Nostrovite

The Nostrovites were a race of shapeshifting humanoids.

Biology
The Nostrovites, in their natural form, appeared to vaguely resemble a human, but with red eyes, sharp claws and teeth, and visible blood vessels. Nostrovites were carnivores, but only ate living flesh and would normally ignore corpses. The females, at least, could create a sticky, web-like structure to keep prey alive until they were hungry again. The blood of the Nostrovites was black.

With the use of their proteus glands, they could adopt other humanoid forms. Though they could mimic appearances, they could not mimic smells. (TV: Something Borrowed)

Life cycle
The Nostrovites, like many other species, had two sexes — male and female. However, they also required a third member: a host. After fertilising the female's eggs, the male would take the eggs and store them in a sack in his mouth. After finding a potential host, the male would bite them and inject the eggs into the host's bloodstream. The eggs would quickly develop inside the host as if the host was merely pregnant with another of its species.

After a while, the female would track down the host and tear them open to release the young. This process was Nostrovite childbirth. The female became very resilient and it was shown that several rounds of bullets had little effect on a female Nostrovite. (TV: Something Borrowed)



History
The Nostrovites were familiar to Jack Harkness. At some point in their history, some Nostrovites discovered Earth and developed a taste for human flesh.

In 2008, a mated pair of Nostrovites came to Cardiff. Though the male was killed by Jack Harkness, it was able to inject Gwen Cooper with eggs. The day after the attack was Gwen's wedding, which she went through with even though the female was still out there. The female followed her to the wedding and took the form of Carrie. It killed Mervyn and captured Toshiko Sato and Banana Boat. Later when it was discovered, the female took the form of Brenda Williams and tried to hold Mary Cooper hostage, but was shot by Gwen.

The Nostrovite survived, and tried to get closer by taking the form of Jack Harkness, but Gwen was able to escape again. Eventually, the Nostrovite eggs were removed by Rhys Williams using the singularity scalpel and the female was destroyed by Jack. (TV: Something Borrowed)

Torchwood website
The series 2 version of the Torchwood website had a preliminary investigation into the Nostrovites by Owen Harper. He speculated that the species has many of the hallmarks of a composite race, and could have taken. He observed hooks in the specimen's DNA, suggesting the species required a genetic sample to shapeshift. He said that "Even when it’s in human form, the brains are located in its abdomen."