Sio is a monotypic genus of fish in the family Melamphaidae, the ridgeheads. The only species is Sio nordenskjoldii, or Nordenskjold's bigscale. It has a circumglobal distribution on the southern hemisphere at depths from 200mto3000mm (700feetto10,000feetm).
The genus names refers to the Scripps Institute of Oceanography, abbreviated as SIO.
This species grows to a length of 12.3cm (04.8inches) SL.
There are no known threats to this species. It is not caught by commercial fisheries.