The Kyan species are most comparable to whales of Earth origin, as both species use vertically-moving tails for mobility and eat local Europan micro organisms for sustenance. These similarities, despite the two species having no interaction between one another, are therefore best explained under Extraterrestrial theory.
Kyan bone is different in composition than most terrestrial organisms. While most Earth creatures have a primarily calcium based bones, species on Europa lack large amounts of this material, and as such tend to have thinner and weaker bones than Earth counterparts. Often, species will substitute bones for strips of cartilage, saving materials while still achieving the desired effect. This currently is the best explanation to why the Kya lack a jawbone.
A typical Kyan skull is primarily cranial cavity, represented by a bulge of the brow located towards the connection of the spine and nervous system. the skull tapers to a point at the front, presumably for improved hydrodynamics and reducing the profile. While the creatures lack a jawbone, a series of cartilage strips connected by muscles allow the Kya to open and close their mouth. Their spines are segmented and bind a rib cage protecting soft organs, much like most vertebrae on Earth. The joints of the forward flippers are largely opposable and flexible, while their rear flippers are less so and primarily used only for mobility. The flippers themselves are primarily very flat bone, measuring between 3 millimeters and 2 centimeters in thickness, thinnest at the rear edge of the bone and thickest at the connection to the Patria, a small bone connecting the flipper to it's 'shoulder'. The two tails on Kya spit roughly two-thirds of their size measured from the tip of the skull with bones that are most humanly similar to the hipbones. Each tail has a total of 5 bones; the two longest connecting the base of the fluke to the spine, and the remaining three forming the fluke itself.
The Kya possess numerous organs, many similar in function to species on Earth, such as gills for breathing and digestive systems for processing food. Their diet is primarily small algae and crustaceans, which they capture using a baleen system, and they excrete through a hole in their mid-lower abdomen. Their gills are located above and behind their cranium, leading to a central heart. Kyan blood is Hemocyaninous (copper-based), accustomed with the low temperatures of Europa's originally colder climate. Kya possess two eyes which are extremely sensitive to visible light, suited to the minimal light of deep oceans. They primarily rely on echolocation and other vibrations for navigation and communication much like terrestrial whales. Reproductive organs of the creatures are also similar to whales and other terrestrial sea mammals.
The Kya, as well as other native species, all evolved from single, unicellular organisms that grew off of the materials from hydrothermal vents (Chemosynthesis), and was the foundation for the ecosystem for millions of years. Despite the early arrival of life early in the moons history, the evolution of many species has been statistically hindered. Because of the increased distance from the sun, and the thick shell of ice that covered the surface of the moon, the amount of radiation reaching the organisms was retarded. This caused the moon biosphere to remain primarily primordial, with single-celled organisms of small multi-celled organisms. As Humans became a starfaring civilization, they noticed that this trait was not exclusive to Europa, and found other worlds that were perfect for life, yet, due to a lack of mutation, were highly primitive.
Another hindrance to the development of life was simply a lack of resources. The moon, while high in oxygen and water, is lacking in base metals and carbon, further reducing the scale and prosperity of life on Europa. This would also seem to explain the reliance on cartalige with most vertebrae species rather than bone, as even small vertebrae rely on other methods of creating skeletons. The Kya are no exception as mentioned above.
More recently, as the moon has been terraformed into a temperate O-Class world, many photosynthetic organisms, most originating from Earth, now live on the surface. Despite this, the Kya very rarely visit the surface, as the warmer temperatures and higher amounts of light can be harmful if prolonged. The removal of the surface increased ambient radiation, however, which could improve and diversify the moons ecosystem, although results have yet to be observed.
After Contact with Humanity
The first missions to Europa started in 1973 with Pioneer 10, the first spacecraft to encounter and fly by Jupiter's moons. The first probe to land on the surface and to break the ice, NASA's Clipper mission, arrived on the surface of Europa in 2026. Instead of drilling through the ice, the Clipper used a large radioisotope thermoelectric generator to melt its way through the ice to reach the ocean surface, Clipper did not encounter any life during it's two year ocean survey, however. Life wasn't found until a series of deep sea explorations were done some 10 years later, in 2039.
Upon first contact, the Kya being the first sentient extraterrestrial ever found by humans, Both races showed an intense curiosity about each other. Communications between the two species was crude at first. This lack of understanding urged humans to create a new language, one that both humans and Kya could understand. This would later be known as the first iteration of Xeno, a product of humanity's attempts to create a universal language between themselves and all other known sentient beings.
On their discovery, they were at an equivalent of a hunter-gatherer stage and appeared to have been that way for many thousands of years. With their contact with humans, they showed a slight interest in what was above their oceans and even allowed for some of them to move and colonize Callisto.
While the Kya are nearly as intelligent as humans, they lack a organized society for 2 major reasons:
The first reason, aforementioned, is of the lack of organic resources. Without such, they lack food and, in turn, population surplus. This makes the creation of a singular society difficult, if not impossible. The second reason is that, because of the naturally slow mutation rate on Europa, the Kya are lacking in the ability to manipulate foreign objects, such as hands allow humans to grip materials. The Kya do have one "finger" on the front of their foremost limbs, which can be used for manipulating, but such manipulation is very limited. This makes creating such necessities as tools and structures extremely difficult. Humanity has offered help often, supplying tools for the species, but only so much progress has been made. As of now, the Kya have multiple small, basic governments in place, but are primarily still hunter-gatherers.
Currently they have little interest in leaving the Sol System. A few very curious Kya have been taken to see the oceans of other worlds in the system and fewer still have visited other extrasolar ocean worlds. The entire population of Kya in the Sol System is around 5 Billion.