#firefly squid

Deep-Sea-cember: It's quite a sight to see firefly squid at night

Deep Sea December 2025: Day 8–Glow

Technically this is the first painting that I’ve done for this art prompt list (it’s currently December 3rd) I appear to have drastically overestimated the amount of time it will take me to complete these drawings!!

Yes!!!! Bioluminescence be upon ye!!!

Ahhhh, it's finally working!! Why is my Internet connection so bad /:

Here's Day 8 of @montereybayaquarium deep sea challenge: glow

Bioluminescence is so fun

This Wet Beast Wednesday is a bright one, because I'm covering an animal that makes its own light. Now only are the firefly squid a rare example of a bioluminescent cephalopod, they are famous for the light show they put on every year. This show is a hot spot for tourists and commercial fishermen alike and I'm here to tell you what it's about.

(Image: a firefly squid. It is a small squid with a transparent body covered in red-brown dots. It has very large eyes. This one is laying eggs, which can be seen as a string of small orbs emerging from beneath its head. End ID)

The firefly squid (Watasenia scintillans), also called the hotaru-ika or sparkling enope, is a small squid, reaching 7.5 cm (3 in) long as adults. They are fairly typical squid in terms of anatomy, with the body divided into the head and mantle. The head contains the brain, eyes, mouth, and appendages. The eyes are very large and adapted for low-light conditions. The mouth opens in a beak (the only hard part of the body) that is surrounded by the appendages. Squid have 8 arms and 2 tentacles. The arms are short and have suckers or hooks (firefly squid have hooks) along the whole length. The tentacles are longer and only have suckers/hooks (firefly squid have both) at the ends. The mantle makes up the rest of the body and contains multiple organ systems. There are fins on the side of the mantle and a siphon used to squirt out water, both of which are used to move. Firefly squid are reddish-brown in color.

(Image: a firefly squid bioluminescing in the dark. The result looks like a bunch of blue dots in the shape of a squid. End ID)

What sets firefly squid apart from most other squid is the presence of photophores: light-producing structures. Firefly squid are not the only bioluminescent squid, but they have the most photophores and theirs have a unique structure. There are three types of photophores on the body. The most numerous cover the underside and produce blue-green light. In addition, there are 5 photophores below each eye and a single large photophore on the tips of the 4th pair of arms. These photophores produce only blue light. The photophores of the firefly squid have a unique rod-shaped crystalline structure in which the light-producing chemical reaction takes place. This makes the light brighter and allows it to be directed in a particular direction. Firefly squid can make a cone of light directed beneath them.

(Image: a group of 6 firefly squid bioluminescing in the dark. Some of them have the large photophores at the tips of their arms lit up, which are especially bright. End ID)

It's not entirely clear what firefly squid use their bioluminescence for. The main hypotheses are that they use it for camouflage, communication, and hunting. Because most of the photophores are on the underside of the body, the squid may use them for a camouflage strategy called counter-illumination. By making the light on their underside batch the color of the sunlight from above, the squid can make itself virtually invisible to predators below. Many animals that live in the deep-sea use bioluminescence for hunting, either using the light to see prey or waiting for prey to approach the light they produce in search of food. The fact that the squid can produce a cone of light may indicate they specifically hunt prey in that cone. The blue-green light they produce may also be used for hunting as many deep-sea species cannot see green light and the firefly squid has pigments in its eyes that indicate it may have color vision. By using blue-green light, the squid could illuminate prey with light the prey cannot see. The possibility that the firefly squid use their light to communicate is a hypothesis that seemingly hasn't been tested yet, though squid are intelligent animals and other species communicate with color changes, so it is possible.

(Image: a firefly squid held in a hand. It is about as large as one finger. End ID)

The behavior of firefly squid is poorly-studied. They are predators who live in deep water along the coast of Japan. Like many deep-sea species, firefly squid follow diel vertical migration, a mass migration of animals from the deep sea to shallow waters in the evening and then back down at morning. Investigation of stomach contents shows that juvenile firefly squid hunt copepods and as they grow larger, their diet expands to include small fish and other squid. Most squid capture prey by keeping their tentacles tucked amongst their arms, then lashing them out to grab prey at high speeds. The most famous bit of firefly squid behavior is related to reproduction. As with other squid, males will place a capsule of sperm called a spermatophore into the female's mantle using a pouch behind the head. The males apparently die quickly after. Unusually, the males reach sexual maturity and mate before the females. The female will hold onto the sperm packet for a few weeks before becoming mature enough to use it to fertilize her eggs. Most squid females will attempt to mate with multiple males, but firefly squid appear to be monogamous. Once they are ready to lay their eggs, the females will migrate to Toyama Bay in Japan to spawn. They spawn at night, releasing long strings of tiny eggs into the water wile glowing. These light shows last for a few weeks in April, but usually peak for a few days. After laying her eggs, the female dies. Their life cycle lasts one year.

(Video: a video of facts about firefly squid that includes some shots of their spawning light display. End ID)

Firefly squid are classified as least concern by the IUCN, meaning they are not in danger of extinction. They have become a major source of income for the Toyama Bay area as they are considered a delicacy and their yearly spawning is a major tourist attraction. Commercial fishermen will spend the nights in April scooping up the squid as they surface to spawn. In addition, lots of the squid will wash up on the local beach where people will go pick them up before the birds and other local animals get to them. The yearly fishing does not seem to have had a major effect on the population, indicating that most spawn before being caught. Firefly squid are traditionally eaten cooked to kill a nematode parasite they often carry that can cause severe stomach problems if not killed by cooking. The majority of research on firefly squid is done on specimens taken during the spawning, leaving a lot of detail about the rest of their lives unknown.