Surviving in the deep sea is no easy task.
With fierce competition, no sunlight, and scarce food and nutrients, organisms at these depths
have developed a number of peculiar lifestyles in response to such an extreme environment.
Nearly all nutrients in the depths are supplied by debris that falls down from the surface waters.
The food webs are sustained only by a slow drizzle of organic particles
and detritus, known as ‘marine snow’.
It isn’t much, but it’s enough to sustain scattered communities of detritivores
who rely on this organic matter for food.
However, on some occasions, the nutrient-poor depths are treated to a feast.
Every now and then, a cetacean carcass will sink to the sea-floor, and draw scavengers from miles
around to feed on the nutrient-rich carrion. This is known as a whale fall, an event that creates
complex localised ecosystems on the sea floor which can support life for decades.
As a whale dies and sinks, the carcass provides a sudden, concentrated food source
in the otherwise food-poor abyss. In fact, when whale-fall ecosystems were
studied in 1988 by a team of researchers at the University of Hawaii, it was estimated that the
sedimentation of a decomposing 40-ton whale carcass provides nutrients equivalent to 2,000
years worth of material that falls as marine snow, over an area of 50 square metres.
In a region of the ocean where food is often just a slow trickle from above,
these events are significant. The body can last for decades. In that time, it creates an island of
life on the sea-floor, supporting an ever-changing ecosystem that blooms out of the benthos.
And as it decomposes, it provides a miniaturised model of ecological succession.
In much the same way as life on oceanic islands sees a change in its communities over time,
the stages of whale decomposition support a number of different scavenger communities over time.
And just like islands, whale-falls are hotspots of adaptive radiation - the process
in which organisms rapidly diversify from an ancestral species into a multitude of new forms.
This is because the new resources provided by whale-falls alter the ways in which deep
sea creatures interact with one another, and open up new niches for organisms to occupy.
As a result, many new species that appear to be specialised to living exclusively at whale-falls,
have been discovered during recent studies.
These findings suggest that whale falls may help to generate biodiversity on the sea floor. Let’s
take a closer look at the complex, fascinating communities that gather at whale falls.
It begins at the surface. Soon after death, the insides of the whale begin to decompose.
As decay sets in, gases cause the body to expand and float up to the ocean surface
where it’s scavenged by sharks and seabirds.
But once these gases escape, the whale’s body begins to sink
into the sunless void of the deep sea.
Once it settles on the ocean floor at depths of a kilometres or more,
it can be classified as a whale-fall. And now begin the 4 key stages of ecological succession,
which each support their own distinct communities of life.
First is the mobile-scavenger stage. After the carcass comes to rest on the ocean floor,
life arrives at the scene in a matter of hours. Mobile scavengers such as hagfish and
sharks are lured by the scent of death, and swim from afar to consume the soft tissue and blubber.
Hagfish use their dental plates to burrow deep into the carcass, or bite off chunks of food.
While they eat, they twist their bodies into a knot, then passing the knot along their
body and force it against the carcass to increase the power of their bite.
This allows them to tear flesh from even the toughest of meals.
The mobile scavenger stage of whale-fall ecosystems is characterised by larger organisms
visiting the carcass, such as this sleeper shark, which can grow to nearly 7 metres in length.
In a matter of months, the carcass is stripped clean down to the bone.
However, in an ecosystem so nutrient-poor as the deep sea,
when something so large falls to the depths, nothing goes to waste.
Now begins the second phase, known as the enrichment-opportunist stage.
Many of the larger scavengers have moved on,
and it’s now the turn of smaller organisms to comb the surrounding sediment for decomposing tissue.
Organic fragments, or detritus, may enrich the sediments nearby for over a year.
The carcass you’re seeing now was filmed in 2019 by the EVNautilus team
at the Davidson Seamount, 3,240 metres beneath the surface of the ocean.
Here, invertebrates have begun to colonise the bones.
Worms, crustaceans and molluscs like these Muusoctopus octopuses
feed on leftover blubber, or burrow into the nutrient-enriched sediment beneath the whale.
Eel pouts pick the bones of any flesh left behind by the mobile scavengers.
Over time, as the organic material is slowly depleted,
the scavenger populations begin to decline.
One year later, in 2020, the EVNautilus team returned to the Davidson Seamount whale-fall.
What they found was a dramatically different scene,
for it had entered the third stage, called the sulfophilic stage.
This is by far the longest stage of a whale-fall, for it may last for 50 years or more.
Many of the bones appear to have disintegrated, and there’s a much smaller density of scavengers.
But although the carcass may appear devoid of life, the remaining skeleton supports a remarkable
community of chemosynthetic bacteria that begin to break down lipids trapped in the bones.
This process release sulphides as a byproduct, which are released up to
eight feet into the surrounding sediment and support a rich mix of dense bacterial mats,
mussels, clams and worms, feeding not on flesh, but on the microbes. Polychaete worms,
like the bone-eating Osedax worms, carpet the whale's bones in a fuzzy red fringe.
These animals have neither eyes nor mouth, but they present reddish plumes that act as gills
and green roots that burrow down into the skeleton. From the marrow,
the worms’ roots extract fats and oils, which microbes living inside the roots break down.
A fourth stage of whale-fall ecological succession has also been postulated. Known as the reef stage,
it involves the solid mineral remains of the whale bones
providing a hard substrate for suspension and filter feeders to inhabit.
Out on the abyssal plain, the soft sediments prevent many sessile organisms
from finding a solid surface to attach to; as a result, hundreds of years after death,
a whale’s corpse may still be able to provide islands of abundance at the bottom of the ocean.
The successive communities that thrive around these whale carcasses
show just how important whale fall events are to the deep sea ecosystem.
The immense biomass of whales, and especially high bone-lips content, allows their carcasses to
support diverse and complex ecosystems that would otherwise not exist in the energy-poor deep sea.
But they are not only important for the survival of existing species,
for the niches they create may play an important part in allowing new species to develop.
For example, an abundance of new bristle-worm species have been discovered at these sites,
which may be uniquely adapted to survive at whale falls.
For species such as these, whale falls may provide evolutionary stepping stones to allow new lineages
to adapt to new extreme habitats like hydrothermal vents and brine pools.