((I realize that these guys are a bit like the Mahlok, but I've had this idea for an alien race for a long time, so I figured 'why not'?))
Species: The Mirran
The Mirran are a somewhat-recently discovered alien species.
The Mirran homeworld was originally discovered by a Victory exploration team 60 years ago. After a cursory examination, they dismissed the world as being unlikely to contain intelligent life, and too difficult to mine for mineral resources, and named it Meruto —an amalgam of Mercury and Pluto, referring to the planet's highly variable environment resulting from its eccentric orbit— before leaving. In the latter, they were more-or-less correct —in the former, not so much.
The Mirran were not discovered for another 20 years, until Yavakaro sent an expedition to try and extract the world's mineral wealth, despite the difficulty, and discovered that the planet was inhabited. Trade was quickly established, and Meruto has since been a great —if somewhat unreliable— source of rare and valuable metals (Platinum, Iridium, and Osmium, among others) to the rest of the galaxy. Another of Meruto's exports has been its people, with many fleeing offworld, whether out of sheer adventurism, a simple desire to escape the planet's numerous internecine wars —or both.
Thus, though still small in numbers among the general galactic population, the Mirran are somewhat widespread, joining in any society that will take them.
Meruto's surface is bare and rocky, and generally looks inhospitable —which it very much is. Though larger than old Terra, it has a much higher concentration of heavy metals in it, and as such is much denser overall, resulting in a surface gravity 1.6x that of Earth. Although larger, Meruto also spins faster, resulting in only a 27-hour day. Despite its higher gravity, Meruto's atmosphere is fairly thin, and contains a mere 6% atmospheric oxygen on the surface (and that only part of the year).
Because of its highly eccentric orbit, the Meruto year (equivalent to 16 Earth-months) is divided into three 'seasons':
During the Bright Season (lasting a little over 4 months), Meruto is very close to its sun, and daytime temperatures can reach over 2400°C; nothing save a Mirran (or possibly a Mahlok) can survive on its (frequently molten) surface —and even Mirran for only brief stretches (maybe an hour at most) at a time, in direct sunlight.
The Twilight Seasons (which last about 1.5 months between each of the other two seasons) are the only time outsiders can survivably land on Meruto.
During the Dark Season (lasting about 7 months), Meruto is very far from its sun, the surface temperature drops to around -215°C, and most of the surface atmosphere liquifies or freezes. Nitrogen snow and puddles of liquid Oxygen cover the landscape.
Meruto's upper crust is swiss-cheesed with caves. The environment in these caves, while still quite variable, is more insulated and consistent than the surface, and so it should be no surprise that this is where life evolved —and, on the whole, continues to live— on Meruto.
The shockingly-human appearance of the Mirran is perhaps the best argument for convergent evolution (or, depending on ones' beliefs, a universal creator) that has ever been discovered. Aside from a few features which make them unmistakeable as anything but, Mirran look quite human: from their body-shape, to their facial features, to having five fingers/toes per hand/foot.
However, there are also some more minor differences. For one, Mirran muscles are more densely-layered than human ones, and so they tend to appear somewhat wiry. In addition, because of their biology, it is practically impossible for a Mirran to get fat.
Due to the higher gravity of their world, Mirran joints are slightly oversized compared to human ones. Also, as a consequence of the heightened gravity, and their cave-dwelling lifestyle, Mirran tend to be a little shorter on average than humans, adult Mirran tending to be between 5'4'' and 5'8'' in height, though an occasional few might reach 5'9'' or even 5'10'' —though these tend to be considered absurdly tall by their own kind; unlike humans, female Mirran tend to match the males in height, weight, and strength. In addition, MIrran ears are slightly larger than human ones, and somewhat pointed —resembling nothing so much as hairless cat ears; they are also capable of moving independently, to better pick up and pinpoint sounds (Meruto's thin atmosphere does not carry sound very well, requiring very good hearing to compensate).
Mirran skin is the most obvious thing that marks them as something other than human. They are capable of changing the color and reflectivity of their skin in a manner somewhat akin to an earthly octopus or cuttlefish, but more dramatically and practically instantaneously. Their skin can change from a blinding mirror-like silver, to an absolute pitch blackness, to either reflect, or absorb, almost all incoming light, respectively. Colors between these two extremes —like various shades of gray, or even white— are also seen, but are prone to changing from moment to moment, reflecting the Mirran's mood and surrounding environmental conditions —mostly the level of light.
Mirran eyes look especially strange to humans. The 'white' of their eyes can shift colors like their skin, and almost always matches their skin tone. Their pupils are ellipsoid —something between round human pupils and slitted cat's-eye pupils in shape— and are highly variable in size; however, their pupils can also change color, and are typically only a little darker than their skin tone, making them difficult, or impossible to see (especially if the Mirran's skin is black at the time), creating the illusion that they have no pupils. Mirran eyes have no discernible irises.
The very fact that life, much less intelligent life, managed to develop on Meruto is a scientific mystery, but Mirran's bodies are well-adapted to their world's extreme and variable environment. Reflecting the makeup of their homeworld, Mirran have a rather high concentration of metals in their bodies (in fact, a Mirran would probably set off a metal-detector stark naked), which play a variety of roles in their biology.
