In every drop of water is a hidden world. Scuba divers can’t see it through their masks; neither can snorkelers swimming among the coral reefs. To really enter this world, you need to look through a magnifying lens. There you’ll see a vast array of vanishingly small plankton, including crustaceans known as copepods. They come in some 13,000 known species, from glimmering-blue sea sapphires to noodle-shaped cod worms. Some roam freely, while others cling to plants or animals. One copepod species can swim into the womb of a gestating shark and attach itself to her calf.