In order to survive the challenging environment of the deep ocean, many organisms have turned
to one of nature’s fundamental ecological associations. Symbiosis. The name for any
kind of a close and long-term interaction between two different organisms. This phenomenon
is found in nearly every ecosystem on the planet - allowing organisms to rely on others
for food, shelter, or to act as a warning for nearby danger. But nowhere on Earth are
creatures more uniquely adapted to relying on others than in the deep sea - a world of
darkness, cold, and intense pressure. Let’s dive in, and take a closer look at the incredible
role of symbiosis in the deep sea ecosystem.
We’ll begin by examining the most common type of symbiosis. Mutualism, which is best
simplified by the phrase, ‘you feed me, I feed you’. You might at first think of
clownfish relying on anemones for protection, in turn providing nutrients through their
excrement. But increasingly, deep sea exploration uncovers some truly unique cases of organisms
depending on one another, forming relationships that ensures the survival of both organisms,
or symbionts, involved. In many cases, a relationship will form between a microbe and a host. The
microbe provides food for the host, while in return, the host provides the microbe with
protection from predators. They both benefit, making this relationship a form of mutualism.
Mutualism often forms between bacteria, and a wide range of host animals including corals,
mussels, and tube worms at deep sea hydrothermal vents and cold seeps. Here, the water that
wells up from below the sea-floor is rich in minerals for the bacteria to convert into
food and energy. To be specific, these compounds are hydrogen sulphide and methane. Both are
toxic to animals, but certain specialised bacteria are able to use the chemicals to
make the organic matter that animals rely on. This process is known as chemosynthesis.
Think of it as an alternative to photosynthesis, but where nutrients are made by converting
chemical energy rather than light energy.
Gigantic, odd-looking tube worms grow in clusters around vents and cold seeps; unusual creatures,
for they have no stomach, gut, or mouth. This is because they instead contain a trophosome,
a large organ containing billions of the chemosynthetic bacteria. The tube worms are specifically
adapted to being a symbiont in this relationship; the stable nature of the vents has allowed
the worms to evolve over millions of years to possess these important physical characteristics
- including a long ‘root’ that collects hydrogen sulphide from the sediment, and ‘plumes’
that collect oxygen from the water. Both the oxygen and the chemicals are transported to
the trophosome, where the bacteria use them and carbon dioxide to produce organic molecules.
Without the tube worm’s trophosome providing them with a constant supply of these key ingredients,
the single-celled bacteria would have a hard time gathering it all in one place.
100% of the tube worm’s nutrition is provided by the bacteria undergoing this process. Symbiosis
is so important to these organisms, that they simply would not be able to survive without
it. Life needs energy. Without bacteria taking refuge within towering tube worms, the worms
would obtain no energy, and life here would be impossible, especially as photosynthesis
is not an option down where sunlight cannot reach.
This example of mutualism is not only vital for the survival of the tube worms and mussels.
It also supports an abundance of other organisms that rely on the tube worms, and thus form
complex communities at vents and cold seeps that simply would not exist if not for the
‘worm-bacteria’ symbiosis. Crabs, eels, isopods, and fishes all use tube worms and
mussels as sources of food, or shelter. These animals are called associates.
To put it simply, the bacteria need to live inside the worms and mussels in order to gain
the ingredients for chemosynthesis. The tube worms need the bacteria to live inside them
in order to synthesise the nutrients and energy they need. And the associates need the worms
and mussels as a source of prey. It all comes back to symbiosis. The key to survival is
working together.
Consider the female anglerfish. Bioluminescence is a vital adaptation to have in the deep,
for standing out is advantageous when it comes to luring the scarce prey population. But
anglerfish cannot produce their own bioluminescence. Instead, they turn to symbiosis, and allow
glowing photobacteria to take shelter in their fleshy lure called an ‘esca’.
But symbiosis does not just mean that both partners have to benefit. In fact, it describes
a variety of close relationships between organisms. Sometimes, one organism benefits from causing
harm to the other, like worms, isopods and copepods infecting host species such as crabs
and fish. This is parasitism. Let’s take a look.
Parasites often get a bad reputation for their role as blood-suckers and vectors for disease.
But parasitism is no less important than mutualism to the deep sea ecosystem. Parasites are key
components of food webs, and overall they can be an indicator of a healthy ecosystem.
They control populations of dominant species, allowing rarer competitors to thrive. Also,
parasitism can lead to mutualism. In shallow reefs, animals with parasites visit ‘cleaner’
stations to have their parasites removed by shrimps and small fish. The small fish get
a meal out of eating the parasites, and the host gets some relief by being rid of the
parasites.
Let’s examine at the case of the Greenland shark. They are often seen with a long, white
parasitic copepod attached to the corneas of their eyes, called Ommatokoita elongata,
which cause blindness in the sharks.
The deep-dwelling, long-living oceanic wanderer is remarkably well adapted to the deep sea;
its large size is an example of abyssal gigantism, allowing it to be more efficient in the nutrient-poor
depths. Its slow metabolism provides a similar benefit. But one thing that the Greenland
shark does not rely on in the deep, dark sea, is eyesight. Unlike many organisms that have
evolved large, light-sensitive eyes to see better, Greenland sharks instead have opted
to rely more on their acute sense of smell to detect prey and carrion. Their eyes are
fairly useless, even if not for the parasite.
This means that while the parasites benefit from living on the sharks, they don’t do
much to hinder the survival of the Greenland shark. Thus, the relationship is potentially
not parasitism at all, but rather it may be an example of the third kind of symbiosis.
Commensalism, when one organism benefits, without helping or harming the other. A good
example of a commensal relationship is found on the skin of whales. The whale reaps no
benefit from the presence of barnacles that attach themselves to its body, but neither
does it suffer due to their presence. The barnacle, however, is a filter feeder, and
makes use of the wandering nature of the whale in order to be exposed to the plankton-rich
waters of the open ocean. This gives barnacles a significant advantage over other filter
feeders that are fixed in one position and must compete for space on the rocks.
Overall, symbiosis is a key ecological association in the extreme deep sea ecosystem. With nutrient
production from sunlight impossible, chemosynthetic bacteria must fill the role of food producers.
Their reliance on other organisms shows that symbiosis upholds nearly all biological communities
in the deep ocean. We find that it allows animals to thrive in habitats and adapt to
live in ways that would not otherwise be possible without their symbiotic counterpart. Through
symbiosis they thrive, through cooperation they conquer. It is an extraordinary phenomenon,
showing off the resilience of life to the extreme conditions of
the abyss.