Gliding silently beneath the waves they turned vast areas of blue water to gold off the northern tip of the Yucatan Peninsula.
Sandra Critelli, an amateur photographer, stumbled across the phenomenon while looking for whale sharks.
She said: "It was an unreal image, very difficult to describe. The surface of the water was covered by warm and different shades of gold and looked like a bed of autumn leaves gently moved by the wind.
"It's hard to say exactly how many there were but in the range of a few thousand...
Fantasea FG7X II
Ikelite Housing for Nikon D500
I-DiveSite Venom 35s
Plan Your Adventure >