Men have long been fascinated by sea creatures. As early as ancient times, Aristotle described crustaceans and mollusks on papyrus scrolls. Today, researchers use methods like citizen science and genetic testing to identify the last species of moonfish. "It's the unknown," explains Erich Hoyt, author of a book on marine creatures. "When you look at the ocean, you might see a dolphin or a whale, but you won't see what is below," he adds. "We only know a part of the number of species living in the depths." Marine animal species are among the most captivating that explorers and biologists have identified, whether for their astonishing appearance or their relentless quest for survival. The abyssal vampire, often called the vampire squid, is a prime example. Despite its name, it is neither a vampire nor a squid. Bruce Robison, a scientist at the Monterey Bay Aquarium Research Institute, explains that the name was given in 1903 by Carl Chun, a teuthologist, who named it Vampyroteuthis infernalis, meaning "vampire squid of hell." "I imagine that the red eyes and cape led him to say 'It looks like a vampire,'" says Robison. "So, he had fun with the name." The abyssal vampire feeds on marine snow, a mix of organic debris, and can spit mucus containing phosphorescent particles to create a luminous cloud and avoid predators. The deceiving moonfish is a recently discovered species. Known scientifically as Mola tecta, it was first identified in 2017 by doctoral student Marianne Nygaard and her team in New Zealand. The name tecta comes from the Latin word tectus, meaning hidden or confined. "I was quite impressed that this fish managed to escape our radar for all these years," says Nygaard. The moonfish inhabits cold waters in the southern hemisphere, including the coasts of New Zealand, Australia, Chile, Peru, and South Africa. Some can weigh up to 900 kilograms. In June, a 2.2-meter-long moonfish was found on an Oregon beach, initially mistaken for the common moonfish, but confirmed to be Mola tecta. The sea cask, or Phronima sedentaria, lives in the twilight zone of the ocean, typically between 200 and 1,000 meters in depth. These small creatures, less than two centimeters long, hunt salps, gelatinous creatures similar to jellyfish. The mother sea cask uses her front claws to eat the inside of a salp, then lives in its empty skin and lays her eggs. After hatching, she moves the salp like a stroller. This cycle continues as the young consume the salp from the inside, accompanied by their mother. "The killing and stealing of another creature's skin constitutes a very beautiful contrast with maternal devotion," says Erich Hoyt. Other fascinating marine creatures include the dazzling Olindias formosa, a jellyfish found along the coasts of Japan, Brazil, and Argentina. It uses multicolored, bright tentacles to attract small fish. With a maximum diameter of fifteen centimeters, this jellyfish alternates between the seabed and coastal waters. Although rare, it sometimes appears in groups when water temperature increases, leading to a rise in population. While its sting is not lethal, it can cause a painful rash and has inspired both works of art and caution for swimmers in Argentina. The great pelican eel, found in the pelagic zone of the eastern Pacific Ocean, is known for its enormous mouth, which can expand like a soap bubble to catch prey much larger than itself. This adaptation helps the eel survive in times of food shortages. Despite its large mouth, the eel mainly consumes small crustaceans. The giant squid, not to be confused with the colossal squid, is the largest known invertebrate. Identified in 1925, it was first found in the stomach of a stranded sperm whale. In 2023, video footage of what may be a baby giant squid was captured in the Australian Ocean, and analyses are underway to confirm this observation.