Somewhat unusually, Mirran blood contains two oxygen-carrying compounds, utilizing both Hemerythrin cells, and free-floating Hemocyanin in the plasma. In addition, Mirran blood is rather thicker than human blood, containing a much greater density of oxygen carriers to make the best possible use of the limited oxygen on their world, despite the per-unit inefficiency of these carriers compared to Hemoglobin. Because of this improved oxygen transport and storage capacity, Mirran can hold their breaths for a very long time —up to half an hour— and, in higher oxygen environments (like any human world or ship) they basically never get out of breath no matter how much they exert themselves. Although, it has been discovered that Mirran born on higher-oxgen worlds tend to develop slightly thinner blood than those born on their homeworld, though they still well surpass humans in terms of respiratory endurance.
Because of this blood chemistry, Mirran are immune to Carbon Monoxide poisoning. In addition, they tend to stop bleeding quite quickly, and likewise heal faster than humans.
When cut, Mirran blood appears blue-violet in color (it's clear when un-oxygenated).
Mirran skeletons are composed primarily of Titanium and Nickel, with other metallo-organic compounds containing rarer and heavier metals layered into the composite structure. As such, their bones are quite a bit denser than humans', and far stronger. In fact, their bones are practically unbreakable (in a falling or hand-to-hand combat sense, anyways), being somewhat superelastic —capable of flexing under high stress, and springing back to shape the moment it's released.
These dense skeletons mean that Mirran weigh more than a human of equivalent height and bulk. And, because of this extra weight, compounded by the higher gravity of their homeworld, Mirran tend to be quite a bit stronger on average than humans —although, again, those born on human worlds are typically less so— and their tougher skeletons mean they can hit harder, proportionally.
In both structure and purpose, Mirran hair is rather similar to fiber optic strands —albeit, far finer than the synthetic ones that humans created for communication purposes. The strands catch and direct light to special light-sensitive cells in a Mirran's scalp. While not literally possessed of 'eyes in the backs of their heads' as some people say, through their hair Mirran are able to sense even minute changes in light around themselves, and hence it is nearly impossible to sneak up on one.
Their hair is shiny, somewhat stiff, and, like the rest of them, tends to match their skin tone —appearing silvery when their skin is light, and black when their skin is. Unlike human hair, Mirran hair grows very slowly, and stops growing after a time unless it is cut; as such, you will never see a Mirran —male or female— with hair much longer than what would be considered medium-length for a human male. In addition, they have no body or facial hair.
Skin, Light Absorption, and Sustenance:
As light is one of the few things that is more than abundant on Meruto (part of the year, anyways) it is no surprise that Mirran evolved to take advantage of it. Their skin contains specialized cells which are capable of absorbing a wide range of the electromagnetic spectrum, including all visible light, as well as down into the microwave range, and up almost to the x-ray range. By some process which is not quite understood, this absorbed energy is then stored in what amount to biological batteries in their chests (just behind and below the lungs, against the back of the ribcage), where it is used to fuel their motions.
This, likewise, makes them immune to most energy weapons, as they can simply absorb the beams. However, their skin can only absorb so much energy so fast, and if the rate is too high, they will shift their skin to a more reflective state. Because they can make their skin almost perfectly reflective, it would require an immensely powerful laser (which could only practically be mounted on a very large vehicle, or a starship) to injure a Mirran.
Mirran skin is thicker and somewhat tougher than a human's (though not dramatically so) and, unusually —in addition to its light-absorbing/reflecting properties— is completely opaque, and a near-perfect insulator, rendering them nigh-immune to both the upper and lower extremes of temperature. Though their skin is extremely insulating, Mirran do not overheat under normal circumstances. Mirran are, in fact, technically cold-blooded, even though they will feel warm to the touch unless utterly starved of light.
If they do begin to overheat, Mirran can modulate their temperature in two ways: the first is to simply make their skin more reflective to take in less light (heat); the second is for them to 'dump' some of their heat into the environment in the form of light. Their skin also contains special photophores, which can be used to re-emit some of their absorbed light —whether as a general glow, or as a coherent beam (i.e., a laser). In addition to its use for modulating their body temperature, this ability makes a phenomenal natural weapon against most any non-Mirran (save a Mahlok). However, it does drain them fairly quickly, and so it typically is only used as a last resort. Also, their ability to focus the beam is limited, and so, likewise, is the effective range (50 meters, at most).
Because they primarily use light to sustain themselves, adults need to eat only occasionally, to provide material to replace dying cells; growing children, and injured or pregnant adults, need to eat quite a bit more, obviously. When deprived of sufficient light for long periods, Mirran tend to become sluggish. And, because the suns of most other worlds are rather dim compared to theirs at its height, offworld their skin tends to stay black or dark gray most of the time to absorb as much light as possible —lighter shades will only be seen if they are very full-up on light.
As water, especially liquid water, is a rare resource on Meruto (in fact, it is considered at least as valuable there as were gold, diamonds, or oil on old Earth —many wars were fought over sources) Mirran likewise can survive relatively long periods (over a week) without it —if nowhere as near as long as with food.
