Written in an accessible and easygoing style, "Citizens of the Sea" describes the ocean and its incredible biodiversity using the theme of champions - mixing obvious categories (like the largest, the smallest, and the deepest-dwelling creatures) with less obvious ones, such as the most social (killer whales and snapping shrimp), the best farmers (damselfish), and the most imitated (bath sponges). The book features dozens of stunning Census of Marine Life photos, images, maps, and other visualisations to help tell each mini-story. The champions are loosely organized in groups according to criteria such as size and shape, habitat and ecology, behaviour, and human connections. The book's themes cross a vast range: the various ocean realms (poles, deep sea, shore, and open ocean), types of organisms (vertebrates, invertebrates, plankton, plants, and bacteria), and modes of scientific discovery (remotely operated vehicles, DNA barcoding, and tagging). The remarkable variety of marine life is at the heart of "Citizens of the Sea", whose author claims that estimates of ocean diversity are too low by a factor of ten. Whether large or small, tropical or polar, predator or prey, the creatures of the ocean have great significance to the environmental health of our planet - and their stories, captivatingly reported and photographed, will enchant a wide audience of readers.