The name cephalopods come from the Greek root words for head and foot. Cephalopods are an advanced form of mollusk (snails, slugs, clams etc). Though many mollusks live in shell, moving around with their single foot, Cephalopods have evolved their one foot into the numerous tentacles that make them so unique. They are also known as ink-fish due to their unique ability to squirt ink out from their siphons.
Siphons are what cephalopods use to propel themselves through the water, and it is also where they expel their waste. The ink is used as a distraction; so any predators will go after it instead of the cephalopod. Most of the ink is actually made up of of the cephalopod’s mucus! In some species it can even cause harm to the sensory organs of the perceived threat.
Cephalopods have three hearts and copper-based instead of iron-based blood, causing it to be blue. They are very curious animals, but tend to stay away from other members of their species. If you haven’t seen the movie My Octopus Teacher, I would highly recommend it to learn more about the behavior of these amazing animals.
I have had my own similar experience with this at Bahia Honda State Park in the the Florida Keys. I saw a Caribbean reef octopus (Octopus briareus) in the marina by our campsite and decided to watch him/her for a while. I put my finger in the water above the octopus and after a few minutes he actually reached out a tentacle and wrapped it around my hand, trying to pull me into the water! It was an amazing encounter. Here is a picture I got afterwards.
Despite being able to change color, all octopuses* are colorblind. Many other Cephalopods are as well. There are over 800 species! Though mollusks can be found in many ecosystems, cephalopods only live in the ocean with only one species tolerant to brackish water (the Brief squid, or Lolliguncula brevis).
Most cephalopods have a cartilaginous (made of cartilage) cranium or shell around their brain, and cuddle fish have a version of hard structure called a cuttlebone that is made of aragonite crystal. You may find these in pet stores where they are commonly used for pet birds to chew on, as a way to supplement calcium.
Squid also have, in a way, an internal exoskeleton. It is similar in shape to a cuttlebone, but instead made of chitin, which is a major material found in arthropod exoskeletons.
Cephalopods are considered the most intelligent invertebrate on the planet. In captivity, they have been known to escape from the aquarium! In one case an octopus managed to make its way across the room into another aquarium with food, then climb up into that aquarium eat, and make its way back into their original aquarium!
You may have heard that octopi have a brain in each tentacle. Not all brains are the same; a brain is a concentrated mass of nerves, and different animals have somewhat different versions of the organ. Octopuses have some of the most well developed brains of any cephalopod, with a centralized system in the head, and prominent axial nerve cords in the tentacles.
Because octopi have no bones, they are able to move their bodies fluidly and fit through about any space larger than their eyes. Squids and cuttlefish don’t quite have this same ability, but some squids have been known to fly!
The Neon flying squid (Todarodes pacificus), is a great example of this ability. They will use their siphons to propel themselves out of the water and flatten themselves out to glide through the air.
*If you’re wondering about the plural for octopus, both octopi and octopuses can be used. Octopi uses a Latin plural, but octopuses is correct in English. I find octopuses more often used in research articles, while octopi is more often used in conversation.
Sources: National Geographic, Britannica, Oxford Research, National Library of Medicine