Deep in the dark – how do you live where the sun doesn’t reach?


December is the darkest month of the year in our latitude. We look forward to Christmas for many reasons, but one of them is that on this night light symbolically triumphs over darkness and the day begins to lengthen. Have you ever wondered what it would be like to live without light? But so in complete darkness? And I’m not talking about its scarcity, a deficit situation to which a multitude of organisms have adapted, even autotrophic ones (such as skiophytes – shade-loving plants). I’m referring to the deep, ink black “though the eye bleed”.

Terrestrial ecosystems never experience this phenomenon, by day thanks to sunlight, by night thanks to stars and the moon. Even on starless nights, it is now in vain to look for biblical Egyptian darkness. In an era of anthropogenic light pollution, when nearly1/4 of the globe’s surface, including nearly half of the United States and nearly 90 percent of the world’s population. Europe, never experience true, deep darkness, to experience it, you have to go down into the depths, towards the guts of the Earth – underground or underwater….

Humans, like most organisms, are adapted to live on a day-night cycle, and light deficiencies negatively affect their functioning and well-being. However, there are organisms in the world whose life cycle runs almost in complete darkness. These are primarily biocenoses of caves and ocean depths. How are their residents coping under these extremely adverse conditions?

Darkness hides the underworld (Jerzy Andrzejewski mod.)

Caves are dark, damp and often dangerous places, yet they are inhabited by many animals that lead strange lifestyles: blind beetles that eat bat droppings, tiny spiders that cast webs like a lasso over their prey, or white salamanders that swim in cave ponds without getting lost. Some of these animals, like bats, sleep in caves during the day, but leave them at night in search of food. These are so-called troglobionts or pseudotroglobionts, animals that find temporary shelter in caves. Others, such as just spiders, shrimps, fish and salamanders, live their entire lives in caves and never come out. It is such animals, called troglobionts, that have developed a whole range of adaptations for living in the dark.

Most animals permanently living in the dark do not have eyes, because why would they have them? In the absence of light, the organs of sight are useless, and evolution has been relentless in eliminating unused structures. Fancy body coloration is similarly useless – most troglobionts are devoid of pigment, white or nearly transparent. An interesting feature is a certain species of freshwater fish. The Mexican mirrorfish(Astyanax mexicanus) has split into two genetic forms. The cave-dwelling form, called the cave-blind or cave-blind, has lost pigmentation and eyes, but terrestrial populations have retained both. Thanks to such characteristics, species of the genus Astyanax are used by scientists as model organisms in the study of the processes of evolutionary adaptation and regressive evolution (the process of adaptation through the simplification or disappearance of structures).

In order to move around in complete darkness without crashing through cave walls, troglobionts had to develop the ability to use alternative senses to sight. Some cave insects get around by using very long and sensitive feelers to navigate, which they use to probe space, like a blind man’s cane. Many insects and spiders have developed structures that allow them to sense minute vibrations and air currents that indicate the approach of prey or predators.

Cave darters(Proteus anguinus), also known as cave salamanders, have electroreceptors that allow them to sense the presence of a predator or prey. Bats, although they have educated eyes, are not the best visual learners and navigate in the dark through echolocation. Some tropical cave fish produce an electric field that works by echolocation in the water, enabling them to sense obstacles, food and predators.

Cave fish is a collective term for fresh- and brackish-water fish adapted to life in caves and other underground habitats. They belong to many families and do not form a monophyletic group – more than 200 scientifically described species of obligate cavefish are known, found on all continents (except Antarctica). Typical adaptations found in cavefish include the disappearance of eyes and pigmentation, and often scales and swim bladder. In general, these fish are extremely sensitive to pressure changes and vibrations, and often have an extremely sensitive sense of smell that helps them navigate. They also never sleep – deep in the caves there is no day or night, so they are constantly active.

The light between the oceans (M.L. Stedman)

The second type of ecosystem besides caves, where light does not reach, is the deep ocean zone (known as abisopelagial). It extends from a depth of about 3,000 to 6,000. m below the surface and very specific conditions prevail there, extremely different from those in the light-saturated and vibrant epipelagic zone (about 200 m thick). First of all, the deep zone remains in absolute darkness. Sunlight does not penetrate below 1,000. meters (bathypelagic zone) even in the clearest water, temperatures are low and pressure is high. The deeper it gets, the worse it gets.