Because of their tough, insulative skin, and unusual blood, Mirran are capable of surviving in hard vacuum, without a suit, for as long as they have air (again, up to half an hour, or more) with little to no ill effect.
Mirran are a two-gender species, and their reproduction is essentially the same as humans', although the gestation time for an infant Mirran is a bit longer —11 months. In addition, they mature more slowly.
Because of the metallic content of their skeletons, Mirran children born on other worlds need special supplements so their bones will develop properly. The fact that some of these metals (which are naturally abundant on their homeworld and inundate all native foods), like iridium, are rare and expensive, even in trace amounts, has caused a group of enterprising (and charitably spirited) Mirran to form a small trading company —Metallo-Vite— which extracts these necessary metals from Meruto, and supplies them in supplement form fairly cheaply to prospective Mirran parents.
Mirran live quite a bit longer than humans on average, up to 160 years, on average. In addition, for reasons not understood, they age rather differently than most other races, displaying almost no physical aging from the time they reach full adulthood (at around 30), until they reach well over 100, whereupon they begin to degrade in a similar manner to humans, if slightly faster.
Isolated to their one world, before a Yavakaro ship looking to mine the planet's mineral resources discovered them and introduced them to the galaxy beyond, their society was rather stagnant. Their entire recorded history prior to contact could pretty much have been copied straight out of a history of the Middle Ages on Earth, the 'governments' a mix of feudal and tribal systems who were often at war with their neighbors. Since the contact, there have been some revolutions, and some republics/democracies have sprung up, while some of the feudal overlords retain their domains. Wars and oppression are still quite common, which is the main reason why those who can tend to buy their way offworld as soon as possible.
There really now exists two Mirran cultures: that of the warlike mess of various governments on their homeworld; and that of the Mirran who've made it offworld into the galaxy, who simply want a better life for themselves and their kind. The latter tend to do their best to blend in with whatever society they find themselves in. Many find work as miners —which they are much desired for, being suited to caves and hostile environments— and others take up the trade of mercenary, putting their kinds' warlike past (and their innate biological skills) to profit; however, there is generally little work a Mirran won't take to get off of their homeworld.
This willingness to do most any job, and their wide distribution amongst the various human and other factions, has garnered Mirran a fair amount of suspicion and distaste —as some wonder where there loyalties really lie.
The Mirran language is rather sophisticated, despite their cultural stagnation. Furthermore, a non-Mirran could never truly speak it (at least not without some form of very fancy prosthesis), as some of it is visual, involving subtle shifts in skin tone and patterning which are practically unnoticeable to the human eye.
Mirran tend to be practical, and aren't very religious. The main deities —if they can really be called that— are a pair: one of light and one of darkness —not surprising given the nature of their world.
The Mirran never developed space travel (the surface of your world being borderline uninhabitable much of the time tends to discourage looking at the stars —not to mention the inability to make almost anything lasting on the surface).
Lacking much else for resources, the Mirran are highly adept at working with stone and metal —particularly superalloys resistant to the extremes of temperature.
Prior to contact, swords and knives were the primary weapons on Meruto. Due to the resilience of Mirran bones, these weapon designs focused on thrusting, so as to slip between the ribs, and tend to resemble either Roman gladii, or later rapiers, from Earth's past. Spears were also occasionally used; before contact, throwing spears were the only ranged weapon the Mirran knew. These weapons are typically solid metal, and a little shorter/slimmer than Terran equivalents to make up for the weight of the dense refractory metals (mostly tungsten and molybdenum) that they are made out of —the only things that can survive the temperatures on Meruto much of the time.
Armor, conversely, never really developed on Meruto; given the metals it would have to be made of, it would simply be too heavy to be practical.
Since contact, the introduction of HPG guns has caused some serious disturbance in Mirran warfare (and the balances of power amongst various states/tribes). However, these still see limited use on Meruto, as they are useless in the melting heat and freezing cold of the Bright and Dark seasons. Those Mirran who have left the homeworld, however, have taken quite well to more modern weapons, though many still retain a blade out of habit.
A weapon truly unique to the Mirran is the light-whip —though humans also refer to them as razor-whips: essentially a long chain —each link being a double-edged blade section— with a spear-tip-like point at the end. A core of flexible, light-reactive material runs down the center of the weapon, connecting each link; this material flexes in response to absorbing certain frequencies of light, allowing the wielder to, by pumping some of their light into the weapon through its grip, control the flexible weapon in ways that would be impossible by simple wrist or arm motion. An experienced light-whip wielder can selectively stiffen sections of the weapon to block attacks, or make it wrap around an opponent's weapon to wrench it from their hands, but It takes a lot of training, and a phenomenal amount of focus. Hence, the weapon is practically useless in the hands of a non-Mirran (or even an unskilled Mirran) —being little more than a clumsy, heavy whip with which they are at least as likely to injure themselves as the enemy.
Light-whips are rare, and difficult to make, and possession of one is considered a mark of great wealth, status, and skill amongst the Mirran. As a result, some Mirran who have left the homeworld have produced improved versions of the weapon: making them out of duranium, and incorporating blazer technology into the design as well.