The firefly of the sea

On spring evenings, within the waters of the Japan you can find the sea lit brighter than the night sky. The cause of these illuminated waters is the tiny cephalopod, Watasenia scintillans, or more aptly known as the firefly squid. At 3 inches long these brightly lit creatures can be found in deep-sea shelf waters around Japan, reaching depths of around 200-600 meters. Their main predator is the Northern Fur Seal, Callorhinus ursinus, who find it difficult to predate due to the squids adaptations.  Watasenia scintillans are also heavily fished, primarily during spawning season where 99.9% of caught squid are egg bearing females. After they have spawned in shallow water, they then die, having lived for just one year. This is known as being semelparous and is common amongst Cephalopods.

During mating season in the spring, W. scintillans migrate and form large aggregations at the surface of Toyama Bay, Japan. They create an incredible annual light show that attracts tourists from all over the world to see. The cause of this spectacular natural phenomenon is bioluminescence.

Watasenia scintillans (Source: Chiswick Chap)


Bioluminescence is the production of light by a living organism, through the use of biochemicals. Organisms range from fireflies who create bioluminescence in their abdomen to cookiecutter sharks, whose tiny glandular organs known as photophores, are intricately hidden amongst their denticles (the rough tooth-like skin of a shark). It is believed that 80% of all organisms that have the ability of bioluminescence are marine organisms.

Bioluminescent dinoflagellate (Souce: Jed via Wiki Commons)

The production of light can be created in two ways; one by a chemical reaction and the other through a symbiotic relationship with bacteria. An example of a symbiotic relationship would be the Hawaiian bobtail squid, Eupryma scolopes, and the bacteria Vibrio fischeri; the squid feeds the bacteria amino acids in return for bioluminescent camouflage, enabling them both to survive. For other organisms who can create their own bioluminescence, the chemical reaction occurring is between Luciferin (a compound) and oxygen. Luciferin oxidises in the presence of oxygen, using luciferase as a catalyst to produce light and create oxyluciferin as a byproduct. Some reactions don’t use luciferase and instead use a photoprotien. These photoprotiens combine with oxygen and luciferins but require another agent, typically an ion of calcium to help produce light.

In the firefly squid, bioluminescence can be found in the photophores. Roughly 800 of these photophores can be found across the ventral (underside) side of the body, ranging from the top of the head to the arms. There are 5 photophores around each eye, known as the ocular organs. The brightest blue light is emitted from collections of three tiny black organs (called the branchial organs) on the tips of the 4th pair of arms. Firefly squid use the luciferin oxidation pathway to generate additional light, via the ATP mitigated process of reacting Coelenterazine-disulfate (a type of luciferase), and dissolved Mg2+, within the photophore.

Bioluminescence is also believed to have a wide range of purposes. Scientists believe W. scintillans could use bioluminescence to communicate and to attract mates. Luminescence can also be used for predatory behaviour, with scientists believing that the squid flash their lights at the depths to attract fish.

The art of camouflage

One important adaptation that W. scintillans has developed using bioluminescence is counter-illumination, or best described as the “art of camouflage”. This is where the photophores match the light wavelengths of the surrounding water column and environment. They do this by adjusting the amount of light coming from their photophores. This means that in the mesopelagic zone where W. scintillans resides, predators hunt often by looking up in the water column to see shadows of prey. With counter-illumination, these shadows aren’t formed due to the camouflage effect, meaning the squid can go undetected. This is also beneficial for the squid when it comes to prey, enabling the squid to sneak up unsuspectingly on its victims, who cannot see it in the water-column above.

Figure 2: A diagram showing the counter-illuminaton of Watasenia scintillans as seen from

Another adaptation that the firefly squid has developed using bioluminescence is the skill of appearing bigger. Their bioluminescent photophores can disguise their shape, meaning if stretched out, they can appear larger, and scare off or confuse predators.

The eye anomaly

Perhaps the most unusual adaptation though is the sight of W. scintillans. Cephalopods are known for their excellent senses, however most cephalopods cannot see in colour as they only have one visual pigment. Scientists have discovered that W. scintillans is the only known exception of this, having three visual pigments. These pigments are located in various locations of the squids retina, allowing it to see different colours in the water column. This has enabled the firefly squid to distinguish between the natural light of the water column, bioluminescence of other species and the light created by other firefly squid.

The firefly squid is a natural wonder, a pretty sight for the masses who flock to see it every year and a great source of interest in the science community wishing to find out just how they’ve adapted to colour vision. If you’re interested in seeing this natural wonder at peak season, take a look at the Hotaruika museum page for more information on tours and more about the firefly squid!

This post was previously published on extrememarine with some minor adjustments, for more posts like this one from my fellow peers, check it out!

Until next time,

Cate x

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