During the Second World War, the seabed began to move. At least, that's exactly what sonar technicians think, who don't understand why the depth of the ocean changes drastically every night. And now, we know that this "false seabed" is the result of a deep scattering layer, made up of millions of animals that carry out diurnal vertical migrations, sinking into the dark mesosphere during the day and approaching the surface at night.
This is a key discovery for our understanding of marine life and one of the most fascinating things Dr Adrian believes we have learned in the ocean. As a senior research fellow at the National Marine Research Centre (NOC) in Southampton, UK, he has focused much of his research on this migratory behaviour. As the head of the international project "JETZON", he coordinates collaborative projects in deep biological research.
As the name suggests, this place is a place with very little light, starting at 200 meters (656 feet) and going to 1,000 meters (3,280 feet). There's too little light here, the plants grow well, but it's acceptable for species that love to hunt, oh yes – the predators here are quite special indeed.
"Honestly, it's really incredible from a biodiversity perspective," Martin told IFLScience: "There's fish, squid, krill and all sorts of prawns. There are plenty of crustaceans here, as well as mammals. For example, sperm whales frequented this mid-water layer [this is the scientific name for the dark mesosphere]. We have a very diverse variety of creatures here. ”
The evolutionary pressures of dim environments have led to some remarkable adaptations in species that live in the shadows. Fish like sailfish and tuna live in the dark, and they have big eyes to hunt, but other animals adapt to the darkness by making their own light.
"A lot of people are aware of the fluorescence that creatures like fireflies emit, but in general, you rarely see the light they emit on surfaces," Martin continued, "There are some estimates that up to 90 percent of complex animals have some degree of luminescence, which suggests that if you live in a place with very little light, you can use the light in many places." ”
"But it's clear that it's a law of the jungle environment, and a lot of creatures in the water are preying on each other or hiding from each other. Some creatures use [glow] as bait – you may have seen a picture of a monkfish with tentacles or a scene from the Finding Nemo movie – but they can also use it to protect their animals. There are creatures that have the ability to surprise an attacking predator by emitting light. ”
Glow is also used by some animals as a way of camouflage. Even in very low-light environments, the silhouettes of some species are still visible against a slightly lighter ocean background. The elongated gill tail (Sigmops elongatus) is one of many light source animals, and when this juvenile develops, it also has glow in its abdomen.
One of the more sensational theories about the fluorescence emitted by living organisms is that some species may use it like a burglar alarm. To prevent predation, animals glow not only to deter predators that are about to attack them, but also to attract larger predators to eat their predators.
"Having the ability to emit light gives you a lot of possible ways to increase your life expectancy," Martin adds, "I would say so." ”
Living in the shadows is hard, and there are many challenges for creatures, but it's also susceptible to change. The NOC's website describes the dark layer as "potentially threatened", and as Martin explains, the driving force for many threats is very, very close to us.
"One of the important factors is climate change, which is directly under a lot of pressure from the perspective of the dark mesosphere, because the temperature of the dark mesosphere changes, and in most cases, it warms. Already low oxygen in ocean waters makes survival very challenging for larger organisms, and our evidence suggests that these areas are expanding in response to climate change and are expected to become larger. This expansion of the hypoxic zone is of particular concern to the dark mesosphere. ”
"There's a lot of discussion going on about how climate change affects plant growth, and phytoplankton are the foundation of the marine food web. Everything depends on phytoplankton to convert carbon dioxide into organic matter, but in the current situation, the amount of phytoplankton growing varies every year, which is in response to climate change. ”
It may sound like a surface problem, but we all know that key discovery during the Second World War that deep-sea life doesn't just operate in the dark mesosphere. Even if climate change does not occur directly in the depths, climate change will have a huge impact on the fertility and growth of surface species, and the alluring jewels and sharp teeth will not be enough to save its inhabitants.
Protecting the dark layer may also be preserving our own species, as this deep-sea area plays a vital role in carbon sequestration – the biggest puzzle Martin hopes to solve.
"One of the big questions that I'm spending a lot of time thinking about now is [how the dark layer helps the ocean store carbon]. I don't think we fully understand how marine life helps store carbon right now, so that's a huge problem for me. ”
"We know that the dark layer is important for storing carbon. We know that about 90% of ocean snow (organic detritus) is reused in the dark mesosphere. It's a very efficient channel, and how carbon is stored in the deep oceans, if carbon is to be kept away from the atmosphere, really needs to be understood. So, we really need to understand how the dark mesosphere affects this huge carbon flux. ”
They look like alien creatures, but they need a home on this planet just like we do.