Two tidal energy and two wave power developers are to fight it out for the Scottish Government's £10 million Saltire Prize as the competition's Grand Challenge phase begins.

The winner will be the one that achieves the greatest volume of electrical output in Scottish waters over a continuous two-year period, using only the power of the sea.

Aquamarine's project deploys its Oyster wave energy converter, a simple mechanical hinged flap connected to the seabed, off the Isle of Lewis.

The other three will be in the waters of the Pentland Firth between the Scottish mainland and Orkney. MeyGen's tidal energy project in the Inner Sound, between the Caithness coast and the island of Stroma, resembles numerous wind turbines but on the sea bed.

Pelamis's device at Farr Point, off the Sutherland coast near Bettyhill, is like a giant metallic snake on the surface which converts the motion from the waves into energy.

ScottishPower Renewables' HS1000 tidal turbine will be at Ness of Duncansby off the north Caithness coast

Deputy First Minister Nicola Sturgeon said: "The Saltire Prize sets a considerable challenge – reflecting the huge potential of harnessing marine energy."