FROM CORAL reefs, viral communities and crustaceans to sharks and rays, the earth’s ocean is the largest ecosystem on the planet. As components of the terrestrial ecosystem, we are generally inclined to assume that living on land is the norm. The scale of the oceanic ecosystem makes this a worryingly self-center
71 per cent of the Earth’s surface is occupied by water, with the oceans housing over 228 450 known animal species, with a possible further few million currently undiscovered. Perhaps the first step is shifting our mindset from an aerial to an aquatic one. It doesn’t take a marine biologist to be fascinated by the novelty and potential of the deep ocean; its inhabitants cause constant surprise. Usually when it is believed that an ecosystem is understood, the ocean produces a “new rabbit from the oceanic hat”.
The physics of water determines much of the uniqueness of the oceanic ecosystem. An incredible 88 per cent of the oceans are deeper than 11 km, with 76 per cent between 3-6 km. This third dimension immediately sets the oceans apart from the primarily two-dimensional terrestrial ecosystem.The three main zones of the oceans are the epipelagic, mesopelagic and bathypelagic, which are measured between the surface and 200m, between 200 and 1000m and between 1000 and 6000m respectively. These zones experience different ecological levels of light intensity. However, the most striking feature that separates an aquatic and aerial ecosystem is the density; at sea level, water has a density 830 times that of air, but its density varies by only 0.8 per cent at each physiological range of temperatures.
The deep ocean, a particularly energy-poor ecosystem, remains inscrutable – despite its significance to life on earth, including the air we breathe. Life on earth thrives from the success of primary producers, namely the production of organic matter from inorganic carbon sources and an external energy source, which, in photosynthesis, is light. Non-photosynthesising microorganisms utilise the energy stored in chemical bonds rather than light, a process called chemosynthesis. Most deep sea organisms obtain their energy from photosynthetic phytoplankton from the near surface and a minority of deep-sea animals rely on chemosynthetic bacteria on the sea floor, where sulphide and methane levels are high.
To understand the oceans, the vast range of organisms inhabiting them, from viruses to the largest of whales must be understood. What adaptations do the organisms of the deep ocean acquire to survive?
If you were to walk around in the dark, your various mechanoreceptors would help you navigate your surroundings and its inhabitants. Each sensation you would experience has an aquatic equivalent: the mechanoreceptors of oceanic animals detect shear between either themselves and the external seawater, or themselves and their internal fluids. The ocean is a noisy place, but perhaps not to a diver. Sounds are distortions in the flow of a fluid (water or air) and are produced when an organism moves relative to the flow. Sound waves travel around 4.3 times faster in seawater than in air. Vibrations in water, of varying frequency, allow detailed communication between individuals, providing it can be detected.
Numerous retinal adaptations have been identified in deep-sea fish. The absence of cone cells, responsible for colour vision that function best in bright light, and an increase in the length of rod cells that function in less intense light. Camouflage, transparency, silvering and bioluminescence are some of the other adaptations acquired by oceanic animals – a whole network of specialised mechanisms allow organisms to thrive and reproduce at various depths of the ocean.
However, climate change and coral bleaching, oceanic acidification, oil spills, pollution, shipping, tourism and over-fishing have already caused mass amounts of damage to our oceans and their ecosystems.
Instead of treating the oceans as the largest living space on Earth, one that also benefits our health and way of life, it has been thought of as an inexhaustible supply of food, dumping ground and transport route.
But it is not too late. It is our duty to protect our oceans, maintain biodiversity and prevent further damage to ecosystems and population numbers. Charities are in action, protecting the oceans and their inhabitants. Oceana, Blue Marine Foundation, Sea Life Trust and the Marine Conservation Society are all protecting the oceans and the many different species and taxa that call them home.