The deep-sea zone is one of the coldest places on Earth – temperatures there are consistently low, ranging from 2-4°C. In addition, there is extremely high pressure, up to 600 times higher than at sea level. Life in such a hostile environment is extremely difficult, and yet it is not at all poor, as we thought just 150 years ago. The reports of the participants of the expedition of the exploring ship “Challenger” (1872-1876) about the “abyssal desert” have been strongly verified by contemporaries, and we write about the richness of life in these inhospitable places in this issue ofWater Matters.

Survival in the deepest parts of the ocean requires extraordinary evolutionary adaptations. It would seem that they would be similar to those found in the darkness of caves. And yet, interestingly, in the deep ocean zones, fish are not blind. Menagerie living in the deep sea usually have well-developed and very sensitive eyes. If sight in the dark is useless, why do these animals continue to invest in producing eyes? The answer is bioluminescence.

Bioluminescence is a remarkable adaptation of many deep-sea species. In different organisms there are different mechanisms responsible for luminescence, but the most common is luciferin oxidation involving luciferase. The reaction was first described in 1887. French biologist Raphael Dubois. By producing their own light, the organisms can communicate, attract prey and even camouflage themselves from predators.

Among the most successful bioluminescent species are the lanternfish, covering about 60 percent of the All deep-sea fish. The bellies and sides of these fish are decorated with light-producing organs and used for camouflage. They also use them for communication. Some 245 species of lanternfish are known, and each boasts its own unique light pattern and flashing pattern – a signature that can help them find suitable mates in dark waters.

However, the best-known, almost emblematic deep-sea species that produces its own light is the anglerfish(Lophius piscatorius), also known as the sea devil. The anglerfish’s light comes from the tip of a rod-like appendage on its forehead. This structure evolved from the spines of the dorsal fin. Its tip is inhabited by a large number of bioluminescent bacteria, whose light the fish uses to lure its prey out of the darkness and capture it. And interestingly, only females have this ability, while males, in order not to starve, bite into the female’s body, basically parasitizing on her in an extremely reduced form.

Other predatory deep-sea fish also use bioluminescent light to spot prey, lure them away, scare off predators or attract a mate. Bioluminescence is used not only by fish, but also by some bacteria, sponges, jellyfish, crustaceans, segmented worms, squid, sharks, and even (though far less frequently) some terrestrial species such as skylarks and honeycreeper. Some of these organisms produce the necessary chemicals themselves, while others, like the referenced anglerfish, rely on symbiotic bacteria. We wrote more about the phenomenon of bioluminescence and the organisms that use it in the July issue of Water Matters.

Deep in the dark - how do you live where the sun doesn't reach? 1

Under Pressure (Queen & David Bowie)

Bioluminescence is an adaptation for living in the darkness of the ocean depths, but not the only one that enables one to function in the extreme conditions there. The crushing pressure of several hundred atmospheres (pressure increases by 1 atm for every 10 meters of depth) is a huge challenge for organisms living in the depths. It’s like putting an elephant on your thumb. In such conditions, having a skeleton or armor made of hard calcium carbonate does not work and forces the reduction of anatomical elements built from it or replacing it with another material.

The deep sea urchins of the family Echinothuriidae have skeletal plates that are so distended that their bodies are as soft as those of teatropods. Instead of sponges with a skeleton made of calcium carbonate, there are glass sponges (Hexactinellida), with a silica skeleton. The bodies of deep-sea fish contain large amounts of a gel-like substance that helps maintain their structure under tremendous pressure.

This reduction of rigid structures means that deep-sea animals often take on a strange form – becoming more flexible, gelatinous or baggy. When taken out of their environment, they undergo decompression, or “swell” as a result of the change in pressure, such as the “blobfish” Psychrolutes marcidus (called the world’s ugliest fish), which looks very different in the deep than when taken out on the surface.

The ability to tolerate the high-pressure and low-temperature conditions of deep-sea habitats is also due to adaptations of organisms at the biochemical level. In deep-sea animals, enzyme activity, cell membrane function or protein stability do not show the disturbances under high pressure that would be observed in organisms not adapted to such conditions.

