When night falls, strange creatures rise from the depths, and the water begins to glow blue. Some ocean phenomena can only be seen at nightfall.
The ocean starts to sparkle thanks to bioluminescence
Dinoflagellates emit blue light when touched, as in this bay of Vaadhoo Island in the Maldives.
You've probably seen pictures like this. Night fell on the exotic island. Waves hitting the shore. The water sparkles with a blue, electricity-like color.
The Internet is very fond of photographs of this magical bioluminescent bay. You may have heard the stories of bloggers who say that everything in life looks less magical. Even so, bioluminescence (in this case caused by a special plankton called dinoflagellates) is a stunning natural phenomenon.
Dinoflagellates emit blue light only when touched, so they can only be seen at the crest of a wave, around boats, and when the oars touch the water. These tiny creatures are the main source of bioluminescence on the ocean surface.
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The so-called bioluminescent bays, such as in Puerto Rico or Jamaica, are the most famous places to observe this glow. Nevertheless, this ephemeral phenomenon can be found in many other places in the ocean, where the density of dinoflagellates is especially high.
Sometimes the population of dinoflagellates grows too quickly, and then a much less beautiful glow is formed, which has a reddish-brown color during the day, also known as "red tide". And some of these dinoflagellates are even poisonous.
An even rarer and more unusual phenomenon than bioluminescent bays is the milky glow of the sea, when the glowing water extends to the very horizon.
The milky glow of the sea has been recorded only a few hundred times since 1915, with most of them concentrated in the northwestern Indian Ocean and near the Indonesian island of Java. This phenomenon is not due to dioflagellates, but rather "bioluminescent bacteria, a large number of which have gathered on the surface of the water," explains Dr. Matt Davis, assistant professor of biology at St. Cloud University, USA, who specializes in bioluminescence.
For centuries, sailors have described the milky glow of the sea as a night, whitish sparkle, similar to a blanket of snow, but scientists have never been able to study this phenomenon in detail.
In 2005, researchers analyzing archived satellite images found that the milky glow of the sea could be observed from space, and that one satellite recorded a vast area in the ocean that had a strange glow for three consecutive nights a decade earlier.
Animals glow in the dark
Bioluminescence, the emission of visible light by the body as a result of a natural chemical reaction, is characteristic of marine animals such as fish, squid and shellfish. In deep waters, most species are bioluminescent and are the main source of light.
In shallow waters, most bioluminescent fish only emit light at night.
"Lantern-eyed fish have a special organ under their eye that they can rotate to emit light from bacteria [that accumulate in this organ], and they use it to hunt and communicate with other animals," says Matt Davis.
Camouflage, protection, hunting are some of the reasons why fish emit light. The squid, for example, uses light in a very sophisticated way. These nocturnal animals have a mutually beneficial relationship with the luminescent bacteria that accumulate in their mantles. With the help of them, at night, squids control their color depending on the brightness of the moon's light, and thus reduce their own in order to disguise themselves from predators.
The light of the moon provokes the biggest orgy on the planet
There is nothing more romantic than moonlight, especially if you are coral on the Great Barrier Reef. One night of the year, in the spring, moonlight provokes the world's biggest orgy. More than 130 coral species simultaneously release germ cells into the water over an interval of 30-60 minutes.
Mass breeding in the Great Barrier Reef is one of the most extraordinary examples of synchronized behavior on earth.
When germ cells, sperm and eggs are secreted, they hover for a second, creating a ghostly reef shape before plunging into an underwater fertilization storm.
Dr. Oral Levy, marine biologist and ecologist and professor at Bar-Ilan University, Israel, has studied this extraordinary phenomenon. “This is a truly amazing phenomenon … we know this event will happen every November, a few nights after the full moon, usually 3-5 days,” he says. "It's always incredible, in particular, I am amazed every time how coral species reproduce simultaneously year after year at the same hour at night."
He adds, “As soon as it happens, it amazes me every time it gets so alive and in sync. It is almost a spiritual experience when you begin to understand the power of nature in all its glory. The light of the moon provokes this phenomenon, as it serves as a synchronizer and a kind of "alarm clock", most likely in conjunction with other environmental phenomena, such as the time of sunset, water temperature and high tide, to signal the time of release of germ cells (sperm and eggs). " It is likely that corals have photoreceptors that determine the phases of the moon, thus knowing when to release germ cells.
Sharks and seals rely on the light of the sky
For many seals, moonlit nights are dangerous. During the winter months, about 60,000 Cape fur seals on Sea Island in Falls Bay, South Africa, are at risk of being eaten by the great white shark, which watches over the fur seals as they enter and exit the water.
According to a 2016 study, it was hypothesized that when swimming at night on a full moon, seals are at greater risk of being eaten by sharks, as bright moonlight highlights their silhouette on the surface, making them easy prey for underwater predators. However, most shark attacks on seals occur at dawn. Researchers who studied the number of attacks at dawn were surprised that predators hunted less in the morning during the full moon. The researchers hypothesized that moonlight, combined with incoming sunlight, could reduce the hunting ability of sharks, and that at this time of day they no longer had an advantage over seals.
Also, the cats could navigate by another parameter - the stars. Common seals (Phoca vitulina) can determine where the pole star is and follow it, research shows. In an experiment using a simulated night sky, the seals swam towards the brightest star.
In the wild, seals need to navigate the open sea to find feeding grounds, which can be hundreds of kilometers away.
Researcher Dr. Bjorn Mok said at the time: "Seals can remember the position of the stars in relation to their feeding area at dusk and dawn when they see both the stars and the earth."
Unusual animals rise to the surface every night
Under cover of night, rare animals migrate to the ocean surface in search of prey. The Humboldt squid is one of the most striking marine animals that can be seen on the surface of the water.
During the day, it usually lives in the depths of the eastern Pacific Ocean in the waters of the deep-sea shelf off the western coast of the Americas, and every night, like many other marine animals, it migrates upward in search of food. Vertical migration is the process where sea animals rise to the surface at dusk and descend again at dawn. This phenomenon is very common.
“The Humboldt squid follows its main prey, the so-called glowing anchovies,” explains Professor Paul Rodhouse, professor emeritus at the British Institute for Antarctic Research and former head of its Division of Biological Sciences. Glowing anchovies, in turn, follow the vertically migrating zooplankton. Since many marine animals depend on zooplankton, "the remaining links in the food chain will follow," says Professor Rodhaus.
“This is a huge daily movement of biomass,” explains Rodhaus. “More than a thousand meters. Some squid migrate more than a kilometer a day."
The Humboldt squid is one of the most stunning species that rise to the surface of the water every night. For its ability to change color and glow bright red, it is called the "red devil". And despite the fact that they are smaller than 13-meter giant squids, they can reach one and a half meters in length. These very aggressive predators grab food with strong tentacles and suckers and tear them apart with their teeth. History has even recorded several attacks on people.
But even such ferocious animals fall prey to even larger predators such as swordfish or shark.
"Of course, they are all active at night to avoid being attacked by larger predators," says Professor Rodhaus. "So, to reduce the risk of being eaten, they must descend into deep, dark waters at night."