Thanks to Wikipedia I can pretend to know what I’m talking about: Captain Nemo, also known as Prince Dakkar, is the inventor and pilot of the Nautilus, a submarine in which he secludes himself from society. In 20,000 Leagues Under the Sea he lives in self-imposed exile and shocks his crew with his commitment to vengeance against the society which wronged him.
He also fights off a giant octopus. At the time, political cartoons and sci-fi stories used an octopus or squid to represent the stranglehold of imperialism and eventually economic monopolies. Jules Verne perhaps borrows the symbol to show Captain Nemo’s battle against the bonds of society.