This is not a run-on sentence. It is not formed from two sentences that have been run together without appropriate punctuation.
The octopus is often considered to be the most intelligent invertebrate and a master of disguise; it can instantly match the colors and textures of its environment to hide from predators.
Is this a run-on sentence?
The octopus is often considered to be the most intelligent invertebrate and a master of disguise; it can instantly match the colors and textures of its environment to hide from predators.
A: no
B: yes