Saturnyn

The Saturnynians were an aquatic species from the planet Saturnyne. The females of the species, if not the entire race, were also known as Sisters of the Water.

Biology
The Saturnynians were a mix of humanoid, fish and crustacean traits. They had a humanoid upper body, a fishlike head with sharp teeth and a lobster-like lower body. (DW: The Vampires of Venice) On their abdomen, they had 4 legs and 2 smaller clawed limbs. On their thorax, they had 2 arms and 4 smaller clawed limbs. These legs were tipped with sharp points, allowing them to tear through their prey. They were covered in scales, which would flatten out to minimize drag in water and would be moulted regularly. (REF: Doctor Who: The Visual Dictionary (2010))

Though they could survive on land, they needed a large amount of water to survive, which they could drain from other creatures. They also had a strong sensitivity to ultraviolet light, being repelled by smaller amounts and being incinerated by large amounts. (DW: The Vampires of Venice)

Technology
When on Earth, the Saturnynians used a variation of perception filter technology to disguise themselves. Though it gave them the appearance of humans, it accidentally gave them vampire-like qualities. Since the technology couldn't hide reflections, and the human brain couldn't process the two conflicting pieces of information, they had no reflections in mirrors and other surfaces. Their fangs would also be seen when they fed due to the human subconscious perceiving danger and attempting to alert the conscious mind.

The Saturnynians were also capable of converting humans into Saturnynians by draining them of their blood and replacing it with Saturnynian blood. They also were capable of controlling the weather, causing storms and earthquakes. (DW: The Vampires of Venice)

History
Some time in the species' history their home-world was besieged by the cracks in the universe, ranging from small cracks to cracks that consumed the planet's sky. At first the cracks appeared to pose no real danger and the aliens were able to observe other worlds through some, and "silence" and the "end of all things" in others. Eventually, however, the true nature of the cracks became apparent and Saturnyne was lost. Deciding to take their chances with the cracks that showed other worlds, the Saturnynians fled to worlds with oceans like their own planet.

One colony made it to Venice in 1580, led by a female known as Rosanna Calvierri along with her children. Only the male children seemed to survive, though there were 10,000 of them. To rectify this problem and save their species, Rosanna lied about an outbreak of the plague, quarantining the city. She then began a process where they took in young girls under the pretense of providing them a better education. However, once amongst them, Rosanna and her son would drain them of blood and replace it with Saturnynian blood, converting them into Saturnynians. Rosanna also intended to to create a tidal wave that would sink the city of Venice, providing them a habitat to live in.

However, their actions unwittingly brought the attention of the Eleventh Doctor, Amy Pond and Rory Williams after they took in Isabella. They and Isabella's father, Guido, investigated the school, enrolling Amy to help them get him. Though she was able to open the trap-door, she was later captured. With the help of Isabella, they were able to escape, though Isabella was dragged back and later killed for insubordination. Rosanna sent the converted females and her son Francesco to stop them, while starting the process to sink Venice. The females were all killed when Guido blew up the house when they attacked, while Francesco was killed when Amy directed sunlight on him. The Doctor was also able to disable the device. With her species doomed and her plans stopped, Rosanna killed herself by jumping into the water with her perception filter still on, with her children not knowing who she was and feeding on her. (DW: The Vampires of Venice)


 * The Doctor later closed the cracks, negating their influence DW: The Big Bang; Presumably, this would mean that the Saturnynians were never lost to the silence, and Rosanna's colony didn't land in sixteenth century Venice.