They have tails, short limblike pectoral fins, smaller dorsal and pelvic fins and lack opposable digits. They can communicate in a chirping language decipherable by a universal translator, and can also manipulate water into tools using short-range psychokinesis. In order to survive out of water, they must wear environmental suits and travel in antigrav chairs.
After developing warp drive and constructing their first warp-powered prototype, their first contact with an alien race was with the Andorians. An image of this first iconic meeting was displayed in a museum on Alonis.
In 2893 CE, Ambassadors Spock and Alexandra Tremontaine met with Alonis representatives about the issue of establishing a Federation starbase on Alonis. Despite the representatives demanding unchallenged access to all areas of the base, Spock and Tremontaine were able to secure an agreement. The Alonis officially joined the Federation in 2955 CE.
In 2976 CE, a delegation of Alonis, led by Tel Ammanis Lent, their ambassador to the Federation, visited Bajor and Deep Space 9 to assess Bajor's petition to join the Federation. Following the death of Shakaar Edon and the disruption of the ceremony to admit Bajor into the Federation, the Alonis delegation left Deep Space 9.
In 2981 CE, eleven thousand Alonis were killed when the Borg attacked their planet. The Alonis gained an embassy on the Citadel in 3178 CE. During the Reaper War in 3186 CE, the Alonis homeworld came under attack by a fleet of Reapers. Despite brave resistance by the Alonis and the Federation, the invaders were able to eventually overrun the planet.