Jellyfish are among the oldest creatures on the planet, yet not much is known about them. Three hundred species have been named, but twice that are thought to exist. They range in size from a millimeter to more than six feet, live for hours or years. Most sting, and one kind can kill a person in less than three minutes. Collecting in massive “blooms,” they literally stop ship traffic and frequently clog intake valves and shut down power plants. And they are “heartbreakingly beautiful.” Juli Berwald, a one-time science textbook writer, became smitten with jellyfish while diving in Israel in 1987. She didn’t plan to pursue them around the world, but she has, and Spineless (Riverhead, $27) is both the story of what Berwald has learned about jellyfish and the story of how the jellyfish made her a scientist.  A wife and mother of two, Berwald found in jellyfish “the intellectual playground …[she] craved,” but scheduled reporting and diving trips around the family’s needs. She nonetheless met with marine researchers in Europe, Japan, and throughout the U.S., and her book examines all facets of jellyfish physiology, from how the animals swim—pulling water, not pushing it, and moving deliberately, not drifting—to how they sting, how they use bioluminescence to communicate, how one species appears every thirty years, like a marine locust, and how another type reverses stages of its life cycle, as if it finds time as fluid as water. Berwald’s driving question is whether jellyfish thrive or suffer in warmer, more acidic seas, and as she presents the conflicting evidence, her book expands into an urgent and illuminating look at the ocean as a complex eco-system beset by climate change.