In the weird and fascinating world of underwater invertebrates, squids and octopuses are two of the most popular entities.
These tentacled beasties share many similarities, so much so that many people have a hard time telling them apart.
The mix-up is easy to understand.
They both have multiple arms, can change the color of their skin, and are said to be quite intelligent.
Squids and octopuses are also distantly related since they both belong to the cephalopod lineage.
However, separated by millions of years of evolving in different directions since their ancestors split up, squids and octopuses have several remarkable differences.
In this post, we’ll explore the distinctions between these two aquatic invertebrates, shedding light on their unique physical characteristics, behaviors, habitat, and reproduction.
Overview of Cephalopods
Squids and octopuses are cousins.
They are both part of a group of marine mollusks collectively referred to as cephalopods (class Cephalopoda).
Other members of this class include nautilus and cuttlefish.
Cephalopoda is the most morphologically diverse and behaviorally complex class within the Mollusca phylum.
There are at least 800 living species of cephalopods found in oceans all over the world.
Cephalopods are exclusively marine, meaning they cannot survive in freshwater.
They all feature a prominent head with a set of arms or tentacles attached to it.
Their name, which means “head-foot,” is a reference to this unique body plan.
All their internal organs are housed within their body cavity (known as the mantle).
Some living cephalopods have hard internal structures.
Cuttlefish have cuttlebone while squids have a pen.
These hard internal shells evolved from the external shells of their ancestors.
However, many cephalopods (such as the octopuses) have no hard internal structure at all.
Cephalopods are considered the most intelligent living invertebrates.
They have a large brain relative to their body size and a complex nervous system.
This allows them to exhibit a wide range of complex behaviors and problem-solving abilities.
Cephalopods are renowned for their impressive learning capacity and have been observed exhibiting sophisticated hunting strategies or even engaging in play-like activities.
Both squid and octopus belong to the cephalopod class.
They share a common ancestry that goes back to about 300 million years ago.
Despite their similarities, living different lives since their ancestral lines split has allowed them to evolve a wide range of distinct behaviors, unique appearances, and different ecological roles.
Although many people have difficulty distinguishing them, octopuses and squids look different physically.
Octopuses have a much bigger head than squids with a rounded profile.
They also have a mantle surrounded by eight arms.
Squids, on the other hand, have a triangular head.
They have eight arms, two tentacles, and two fins on their head.
Most octopuses lack fins, but a few deep-water species may be exceptions.
Both cephalopods have two eyes but differ in the shape of their pupils.
Octopuses have rectangular pupils in their eyes, while squids have circular pupils.
The octopus is completely soft-bodied.
They do not have a shell (both internally and externally) or any other hard body parts.
Squid, however, has a stiff internal structure known as a pen or gladius.
This acts like an internal backbone that supports the squid’s body.
There’s also a huge size disparity between squids and octopuses.
The smallest octopus species is the star-sucker pygmy octopus, which has a total body length of about 1.5 centimeters (0.59 inches).
Conversely, the biggest octopus species is the giant Pacific octopus, with an average length of about five meters (16.4 feet).
The smallest squid (southern pygmy squid) is about the same size as the smallest octopus species, with an average length of about two centimeters (0.8 inches).
On the other end of the scale, squids grow significantly bigger than octopuses.
The colossal squid is the largest squid species and also the largest of all cephalopods, with reports of individuals measuring up to 20 meters (over 60 feet) in body length.
The physical distinctions between these two cephalopods highlight the evolutionary adaptations they have each undergone, which allows them to thrive in the marine environments where they’re found.
Tentacles and Arms
Like all cephalopods, squids and octopuses have multiple appendages.
This is one of the things people often mix up about them.
For starters, the octopus’ appendages are called arms and not tentacles.
Although they look similar, tentacles only have suckers at the tips, while arms are completely covered in suckers.
All eight appendages of the octopus are covered in single or double rows of suckers, which means they’re all arms.
Squids have eight arms like this as well but also have two tentacles.
Squids use these two specialized tentacles (which are longer than the other appendages) to capture prey, which they tear apart with the sharp beaks in their mouths.
The octopus’ arms are more flexible than those of a squid.
They use these arms to walk on the sea bottom and can also handle or manipulate objects in their environment with them.
