without this cycle. It helps to maintain the health of the Earth itself
by forming a massive exchange of nutrients, oxygen and carbon that sustains life both
above and below the waves.
Next time, we’ll examine the trans-oceanic seasonal migrations undertaken by larger animals
in response to changing seasons and food availability,
from the infamous journeys of whales, to the
treacherous migration of sardines.
To find out more about the deep sea, and to visualise just how deep some of these creatures
live, visit the ‘exploring the zones’ page over on our Deep Sea Hub, where you can
scroll down through a scale model of our oceans.