The species itself originates on a planet covered in a shroud of darkness. Because of this, sunlight is lethal, and causes them to burn up.
Avis is the god of the Krill people.
In September 2419, a Krill destroyer attempted to seize quantum field technology developed by the science station on Epsilon II. Their efforts were thwarted when the Planetary Union vessel USS Orville triggered the device aboard the destroyer, causing it to break apart and explode. One survivor, who had boarded a shuttle to the Orville, remained in custody while his ship was lost.
The captain of a decoy USS Bleriot concocted by the Calivon claimed that their vessel had been raided by a Krill destroyer as part of their ruse to lure the Orville. Krill were infact among the captive specimens kept by the Calivon in their zoo.
A Krill destroyer attacked the colony transport USS Druyan, forcing the Orville to leave its watch at the Dorahl Bioship to intercept them. Thanks to daring manoeuvres executed by John LaMarr, the Krill ship was destroyed with only minor casualties inflicted on the Druyan.
Soon after, the Orville received a distress call from the new colony on Kastra 4, which was being bombarded from orbit by a Krill battlecruiser called Kakov, the third Krill attack in a month. Initially attempting to engage the Kakov directly, the crew determined that the Orville was no match for the Krill warship. The Orville then skimmed the planet's atmosphere in order to use the contrail to obscure Krill sensors, before heading straight away from the planet and launching the ship's entire complement of torpedoes at the pursuing Kakov, whose sensors were still being obscured and deflectors weakened by the atmosphere. The Kakov was obliterated by the barrage, although one of the ship's shuttles survived intact. (TO episode: "Krill")
Admiral Ozawa ordered Captain Mercer and Lieutenant Malloy to use the Krill shuttle to infiltrate another Krill vessel and obtain a copy of the Krill holy text Anhkana. Thanks to Kaylonian holographic technology, Mercer and Malloy managed to rendezvous with a Krill destroyer called Yakar, while disguised as Krill crewmembers "Chris" and "Devin". Their mission plans changed upon discovery that the Yakar was heading to the farming colony on Rana 3 in order to test their prototype neutron field generator on it. Mercer and Malloy realized that the Krill were sensitive to ultraviolet radiation and managed to overload the ship's lights to produce level lethal to the Krill, wiping out the entire crew, save for a teacher named Teleya and her students. Mercer and Malloy then piloted the destroyer to rendezvous with the Orville in order to hand it over to the fleet for study. (TO episode: "Krill")