Whoever takes a swim along Puerto Rico's shores at night will witness a special show. The water starts to glow in a bright blue and green. The effect is called bioluminescence - it's our natural phenomenon of the week.
Image: Colourbox/P. Grebenkin
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When people flock to the beaches of the Caribbean island of Puerto Rico at night, they do so for a reason: The water immediately surrounding swimmers starts to glow a mystical bluish-green. The light glows stronger with each movement.
Puerto Rico is said to be the place to go to watch the water glow. Nowhere else in the world is the concentration of microscopically small algae greater. And those algae, or plankton, are bioluminescent - meaning, they glow in the dark.
Illuminating
The single-celled organisms are called dinoflagellates. One could also call them the fireflies of the sea.
They can be found all over the world. But it is only in a place like Puerto Rico, where there are so many, that you can see them glow so strongly.
And there can be many, when conditions like water quality and nutrient supply are just right. If colonies reach 100,000 single cells per liter of water and the sun is strong throughout the day, the dinoflagellates can be charged - just like a solar lamp - and glow the night away.
Image: Colourbox/P. Grebenkin
If some troublemaker appears, the plankton try to scare the intruder away. In the case of fish, this could work - with humans, obviously not.
Light source
The light is produced through a very simple chemical reaction. When oxygen and the biological substances luciferin and luciferase (an enzyme) come together, energy is generated, which is released as light.
But the light can be different depending on the bioluminescent species present. Various colors are possible. While aquatic species often glow blue or blue-green - because this color can be seen best underwater - fireflies on land appear yellow.
The black dragonfish has the ablity to produce its own light. This is advantageous because many animals cannot see red light in the depths of the ocean. The light works instead as an invisible searchlight - the creature can locate its prey, but does not attract attention.
A pretty slick ability.
More from below the surface...
Deep secrets of the Mediterranean
It's holiday time along the Mediterranean coast. But do you know what's lurking in the water when you take a cooling dip? Amazing creatures share your bathing space, but beware, not all are pleasant.
Image: picture-alliance/dpa
Fried egg jellyfish
With such a culinary name and its astonishing appearance, sea lovers probably do not need any further argument to be amazed. To top it off, it seems that this jellyfish's sting has very little or no effect on humans. In contrast to most jellyfish, this one can move on its own, and, surprisingly, it is one of the most common jellyfish in the Mediterranean Sea.
Image: picture-alliance/dpa
A vast fauna down there
When you feel something unfamiliar touching your toes, it might be comforting to think it's just plants - but that's not always the case. One of the plant-like creatures living on the bottom of the sea is this yellow cluster anemone, whose polyps can have up to 36 tentacles each. In the Mediterranean, this species often appears in dense agglomerations close to sponges, tunicates and algae.
Image: Public Domain
Look but don't touch
Although it's very attractive for divers, this marine worm of about 15 centimeters - which can reach up to 35 centimeters - does not like to be disturbed. If so, its bristles can penetrate flesh and produce an intense irritation to the skin. Its vivid combination of colors is a warning for those in the know, but in case the message is still unclear, its name spells it out: bearded fireworm.
Image: Public Domain
Flying under the water
The 'wings' of the flying gurnard are, in reality, very large pectoral fins, which enable the fish to glide above the water for short distances. Disappointingly, the flying gurnard does not fly as such; it is mainly a bottom-dwelling fish and only extend its fins when troubled or in need of scaring predators. This produces a beautiful spectacle of bright and fluorescent colors.
Image: Imago
Would you guess that?
This three-centimeter flatworm is one of the enigmatic wonders that nature has to offer us. Its oval, flat body has a blue tone, a yellow central longitudinal band and dozens of white stripes - it can also appear in pink. Two tentacles on its head are reminiscent of a small mammal's ears or to a snail's horns. Its diet, sexuality and regeneration methods are also part of its unusual features.
Image: picture-alliance/Hippocampus/Bildarchiv
The charming, but foolish, turtles
Some of the creatures living here are far more famous - but no less outstanding. The loggerhead is the most common sea turtle in the Mediterranean - but tourism and getting caught in fishing nets mean it's endangered. Their large heads, which support powerful jaws and enable them to feed on hard-shelled prey, justify their name. But in some languages they are known as the 'foolish turtles'.
Image: cc by ukanda 2.0
'Turbot' is watching you!
Even with newly purchased professional goggles, it would be hard to notice this fish. But it is somewhere there, just under our feet. The turbot is a flat fish with both eyes on the left side of its - surely flat - head. It likes blending in with the sand and mud on the sea bed, but despite its camouflage, it is sought by fishermen around the world, as it is considered a highly-prized species.
Image: Imago/Blickwinkel
Much more than a scary presence
There are not many natural dangers in the coastal areas of the Mediterranean Sea, and that might be the reason why jellyfish create such a panic. But despite the bad rap these annoying bathing companions receive , they are actually remarkable animals. As an example, the compass jellyfish has 24 impressive tentacles and, upon maturity, it changes sexual functions from male to female.
Image: cc by Francesco Crippa 2.0
Here comes the star
The spiny starfish can reach an impressive diameter of 70 centimeters, even though the central disk is relatively small. Each of its five arms has three longitudinal rows of purple spines, each surrounded by a white cushion. Despite being native to the eastern Atlantic Ocean, its habitat range extends from Iceland to the Mediterranean Sea and the Azores.
Image: picture-alliance/OKAPIA KG, Germany
Aquatic scorpions
Not everything is peace and love under the Mediterranean Sea. The small red scorpion fish is a very venemous species due to its sharp spines coated with mucus. It is mainly found in rocky littoral habitats, even though sometimes it can be found up to 700 meters under the sea. It is carnivorous and feeds on small fish and crustaceans - of course, one would expect no less from a scorpion fish!
