Cephalopods have one of the longest and most fascinating histories of any animal on the planet. Separated from us by up to three-quarters of a billion years of evolution, there are still some remarkable similarities.
Squid and octopus are just two orders of cephalopods, from a once vastly branched lineage of cephalopods, which even today spans seven orders across all of the ocean’s marine habitats.
These are the most iconic and familiar cephalopods, and despite being separated by hundreds of millions of years of change, they share some striking similarities.
So, how do we tell them apart? Let’s take a look at the differences between octopus and squid.
1. Squid and Octopus took different paths
Octopus and squid are both molluscs, like your common clam or garden snail. They might not look like snails at first glance, but when you see their cousins the Nautilus, this becomes a little more apparent.
These are three examples of cephalopods, a class of mollusc that also includes cuttlefish, and they likely diverged sometime around the beginning of the dinosaur’s reign, 250 million years ago during what’s known as the Mesozoic Marine Revolution, and some suggest they could have split as far back as 300 million years ago.
Squid and octopus represent a relatively recent diversification in the positively mind-blowing history of cephalopods, having evolved from animals with external shells and subsequently internalising theirs.
The result is the branching, hand-shaped octopus with a bulbous head – optimised for walking along the sea bed; and the more streamlined, pointed squid, a nimbler and more able pelagic hunter.
And this shape has a lot to do with what happened to their shells.
While all that’s left of the ancient shell of the ancestor of all octopuses are a pair of fragments called stylets, the shell of a squid is a lot easier to spot.
2. Squids still have their shell
Inside an octopus mantle is a pair of tiny, bony fragments called stylets. These are all that’s left of a shell that would have covered the entire body of its ancestors. Over time, these shells became internalised, and in octopuses, they’re all but gone.
But inside the squid, there is a far more conspicuous apparatus called a gladius. This long, stiff, proteinous mass is what’s left of the shell that belonged to the ancestors of squids.
No longer chambered, it functions to provide rigidity to the otherwise soft-bodied animal and leverage for muscular contractions that give it speed when swimming. 1
3. Squids are the largest (and the smallest)
There are hundreds of octopus and fish species spread out all over the world, as well as some deep-sea cephalopods that are not quite one nor the other, and they come in a huge range of shapes, sizes and colours.
But the biggest of them all are the colossal and giant squids. The giant squid, when fully extended, can reach around 14m long, and this is from the rare specimens that have been recorded – it’s possible these abyssal giants can grow even larger.
The colossal squid is shorter in length, but around twice the mass of the giant squid at up to 500kg.
The largest octopus known is the Giant Pacific octopus which is no slouch either, maxing out at around 5 meters long, and about 50kg.
But at the other end of the spectrum, the pygmy squid tops out at around 1cm long and weighs as little as 0.02 g. The smallest octopus isn’t much bigger, however, and tops out at a gram in weight. 2
4. Octopuses don’t have any tentacles
It’s common to refer to what’s coming out of an octopus’s head as tentacles, but they’re actually known as arms. And this might seem like an arbitrary distinction, but it appears that, despite being separated by around 600 to 750 million years of evolution, octopuses and humans grow arms using the same set of genes.
Arms in cephalopods are defined as sucker-covered appendages, of which the octopus has eight including the seven-arm octopus, Haliphron atlanticus, who’s just trying to be different by hiding one of them.
The word tentacles is usually reserved for elongated appendages with suckers just on the end of them. And by this definition, squids have just two. Whether or not tentacles are a subcategory of arms appears to be a bit subjective (but likely the case), but what is certain is not all arms are tentacles.
But, if a cephalopod is wrapping its tentacles around you, it might not be a squid, either, as cuttlefish, nautiluses and vampire squids (not real squids) all have tentacles, but you can be sure it’s not an octopus. 3
5. Squid are terrifying (even if you’re not a crab)
Octopuses tend to spend most of their time on the sea bed. They’re not as rigid as a squid, and more cumbersome when swimming, so they’re more specialised at patrolling the ocean floor and eating shellfish and crustaceans. Their long arms are excellent at suckering onto a clam and prying it open.
Squid are faster, and much more aggressive predators of, well, anything they can get their arms around. They don’t really spend any time on the ground, instead, they use their superior agility to dart around in the open ocean, killing things.
And when these squid are large enough, even humans are on the menu. Humboldt squid are 2-metre-long powerful cold-water pack hunters and they’re notoriously aggressive.
These muscular squids have attacked, and likely killed, many people, and divers need to wear chainmail to protect their flesh from the squid’s large beaks. The largest octopus in the world is only really a danger when it pulls off a diver’s breathing apparatus out of curiosity, but it isn’t generally aggressive at all.
6. Squids swing
Octopuses are relatable in their mating habits, too. Like humans, they tend to pair off, the male inseminating the female in a cave by handing her a bag of sperm which she stuffs into her reproductive ‘pocket’, after which he promptly dies.
Ok, it’s not entirely like humans, but it more resembles the biblical tradition than the squid’s method.
Squids gather in huge mating orgies, in what’s called a “scramble-type” system, and it’s just sperm packets and explosive injections all over the place. Come to think of it, this is not unheard of in human societies, either.
7. Octopuses are better mothers
One thing they both share, however, is that mating is usually their final act. Mother octopuses will gracefully waste away, wafting oxygenated water over their new eggs before succumbing to starvation not long before they hatch.
Squid follow suit, but with no such brood care (there are some exceptions to this). They usually just lay a bunch of eggs all over the place before expiring.
Some float, some sink, but they all expire after reproducing, which is one of the most tragic evolutionary pranks in the animal kingdom.
8. Octopus are smarter
This one is a little unfair, because there’s not enough squid science to make a conclusive decision on it, and the largest squids have barely been witnessed alive, let alone studied in their natural habitats, but one thing we do know is that octopuses are highly intelligent.
Note: it’s also unfair on account of much of squid communication being colour and light-based, which is so alien to our understanding of communication that we will be inherently biased against recognising it.
But back to octopuses, perhaps it’s safer to say they’re relteably smart. Captive octopuses show signs of frustration, and consequently, mischief, personality, and everything you’d more readily assume could come from a puppy or particularly charismatic human.
This makes the story of their short lifespan and one-time reproduction a tear-jerker of Shakespearian proportions, especially for those who become close to them – something which is very easy to do with an animal as interactive and curious as an octopus.
Final Thoughts
So there you have it – octopus versus squid!
Octopus and squid are both gangly, jelly-like cephalopods with suckers and jet propulsion, but they’re not as similar as they might seem on closer inspection.
Octopuses, as the name suggests, have eight arms. Squids have ten – two of which are tentacles.
Squids patrol the open ocean, hunting and mating in groups. Octopuses are sea-floor cruisers, solitary hunters of slow-moving animals.
Both are intelligent and fascinating creatures in their own right, but octopuses have a recognisable personality, something that evolved independently from mammals yet emerged as eerily similar.
But both are surviving lineages of one of the most dominant and ancient groups of animals the world has ever seen: the cephalopods.