The class, Cephalopoda, falls under the phylum, Mollusca. While cephalopods are not commonly kept in home aquariums, they remain an extremely diverse and interesting group. Many of the cephalopods are very short-lived (octopus), many are very difficult to keep alive in captivity (squid), some have very special requirements (nautilus), many have venom, and the octopus has the added difficulty of even keeping it inside a tank.

Let's take a brief look at this group, all of which are marine animals.


The Class, Cephalopoda, diverse and interesting

These sea creatures within the class, Cephalopoda, are considered to be among the most intelligent invertebrates on earth. The two subclasses of cephalopods are Nautiloidea, which has nautiluses and Coleoidea or Dibranchiate, which includes cuttlefish, octopuses, and squids. Unlike other mollusks, members of this, Coleoidea, subclass do not have exoskeletons.

Cephalopod taxonomy is unstable and frequently changing with the discovery of new creatures like the frilled Pacific octopus in 2017, which was previously considered to be just a giant Pacific octopus.

Octopuses have eight tentacles, and both squid and cuttlefish have 10 tentacles, eight short ones and two long ones. Squid are considered the fastest swimmers of octopuses, squid, and cuttlefish.

Stealthy and efficient predators

Many cephalopods are stealthy hunters who apply sophisticated tactics thanks to their big brains. They often display a deep understanding of their environment and deploy hunting techniques that will leave you amazed.

An octopus.

This is a royalty-free image from Pixabay.

Hiding in plain sight

Consider cuttlefish that mimic the shape of more harmless creatures like crabs and walk along the sea floor to avoid scaring fish away. When they get close enough to their prey, they strike at lightning speed.

Another sinister trick that some octopuses and cuttlefish use is camouflage and mimicry of the environment. They can adopt the texture and color of reefs, seaweeds, or the sand, becoming almost impossible to discern. Then as fish swim or crabs pass nearby, the octopus or cuttlefish shoots its lightning-quick tentacles and grabs the prey.

Sly squids

Squids are probably the most villainous though. One species, Grimalditeuthis bonplandi draws the prey to itself. Unlike other squids, this species has weak tentacles that resemble clubs. When hunting, the ends of the tentacles glow, attracting a crowd of curious sea creatures like crabs and fish. These unsuspecting animals probably think they have found an easy meal right before the squid pounces.

Two squid.

This is a royalty-free image from Pixabay.

Subduing the prey

Other squids hunt using the stealth approach almost like their octopus and cuttlefish cousins. Using its camouflage to get close to prey before shooting their tentacles to grab the meal.

Most squids, cuttlefish, and octopuses are efficient. After capturing their prey they inject them with venom that causes paralysis to avoid fighting. This also allows them to take prey somewhere quiet where they can have a peaceful meal.

Mild mannered cephalopods

In an odd twist, the vampire squid is actually the least terrifying; a name like the monk squid is probably more fitting. It gets its name only because it lives in the dark depths of the sea where there is little oxygen and few predators.

It lives frugally.

A vampire squid is less of a predator and more of a detritivore; feeding on marine snow and the occasional unfortunate crustaceans that may have been hitchhiking on the marine snow. This type of squid hunts by stretching a sticky tentacle that can go up to 10 times its body size. The tentacle traps the marine snow and mucus falling from above.

Nautiluses live a somewhat similar lifestyle, mainly scavenging on dead fish that sink to the bottom of the sea. They opportunistically kill small prey that wanders close to them.


This is a royalty-free image from Pixabay.

Smart octopuses and cuttlefish

One video shows octopuses using abandoned shells or coconut as shelter to hide from predators or prey. It sits on one shell and covers itself with the other, then lies in wait until a crab comes close. Then the octopus gleefully snags it and hurries off to enjoy its latest catch.

The octopus can carry around the shells, knowing they will need to use them again in the future.

An octopus "borrowing" a shell.

This is a royalty-free image from Pixabay.

Bluffing octopuses

When cephalopods encounter threats, they use several defensive tactics. Some species of octopus such as the mimic octopus can take the form of a predator’s feared enemy to protect itself.

Sea snakes eat damselfish, and damselfish know this, and fear the snakes. More importantly, the scheming octopus knows it too. An octopus was caught on video burying its body, except for two tentacles that changed color and pattern to look like the poisonous sea snake when under attack.

This is an example of an instance where, not only does the octopus mimic, but it knows the enemies of its enemies and uses that knowledge to avoid predators.

Cuttlefish drag act

When cuttlefish become adults and want to mate, males compete for females. Smaller male cuttlefish want to mate, but they do not want fights against stronger adversaries.

