This is a pelagic octopus, swimming through the ocean far above the sea floor. Lots of predatory fish will be looking upward from the depths, trying to discern the shadow cast by prey. For the Glass Octopus, this means orientating their body so that their decidedly opaque digestive gland stands vertical in the water column. That way, it casts the least possible shadow. Oh dear. Don't we all want our belly to cast the least possible shadow?