Today we're going to narrow our focus from cephalopods down to just octopuses. Even more specifically, we're going to look at benthic octopuses. Alright, maybe we need some quick definitions here.
Cephalopods are marine molluscs that are bilaterally symmetrical, have a prominent head and have a set (or more than one set) of arms or tentacles.
Octopuses have all the qualities of cephalopods but are further narrowed down to those with eight arms.
Benthic octopuses are octopuses that live in relatively shallow water and spend the majority of their life within the proverbial arm's reach of the ocean floor. When you picture an octopus in your head, you're almost certainly picturing one of the benthic octopuses, since they make up the majority of the known octopus species.
Alright, now that we've got our terminology down (I'd hate to be told I'm wrong because I'm using unspecific terminology), here's your fun fact: Benthic octopuses have no bones, and the only parts of their bodies that are harder than a flexed bicep are their beaks and their tongues (a very few are thought to have small, internal vestigial shells, but their classification as benthic octopuses, due to these shells, is debated). This means that the octopus is barely limited by its anatomy when it comes to where it can and cannot go.
We humans are limited in where we can go by a couple different structures: Our skull, our hips and our long bones (femur, humerus, etc.). None of them can compress or fold, so if they can't fit through an opening, neither can we. We're also limited by our need to expand and contract our ribcage to breathe, so we can experience something called mechanical/compression asphyxiation if we can't expand our ribcage again after we've exhaled. It's how constrictor snakes kill, and how crazy doctors die in chimneys. What are the limits on octopus trying to squeeze through a narrow opening? Its beak and its brain. If both of those can fit through the opening, then so can the octopus.
Alright, so how much compression does that give them? Check out this 275kg octopus squeezing its way through an opening the size of a U.S. quarter.
Keep that video in your mind when you go to sleep tonight, and don't forget: A U.S. quarter has a 24.26mm diameter and the average human trachea has a diameter of 24mm. There's plenty of room in your throat for a 600lb octopus to squeeze into.
Sources:
Tree of Life web project Octopodidae page
Wikipedia page on Octopuses
TheCephalopodPage.org
Wikipedia article on U.S. coin sizes
Wikipedia article on the Human Trachea
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