Transparency is an intriguing physical trait found throughout the animal kingdom, and in oceanic species, especially; as pointed out in this excellent overview of transparent animals by Scientific American's Sönke Johnsen, "almost all open ocean animals not otherwise protected by teeth, toxins, speed or small size have some degree of invisibility."
While being partially or totally see-through is thought to help many creatures evade predators, there's a lot about transparency, including its purposes and its underlying mechanisms, that we still don't understand. (For example, some scientists speculate that glass frogs — a variety of partially translucent frogs native to South America — evolved transparency to help regulate the function of their internal organs, though they've yet to confirm this hypothesis.)
In this video by Nature Education's Creature Cast, Riley Thompson explains some of the science behind animal invisibility. Cool insights abound — it's a most excellent way to spend three minutes of your day.