Cuttlefish are Cephalopods, along with squids and octopi. (Wikipedia says it should be octopuses, but that just sounds wrong to me, so I'm sticking to my guns.) Before discussing the origin of cuttlefish, I will point out that Cephalopod comes from Greek roots meaning head-foot, suggesting the Greeks believed octopi use their heads for feet, an inexplicable conclusion given the fact that they are just dripping with tentacles. Perhaps Cephalopod refers to the way the "feet" grow out of the head, but where else would they grow?
In any case, the name cuttlefish is much less perplexing. The cuttle bone, a unique feature in the animal kingdom, is a porous internal shell that the cuttlefish uses to regulate its buoyancy by altering the ratio of gas and liquid therein. Even octopi and squid don't have cuttlebones. How they regulate buoyancy is their own business. As far as the -fish part of cuttlefish, this comes from the fact the ancients were much more broadminded about what constitutes a fish than we are today.
Cuttlefish have so many odd-ball traits, it's hard to know where to begin with them. Their blood is blue-green, because instead of using iron-containing hemoglobin like us red-blooded vertebrates, they use copper-containing hemocyanin. Cuttlefish also have three separate hearts, one heart for each set of gills and one for the rest of their bodies. This is what they get for not having hemoglobin like the rest of us.
The most remarkable thing about cuttlefish, however, is their ability to change color. In fact, they are sometimes known as chameleons of the sea because of this, but chameleons can't hold a candle to cuttlefish. Description cannot do the cuttlefish justice: I attach a video of one "hypnotizing" its prey.
As you watch, you may think bands of light and shadow are shining on the cuttlefish from above; nope, that's all cuttlefish. I've got my doubts that crabs can be hypnotized by anything, but if they can, it's certainly a cuttlefish. One more bizarre thing about cuttlefish, is they can do all this and yet evidently colorblind, as has been demonstrated in scientific research by Lydia Mathger, Alexander Barbosa, Simon Minor, and Roger Hanlon. How cuttlefish are able to so thoroughly camouflage themselves without seeing color is a mystery. Another mystery is what led Mathger et al to suspect cuttlefish were colorblind in the first place. This is like investigating eagles to see if they are flightless.
Cuttlefish can not only fool prey and predator, but each other. The ratio of male to female cuttlefish is approximately five to one, which is something God did to ensure life is interesting for cuttlefish. During mating season, the male cuttlefish must jealously guard its mate from potential intruders. Some, however, sneak in anyway, by tucking their extra tentacles (males have eight, and females have three) under themselves, changing their coloration to a more feminine hue, and even pretending to carry an egg-sack, and otherwise doing anything to give themselves "female" traits and thus to appear harmless.
Among humans, this is known as "being a good listener."