Suckers and Hooks
Squids have cup-like suckers arranged along the length of their arms and tentacles.
The shape and size of these suckers vary from one species to the other.
However, for most species, the suckers are typically lined with small, toothed rings on the outside, while the inner surface has a central chitinous tooth.
This tooth can be quite sharp and can dig into soft-bodied prey.
Some squid species also have small hooks or “sway bristles” on the ends of their tentacles.
These hooks help the squid latch onto the soft tissue of their prey effectively, enhancing their ability to hold on to slippery prey like fish, which forms the bulk of their diet.
Octopus arms are lined with suckers too, but they do not have toothed rings or hooks like that of the squid.
Instead, the suckers have a smooth inner surface that holds onto prey through suction.
The octopus’ suckers are also highly sensitive, and it uses them to interact with their surroundings.
So unlike the squid suckers that are only used for capturing prey, octopus suckers serve a sensory function.
They use it to determine the texture, shape, and composition of the objects they touch.
Octopuses are also known for their impressive manipulation skills, and the suckers on their arms play an important role in this.
They can use their arms to perform intricate tasks like opening up a shellfish or holding and manipulating objects with remarkable precision.
Squids and octopuses swim by jet propulsion.
This type of movement involves sucking water into their mantle cavity and expelling it quickly through a narrow siphon.
Both cephalopods are flexible and agile swimmers.
They can swim in any direction and alter their course quickly when they need to.
However, squids are better swimmers compared to octopuses.
They live in the open ocean and have a streamlined body engineered for speed and efficiency.
Octopuses are benthic animals, and they tend to use their arms to crawl over the ocean floor, exploring the nooks and crannies of the sea bottom.
Squids also have fins on their head, which further assist with propulsion and steering in the water, especially when swimming at low speed.
They wrap their fins around their body when they need to move quickly.
Most octopuses don’t have fins like this, except for some deepwater octopus species.
Since octopuses spend more time near the ocean floor, their diet mainly consists of bottom-dwelling marine animals, especially crustaceans like crabs, lobsters, shrimps, and clams.
They are opportunistic hunters that rely on stealth and ambush when hunting prey.
Octopuses are completely soft-bodied.
This lack of rigid internal skeleton allows them to squeeze into crevices in the sea bottom and lie in wait for prey to swim by.
They also use their ability to change color and texture to blend seamlessly with their surroundings.
When a prey swims close, the octopus grabs it with its suckers.
Most species produce powerful toxins that can immobilize prey.
Squids, on the other hand, feed mainly on fish and crustaceans.
They can chase prey actively thanks to an elongated body, powerful fins, and a quick burst of speed.
Unlike octopuses, squids don’t have venom.
Their arms and tentacles are lined with sharp tooth rings and hooks that help them hold on to slippery prey.
The squid’s sharp beak can also cut through the flesh or exoskeleton of tough prey.
Cephalopods, in general, are known for their complex and varied communication methods.
They mainly use visual signals, including changes in skin color and patterns, to communicate during courtship or when showing aggression towards intruders.
Octopuses may also communicate using postural changes in their arms
Squids are always traveling through the open sea.
Some species move together in schools that may contain several individuals, while some are solitary.
Octopuses are less gregarious.
They mostly live alone in dens on the ocean floor.
Squids and octopuses have similar defensive strategies.
When threatened, they release a jet of inky fluid into the water.
The cloud of ink blocks the attacker’s vision, providing a distraction for the cephalopod to get away.
They can also propel their body quickly through the water, which can be valuable in evading potential threats.
Octopuses have a few special tricks they employ to escape predators that squids don’t.
They can eject one of their arms from their body to distract the predator, giving the octopus enough time to get away.
Some species are also capable of advanced mimicry, which involves imitating the appearance and behavior of other marine animals.
When all else fails, octopuses can inject venom into their attacker.
In fact, the blue-ringed octopus is considered one of the most venomous animals in the ocean.
They have been known to inject venom into their eggs, which allows them to generate their own venom even before hatching.
Habitat and Distribution
Squids are pelagic animals, meaning they live in the open ocean.
They can be found in both shallow and deep water ecosystems.
While some species live close to the surface, others are deep-sea dwellers.
