#adaptation physiology

When we’re talking about light, we’re talking about the electromagnetic (EM) spectrum. It’s the full range of EM radiation, and organised by either frequency (in hertz/Hz) or wavelength (in nanometers/nm). The spectrum itself is then divided into bands, with dif names for the waves in each band, and from low to high (frequency) these are radio waves, microwaves, infrared, visible light, ultraviolet, x-rays and gamma rays. We’ll only talk about a couple of these, don’t worry, and I won’t complicate things too much.

So light/visible light/visible radiation is basically what we (humans) can see, and spans the wavelength range of 400-700 nm. As you can see from the image below, it’s flanked on the left by the UV band (high side of the freq spectrum and lower wavelength) , and on the right side by the IR band of the EM spectrum (on the lower frequency but higher wavelength).

When it comes to IR, the only important thing to remember here is that red is on the right side of that spectrum (at the far end of the human visible light spectrum) and that IR begins with waves that are just longer than those of red light. This means that red is at the long wavelength end of the visible light spectrum and has a dominant wavelength of approximately 625–750 nanometres. Also important is that IR be sensed as heat, even though it is invisible to the human eye.

Now as for ultraviolet light, usually simply known as UV, is electromagnetic radiation of wavelengths of 10–400 nm, is present in sunlight and constitutes about 10% of the total electromagnetic radiation output from our Sun. We usually put the lower end of the visual spectrum at 400nm, but that’s not a hard limit, and shorter wavelengths just become less and less visible in that range, and a lot of animal species can see near-UV than humans can’t.

An interesting tidbit here is that your eye is basically just a giant receiver, it captures this optical radiation across the visual spectrum, which causes the electrons inside a molecule to go bonkers, which then leads to a change in the bonding chemistry of that molecule, and that change is interpreted and sent off to our brain as a signal (aka what we see -very very broadly, apologies for any inconsistencies, it’s been ages since I’ve actually had any of this in college). At the lower end of the EM spectrum (at the IR band), we stop seeing things when the longer wavelengths no longer have the ability to excite those photons and trigger a reaction. Same thing goes for the other side of the spectrum (UV), where the internal architecture of the eye steps in, and UV light is absorbed by the cornea below 360 nm and the internal lens below 400 nm. Furthermore, the rods and cones located in the retina of the human eye cannot detect the very short (below 360 nm) ultraviolet wavelengths and are in fact damaged by ultraviolet. Fun fact, this is why some people who are born without a lens in the eye (who can still see since the lens is only responsible for about 30% of the eyes' focusing power) say they can ‘see’ UV light, as do patients who’ve recently had cataract surgery). Birds have a fourth color receptor for ultraviolet rays; this, coupled with eye structures that transmit more UV, gives smaller birds "true" UV vision – remember that, I’ll circle back to it later.

Many animals with eyes that do not require lenses are able to detect ultraviolet (such as insects and shrimp -who here hasn’t heard or seen the shrimp color post come across their dash? They’re not wrong, because shrimp are tetrachromatic, meaning they have more chromatophores than we do and thus see a larger spectrum of shades and colors). And they do this by quantum photon-absorption mechanisms, in much the same chemical way that humans detect visible light. The human eye only uses three types of cones to sense light in three bands of color (meaning we’re trichromatic). The biological pigments of the cones have maximum absorption values at wavelengths of about 420 nm (blue), 534 nm (bluish-green), and 564 nm (yellowish-green).

Though there are quite a few animals that are sensitive to various types of infrared, not all of them do it by means of this so called quantum-absorption where molecules get excited and we capture that change and interpret it. Infrared sensing in snakes, for example, depends on a kind of natural thermal imaging, in which tiny packets of cellular water are raised in temperature by the infrared radiation. EMR in this range causes molecular vibration and heating effects, which is how these animals detect it. I hadn’t encountered prey sight yet in my Warhammer readings, but I’d be interested in knowing what exactly it entails and what type of IR and/or thermal imaging it’s employing. @ladyalisette mentioned this in her response to @nereidof40k's post, which inspired me to dig a little deeper into the topic since I'm always down to geek out with a fellow biologist.

