Atlantic bluefin tuna

In open water, the relief vanishes and a school of large silver silhouettes crosses the field of view. Swimming is part of the bluefin tuna’s breathing: movement forces water across its gills. Counter-current blood vessels retain some of the heat produced by metabolism, keeping the swimming muscles warmer than the surrounding sea.

What a diver sees near the surface is only a fraction of its vertical range; the species has been recorded down to 985 metres. In the western Mediterranean, around the Balearic Islands, surface circulation also helps retain particles within a major spawning area.