In the middle of the Pacific Ocean, Gonzalo Mucientes has discovered an invisible line in the sea that separates male mako sharks from females. The line runs from north to south with the Pitcairn Islands to its west and Easter Island to its east. On the western side, a fisherman that snags a mako will most probably have caught a male. Travel 10 degrees of longitude east and odds are they’d catch a female. This is a shark that takes segregation of the sexes to new heights.

Mucientes and colleagues from Spain, Portugal and the UK spent four months aboard a Spanish longline fishing vessel.  Amid more typical catches, the boat often snagged shortfin makos and blue sharks.