Glass octopus in the Atlantic Ocean off Cabo Verde
See-through cephalopod

World Octopus Day

One of the least understood of sea creatures, this glass octopus lives in the depths of the ocean where sunlight can't reach, about 3,000 feet down. Glass octopuses are rarely seen and difficult to observe, but they're a great example of the diversity of the order Octopoda, which we're celebrating today on World Octopus Day. The observance comes around, appropriately, every Oct 8.

The more we learn about octopuses, the more fascinating they reveal themselves to be. They're among the most intelligent and behaviorally diverse creatures in the sea. Octopuses can remember and recognize individual humans and are among the very few animals known to be able to use tools. Some, like the giant Pacific octopus, are enormous, and some are tiny. They live in a variety of habitats, from tidal pools to the deepest abyss of the ocean.

In the darkness where they live, glass octopuses are nearly invisible, which helps them elude predators like bottlenose whales. Only their eyes give them away. The species is relatively small, with a mantle (the bulbous 'head' that houses the animal's organs) about 4 inches long. And their lives, like those of most octopus species, are brief—just a few years. They grow quickly, mature early, reproduce, and die soon after that, never seeing the light of day.