The

deep sea can be a lonely place. The dark expanse appears endless, and life seems sparse and

isolated. However, deep sea biodiversity relies on these scattered organisms interacting in

order to survive, whether they’re working together in symbiosis, scavenging, being predated,

or parasitising a host animal. But there is one ecological interaction that does more

than any other to influence organisms to change and diversify, and thus plays an important

role in the success of deep sea communities. Let’s take a closer look at competition

in the deep sea, and how it shapes this unique ecosystem along with its inhabitants.

Surviving in the deep sea is a challenge. Here, the low oxygen levels and scarce nutrients

means the resources that organisms require are hard to come by. As a result, there is

conflict between animals that strive for the same resources in the same place. In other

words, competition is fierce.

Animals throughout the oceans each belong to different trophic levels of the food web.

These are the hierarchical layers in an ecosystem, each made up of organisms with the same feeding

behaviour, whether it be as producers, predators or prey. This is important considering much

of the marine world’s competition takes place between animals belonging to the same

trophic level. For example, sharks compete with other top predators like killer whales,

with both requiring abundant space and similar prey to eat. Grazers compete with other grazers

for space on the rocks. Producers, like plants, compete with other producers to procure the

most sunlight. In the shallows, this kind of competition can lead to there being a huge

diversity of creatures, simply because nutrients and other resources are abundant with plenty

to go around.

But in the depths of the open ocean, the far more limited resources mean only a small number

of niches can exist. A niche is the ecological role or way of life that a species can occupy,

and with only a few of these available in the deep, there is greater competition between

different species trying to fill the same niches. This explains why the deep sea has

so much competition, for animals must share the ecosystem with other competing species

all trying to consume the same limited resources.

But competition does not always arise in the same way. In fact, it comes in two different

forms. First is interspecific competition, which occurs between members of different

species. The second form of competition, intraspecific competition, occurs between members of the

same species. If you struggle to remember which is which, consider that the word international

means between different nations.

Some of the best deep sea communities that demonstrate interspecific competition are

found on the very bottom of the ocean. Out on the abyssal plain, space may be abundant

for benthic communities, but the lack of food or shelter means their chances of survival

are low. So instead, many creatures congregate in high abundance around certain hotspots

of activity, from sunken whale carcasses, to chemically rich brine pools and superheated

hydrothermal vents. Places where energy is produced and food is bountiful. These are

ecosystems that boast high species richness, but where the rocks are overcrowded with competing

organisms all attempting to position themselves in the best spot, or scavenge what they can.

This has lead to examples of symbiosis arising, allowing organisms to live in comfortable

co-existence with other species, while relying on them for survival. But this is only the

case when organisms have something to gain from one another. For example, at hydrothermal

vents, giant tube worms rely on the energy produced by chemosynthetic bacteria that convert

dissolved chemicals in the heated water into nutrients. A different species, the yeti crab,

also relies on the same bacteria for the exact same reason. So while there may be symbiosis

between the animals and the bacteria, there is simultaneously competition between the

crabs and tube worms for space on the rocks, and for possession of the bacteria. This is

an example of interspecific competition, and it’s not just tube worms and yeti crabs

that compete over the same bacteria. Deep sea shrimps, mussels, and squat lobsters all

fulfil similar ecological niches, and so all are competing for the same resources in the

same place.

Often, interspecific competition can lead to the extinction of one of the species competing.

The organism that is less well adapted may lose out on the resources it needs, meaning

members of that species are less likely to survive. They become out-competed. This is

because species with identical niches also have identical needs. The idea that in a stable

ecosystem, no two species can have exactly the same niche and stably co-exist, is known

as the competitive exclusion principle. But when this doesn’t lead to extinction, interspecific

competition instead causes specialisation of the different animals. A phenomenon called

resource partitioning occurs, where species with overlapping fundamental niches evolve

different adaptations. It helps the species coexist because there is less direct competition

between them. This is what occurred at hydrothermal vents to make them so stable. The competing

crabs, worms and shrimps may all be in pursuit of the same resources, but they have developed

very different ways of acquiring them. As a result, each of these species are utilising

only a part of the wider niche they could have used if they were the only species in

the ecosystem. Let’s take a look at the ways they’ve changed in order to co-exist.

Yeti crabs farm the bacteria in filamentous colonies on their bodies, reducing the pressure

on the crabs to compete for space with other species like the shrimps. The crabs are able

to move around and take the bacteria with them. Contrastingly, the tube worms are sessile,

meaning they are fixed in one place and cannot move. Their competitive advantage arises from

their ability to store the bacteria within their tubes, effectively holding them captive

and benefitting from all of the nutrients they produce. Another denizen of deep sea

vents, the Pompeii worm, farms bacterial colonies in a similar fashion to yeti crabs, but still

behaves very differently as it inhabits locations on the vent structures that are far hotter

than those that the crabs can tolerate. In a way, the worms and crabs have become behaviourally

isolated from one another.

The different adaptations and behaviours of these creatures means that can all share the

same bacteria without getting in each others way. So unlike in the open ocean where low

resources lead to low diversity, the opposite is the case here, and the abundant resources

at deep sea vents allow the ecosystem to boast high diversity due to competition driving

these species to specialise in different ways.

In the deep sea, conflict between individuals of the same species is the most common form

of competition. This is because the large distances and vast expanses of the depths

leads to geographical isolation of competing species, therefore reducing competition as

they simply migrate to different locations where they can fulfil the same niche without

disturbing one another. But for members of the same species, conflict is unavoidable.

Whenever animals fight over territory or attempt to court potential partners prior to mating,

they are competing. And these interactions are just as important as the ones discussed

in Chapter 2.

Group hunting techniques demonstrate how intraspecific competition is a basic factor in natural selection.

At night, groups of up to 40 Humboldt Squid swim upward to prey upon schools of lanternfish. While

they may be working together to corral their prey, they are also in direct competition

with one another. Those that are stronger and faster are likely to catch more fish.

In fact, they have been known to turn to cannibalism, with larger Humboldt Squids picking off the

smaller members of their own school. In a brutal display of survival of the fittest,

this leads to the evolution of better adaptations within the species, as those with the advantageous

characteristics will out-compete their lesser rivals and be more likely to reproduce. In

other words, the best competitors are the ones who survive and get to pass on their

genes.

Competition is a very important part of understanding the ecology of the deep sea. Many aspects

of this ecosystem are influenced by these interactions. The ways in which animals compete

with each other determines how species are distributed, as well as community structures,

food webs and population dynamics. And over time, these interactions lead to behavioural

and physical adaptations in a species that shape its evolution. On the sea floor, resource

partitioning allows one niche to be shared among many organisms. In a similar fashion,

geographical isolation in the midwater zone allows an abundance of pelagic inhabitants

to co-exist. Together, these processes and principles demonstrate how important competition

is to the biodiversity of the deep sea.

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