The deepest-dwelling squid has been observed at a depth of up to 6,200 meters.
Most species are adapted to an active swimming lifestyle and are frequently found in parts of the ocean with strong currents.
Squids are also tolerant of different temperature and depth ranges.
Some species have been known to migrate vertically in the water column.
These species move to shallower depths to feed at night and descend to the cold depths of the ocean during the day to avoid predators.
Octopuses are benthic animals, meaning they’re mostly found on or near the sea floor.
They’re mainly found on rocky reefs, coral reefs, or sandy and muddy sea beds.
Their soft body allows them to squeeze into crevices in the sea floor, which they use as a den for shelter and protection.
Both of these cephalopods have a global distribution, with different species living in various oceanic regions.
They share most of their range, but squid are adapted to a broader range of temperatures and depths compared to octopuses.
Octopuses pair up to mate by following chemical cues in the water.
They’re mostly solitary but will seek out each other when it’s time to mate.
During mating, the paired-up male and female octopuses find a safe and secluded area.
One of the octopus arms is modified to form a hectocotylus, which contains his sperms.
The male transfers the sperm into the female’s body using this arm.
Male octopuses only mate once; they stop eating right after mating and are often dead within a few weeks.
After mating, the female octopus seeks out a suitable location to lay her fertilized eggs.
She can lay up to 30,000 eggs in strings that resemble decorative holiday lights.
Octopuses are dedicated parents.
She protects her eggs by fending off predators and cleaning up the den until they hatch.
The incubation period can be anywhere between 30 days and an entire year, depending on the species.
Most octopus species stop eating during this period as she does not leave their eggs to hunt.
Consequently, the female typically dies after the eggs hatch.
Squids do not pair up to mate the same way octopuses do.
Instead, they first form large groups, which may include several individuals.
The process of pairing up involves elaborate courtship displays to attract a suitable mate.
Males swim rapidly in circles and may exhibit elaborate changes in color and body pattern.
As time passes, the females join in this elaborate display, and the large group soon starts to pair up as females select suitable mates.
The pair then adopt a head-to-head position and may lock their jaws.
The male squid uses his modified arms (heterocotylus) to transfer his gametes (encased in a spermatophore) to the female.
The female may keep the spermatophore until conditions are more favorable or fertilize the eggs immediately.
Squids don’t lay eggs in dens like octopuses do.
Instead, they attach their eggs to rocks or corals.
Once the eggs are laid, the squid swims off, leaving the eggs to survive and hatch on their own.
Octopuses generally have a short lifespan.
They mostly live for about one to three years on average.
Squids live slightly longer, with a lifespan of about nine months to up to five years.
Different cultures around the world cook squids and octopuses as a part of their menu.
Squids have a mild, slightly sweet, or nutty flavor, and the texture is firm and chewy.
Octopus meat is more tender than you might imagine.
The flavor is often described as slightly sweet, with a hint of brininess.
On menus, squid meat is often served as calamari.
All parts of the squid, including the arms and tentacles, are edible.
They’re often added as ingredients to stir-fries, noodle dishes, and rice in many Asian cuisines.
Squid ink is also edible and is often used as a coloring in making black pasta or added to soups, risotto, or paella.
Only the hard parts of the squid, such as the beak and gladius (pen), are excluded.
Octopuses are consumed as food by cultures all over the world, too.
It can be served pickled, and this is called octopothi in Greek restaurants.
In parts of Asia such as Japan, octopus is used in making sushi or takoyaki.
Some people also consume small species of octopus alive as a healthy food.
With around 300 species of each, octopuses and squids are among the most diverse invertebrate groups living in the world’s oceans.
Despite being close cousins, these two cephalopods differ remarkably in terms of their physical attributes, habitat preferences, and behavior.
Squids have an elongated body, while octopuses have a more rounded profile.
They also differ in the number of arms and tentacles.
While squids have eight arms with two tentacles, octopuses only have eight arms.
The squid’s arms and tentacles are also equipped with hooks and tooth rings that help them capture prey effectively.
You’re more likely to find squids in the open ocean, but octopuses prefer benthic habitats such as rocky reefs and seagrass beds.
Understanding the differences between these two cephalopods will make it easier to differentiate between them and appreciate the diversity and ecological significance of these cephalopods.