I actually didn't comprehend that Sekto was the octopus in the river until the second time I watched the movie. I thought the swimming creature was a spy or something, and that the Old Steef had simply been corrupted. Then the possibility of a parasite dawned on me, I looked on the Internet and saw people discussing it, and then I watched the ending vid again and it made perfect sense. That's when I noticed the marks on the Old Steef were from suction cups; my first impression was that he had just ended up like the Emperor on Return of the Jedi.
As for Sekto fitting on the Steef's head, I think it wouldn't be too difficult. Given that octopi have no bones, you'd be amazed at what they can squeeze into or stretch over.
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