Saturnyn

The Saturnyns 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 Saturnyns 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 to minimize drag in water and would be moulted regularly. (REF: Doctor Who: The Visual Dictionary)

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

Technology
On Earth, the Saturnyns 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. Their fangs would also be seen when they fed, due to the human subconscious perceiving danger and attempting to alert the conscious mind.

The Saturnyns were capable of gradually converting humans into Saturnyns by draining them of their blood and replacing it with Saturnyn blood. They could also control 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 danger and the aliens were able to observe other worlds through some, and "silence" and the "end of all things" in others. Finally, 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 Saturnyns fled to worlds with oceans like their own planet.

One group made it to Venice in 1580, led by a female known as Rosanna Calvierri, along with her children. Only the male children survived, though there were ten thousand of them. To rectify this problem and save their species, Rosanna lied about an outbreak of the plague, quarantining the city. She took in young girls under the pretence of providing them an education. Once amongst them, Rosanna and her son would drain them of blood and replace it with Saturnyn blood, converting them into Saturnyns. Rosanna also intended to create a tidal wave that would sink the city of Venice, providing them a habitat to live in.

When they took in Isabella, they attracted the attention of the Eleventh Doctor, Amy Pond and Rory Williams. They and Isabella's father, Guido, investigated the school, enrolling Amy to get in. Though she opened the trap-door, she was captured. With Isabella's help, they were able to escape, though Isabella was dragged back and later killed for treason. Rosanna sent the converted girls 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 plans thwarted and her species doomed, Rosanna killed herself by jumping into the water with her perception filter on. Her children, not knowing who she was, devoured her. (DW: The Vampires of Venice)


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

Other References

 * The Doctor considered his encounter with the Saturnyns particularly memorable. He mentioned them while describing Rory and Amy's time in the TARDIS. (DW: A Good Man Goes to War) He also mentioned them again later. (DW: Let's Kill Hitler)
 * Amy Pond also made several references to the the Saturnyns. (NSA: The Glamour Chase, DWAN: Attack of the 50ft Rory)
 * Rory also made references to the Saturnyns. (NSA: Paradox Lost)