Inkfish, as opposed to other cephalings, have very unique types of ink. As organisms, they are very dependent on their ink when it comes to a lot of their natural behaviors, such as hunting, moving and climbing. How they co-evolved with their ink and became such complex creatures is an interesting topic.
The evolution of cephaling ink starts from their ocean phase, when cephalopods were first using ink largely as a defense mechanism while escaping. In that sense, the usage of ink stayed largely the same throughout the evolution of inkling species. The largest difference came in the sheer amount of ink that inkfish would produce, as well as the fact that it would become an important asset in their camouflage and locomotion. Additionally, the usage of ink in modern inklings became much more aggressive and associated with attacking as a follow-up.
[Inkfish ink comes in a spectrum of colors that can be controlled at will and has great cultural and personal significance. It is a common misconception that inkfish ink always matches the color of the tentacles. This is often the case due to instinct and camouflage purposes, but the chromatophores around the body don't necessarily always match the ink color.]
Inkfish ink is created and stored in a large ink sac, sometimes taking up half of an inkfish's total volume, or even more depending on the species and their body mass. On average, the amount of ink that inkfish hold in comparison to their total body mass is more than ten times that of non-inkfish cephalings with functional ink sacs, and the production of the ink is near-constant. As ink expands with contact to air, ink usage in inkfish is very efficient, as just a few droplets can form into a sizable cloud when spat out.
The basic consistency of inkfish ink is relatively viscous, much like paint, but it also quickly becomes sticky when in contact to air. There is however a lot of variety in ink consistency based on chemical composition, amount of mucus, hydration level and the health status of the inkfish. Ink might be abnormally thin or runny in inkfish that are sick, or it might be too viscous to really flow in other cases of illness or bad dehydration. Usually, inkfish will adjust the consistency of the ink by small increments based on the use case - thin ink that scatters into the air is perfect for smokescreens, whereas more viscous ink is better for marking vertical surfaces or tagging prey.
Different colors of ink often have different chemical consistencies. Contrary to popular belief, these differences have quite little to do with the color itself. Colors are however used to differentiate shoals and teams, and any one group will typically use a specific color to associate to their ink and territory. Ink is relatively harmless to the inkfish and any other inkfish that is currently adapted to the specific type of ink. This adaptation can be changed and configured even at pretty fast paces, and inkfish with the same adaptation will produce ink that is a very similar chemical consistency and color to others in the same group. In modern ink battles, the ink color and consistency is usually configured so players adapt to a very specific type of ink picked to be extremely different to the enemy team’s ink, and exact color may be configured by ingestion of short-lived ink dye to ensure uniformity.
Ink is a very efficient weapon against other creatures. Bright ink sprayed on top of a predator will not only slow them down and potentially obstruct their senses, but it will also make them stand out as a target for other predators or prey that can act accordingly to their presence and lack of camouflage. Ink is especially a powerful weapon against birds of prey, as a direct shot of ink often glues their feathers down and allows the inkfish to emerge victorious. Most small animals get stuck even in shallow ink, which makes it an efficient passive hunting ground, while inkfish themselves coat their bodies with their ink and are able to move on top of it quickly.
Interestingly, inkfish ink of different consistencies is quite harmful to other inkfish with a different ink consistency, and too much contact with ink of a different consistency causes the ink system to trigger a rejection that ejects all the ink out of the body.
Among cephalings, a number of species have functioning ink sacs, but only the largest collective group of inkfish utilize large-scale dependency on their ink and sport massive ink reserves and systems to evenly distribute this ink around their bodies. The ink sac is an inkling’s largest internal organ, taking up roughly 70% of the space in their organ sac - or even more at full capacity. The ink system, including the ink network pulsing ink throughout their bodies, accounts for a large fraction in total body weight in most inklings. Fortunately, higher ink amounts translate to higher ink pressures and stability, so the support structures of inkfish scale with the amount of ink present instead of struggling with sudden ink weight.
In inkfish, the ink is produced from water content, waste products, nutrients from consumed food, and secretions from glands in the ink sac. It is produced and stored at high pressure within the ink sac’s larger section often referred to as the ink reserve, and pumped throughout the body through various “ink hearts” around the body. This ink is kept in production nearly at all times, with increased production during periods of high activity, such as sports, with increased metabolic rate and hormonal activity. Ink production is at its highest during emotional highs, such as situations of high stress, fear or excitement, in which case ink may leak and spray out of the skin. It is at its lowest during general inactivity, and slows down considerably when the ink sac is full. Ink production naturally slows down with age, but may also slow down to a crawl or stop completely due to prolonged dehydration, starvation, various other health ailments and periods of dormancy, such as hibernation.
[Comparison of ink systems and ink sac size in a non-inkfish cephaling (left) and an inkfish (right). In non-inkfish cephalings with ink sacs, the ink sac is typically a very small organ at the tail end of the digestive system. In inkfish, the ink sac is much larger and ink itself has a central role.]
