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In the depths of the ocean, going alone is rarely an option. The harsh environments of
this seemingly infinite void are an imposing challenge for any lone organism to face, as
the dark conceals many dangers, while dazzling lights deceive unwitting prey.
For this reason, animals in the ocean tend not to spread out. Instead, wherever possible,
they come together in their masses to create abundant patches of life in the lonely depths.
Hotspots of biodiversity, in stark contrast with the otherwise barren stretch of darkness.
Safety in numbers is a fool-proof survival strategy that increases their chances of finding
food, evading predation, or having reproductive success in the endless blue.
From vibrant coral gardens and scavenger communities,
to expansive blooms that cloud the ocean surface,
these are the animal aggregations of the oceans.
From zooplankton to top predators, aggregations are an incredibly common phenomenon in the
pelagic zone due to the challenges of living here. Strong winds, towering waves, and vast
empty regions of low prey availability mean animals can either adapt to life as solitary
migrators, with streamlined bodies and powerful tails for covering large distances in search
of food, or they can gather together and find sanctuary in huddled congregations of life.
They can migrate, or aggregate. Creatures that chose the former option, like the ones
we discussed in the previous video, often need to journey for thousands of miles each
year between breeding and feeding grounds.
But for some, this simply isn’t an option.
Many open ocean drifters come together not of their own accord, but due to the movement
of currents gathering them into biodiverse swarms of life.
Beneath the waves, current-induced blooms of jellyfish get swept together by colliding
currents. Here, they coincide with a rich supply of plankton, on which they gorge themselves
until the entire food source is depleted. What follows is a mass die-off in the jellyfish
swarms, and another remarkable phenomenon takes place.
Their corpses begin to sink en-masse, triggering a major nutrient cycling event that provides
abundant nutrition to benthic communities on the sea-floor.
These jellyfish blooms demonstrates how animal aggregations that form rapidly can sometimes
strip one region of its nutrients, and in death export them to another.
The coordinated behaviour of fish takes pelagic aggregations one step further. Rather than
being a product of currents, it is the behavioural adaptations of fish that leads to them forming
two kinds of aggregation.
Shoals and schools. In a shoal, they swim together loosely and without much organisation.
But in a school, the movements of fish become mesmerising. Shimmering, turning, contracting
and expanding in unison, hundreds or thousands of individual sardines move together as if
performing a well-rehearsed ballet.
A seemingly choreographed display of dramatic flourishes, made possible by their incredible
ability to coordinate their body positions and synchronise their movements. All in the
name of survival. Remarkably, there’s no leader. These schools demonstrate a phenomenon
called self-organising, in which each fish
maintains a zone of repulsion with their neighbours.
The dance works as each fish only needs to coordinate with individuals nearby, creating
a wave-like movement made up of thousands of individual movements.
The effectiveness of schooling behaviour is demonstrated by these lanternfish. The display
visually confuses predators like the voracious Humboldt Squid, making it harder to single
out any individual fish and giving them a better chance of survival than going it alone.
But although there may be safety in numbers, the mass swarming of pelagic prey also serves
to draw predators out from the depths with the promise of a meal.
Every night, aggregations of zooplankton rising from the depths triggers the largest marine
gathering of all.
A host of vertical migrators amass near the surface, from opportunistic cephalopods to
filter feeding behemoths like the megamouth shark, all drawn from the twilight zone by
the prospect of a plankton platter.
Among them are krill, gathering in swarms of up to 60,000 individuals per cubic metre.
In total, 5 billion metric tonnes of marine organisms take part in this nightly migration,
and unwittingly play a very important part in supplying deeper waters with nutrients.
As these migrators descend once more as the sun rises, they defecate, releasing organic
material in the form of large fecal pellets that sink quite rapidly.
By defecating at depth rather than the surface, the pellets make it to the bottom much faster,
and the efficiency of this nutrient transfer is increased. Benthic animals, confined to
life near the bottom, cannot participate in this migration.
They instead rely on the pellets. So, just like jelly-falls, the vertical migration ends
up nourishing the benthos and allowing life to thrive at the very bottom of the ocean.
Down in the depths of this hidden world, these creatures of the sea floor form aggregations
of their own.
Some of the
decaying corpses of jellyfish and the fecal pellets of vertical migrators settle on the
sea floor. It’s not much - only a small proportion makes it all the way to the bottom
- but it’s vital for any of the animals living here. Groups of scavengers looking
for a meal arrive very quickly, forming carrion communities on the abyssal plain.
