The Saturnyns, Saturnynes or Saturnynians, were an aquatic species from the planet Saturnyne. The females of the species were also known as Sisters of the Water.
Though they could survive on land, they needed a lot of water, which they could drain from other creatures. They also had a strong sensitivity to ultraviolet light and were incinerated by large amounts.
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. (TV: The Vampires of Venice)
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 Saturnyns were able to observe other worlds through some and "silence" and the "end of all things" through 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, Rory Williams, and Isabella's father, Guido. Amy went undercover and enrolled into the school, but was captured after she opened a trapdoor to let the others in. With Isabella's help, they escaped, though Isabella was dragged back and later killed for treason. Rosanna sent the converted girls and her son Francesco to stop the Doctor and his friends while she started the process to sink Venice. The female Saturnyns were all killed when Guido blew up his house when they were attacking, and Francesco was killed by Amy.
The Doctor disabled Rosanna's weather device, thwarting her attempts to sink Venice. With her plans stopped and her converted women dead, Rosanna Calvierri came to the realisation that her species was doomed when her version of a perception filter would not deactivate. With only male Saturnyn in the canal, she was the last female - and her children would not recognise her upon returning to the water. Despite the Doctor's attempts to convince her not to do so, Rosanna jumped into the water and her children devoured her. Rosanna's last words had indicated that her children and indeed her entire race, would then die, as she asked if the Doctor could "deal with another dead race on his conscience". (TV: The Vampires of Venice)
The Doctor considered his encounter with the Saturnyns particularly memorable. He mentioned them while describing Rory and Amy's time in the TARDIS. (TV: A Good Man Goes to War) He also mentioned them again later. (TV: Let's Kill Hitler)