The ink system of inklings pumps ink throughout the body and recycles it around the system. Inklings constantly pump ink across their bodies through networks of tubes that are more directly known as ink veins. These ink veins come in several types: ink ducts or glands, major ink veins and minor ink veins. Ink flows out of the ink sac and passes through ink hearts - muscular parts of the system that keep ink flowing around at a high pressure. Major ink veins carry large amounts of ink in general parts of the body, and are the primary arteries in which the ink circles around - these form what is sometimes referred to as the “ink skeleton” as the pressurized ink in the system plays an important role in the locomotion and stature of inkfish. Minor ink veins split off from the major veins, delivering ink deeper into their general areas of function, where they further split into smaller veins. Minor ink veins are the thinnest type of ink vein, transferring ink to muscles and ink glands on the skin.
The ink glands or ink ducts of an inkling discharge ink onto the skin to provide defense and a coating of ink to help the creature blend into its own ink and slide on top of it. In bipedal form, the glands are only activated by heightened emotions, physical activity, discharging of ink, contact with ink, exceptionally hot weather and most commonly changing into one’s swim form. Otherwise, they are typically dormant in an inkling’s bipedal form save for if the inkling is in an aggressive or otherwise stressed or threatened state, or if they have recently turned back from their swim form.
The ink circulation system recycles ink that is cycled out of the body. While ink is almost constantly discharged from the ink glands onto the skin in swim form, it is also absorbed right back into the ink stream. This enables the inkling to have a near-constant, thin ink coating with a very modest loss ratio. In swim form, the ink coating makes inkfish barely detectable at a glance from ink on the ground. In bipedal form, these ink glands are largely closed off through the majority of the body to avoid loss for when this ink flow is not needed, and only let through minimal amounts of ink to keep the skin from drying.
Probably the most notable thing about inkling ink is the sheer range of colors that it can take. There is a common assumption that the color of the ink is what causes the different reactions between inkfish inks, but this is not the case. Different types of inkfish ink have different chemical consistencies, and color is only a secondary way to distinguish affiliation to a shoal. This means that two completely different ink colors can technically have the same consistency aside from pigments, and two identical colors can have completely different consistencies and react badly.
The ability to change ink colors is almost as seamless and flexible as the color change of the skin, being able to be changed at will and in mere minutes. As inkfish evolved and started moving on land, ink quickly became a way to mark territory, and thus naturally evolved into a very versatile asset. Brightly colored ink has multiple advantages; it is easy to spot in the environment, even in lower light levels. A lot of predators are also naturally turned away from bright colors, as they typically entail toxicity; so inkfish adapting their ink- and skin colors to be very bright worked in their favor in more ways than one.
When talking about changing one’s ink color, there is often a misconception that an inkfish has one “natural” ink color, which they do not. Ink colors can be changed any time at will, and full color change across the ink system can occur in minutes. Ink color is also not necessarily tied to the color of the tentacles, but is often matched for consistency’s sake. It is true that an inkfish will typically sport one color for their early life, but this is a result of juvenile inkfish mirroring the colors of their closest caretakers and shoal.
Ink color has very interesting chemical attributes when it comes to mixing with other colors. When two ink colors mix together, the two colors usually do not mix together; both colors will stay their own distinct colors without blending at all. The exceptions to this are cases where two inks of the same chemical composition have different colors, in which case the inks will mix together much like paint.
[A demonstration of some of the ways inkfish passively lose ink (purple) and how they gain back hydration to produce more ink (green). Most ink is lost through passive ink flow through the skin and evaporation or ink soaking into textiles instead of being reabsorbed.]
Because the constant, passive mass production of ink is a large cost in water, inklings are quite dependent on staying hydrated and taking in lots of water to be able to keep up the pace. To counter this weakness, the inkling body is efficient at retaining water, and extra water is not only absorbed from the environment but also from additions to their diets new to inkfish- fruit high in water content is consumed primarily for hydration purposes and secondarily for the quick energy in sugars. Inkfish skin is also notably thin and porous, resulting in some unwanted hydration loss, but giving inkfish an edge in humid environments where large amounts of their hydration needs are absorbed straight through the skin. This however has the additional effect of making them vulnerable to chemical impurities in the air as well as being submerged in too much water, which will flood into the ink system and flush out ink.
Ink is constantly created in the ink sac and circulated around the body through the ink veins and glands scattered just underneath the skin. The ink glands of inkfish both expel ink onto the body and absorb it back in, keeping the skin hydrated and protected while maintaining a modest ratio of ink loss. Inkfish are also able to absorb ink back into their bodies off of the ground or through external sources, such as Spawn Points, as well as easily accept ink from shoalmates with the same ink type in cases of low ink*. These adaptations to recycle and utilize ink from outside sources have helped to make inkfish sustainable and efficient ink users.
(*This is what is happening in Salmon Run when teammates have to spray a dead teammate with ink. The Lifesaver functions as a small dish to collect a bit of ink in, just enough to help the downed teammate regain enough ink to get back on their feet.)
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