They were notable for their many limbs and their two curved canines. They constantly needed to either consume or come in contact with water and this included moisture in blood. If ever they used perception filters, their teeth would remain and they would smell vaguely of fish.
They also resemble deep dwelling sea life in the respect that they have a great sensitivity to light and while they are able to travel in daylight, exposure to direct sunlight will cause them to explode.
The species used perception filter technology, which they used to blend in with the human population of Venice. However, this camouflage was not perfect and gave rise to vampire-like qualities among those in the school. Since the technology couldn't hide reflections, and the human brain couldn't process the two conflicting pieces of information, anyone who saw a member of the species near a reflective surface saw a human without a reflection. Their fangs would also be seen when they fed due to the human subconscious perceiving danger and attempting to alert the conscious mind.
Some time in the species' history their home-world was besieged by the cracks in the universe, ranging from small cracks to cracks that consume 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. The species itself was not lost. Deciding to take their chances with the cracks that showed other worlds, the Saturnynians fled to worlds with oceans like their own planet. A colony arrived in Venice though it is unknown if any other members of the race made it to other worlds.
This colony of Saturnynian's were led by a female known as Rosanna Calvierri along with her son both of whom made a secret base in Venice on the planet Earth after falling through the crack in time that led them to live in an earlier period in the world's history. These two were not the only Saturnynians as there were at least 10,000 further males that lived beneath the waters but they were largely devoid of any female breeding stock. To rectify this solution and save their species, the began a process where they took in young girls in their human guises under the pretense of providing them a better education. However, once amongst them, Rosana and her son would bite their necks where they would first replenish their own fluids as well as possibly convert the human into a Saturnynian hybrid with the process eventually turning them into a full breed member of the species.
Whilst this was happening and they were developing a breeding stock of human converted Saturnynians, Rosana also intended to use an advanced alien device to create a tidal wave that would sink the city of Venice beneath the water this providing her children a habitat for them to share the Earth as well as continue their civilization. However, their actions unwittingly brought the attention of the Eleventh Doctor and his Companion Amy Pond. Over the course of events, the female breeding stock was killed in an explosion and the device was destroyed as well as her human disguised son was killed by exposure to concentrated sunlight. This led to Rosana willingly committing suicide by keeping her human disguise on as she jumped into the water with her children unable to tell that it was their mother who they had feasted on. Her last words to The Doctor that all that was required was the loss of one city for the survival of her race. Furthermore, she stated that she hoped that he would remember them as she fell into the water to her death thus leading to the demise of the Saturnynian colony on Earth and possibly leading to their extinction.(From the Vampires Of Venice 2010). via