Heptapod

Heptapods are a sapient, space-faring extraterrestrial species. After first contact with Humans, two Heptapods, nicknamed Abbott and Costello, attempted to communicate with Dr. Louise Bank and Ian Donnelly from their ship in Montana.

These aliens are enigmatic creatures, which have come to Earth for reasons left vague. Nevertheless, it's been determined that they are benevolent beings, whose ultimate goal is to unite humanity as they will need the Humans' help in the far future.

Biology
As the name suggests, a Heptapod has seven limbs, whose arrangement makes the lifeform heptaradially-symmetrical. On Earth, they never leave the confines of their spaceships, perhaps due to them being unable to breathe Earth's atmosphere, and instead leave it to the Humans to initiate first contact.

They have jointed, arthropod-like limbs, each ending in a point that can split open into a seven-pointed starfish-like "hand". When walking, they appear to put five limbs ahead and the remaining two behind as they move. They have a dull-grey, fleshy-looking skin with a rough texture, and they seem to have no visible eyes or mouths. However, several orifices are visible on Costello's upper body. Their physique seems to vary noticeably among individuals, as Costello is chubbier than Abbott.

Although this is initially not apparent to the Humans who participated in first contact with this species, the seven appendages only form a small lower portion of the Heptapod's body, with a large upper section that resembles the diamond-shaped body of a giant squid, and a head-like protrusion at the top. Seen in silhouette, the upper body of the Heptapod resembles a vaguely humanoid figure, eerily reminiscent of images of a hooded Grim Reaper.

Communication and language
Heptapods use low-pitched, thrumming vocal communication that is unpronounceable by Humans (classified as Heptapod A), and a written script (Heptapod B). The vocal and writing components are unrelated. Heptapod B does not represent sound.

Their written script consists of circular shapes created with an ink-like substance. The circular shapes are modular, being comprised of various sub-sections conveying different meanings or concepts. Heptapod B can induce a non-linear perception of time for those who intimately learn and understand the language.

Technology
Heptapods are more capable than Humans when it comes to spaceflight. When they make first contact with humanity, they use radio to transmit their vocal recordings.

Heptapod ships (referred to as "Shells") are 450-meter-tall vehicles composed of a black, stone-like material of unknown composition. The Shells are oval and somewhat flattened in shape, resembling an immense, obsidian concave lens. They completely lack any surface features; there are no visible thrusters or steering mechanisms, nor are any emissions whatsoever detectable from these Shells. There is, however, a small vertical corridor leading to a transparent glass wall, through which the Humans communicate with the Heptapods.

The interior of the ship is made of a white, glass-like material with a serrated texture. The inner atmosphere (presumably the atmospheric conditions found on the Heptapods' homeworld) is thick and misty, and is dense enough to have properties between a liquid and a gas. Through this medium the Heptapods can fly through the air, and project their inky logograms within the mist. Louise was able to survive for several minutes in the Heptapods' environment.

The ship is also able to send out a small, twelve-foot-tall cigar-shaped craft, which emerges from an unknown opening. Costello uses this craft to bring Louise into the ship.

When departing from a planetary surface, Shells do not actually move away from it. Instead, they gradually become transparent until they vanish completely.

Appearances

 * "Story of Your Life", by Ted Chiang (1998)
 * Arrival (2016)