Selachian

The Selachians, formerly the Ockorans, were a race of aquatic warriors.

Biology
The Selachians resembled humanoid salmon from the waist up, with thin, straight mouths, narrow unblinking eyes, a wavy undulating crest, and gills. To fit into their battle-armour, Selachians had the lower half of their bodies amputated. (PDA: The Murder Game)

Culture
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.

Ockorans had a strong sense of pride and would refuse to show weakness or fear. They would rather die than give into their enemies. Conversely, their weaponry, armour and name was specifically chosen to cause fear in the enemy.

Ockoran hierarchy was shown by stripes painted onto their battlesuits, going from top-left to bottom-right. The Supreme Leader's suit had black stripe edged with red, while the next level down had blue stripes. (PDA: The Final Sanction)

Technology
The most recognisable technology used by the Selachians was the Selachian battlesuits. These battlesuits were large, bulky things that gave the Selachians 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. (PDA: The Final Sanction)

The Selachians were one of the few aquatic races who evolved to have Level Two technology. (PDA: The Murder Game)

They had thinker-weapons comparable to the organic ships of the Zygons and the Faction Paradox's shrine. (EDA: Alien Bodies)

Though the Selachians didn't naturally choose to use advanced technology, after the actions of the Kalarians they used the the technology of many other races. They also learned from their battles. Selachians commonly used organic technology. (PDA: The Final Sanction) They were known for their advanced technology and the Sixth Doctor believed that their weaponry was "phenomenal". (MA: Killing Ground)

Selachian ships were black, tube-like structures with three anti-gravity discs on the underside. Selachian ships were flooded and the engines would only work underwater. (MA: Killing Ground)

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. (PDA: The Final Sanction) They also used thermosystron bombs. (EDA: Alien Bodies)

History
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. (PDA: The Final Sanction) 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. (DWA: Peril on the Sea) The Ockorans eventually developed armour that allowed them to breathe on land, and 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 Kalarians, the Ockorans set out into space to follow them. They conquered Kalaya within six months and three 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. (PDA: The Final Sanction)

The Ockorans adopted the name "Selachians" after studying other worlds and learning that sharks were the most commonly feared marine creatures amongst humans. (PDA: The Murder Game)

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. (EDA: Alien Bodies)

The Selachians left Ockora around 2104 and conquered Kalaya in the first six months and three more neighbouring worlds within a year. (PDA: The Final Sanction)

In 2146, the Selachians attacked Hotel Galaxian. (PDA: The Murder Game)

In 2191, the Cybermen stole a Selachian ship. (MA: Killing Ground)

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. (PDA: The Final Sanction)

The Selachians once invaded the Earth colony Rho Priapus. The planet was later taken back by the Space Marines. (EDA: The Janus Conjunction)