Hank has only seven tentacles because the animators realized they could not fit eight onto his body. His backstory was rewritten to account for the missing limb. For similar reasons, in the classic sci-fi film It Came from Beneath the Sea (1955) special effects genius Ray Harryhausen was only able to create a stop-motion giant octopus with six tentacles.
The license plate on the truck is CALA113. "A113" appears in all Pixar movies, as a reference to the Cal Arts room where many of the animators of Pixar Studios attended.
Hayden Rolence replaces Alexander Gould as the voice of Nemo, due to Gould having outgrown his original role since Finding Nemo (2003), though Gould does have a cameo as Passenger Carl. Bennett Dammann replaced Nicholas Bird as the voice of Squirt for a similar reason.
The setting of the film was changed from an aquatic park to a Marine Biology Institute after the controversial documentary Blackfish (2013) was screened for the crew of Pixar.
Hank is an octopus, and therefore he can survive out of water for some time, squirts ink when he's scared, and can change color for camouflage. Hank's behavior is actually very similar to that of a real-life octopus: octopuses are notoriously hard to keep in captivity, as they are intelligent escape artists that are known to leave their tanks and wander around on land. His camouflage ability not only has him changing color, but also his shape: a technique used by real-life octopuses (especially the mimic octopus) to escape predators or mimic more dangerous animals for defense.