Cephalopods, Earth’s first truly substantial animals, are still among us: Their fascinating family tree features squid, octopuses, nautiluses, and more. The inventors of swimming, cephs presided over the sea for millions of years. But when fish evolved jaws, cephs had to step up their game (or end up on the menu). Some evolved defensive spines. Others abandoned their shells entirely, opening the floodgates for a tidal wave of innovation: masterful camouflage, fin-supplemented jet propulsion, and intelligence we’ve yet to fully measure. In Monarchs of the Sea, marine biologist Danna Staaf unspools how these otherworldly creatures once ruled the deep—and why they still captivate us today.