[The following essay contains SPOILERS; YOU HAVE BEEN WARNED!]
When questioned about the underlying themes of Gyo, author Junji Ito half-jokingly replied (paraphrased):
Well, I realized that sharks aren’t very scary out of the water, so I decided to give them legs.
But despite this B-movie premise and its predominantly tongue-in-cheek tone, the manga features a wealth of subtext. Consider, for example, that the “walking fish” are the product of biological weapons research (possibly; Ito deliberately leaves their origins ambiguous); both the devastation that their invasion wreaks on the environment and the degeneration that it causes in the human body are unnervingly reminiscent of the results of nuclear warfare.
The true source of the series' horror, however, lies somewhere significantly more existential. The aquatic creatures that drive the conflict are not malicious—indeed, they’re technically dead by the time they reach land. The robotic “limbs” that enable their locomotion are, in fact, parasitic in nature, injecting their hosts with a germ that induces increased gas emission—thereby transforming them into flesh-and-blood fuel tanks that propel the machines, further spreading their own infectious fumes in a self-perpetuating cycle.
And people are not immune to the contagion. Sentience, consciousness, and free will are irrelevant; as far as the automatons are concerned, “life” is simply energy, existing only to be consumed and discarded like so many disposable batteries.
And that is the techno-organic nightmare at the heart of Gyo: the haunting notion that mankind can be reduced to an expendable resource.