Gillies were a faction of humans that had been bio-engineered to live underwater. Due to the different pressure properties of liquids and gases, this enabled them to crew short-range, high-g intra-system ships.
Most gillies were just normal people or labourers that worked in submarine conditions, with barely more than gills on their chests distinguishing them from baseline humanity. Some gillies were mentioned as being so adapted to a submarine environment they could not breathe or function in air any longer and had to be transported in huge robotic tanks.
- Revelation Space (2000) - mentioned in passing
- Galactic North short story anthology (2006)
- A Spy in Europa - explored in greater detail, including their origins
- Denizens - A bio-engineered species specialised for life underwater, derived from a human basis.