Biology:
Yij'u possess six arthropodic legs 3 feet in length for locomotion both in and out of water, with four additional limbs suited for more complex and finer movements such as self-grooming, seizing prey and reproduction - openings to the genital tracts are found on two of these limbs. These rigid, chitinous tentacles are approximately 2.5 feet long and can be folded into the body for protection against predators, with a series of hook-like grips spread across them. Two pincer-like appendages, barely a foot in length, also protrude for the purpose of feeding and foreplay.
Yij'u are coated in a metallic, chitinous exoskeleton that serves as a natural armour against predators and an evolutionary response to Juul's weak magnetic field and its subsequent exposure to radiation from Xuul and Xaiju. This exoskeleton is not dissimilar to that worn by Earth-based arthropods and crustaceans, with Yij'u periodically moulting segments to allow for growth. Infant Yij'u are born with a partially formed exoskeleton, though it is not fully hardened for the first months of life and does not offer the same protection against solar hazards. If a limb is amputated, a Yij'u can regrow it though moulting, though the process will take several years depending on the subject's environment, age and personal health.
Though far from humanoid, Yij'u possess a section of sensory organs analogous to a head. Their 'faces' are resemblant of broken, crystalline structures, unique to each Yij'u, where many bioluminescent eye 'panes' allow them to send and receive lightwaves from one another and perceive ultraviolet light. They are also capable of producing high-frequency noises and clicks for communication and echolocation, much like whale song. They feed via a series of mandibles lined with teeth designed for breaking through soft tissues and weak shells.
Overall, fully matured Yij'u have an average core body length of 3.5 feet, which extends to 6 feet when their appendages are outstretched, such as when they are swimming in aquatic environments. Using their arthropodic limbs, they can propel themselves through deep waters (with the aid of an internal gas pocket for ballast and pressure regulation) like cephalopods and move along the sea floor and surface. This allows them to exist at a water pressure that would crush many 'soft' humanoid species and has given them a reputation as a robust, hardy lot.
Though having evolved for aquatic environments, generations spread across their homeworld followed by subsequent technological and medication advances have allowed the species to adapt to living on the surface. Yij'u cell clusters are able to adapt based on the environment in which they are raised; an individual raised at surface levels will develop gills better suited to drawing oxygen from the air and stronger limbs, while ocean dwellers will be better suited to higher water pressures and diving. This has, to an extent, led to a marked divergence in the species based on these ethnic grounds.
Yij'u are viviparous, becoming fertile during a seasonal period timed to when their homeworld's star Xaiju is at its least active phase. Following a gestation period of 7 months, traditional Yij'u would give birth in rocky pools, shallow coves and undersea caves close to the surface to avoid both infant predation and exposure to solar flares from Xaiju. Although infants will frequently shed and regrow segments of their exoskeleton over the course of several months, they are not fertile until 14-19 years old and will not stop slowly growing until they are 26 years old, at which point they are considered biologically and socially mature.
Since achieving technological singularity, Yij'u lifespan has appeared to reach a peak of 175 - 215 years, with few exceptions. Outside the confines of their homeworld, colonies and vessels, many surface-dwelling Yij'u operate in arthropodic exosuits that facilitate mobility even in hostile environments.
Culture: Early recorded language came in the form of braille-like etchings on stone, bones and shells, while later cultures were able to use bioluminescent secretions from other creatures to convey writing and artwork. Yij'u are also known to engage in scarification of their exoskeletons, often imprinting them with bioluminescent ink as a form of body paint. The phrase "Yij'u" itself is a loose translation of the phrase "Cloud Hunters" by which early Yij'u characterised themselves, for wherever they surfaced the clouds remained above them.
