Jovian (The Jupiter Plague)

Jovians are a hive-minded race of aliens that inhabit the icy surface of the planet Jupiter. While they originally had no awareness of the universe beyond Jupiter, they are highly intelligent and technologically adept, and upon discovering Earth and humanity, the Jovians experimented on them with devastating results.

Anatomy & Physiology
Jovians are vaguely humanoid in general size and shape, but they are much thicker and stockier due to Jupiter's high gravity. Their skin is thick and elephant-like in appearance and texture. Their heads are "saurian" in shape and are covered in heavily wrinkled flesh. Jovians can extrude tentacles and eyestalks from their bodies, but these may be biotechnological devices rather than actual body parts. These tentacles are much harder and stronger than steel, but are damaged and corroded by oxygen.

Biochemistry
Jovian biochemistry is never explained in detail, but it is known to require ammonia, since Jovian tissue exposed to terrestrial atmosphere smells strongly of ammonia. Ammonia probably functions as a biochemical solvent. Jovians live in a hydrogen-helium atmosphere, presumably breathe hydrogen, and are severely burned by free oxygen.

Hive mind
The entire Jovian species has a single collective mind. Individual Jovians are like cells to the hive mind, which learns everything that any Jovian experiences and treats each Jovian as an expendable appendage. The hive mind is extremely intelligent and fast-learning, enough to develop an expert-level understanding of human biology, technology, and language after only a few weeks of interaction.

Environment
Jovians inhabit the surface of Jupiter, an environment dramatically different than that of the real-life planet.

In The Jupiter Plague, at least some of Jupiter's interior consists of ice, most of which lies beneath an "ocean" of atmosphere liquefied by pressure. Some of this ice forms mountains that rise above the liquid layer and into the lower depths of the gaseous layer, about 200 miles (320 km) beneath the clouds. The Jovians inhabit at least one of these islands, a formation called "the Reef" by the humans who discovered it - they may or may not have access to other such formations.

Conditions on the Reef are extremely hostile by Earth standards. The atmospheric pressure is perhaps a hundred times greater than on Earth, and the hydrogen-helium air is bitterly cold, just above -150 degrees Celsius. The thick, cloudy atmosphere renders the Reef all but lightless. Other than the pressurized hydrogen ocean around the Reef, the only liquid is ammonia/methane rain.

While they are otherwise highly durable, Jovians are completely dependent on this cold, high-pressure, anaerobic environment. Contact with free oxygen burns and corrodes their tissue, and fully immersing a Jovian in Earth's atmosphere instantly and violently kills it.

Society & Behavior
Jovians exist in a hive mind that encompasses the entire species. The hive mind places no value on the lives of individual Jovians, since each Jovian is only an expendable part of the whole. Prior to discovering humanity, Jovians had never encountered or imagined any other form of intelligence, and when they encountered humans they treated them as hive-mind components like themselves. Thus, while the Jovians are curious about humans and seem to bear them no real ill will, they are extremely dangerous, since they see no issue with killing people in order to understand humanity.

Because of their disregard for individual lives, Jovian contact with humans was initially violent and destructive. During first contact, the Jovians slaughtered the crew of the spaceship that landed on the Reef, then sent it back to Earth carrying a deadly plague - it is unclear why they did this, but it is thought to have been an experiment of some kind. While this "experiment" killed thousands of people, the Jovians apparently had no real hostile intent, since they readily handed over a cure and could easily have wiped out mankind with a much deadlier disease if they wished.

Science & Technology
Since Jovians have no access to metal or fire, their technology is exclusively biological in nature. They can use biotechnology to do nearly anything that human technology, constructing advanced and versatile biological tools. For instance, they were able to anchor the human spaceship Pericles to the Reef using a powerful magnetic field, apparently generated by their biotech. To interact with humans, they use highly versatile tentacles, which bear eyes and barrel-like containers - however, these tentacles may actually be part of the Jovians themselves. Jovian biotech is mostly based on the same cold and anaerobic biology that the Jovians themselves use and is thus vulnerable to oxygen, but the Jovians developed a clear "skin" to shield themselves and their technology from oxygen.

Due to the Jovian hive mind's superhuman intelligence and learning capacity, Jovians can very rapidly adapt to new experiences and develop new technologies to deal with them. They quickly figured out how to manipulate Earth biology as easily as their own, a skill that they used to create both a lethal virus and a molecule that could cure not only that disease but (with modification) any other. They also quickly adapted to human spacecraft, hijacking the Pericles and modifying it to contain a pressurized chamber for a Jovian to live in.

Despite their great intelligence and highly advanced technology, the Jovians had no idea that a universe existed beyond Jupiter before humans told them. This was because the thick, dense clouds above the Reef obscured the stars, the planets, and even the Sun.

Trivia

 * The Jovians first appeared in Harry Harrison's Plague from Space, published in 1965. Plague from Space was republished and expanded in 1982 as The Jupiter Plague, the title under which it is now available.
 * Nothing like the Reef actually exists on Jupiter. It has no solid surface, nor any other solid components except its core.  Additionally, Jupiter's atmosphere grows steadily hotter with depth - at the Reef's stated depth, Jupiter is hotter, not colder, than Earth.