Octopus 1758, genus name of a type of 8-armed cephalopod mollusks, from Gk. oktopous "8-footed," from okto "8" + pous "foot." Proper plural is octopodes, though octopuses probably works better in English. Octopi is from mistaken assumption that -us is the L. noun ending that takes -i in plural.
From National Geographic, we learn that the photo was a happy accident, snapped by explorers-in-residence during their expedition to Gabon. They were in a research vessel anchored 52m off Loango National Park on the coast of the African continent. Steve Firman was piloting an ROV and used the manipulator arm to bring a large seashell on board. Out crawled an octopus, so they put her and the shell in a tank of seawater. The eggs she had just laid inside were hatching and streaming out by the 1,000s! Enric Sala took photos before releasing them back into the sea.