The ocean is a strange place after dark

Moonlight triggers the world’s biggest orgy, strange creatures emerge from the depths, and waves glow blue. Some phenomena in the ocean can only be witnessed after dark.
1. Bioluminescence makes the sea shimmer

Dinoflagellates emit blue light when they’re disturbed, like at this bay on Vaadhoo Island in the Maldives (Credit: Naturepl.com/Doug Perrine)

You may have seen the pictures.
It’s night-time in an impossibly exotic location. Waves are breaking on the beach. The water is sparkling with electric blue lights.
The internet loves an image of a magical-looking bioluminescent bay. You may also have seen travel bloggers bemoaning the real event as not quite living up the hype.
Even if the latter is true, bioluminescence (in this case usually caused by planktonic organisms called dinoflagellates) is a pretty amazing natural phenomenon.
Dinoflagellates emit blue light when disturbed, which is why they can be seen sparkling over wave crests, around boats or when a hand or paddle runs through them.
These tiny creatures are the most common source of bioluminescence at the ocean’s surface.
So-called bioluminescent bays such as in Puerto Rico and Jamaica are among the best-known places to witness the glow. However, the ephemeral phenomenon can be found throughout the ocean where there are dense gatherings of dinoflagellates.
Sometimes dinoflagellates’ population increases rapidly causing blooms, which by day are coloured a less attractive red-brown, sometimes known as red tides. And some, but not all, of these red tides are poisonous.

These tiny creatures provide the most common source of bioluminescence at the ocean’s surface (Credit: Naturepl.com/Martin Dohrn)

Even stranger and rarer than bioluminescent bays are “milky seas”, where continually glowing water stretches for as far as the eye can see.
Milky seas have only been seen a few hundred times since 1915, mainly concentrated around north-western Indian Ocean and near Java, Indonesia.
They are not caused by dinoflagellates, but are thought to be the result of “bioluminescent bacteria that have accumulated in large numbers near the surface”, explains to Dr Matt Davis, Assistant Professor of Biology, St. Cloud State University in the US, who specialises in bioluminescence.
Reports by sailors over the centuries have described milky seas as a nocturnal whitish glow like a field of snow, but scientists have had little chance to investigate the phenomenon first-hand.
In 2005, researchers analysing archived satellite images found that milky seas could be seen from space and that one satellite had captured images of a huge area of ocean that had displayed the strange glow for three consecutive nights a decade earlier.
2. Animals glow in the dark

Bobtail squid have a symbiotic relationship with bioluminescent bacteria (Credit: Naturepl.com/Jurgen Freund)

Bioluminescence, the emission of visible light by an organism as the result of a natural chemical reaction, is common among marine life such as fishes, squid and molluscs. In the deep sea most species are bioluminescent, where it is the main source of light.
In shallower waters, most bioluminescent fish display their lights at night.
“Flashlight fishes have a specialized pouch under their eye that they can rotate to expose the light emitted from these bacteria, and they use this glow at night to hunt for food and communicate,” says Dr Matt Davis.

Flashlight fishes have a specialised pouch under their eye that use to expose bioluminescent bacteria (Credit: Matt Davis)

Ponyfish emit light from the bioluminescent bacteria housed in a pouch using transparent muscular shutters, to communicate, he explains.
Camouflage, defence and predation are among the variety of reasons fishes are thought to emit light.
For example, bobtail squid have an ingenious way of using lights. These nocturnal animals have a mutually beneficial relationship with luminescing bacteria that live in a mantel cavity on its underside. At night the squid control the intensity of this light to match the moonlight, and can reduce their silhouette to camouflage themselves from predators.
3. Moonlight triggers the planet’s biggest orgy

Mass spawning on the Great Barrier Reef is one of the extraordinary examples of synchronised behaviour on Earth (Credit: Naturepl.com/Jurgen Freund)

