Life in the deep sea is unique for a reason. Here, pressure is immense, waters are cold,
and below 3,300 feet down, there is no sunlight. Creatures that live beyond the twilight zone
spend their lives in an expanse of darkness. But although there may be no sunlight in the deep sea,
many organisms have used this feature to their advantage, developing a remarkable
adaptation in order to communicate with one another, hunt, and confuse predators.
These twinkling, flashing, pulsating lights are caused by the process of bioluminescence - the
result of a chemical reaction that produces light energy within the body of an organism.
It is a common feature of many marine organisms - a single adaptation uniting
vastly different creatures, from bacteria and algae to cephalopods and sharks.
There are 1,500 species of fish alone that are able to luminesce. And the fact that so many
different organisms have this ability is evidence of convergent evolution - in which bioluminescence
has evolved multiple times on at least 40 separate occasions. This suggests that down in the deep
sea, it is a highly beneficial adaptation to have, contributing to the success of organisms
by allowing them to communicate, find a mate, and attract prey, among other useful behaviours.
More than half of the ocean's jellyfish and ctenophore species produce bioluminescence
in some form. This comb jelly moves by beating its shimmering, hair-like cilia,
and can produce bright flashes to startle predators.
Their relatives, siphonophores, release glowing particles into the water to confuse the predator.
One particular siphonophore variety, known as Erenna sirena,
lures its prey by wiggling glowing bioluminescent lures.
The spindle-shaped body of the firefly squid is covered in bioluminescent organs called
photophores that emit a blue-green light. In the case of these shimmering cephalopods,
they utilise the adaptation to attract prey such as small fish down in the gloomy mesopelagic zone.
But they also demonstrate another evolutionary phenomenon. Counter-illumination, a form of
camouflage in which they light up in varying intensities to match the ambient light around
them. For example, they glow brighter as they ascend to shallower waters at night.
The colour they emit matches the faint glow of moonlight from the surface, meaning they
can blend in with their surroundings whether it is dark or light, and hide themselves from predators.
In much of the open ocean there is no concealment, so animals must adapt to be able to hide in the
wide open. This unique skill is helped by the fact that firefly squid are the only cephalopods to see
in colour, so they are able to distinguish between the ambient light and their own bioluminescence.
Firefly squid gather at the surface in large numbers from March until May during their
spawning season. Here at Toyama Bay in Japan, they bathe the beaches in an eerie blue glow,
lighting up particularly bright in order to attract a mate.
However, not all bioluminescent creatures are able to produce their own light themselves.
A common example of this is the case of the female anglerfish. Unlike some creatures, they rely on
a symbiosis with bioluminescent bacteria called Photobacterium, which move into the anglerfish's
esca, or lure, potentially by entering through a pore that develops in adult anglerfish.
The fish provides protection for the bacteria. In exchange, the bacterial symbiont produces the
bioluminescent glow that the anglerfish could not otherwise produce. Thus, we find these fish
displaying glowing bacteria-filled orbs that protrude from their bodies, which lead wandering
prey astray with eery lights towards their doom, much like the mythological will-o-the wisp.
Finding a mate can be hard in the vast, dark depths, so the far smaller male anglerfish
also makes use of the luminous bacteria within the females
as a means of locating them in expanse of the deep sea.
In most cases, however, an organism itself will contain the chemicals needed to luminesce.
In order for bioluminescence to occur, a species must contain a molecule called
luciferin. When luciferin reacts with oxygen, it produces light.
Organisms are able to regulate their chemistry in order to control when they light up,
as well as how bright and what colour. This is useful as different situations
will benefit from differences in the way the light is expressed.
This is the green bomber worm, a tiny polychaete worm that uses light to startle predators by
dropping a bioluminescent bomb, exploding its light-producing organs when it feels threatened.
One common feature that is present in almost all bioluminescent organisms,
is the colour of their lights. Blue.
But it isn't mere coincidence that all these creatures have evolved to emit the same colour
light. It is because blue light travels furthest in water, with its intermediate wavelength.
Visible lights with very long wavelengths, like red, or very short wavelengths, are absorbed
faster and filtered out. The very same phenomenon is responsible for the ocean appearing blue.
Some creatures can use this feature of different coloured light to their advantage,
like the bloody-belly comb jelly, which lights up red to make itself invisible to predators.
Furthermore, the fact that most bioluminescence is blue in the deep sea
is linked to the vision of deep-sea organisms, many of which do not have colour vision,
but rather a single, blue-sensitive, visual pigment. Thus, having eyes that are highly
sensitive to blue light is useful for detecting luminous prey, as well as potential mates.
This explains the large eyes found in many deep sea creatures, like
the barreleye - an adaptation that enables them to absorb as much light as possible.
Overall, bioluminescence is an adaptation that drives much of the processes in the deep sea,
for it used by many creatures flashing and glowing for many different reasons.
Used for repelling predators, luring prey, seducing potential mates,
bioluminescence underpins a coruscating dance of life, death and sex in the deep sea.