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Squid vs Octopus: What’s the Difference?

Squid vs Octopus: What’s the Difference?

Written by Scuba.com
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Updated on December 14, 2022
octopus
TAGS: squid

There’s a reason why squid and octopus are often confused with each other. For one, when you compare squid vs octopus, they are both classified as cephalopods, with heads and arms that they use for swimming and catching prey. But while they may resemble each other, they do have their physical differences and unique hunting methods, not to mention habitat preferences.

So what is the difference between a squid and an octopus? Let’s find out below.

Squid vs Octopus

Squids close up shot
Image via Shutterstock

To help you understand the differences between squid vs octopus better, we’ll describe their differences and similarities when it comes to biological classification, anatomy, size, lifespan, habitat, and hunting behavior.

Biological Classification

Both squids and octopuses are cephalopods, which means “head-footed” as they have no real feet and their head is their most prominent feature. They’re also both invertebrates and mollusks, and they both have eight arms. And, unlike other mollusks, neither one has a shell.

Anatomy

Squids and octopuses are built similarly, but you can tell they’re different creatures just from their appearance. The octopus has a round head and eight arms that radiate from it like the spokes of a wheel. The squid has a triangular head, eight arms in the rear, two extra long tentacles, a long body, and a bony plate called a “pen” that’s a vestigial remnant of its mollusk shell.

You’ll also notice that a squid has two fins on their head, while the octopus has no fins at all—except for some deep-water species like the Dumbo octopus, which was named for the ear-like fins on its head that resemble the large, floppy ears of Disney’s Dumbo (the elephant).

Both squids and octopuses have suckers on their arms, but the two extra tentacles on squids are equipped with specialized hooks and sucker rings with teeth attached.

The eyes of the octopus and squid are also placed differently. Octopus eyes protrude from the top of the head, bulging near the tentacles. Squid eyes are embedded in the head between the tentacles and mantle of the body.

Interestingly, both have squirt ink and copper in their blood. The copper is what helps transport oxygen within the animals’ bodies and makes their blood blue.

Internal shells

The internal makeup of octopus and squid is also unique to each cephalopod. One of the distinct differences is in the internal shell, or “pen.” The majority of octopus species have no hard internal shells or bone structures, allowing them to squeeze themselves in and out of tight places along the reef (and often in aquariums, baffling the staff until they see it for themselves!).

Squids have actually retained a remnant of their ancestors’ shell inside their body, a feather-shaped structure called the gladius. It is used for internal support for the mantle and as an attachment point for muscles, and can very widely in shape and length, depending on the species. In some species of squid, the gladius extends far past their fins, supporting a trailing, tail-like appendage.

Arms

Octopuses come in a variety of colors, and have eight arms which have suckers all along. Whereas squid have eight arms, plus two longer tentacles for catching fish in open waters, and a rigid backbone structure. The arms of an octopus are more flexible than those of a squid, allowing them to walk, hold things, and move things.

Size

Many believe squids are the much smaller kind. Surprisingly, squids normally grow between 60 centimeters to nearly 20 meters long—although the smallest species of squid, the sepiolid, is under an inch long. Octopuses, on the other hand, only grow anywhere between 1 centimeter to 9 meters.

Lifespan

Another key difference between squids and octopuses is their life expectancy. Considering their size, both have a pretty decent lifespan. Squids normally live between 9 months to 5 years, while octopuses can survive anywhere between 1 to 3 years.

Habitat

Squids can be found in various areas of the open sea, from shallower waters to the darker depths of the ocean. On the other hand, octopuses prefer the seafloor level, whether it be shallow or deep waters. In fact, this is why you’ll find octopuses living in rubbish that have sunk to the bottom of the ocean floor.

octopus in the water
Photo by Diane Picchiottino on Unsplash

Hunting Behavior

Due to their anatomical differences, their hunting techniques can vary. Squids swim in the open ocean in groups and use their two extra tentacles with sucker rings to give them a little extra reach and help catch fish and shrimp, which they’ll then eat in chunks.

Octopus, on the other hand, stay on the seafloor and eat bottom-dwelling crustaceans, because their soft bodies make them vulnerable to predation. They grab, pierce, and inject poison into their prey to paralyze it. They will then dissolve and loosen the prey’s meat using their saliva before ripping it apart using their beak and mouth.

Some squid will expel ink clouds when they feel threatened, whereas octopus will change color and camouflage to avoid danger.

Mating Behavior

Male octopuses have a specialized arm (the hectocotylus) which transfers sperm to a female, whereas squid mate in large groups.

Socialization

With the exception of mating, octopuses are solitary animals, whereas squid can live in schools, or alone. Squid will travel in large groups and live in the open ocean, whereas octopuses will live alone in a seafloor den.

Squid vs Octopus Comparison Table

 SquidOctopus
 Squid on white backgroundOctopus on white background
ClassCephalopoda
OrderTeuthidaOctopoda
Species298300
PluralSquid (collective); Squids (2 or more species)Octopodes; Octopi; Octopuses
Found inSaltwater, open oceanSaltwater, dens of the seafloor
NatureSolitary or living in schoolsSolitary by nature
AnatomyMantle, head, 2 fins, 8 arms, 2 tentacles with hooks and sucker ringsMantle, head, 8 arms with 1 or 2 rows of suckers without hooks
ShellHas a bony structure called a pen, which serves as its flexible backboneNone
Fins2 on their headNone, except for rare few found in deep water
SizeUp to 60cm (average squid); 13 to 20m (giant squid)1cm to more than 5m
Life Span9 months to 5 years1 to 3 years
Hunting BehaviorCatches food with two long tentacles and eats in chunksGrabs and pierces into prey to inject paralyzing venom, dissolves and loosens meat with saliva, rips prey apart using beak and mouth
Food SourceFish and ShrimpBottom-dwelling crustaceans
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