The Ockorans were an aquatic species.
Ockorans looked partly humanoid, with a humanoid upper half and a fish-like tail. They had bright pink skin, black eyes, thin mouths, no noses and white wavy crests instead of hair. Their upper body was lightly built, with thin arms with three webbed fingers. Despite their weak appearance, Ockorans were dangerous fighters and much stronger than they looked. Their lower half was like a fish, with a scaly tail and fins. Ockorans had three gills down the side of their body and were unable to survive out of the water, suffocating in minutes. Male Ockorans could be identified by their lighter coloured crests, as well as differently shaped gills. Ockorans had a long lifespan, over hundreds of years. Ockorans had violet-coloured blood. Oddly enough, the Ockorans had a slight genetic resemblance to the Dolphins of Earth, as well as to the Kalarians of Kalaya.
The Ockorans were known for their advanced technology and the Sixth Doctor believed that their weaponry was "phenomenal". The Ockorans were one of the few aquatic races who evolved to have Level Two technology. After their wars with the Kalarians, the Ockorans started developing technology and taking it from other species. Ockorans commonly used organic technology. They had thinker-weapons comparable to the organic ships of the Zygons and the Faction Paradox's shrine. Ockoran ships were black, tube-like structures with three anti-gravity discs on the underside. Ockoran ship were flooded and the engines would only work underwater. The most recognisable technology used by the Ockorans was the Selachian battlesuits. These battlesuits were large, bulky things that gave the Ockorans the appearance of humanoids. Ockorans had to have studs implanted into their nervous system and have their tails removed in order for them to use the battlesuits. They had plasma weapons attached to their arms, which were built for fear rather than power (though it was still a dangerous weapon). Their helmets were shaped and painted to look like sharks in order to intimidate their enemies. Later battlesuits were equipped with nanites to repair damage and to counteract the nanites of the Terran Security Forces. Ockoran transports looked like armadillos with tank-treads. They were organically based and responded to telepathic commands from Ockorans. They could spit nerve gas and, when the Terran Security Forces counteracted that, acid.
Ockorans also had a wide variety of technology that was used during the war with the Terran Security Forces. They used special spores underwater that would burn when exposed to air. Ockoran also used the Cloak, which could blind and capture enemies. They left organic weapons on Kalaya that would activate once they left and would attack the nervous systems of any surviving humans. Ockorans made a tough rope out of kelp. They also used thermosystron bombs.
Ockorans lived in cities carved out of the rocks at the bottom of the sea. There were openings all around the buildings for the Ockorans to swim into. Ockoran's language was made up of singing and though it was hard to translate, it was considered to be very melodic. The Ockorans believed in a being called the Great Mother, a being who was the mother to the Ockorans and who guided them through the world. They also believed that the land (called the Second World) was where the wicked souls went and the inhabitant of the land were jealous of the Ockorans. Legends state that the Great Mother was killed trying to protect the Ockorans from the Kalarians, which led to an intense hatred of all air-breathers.
How much of this myth was based on Ockoran history is unknown.
At some point before the 22nd century the Ockoran's home-planet of Ockora was discovered by the Kalarians and chosen as a vacation spot. Though the Kalarians knew of the Ockorans, they believed they weren't intelligent and hunted them for sport. Likewise, Arix set up cruise ships for people who wanted to hunt them. The Eleventh Doctor was able to stop her and bring her to Galactic Security. Eventually the loss of the Ockorans were too great and they developed their battlesuits to fight back. They emerged from the oceans during a culling festival and took their revenge on the Kalarians. Their homeworld was cordoned off and the Ockorans were eventually forgotten. (PDA: The Murder Game) In order to take revenge on the air-breathers, the Ockorans set out into space following the Kalarians. They conquered Kalaya within 6 months and took 3 more planets within a year. They soon spread out, conquering more planet, trading with other species and forming the Selachian Empire. They soon became a major power on the galactic markets.
By 2069, the Selachians had encountered humans. They were featured in the Eye-Spy Book of Alien Monsters, which was handed out to UNISYC personnel as potential threats.
The Selachians conquered Rho Priapus, but they were driven off by the Space Marines.
In 2146, the Selachians attacked Hotel Galaxian.
In 2191, the Cybermen had stolen a Selachian ship.
For decades, the Selachians and the humans had traded with each other. Eventually, towards the end of the 22nd century, one deal went particularly bad, causing the humans to declare war on the Selachians. The war went badly for the Selachians and by 2204 they had been pushed back to Ockora. Though they had taken prisoners on planets they were forced to leave, such as Molinar and Kalaya, this wasn't enough to save them and Ockora was destroyed by General Wayne Redfern when he dropped the G-bomb.
After the meddling of the Second Doctor, the Supreme Leader and 40 Selachian soldiers were able to escape the planet's destruction and get on board the Triumph. They killed everyone on the ship, but the other G-bomb was accidentally set off, leading to the formation of another black hole.