So what do they do?

In one instance, a male cuttlefish showed a pattern mimicking a female on the half of its body that was visible to other males. On the other half of its body, it showed its male patterns to woo the female!

Flamboyant cuttlefish, Metasepia pfefferi

This is a royalty-free image from Pixabay.

Disguises galore

Members of the subclass, Coleoidea, have advanced defensive mechanisms thanks to their big brains. Scientists have taken videos that show cuttlefish and octopus changing both the texture and color of their skin to mimic the sea floor and reef formations so they can throw off predators or ambush prey.

Animals in this subclass can change their colors quickly because their brains control chromatophores--the cells that determine outward appearance.

Squid adopt a different approach to disguise.

Their skin behaves in a way that lets light pass through to minimize detection. When accosted by prey, some types of squid leave a bioluminescent arm behind as they flee to confuse the predator.

Swimming squid.

This is a royalty-free image from Pixabay.

Taking flight

Cephalopods can use their jet propulsion capabilities flee from predators. Some also direct jets of water to the sea floor to scatter sand and unearth prey.

How does cephalopod jet propulsion work?

First, the cephalopods take water into the mantle through an opening in their heads. They then increase pressure while closing all orifices. When they want to sprint, they force the water through their funnels at high pressure that propels them in the opposite direction. These animals also control the direction of propulsion by aiming the funnels.

Thanks to their torpedo-like shapes, squids can achieve speeds like 25 miles an hour which helps them flee from predators. That might not be enough; squid is a favorite meal for sperm whales. Nautiluses have shells that protect their succulent bodies from predators.

Deploying ink

When fleeing, members of subclass, Coleoidea, (except for octopods in the suborder, Cirrata or Cirrina) may leave a cloud of ink.

The ink serves as a smokescreen that gives the animal time to flee. That short time is precious because it can allow the animals to either find a hiding place or change appearance to avoid capture.

The ink is released in a formation that mimics the shape of the cephalopod, sometime this confuses predators who go after the ink!

During the 2018 winter Olympics in Korea, ice cream flavored with squid ink became a fan favorite. Foods flavored with squid or octopus ink are seen regularly on the menus of high-end restaurants.

How clever are octopuses?

One test showed the amount of ingenuity that octopuses could exercise. Scientists put a crab was put in a jar and closed it using a lid. The octopus opened the lid and ate the crab in very short time.

In fact, octopuses are some of the hardest sea creatures to keep in tanks and aquariums. They are adept at looking for ways to get out.

Octopuses can weasel through small openings thanks to their soft bodies; their beaks are the only hard part of their bodies.

Consider Inky, the octopus who got out of his tank in New Zealand through the overflow drain. He then slithered across a room, crawled into a drainpipe and slid back to the sea. The drainpipe was a mere 6 inches wide in diameter. The news made headlines all over the world.

When scientists put these animals in tanks that had an opening that was too small, the octopuses tested the openings with their arms and did not try to leave the tanks. This shows they are unlikely to get stuck because they know their size. Octopuses and cuttlefish have also been known to solve and recall solutions to mazes.

The blue-ringed octopus, genus, Hapalochlaena, considered one of the most venomous marine animals in the world.

This is a royalty-free image from Pixabay.

Live fast die young

Most members of the subclass, Coleoidea, grow rapidly, live approximately three years and mate once in their lifetimes. They often die shortly after laying their eggs. Some few species may reproduce twice in their life span. Among some species of octopuses, the females take care of eggs, not leaving them even to eat until they hatch. The mothers then die of hunger and exhaustion shortly afterward.

In 1977, scientists found out that this behavior is triggered by the optic glands. When scientists removed these glands, the octopus stopped brooding and ate again, prolonging its life.

Octopuses, squid, and cuttlefish are distinguishable by their lack of external shells. However, squid and cuttlefish have small internal shells that help their bodies remain more rigid, allowing them a more streamlined shape that makes them quicker swimmers than octopuses.

Nautiluses outlive them all

Subclass nautioloidea has the laid-back members of this class, but they too have something to boast about. Rather than the live fast, die young lifestyle of members of coleoidea, this subclass boasts a long life span that goes up to over 20 years.




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Author Profile: David Kimutai

David loves visiting the ocean, and his favorite saltwater aquarium fish is the dwarf angel, genus Centropyge in the family, Pomacanthidae. He is a freelance science writer and digital marketer living in Nairobi, Kenya.

His love for aquariums started when he was young, when together with his brothers, they fished a trout from a local stream and kept it alive at home for days before releasing it.

His forum name is Davidangelfish