Weed creature

A weed creature was an intelligent, semi-mobile form of seaweed that once inhabited the Atlantic Ocean near the coasts of England.

Biology
Weed creatures fed on gas and had stingers. A weed creature could also release poisonous gas or a foam that allowed it to mentally control those it touched, who in turn gained the ability to exhale gas. It was vulnerable to oxygen and high-pitched sound. (TV: Fury from the Deep)

History
Weed creatures were sighted by mariners as early as the 18th century, but the Seventh Doctor once described them as "primeval", existing since the days of the dinosaurs in Loch Ness. (PROSE: Christmas on a Rational Planet)

In the late 20th century, a weed creature was sucked up by an off-shore Euro Sea Gas rig, and, in the space of a few days, took over the crews of the off-shore rigs. It later took over Oak and Quill, two engineers at the refinery commanded by John Robson, and used them to commit various acts of sabotage around the refinery. It also took over Maggie Harris, Robson and Van Lutyens.

The weed's plans to conquer Earth were defeated when the Second Doctor used a recording of Victoria Waterfield's screams to destroy its nerve centre. (TV: Fury from the Deep)

John Smith mentioned "intelligent seaweed" to Joan Redfern in passing. (PROSE: Human Nature)