The deep sea has a range of unique conditions which have made it the ideal place for the evolution of bizarre organisms with otherworldly appearances. Complete darkness, cold temperatures, and high pressure combine to make this ecosystem one of the harshest to live in. For organisms to withstand these conditions they have had to adapt both physiologically and morphologically. While the number of organisms in the deep sea is actually relatively low compared to other marine ecosystems, it does have high levels of endemism which means that of the species that do exist there, most are found to exist only there. Given that their adaptations are highly specialised to cope with these extreme conditions, this trend makes sense. Unlike other marine environments, the conditions in the deep sea do not undergo many fluctuations. While a shallow marine environment has day and night cycles influencing photosynthesis and water temperature, the conditions in the deep sea, although extreme, are quite stable.
For the most part, deep sea ecosystems depend on energy from other overlying ecosystems since there is no light for primary production to take place via photosynthesis. Its reliance on dead organic matter from the photic zone, where light penetrates and photosynthesis takes place, makes the deep sea an important component of ocean nutrient cycling. Detritivores in the deep sea feed on “marine snow”; particles of organic matter which originate from the upper water columns. The smallest detritivores are referred to as zooplankton, and many of them are larval forms of invertebrates, such as crustaceans and molluscs. Sea cucumbers are an example of larger detritivores, known as deposit feeders. These animals are echinoderms, an animal group which also contains starfish and sea urchins. Like all echinoderms, they have a water vascular system that allows them to move using hundreds of little tube feet. As they crawl along the sea floor, they graze accumulated deposits of marine snow. Their slow movement yields the advantage that there is little energy demand, and so they can cope well with the limited food supply. It also makes them vulnerable to predation, however, and so they have evolved the capacity to release parts of their internal organs which stick to predators, so as to deter them.
Adaptations in deep sea predators are also tailored towards minimising energy usage. This tells us that within the deep sea context, where food is relatively limited, the benefits conferred by this strategy outweigh those that could be had by being an active predator. Predatory deep sea fish are a good example of this, given that their shallow marine counterparts live far more active lifestyles. Indeed, anglerfish who have the characteristic light-emitting esca to lure prey are a good example of how predators have also evolved adaptations to conserve as much energy as possible. The long and thin teeth in the large gaping mouth of an anglerfish also contribute to conserving energy since this means that once the prey has been caught, there is no chance of it getting away. Another factor which would hinder active predators is the cold temperature which reduces metabolic capacity and, ultimately, capacity for prolonged chase of prey. “Colossal squid who can reach lengths of 16m are another well known example of deep sea gigantism.”
“Colossal squid who can reach lengths of 16m are another well known example of deep sea gigantism.”
Hagfish are amongst the larger scavengers in the deep sea. Most interestingly perhaps is their place in the family tree of fish and more generally vertebrates. Although they do have a skull, they lack a spine and they have only recently been classified as vertebrates due to recent studies suggesting that their rudimentary vertebrae had previously been been more developed in their ancestral form. In relation to other fish, they belong to a primitive branch which lack jaws, known as agnathans. Instead, hagfish have two sets of teeth on either side of their tongue that they use to tear off chunks of flesh from carcasses which have descended to the sea floor. The availability of such carcasses is unpredictable, and so, they too are well adapted to minimising energy usage. A whale carcass, for example, will attract huge numbers of hagfish who will consume all the soft tissue, leaving behind only bones. Invertebrates and bacteria will then proceed to break down the bones, ultimately continuing the process of nutrient cycling. Although their food is scarce, it is usually quite abundant when it does arrive. By gorging on it when it is available, hagfish can survive for months without feeding.
A long lifespan in deep sea animals, such as the hagfish, is sometimes coupled with the phenomenon of deep sea gigantism in certain invertebrate animal groups. For example, giant isopods are far larger than their relatives, such as woodlice, and can reach the size of small dogs. Colossal squid, who can reach lengths of 16m, are another well known example of deep sea gigantism. The reason behind this phenomenon is not fully understood, but it is thought to be in part due to the increased energy efficiency which is conferred by being larger in size. Many of these animals have slow metabolisms and so have a low food intake. The way in which the cells of invertebrates develop at low temperatures and high pressures are also thought to contribute.
When looking at the deep sea, we see a system which is largely dependent on energy from the surface and the ecosystems driven by the sun. This is true for the most part, but at deep mid-ocean ridges, where tectonic plates are drifting apart, we actually see some level of primary production of energy within a deep sea system. Here, heat from the earth’s crust emerges through black or white smokers in the form of hot vapour together with essential elements. This combination of heat energy and chemicals sustains populations of thermophilic microorganisms who thrive at temperatures close to or in some cases over 100℃. In the last few decades, the theory that life could have originated within the hydrothermal vent system has gained traction. Unlike black smokers, white smokers are usually found on the outer edges of mid-ocean ridges, and so, have lower temperatures. These lower temperatures, coupled with the presence of certain minerals, could feasibly have led to a series of geochemical and, ultimately, biochemical reactions, once stable organic molecules had been formed by the catalytic effects of these minerals acting as basic enzymes.
The existence of these microorganisms in the deep sea hydrothermal vents was only discovered in the 1970s when improved technology marked the beginning of deep sea exploration. Since then, our knowledge on these systems has greatly improved, and we know that the presence of this primary production through the microorganisms supports a whole ecosystem, independently of the sun’s energetic output. Tube worms, who are a type of annelid (like earthworms) are found attached to hydrothermal vents, and they too derive energy by metabolising the nutrients released from the vents. Rather than having a stomach, these worms have a internal organ known as a trophosome, which contains symbiotic bacteria capable of oxidising hydrogen-sulfide or methane to produce energy.
“If we continue to exploit their resources unsustainably many of these will be gone before we have even discovered their existence.”
Like many other marine habitats, the deep sea and its unique biodiversity is under threat by human activity. Deep sea trawling can lead to the destruction of deep sea corals, some of which are 4,000 years old. The slow growth of many deep sea organisms means that major disturbances can cause lasting damage which take years to recover. As our technology improves alongside the prospect of deep sea mining becoming a reality, these ecosystems will be at even greater risk. As in many other cases of resource exploitation, the question which will have to be answered is whether the fact that we can do it warrants doing it. Taken in a wider context, our relationship with the ocean has, until now, been largely one sided, where we alone have reaped the benefits to the detriment of its health. The oceans still have countless secrets left to be unravelled but if we continue to exploit their resources unsustainably, many of these will be gone before we have even discovered their existence.
By skimming the surface of the complex dynamics that characterise this unique system, we can begin to appreciate just how wonderful the products of evolution can be. The capacity of life to not only thrive but maybe even emerge from the deep sea reminds us of the biological potential of evolution, capable of giving rise to organisms tolerant of even the most extreme conditions under the right selective pressures. Through an evolutionary lens, the deep sea is just another ecological niche, where a combination of abiotic and biotic factors interact to form further potential for life to develop. The phrase “desperate times call for desperate measures” comes to mind when looking at the deep sea, a place which, although hostile in many aspects, also has a wealth of resources available. Therefore, the deep sea has been, and continues to be host to ingenious biological solutions to deal with very real problems.