Invertebrates make up more than 90 percent of all living beings on Earth, and underwater, their diversity is apparent. Sea invertebrates include crustaceans, mollusks, jellyfish, and a host of other sea creatures that are integral to marine ecosystems and have important uses for humans, too. This colorful, photo-packed lower elementary title introduces readers to the most common sea invertebrates. "Compare and Contrast" boxes encourage readers to identify what traits these animals share and what makes them unique. Vocabulary boxes define difficult terms, and "Think About It" boxes invite readers to think critically beyond the text.