Ravenous hagfish strip the flesh of larger meals, while sea cucumbers sift through the
sediments to feed on smaller particles.
But where these carrion communities are most prolific, are around regions of abundant material,
like whale-falls.
A sunken whale carcass provides the benthos with a sudden concentrated source of food,
which animals make sure to take advantage of.
Drawn by the scent of death, crustacean hordes like these pink amphipods amass to feast on
the whale’s flesh. Some organisms have become wholly reliant on whale-falls for survival,
like bone-eating osedax worms which burrow down into the whale’s skeleton and feed
on the enclosed lipids. Unlike the open ocean where fish aggregate for protection, carrion
communities assemble to share a meal.
It’s not just mobile scavengers that aggregate on the sea floor. Sessile animals, meaning
those that are fixed in one place, tend to settle in far-reaching communities.
Crinoids cling to any solid surface they can find, where they feed by filtering sinking
organic material out of the water.
In their thousands, ghostly grimpotethuis octopuses
tuck themselves away in crevices at the base of a seamount to lay their eggs, attracted
by warm seeps in the seafloor. Cold-water corals are responsible for creating one of
the largest of all deep-sea aggregations.
Expansive coral gardens, forming hotspots of life on continental shelves and slopes,
and the upwelling zones of monumental ocean seamounts where they are bathed in nutrient-rich
currents.
Seamounts are of significant importance to all kinds of marine life, for they are one
of the only locations where seafloor aggregations collide with open ocean swarms.
The concentrated upwelling of nutrients that rise from the ocean floor fuels productive
hotspots for both sessile communities, and migratory fishes.
Even pelagic wanderers that tend to go it alone are drawn here, unable to resist the
abundant food. In fact, seamounts are common feeding grounds for solitary sharks and migrating
whales, who emerge out from the endless blue to feed.
Returning to the sea floor, corals are just one example of the deep sea’s many reef-builders.
Alongside sponges and mussels, they belong to a collection of crucial sessile organisms
that provide food and shelter, and bring structure and complexity to the otherwise featureless
abyssal plain.
Sponges form their own aggregations, called the sponge grounds. Here, a high diversity
of symbiotic life can be found inhabiting the intricate structures of these pale deep
sea sponges, nourished by the nutrients the stalked sponges pull out of the water column
and bring to the seafloor.
Squat lobsters, sponge-dwelling shrimps, and crabs all seeking refuge from predators. This
larger deep sea crab carries a sponge in its dorsal legs as a crude form of camouflage,
while granting the sponge access to
nutrient-enriched currents.
This relationship highlights another way that coming together increases an animal’s chances
of survival. By forming communities, animals can form symbiotic associations with one another,
and can adapt in ways that would not be possible without the presence of their symbiont.
These Ophiocreas brittle stars form a close association with cold-water corals. The corals
provide a perch that grants the brittle stars access to faster, more nutrient-rich currents.
In return, the brittle stars may prevent parasites from damaging the coral.
An excellent example of non-obligate symbiosis, meaning both species can survive without the
relationship, but coming together allows them to thrive rather than just survive.
We’ve discussed the symbiotic associations of chemosynthetic bacteria with deep sea organisms
many times in this series, but not yet considered how this influences the formation of aggregations.
Around hydrothermal vents, the presence of these bacteria converting dissolves chemicals
into nutrients creates a diverse and abundant assortment of life. This is a zone of primary
production, where instead of sunlight, it’s chemicals that are used to form organic material.
Thus, we find some of the most peculiar of all aggregations. Tube-worms cluster in the
sediments.
Mussel beds of epifaunal bivalves encrust the rocks, building an extensive reef
on which many other organisms rely, like these predatory crabs. This goes to show that regions
of productivity are especially important when it comes to the formation of aggregations.
Aggregations are an important feature of marine ecosystems. Arising in areas of productivity,
stability, or high nutrient density, the large-scale
assembly of animals creates dynamic patches
of life.
Here, increased survival rates directly influence biodiversity in both the midwater, and for
the benthos, for although these aggregations are physically isolated, the action of open
ocean aggregations directly nourishes those on seafloor. It is a prime example of how
a behavioural adaptation can be a powerful tool for survival.
So, whether it be for protection
from predators, foraging or scavenging, sticking together is a crucial strategy for an incredible
array of marine species.