There is nothing more romantic than a moonlit night, especially if you are a coral on the Great Barrier Reef off Australia.
One night a year in spring, the biggest orgy on earth is triggered by lunar light.
Over 130 coral species simultaneously release their eggs and sperm into the water during a window of just 30-60 minutes.
This mass spawning event might be the most extraordinary example of synchronised behaviour in the natural world.
When the gametes – eggs and sperm cells - are released they hover for a moment, forming a ghostly replica of the reef’s shape, before dispersing into an underwater blizzard as the sperm fertilise the eggs.
Dr Oren Levy, a marine biologist and ecologist and Professor of Life Sciences at Bar-Ilan University, Israel, has studied this extraordinary event.
“This is really fascinating phenomena…we know this event is going to happen a few nights after November's full moon each year, three to five [days] post full moon,” he says.
“[It is] always amazing, in particular I am so amazed how each of the coral species year after year spawn at the same hour of the night.”
He adds: ”Once it happens it is always so exciting to see how everything is becoming so live and synchronised. It is almost [a] spiritual event and you understand the power of nature in its best.”
Moonlight triggers the phenomenon by acting as a synchroniser or “alarm” probably with other environmental signals such as sunset timings, water temperature and tides to cue the time of the gamete [egg and sperm cells] release, explains Dr Levy.
He adds that corals seem to possess photoreceptors that detect the phases of the moon, which helps with the “fine tuning” of the gamete release.
4. Sharks and seals rely on celestial light

Just when you think it's safe to go into the water... great white sharks hunt at night too (Credit: Naturepl.com/Chris & Monique Fallows)

For some seals, moonlit nights spell danger.
During winter months, the 60,000 cape fur seals on Sea Island in False Bay, South Africa run the gauntlet of being picked off by great white sharks patrolling the seas when they enter and exit the water.
One study in 2016 hypothesised seals swimming at night during a full moon are at more risk of being eaten by a shark since bright moonlight silhouetting them against the surface makes them an easy meal for predators lurking below.
However, most shark attacks on seals happen just after sunrise. Researchers behind the study, which measured shark attacks at dawn, were surprised to find seals were much less likely to be predated at this time of day if there was a full moon.
The researchers theorised that lunar illumination combined with emerging sunlight may decrease the stealth ability of the sharks and that the advantage switched from sharks to seals as night turned to day.
And seals may rely on another celestial feature to navigate - the stars.
Captive harbour seals (Phoca vitulina) are able to locate a single lodestar and steer by it, researchers have shown.
During a test using a simulated night sky, seals swam towards the brightest star and could orientate themselves when the stars were swivelled around.
In the wild, seals need to navigate the open ocean to find foraging grounds that may be separated by hundreds of kilometres.
Researcher Dr Bjorn Mauck said at the time: "Seals might learn the position of the stars relative to foraging grounds during dawn and dusk when they can see both the stars and landmarks at the coast."
5. Strange animals come to the surface every night

Humboldt squid are among the most striking creatures to surface every night (Credit: Naturepl.com/Franco Banfi)

Under the cover of darkness rarely seen creatures migrate to the ocean’s surface to feed.
The Humboldt squid, also known as the jumbo squid, is one of the most eye-catching marine animals you can see lurking in surface waters.
By day the squid lurk in the deep waters of the Eastern Pacific Ocean along the deep shelf that runs off the west coast of the Americas and every night they are one of the many ocean animals to migrate upwards to find dinner.
Vertical (or diel) migration - when ocean animals swim to the surface at dusk and disappear down again at dawn – is extremely common.
“What [Humbioldt squid are] doing largely is following their main food item, which is the so-called lantern fish,” explains Professor Paul Rodhouse, an Emeritus Fellow for the British Antarctic Survey (BAS) and former head of the organisation’s biological sciences division.
In turn, lantern fish follow vertically migrating zooplankton.
Since zooplankton are depended on by so many ocean animals, “the rest of the food chain will be following on after it,” says Prof Rodhouse.
“It is a huge movement of biomass every day,” says Prof Rodhouse. “More than a thousand metres. Some of the oceanic squid probably migrate over 1000m every day.”
He adds that almost all pelagic species (animals that live in the water column not near the bottom or shore) that can swim make the journey.
Humboldt squid are among the most striking creatures to surface every night. Their ability to change colour and flash bright red when agitated has earned them the nickname “red devils”. Although much smaller than their cousin, the 13m-giant squid, they can reach a length of about 1.5m (almost 5ft). Highly aggressive predators, they capture prey with strong tentacles and suckers and tear into it with powerful beaks, and have reportedly occasionally attacked humans.
But even ferocious Humboldts are preyed upon by bigger predators such as billfish, swordfish and sharks.
“Of course what they are all doing [by being active at night] is avoiding predation by the top predators,” says Prof Rodhouse. "The big predators that are visual predators and which stay in the surface waters and see their prey.”
“So they’re all… reducing the risk of being preyed on by going down into deep, dark waters at night.”

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