Why is Luka Megurine a octopus?

All the Vocaloid packages have an object/food which resembles them. Megurine Luka is usually a Tuna, but along the lines of sea food. It goes with octopusses. This could also be because of her hair.

But to be honest the safest answer for this question is that it is a meme, and it came from a fad.