Across generations, Yij'u have fragmented into different societies and cultures as their species colonised the surface of their world and those beyond it. On Juul, one of the key divides is that of water-dwelling cultures which have formed deep beneath the ocean, often close to the seabed, whereas land-dwelling cultures were those that lived along shallow coastlines, rivers, lakes, marshland and, eventually, further inland. It was the surface-bound cultures, forced to adapt to hostile surface conditions, that achieved many of the technological innovations and discoveries that would propel the Yij'u to an industrial era.
Due to constant cloud formations and the changing tides, few Yij'u ever saw beyond the skies of Juul. It was not until the land-dwelling cultures had gained a foothold that they had identified other objects in the sky apart from Xaiju, sparking the study of astronomy.
Though pre-industrial waterborne cultures had been at a technological and scientific disadvantage compared to their landfaring kinsmen, they had adequately compensated for it with cultural enrichment; art, philosophy and poetry, to the extent that technological enlightenment was equated with brutalism by some philosophers.
A recurrent theme in Yij'u belief systems is the notion that they are an element of the natural order and will return to it upon death. Water-dwelling cultures would often lay their dead to rest at the seafloor, where wildlife would consume soft tissues and scatter their remains as a means of returning to the world. Evidence of these 'sea burials' is present across Juul's oceans in the form of millions of shells littered across the ocean floor, dating as far back as 27,000BCE. This practice continued even as land-dwelling cultures formed apart from the ocean, though the lack of carrion-eating wildlife in these regions led to the practice of ritual cannibalism by relatives and associates of the deceased.
As the Yij'u have moved into space, these funerary rites have often been continued via other means; some choose to have their remains deposited into the atmosphere of the gas giant their homeworld orbits, others on their colonial worlds and their deep, briny seas.
While other cultures may look upon this as a rather morbid practice, there is very little taboo towards the presence of the dead. In fact, it ties into the deep sense of continuity that the Yij'u have with their species' history and identity - most Yij'u are able to trace their lineage back thousands of years.
Yij'u are traditionally monogamous and will pair for life until a partner dies or is otherwise separated for an extended period of time - but even so, many will refuse to do so. Cultural and legal penalties for wilful abandonment of one's partner and offspring are often severe - as are the penalties for infidelity. This stems from the prehistoric period where this was a simple necessity to survive; one parent would guard and tend to their offspring while the other would be expected to procure resources. Concepts of immediate family relations; parents, siblings, forebears and so forth are consistent across Yij'u culture.
Yij'u architecture is not known for achieving great heights; their preference is for the subterranean with great cities carved out of caves and aquifers. This was largely a practical measure; the volatile climate of their homeworld made tall structures at greater risk of collapse and even after overcoming the engineering challenges involved, a preference for traditional architecture remains. Immediate surveys of the surface would not indicate Juul as the capital of an interstellar civilisation so much as one at the pre-interplanetary stage.
Homeworld:
A rocky world of approximately 1.57 Earth masses and a surface gravity of 1.42Gs, Juul is the third moon of Xuul, a gas giant of approximately 0.95 Jovian Masses and 1.17 radii its size. Juul is tidally locked to Xuul and thus remains subject to its powerful magnetosphere and exposed to radiation on a daily basis, which has resulted in most of its native species evolving with an exoskeleton or other countermeasure against radiation exposure.
Juul has a nitrogen, argon and oxygen based atmosphere with a surface pressure of 1.4 ATM - marginally denser than Earth's, with surface temperatures ranging from −60°C to 15°C. 89% of its surface is covered in water, with the remaining land surface characterised by frigid crags, rocky marshland and tundra. Tidal forces from Xuul generate powerful weather extremes; windstorms and tidal surges that scour the surface on a daily basis. Photosynthetic organisms have largely evolved to be aquatic, or otherwise capable of withstanding extremes of both climate and star - leading to the spread of oxygen producing algae and tree analogs with flexible roots adapted to surviving windstorms and solar flares that breach Xuul's magnetosphere.
92% of the moon's native biota are aquatic, with a thriving marine biosphere. Those species that are landfaring are typically short and stout, with comparably few airborne species.