Now, let’s talk about the adaptations that’d happen in case of a perpetually low-light environment such as Nostramo. Adaptation itself is already a term in visual physiology, and very apt in this case since it refers to the ability of the retine to adjust to light levels. In humans, rods (not cones) are responsible for night vision, since cones only function at higher light levels, and our night vision tends to be limited mostly because there’s less resolution and color differentiation is almost impossible, instead being limited to shades of gray (can we say that without thinking of 50 shades? No? just me? *sigh*). But, we *can* see in the dark, even though we don’t do it super well. So when we transition from light to dark, our eyes undergo what’s called natural dark adaptation, and it’s a slow process which can take from 45 min to two hours. During that adaptation, the eye adjusts from high to low luminescence ‘settings’, meaning that they increase their sensitivity (by several orders of magnitude, it’s actually quite impressive). This adaptation period is different between rod and cone cells and results from the regeneration of photopigments to increase retinal sensitivity. I believe @nereidof40k already mentioned rhodopsin in her post, and that comes into play here. Rods (and cones) don't work on their own; they are inert, and that particular type of nerve cell needs a chemical to enable their function -Rhodopsin. The body does not produce this chemical in daytime, and it takes a very low light level sensed by the eyes to produce this chemical. When the light is detected at a low level for 20 minutes or so, the body starts producing rhodopsin and night vision starts setting in. A key trait of this feature is that rhodopsin is photoreactive, meaning it reacts to light, and it only takes a few seconds of bright light to cause the rhodopsin to decay into two parts with a photosensitive reaction, and the rods stop working. Then the cycle starts again, meaning your eye has to start the process of adjusting to the dark all over again. It is an interesting trait that deep red lights do not trigger the neutralization of the rhodopsin, so astronomers and safety officials use red lights for night lighting to allow night vision to continue and work in low-lighting conditions. However, unless the light is monochromatic like a laser, even red light has elements from other colors, which means that even a bright red light can reduce the rhodopsin, and so a dim red light is best for maximizing after-dark eye behavior. It has also been suggested that that since stars typically emit light with shorter wavelengths, the light from stars will be in the blue-green color spectrum. Therefore, using red light to navigate would not desensitize the receptors used to detect star light. This ties in nicely with @ladyalisette ‘s statement that the red end of the spectrum tends to travel further in space.

It also works nicely with what we know of Nostramo, seeing as it’s sun is ‘dying’ and orbited a distant red giant. As objects grow hotter, they radiate energy dominated by shorter wavelengths, changing color before our eyes. Now our own Sun produces more yellow light than any other color because its surface temperature is high at 5,500°C. If the Sun's surface were cooler —say 3,000°C—it would look reddish, like the star Betelgeuse. If the Sun were hotter—say, 12,000°C—it would look blue, like the star Rigel. If the star is dying, the temp would be lower, and the light it gives off would be redder, meaning that if any sun were to reach Nostramo at all through the toxic smog layering the hive city, it would be on the red side of the EMR spectrum anyway. It's atmosphere blocked out the visible spectrum of light, leaving the planet in almost complete darkness. However, the planet isn't noted as an ice world or volcanic, meaning that enough sunlight still reached it, though it might not be visible to the human eye, and it would be safe to assume that Nostramans have adapted to that reality, and their physiology would have evolved measures to survive in that kind of environment over time.

With regards to vision, there’s three main kinds, namely scotopic, photopic and mesopic. Photopic vision is the vision of the eye under well-lit conditions. Scotopic vision is the vision of the eye under low-light conditions. Mesopic vision is also called twilight vision, and it’s a combination of photopic and scotopic vision under low-light (but not necessarily dark) conditions. Now, evolutionarily speaking, there’s advantages and disadvantages to maxing out all your stats in one area, but trying to do both is usually quite ‘expensive’ in the terms of resources it takes to develop a sense. So as per usual it’s about finding the balance in order to give yourself the edge when it comes to passing on your genes. In the human eye, cone cells are nonfunctional in low visible light. Scotopic vision is produced exclusively through rod cells, which are most sensitive to wavelengths of around 498 nm (blue-green), and are insensitive to wavelengths longer than about 640 nm. Under scotopic conditions, light incident on the retina is not encoded in terms of the spectral power distribution. Higher visual perception occurs under scotopic vision as it does under photopic vision. Now, cone photoreceptors are -haha, you guessed it- conical in shape, and there are three types of cone photoreceptors, each being sensitive to a specific wavelength of. The various cone cells are sensitive to either short wavelengths (blue light), medium wavelengths (green light), or long wavelengths (red light). Only relevant thing to keep in mind here is that in the language of optics, red is the color evoked by light that stimulates neither the S or the M (short and medium wavelength) cone cells of the retina, combined with a fading stimulation of the L (long-wavelength) cone cells.

Seeing as our night vision is largely dependent on rod cells, which are more sensitive towards the lower part of the wavelength spectrum, not the higher end (where red and IR are), I’d say that it would be ‘easier’ evolutionarily speaking to just make Nostramans sensitive to UV light, perhaps by adding that fourth color receptor other animals already have, and maybe some other physiological changes like the lens being less susceptible to UV damage.  The main physiological adaptations to such an environment would be (1) having a larger eyeball, (2) a larger lens, (3) a larger optical aperture (the pupils could expand to the physical limit of the eyelids, hence leading to the near-black eye look), (4) a higher density of rods rather than cones (or rods exclusively) in the retina, and (5) a tapetum lucidum (which is a reflective structure that is responsible for this superior night vision as it mirrors light back through the retina exposing the photoreceptor cells to an increased amount of light). I’d definitely say there’d be advantages too when it comes to heightening their ability to see parts of the IR spectrum based on the planetary conditions on Nostramo, so it could definitely be a case of having both adaptations. If we want the human eye to perceive the wavelengths above/below the visible EM spectrum, there would be the need to reassign (or add a new type of) cones to generate a response stimulus at those wavelengths, and we’d have to define how broad we would want the response to be. It would be important to define how sensitive it is (the smaller the wavelength range it detects, the more sensitive it is, but the less it sees overall). Another interesting change could rhodopsin becoming less photoreactive, or it's metabolism being quicker so as to allow quicker adjustment in low-lighting conditions.