Dark everywhere, hungry everywhere… (inspired by Mickiewicz)

Lack of light prevents photosynthesis, so both underground caves and abyssal zones of the oceans are devoid of primary producers. Some microorganisms that inhabit the depths and caves have the ability to produce organic compounds using energy from chemical reactions, without light, the so-called “organic compounds”. chemosynthesis. These organisms, called chemoautotrophs, have the ability to oxidize hydrogen sulfide, hydrogen and methane, readily available, for example, in mineral-rich waters from thermal chimneys. Also in some caves, bacteria have been discovered that use sulfur as an energy source.

However, this does not change the fact that these ecosystems are very nutrient-poor and almost entirely dependent on allochthonous matter, i.e. what falls in, falls off or sends in from outside. Animals of these sparse environments have developed various strategies for acquiring food, including filtering fallen organic debris, scavenging or predation (often using just bioluminescence). They have also developed the ability to survive long periods of famine. They save energy by reducing activity and slowing metabolism. Some species can survive by taking food only a few times a year. Crustaceans of the shovelnose cluster (Remipedes) can live for months without food, using energy stored in the form of fat globules.

Reduced metabolism, slowed growth rate and reduced nutritional resources promote the so-called “food security”. Deep Sea Gignathism, or the attainment of huge body size by animals. We are all fascinated by stories about giant squid, sea spiders (stump spiders), amphipods, clams, crabs or octopuses of unprecedented size anywhere else (we also wrote about the largest marine animals in the November issue of Water Matters). Interestingly, despite similar conditions, the phenomenon of gigantism is not observed in caves.

Scientists point out that being bigger in deficit conditions is more efficient (according to Kleiber’s principle, linking body weight to metabolic rate) and provides a huge advantage. Larger animals can move faster and farther to find food or a mate. They have a more efficient metabolism and store supplies better. Large predators, when presented with more food (such as carrion drifting from the surface into deeper zones of the ocean), can consume more at one time, providing energy for a longer period of time.

Also, low temperatures in the deep sea and the associated slowed metabolism of animals can stimulate gigantism (according to Bergmann’s principle that animals get bigger the colder the climate they live in). In this ecosystem, organisms often grow and mature very slowly, but they also live a very long time, and their growth continues throughout their lives. For example, the Greenland shark(Somniosus microcephalus) can be more than 7 meters long and weigh up to 1.5 tons, but it takes centuries of its life to achieve these parameters. These fish grow about 1 cm per year and do not reach sexual maturity until they are about 150 years old. Among other things, they owe such a long life to the lack of predators in the deep sea.

Why do we need this knowledge? Implications for science and technology

Understanding how creatures survive in the darkest corners of the Earth is of great importance for the development of various scientific disciplines and technological advances. The study of unique life forms in the deep-sea zone provides information on the adaptations of organisms to extreme environments over millions of years. This information can help us better understand evolution, including the phenomenon of convergence, that is, the development of similar solutions in unrelated organisms. Knowledge of adaptive mechanisms may prove to be the key to developing new formulations and treatments. The specific biochemical properties of deep-sea zone organisms offer great potential for biotechnological advances. Scientists are constantly discovering new compounds and enzymes that can find applications in various industries, including. In medicine and renewable energy.

Some of the adaptations may help invent drugs for human diseases. Recognizing the sleep mechanisms of cave animals, in which sleep activity runs differently than in organisms that function on a diurnal cycle, can help develop therapies for sleep disorders. Similarly, by studying how these animals lost their eyes, we can learn how to treat blindness. Since many animals of dark and cold environments live longer than animals of the Earth’s surface, the study of their physiology can help understand the aging process and even discover ways to extend life.

Observation of the way deep-sea organisms move in their environment is stimulating the development of new robotic technologies. Such innovations could improve underwater exploration and help us better understand the intricate ecosystems of the deep ocean, their trophic networks and the interconnectedness of species. The deep sea plays a key role in the carbon cycle and the overall state of the Earth’s waters. Unraveling its secrets can contribute to a better understanding of climate change and the importance of protecting these ecosystems.

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