An octopus is not a fish. While both are aquatic animals, they belong to entirely different biological classifications. Octopuses are marine invertebrates, lacking a backbone and internal skeletal structures found in fish. This fundamental anatomical difference places them in distinct branches of the animal kingdom.
Why It’s Not a Fish
The primary reason an octopus is not classified as a fish is its fundamental biology. Fish are vertebrates, possessing a backbone, while octopuses are invertebrates, lacking such a bony structure. This absence of a backbone allows octopuses to squeeze their soft bodies through incredibly small spaces.
Fish use fins for locomotion, propelling themselves through water, while octopuses primarily move by crawling with their eight arms or using jet propulsion by expelling water from a siphon. Another distinction lies in their external coverings; fish are characterized by scales, which octopuses do not possess, instead having soft, unscaled skin. While both respire using gills, their respiratory systems differ. Fish have fins as their primary appendages, unlike the octopus’s eight arms, which are equipped with suction cups.
What an Octopus Is
An octopus is a marine invertebrate belonging to the phylum Mollusca. This phylum includes soft-bodied animals like snails, slugs, clams, and oysters. Within Mollusca, octopuses are part of the class Cephalopoda, a name that means “head-foot,” referring to their prominent head and arms that extend directly from it.
This class, Cephalopoda, also encompasses other well-known marine creatures like squids, cuttlefish, and nautiluses. Cephalopods are characterized by their developed nervous systems, complex behaviors, and a modified foot that forms their arms or tentacles. Octopuses specifically belong to the order Octopoda, distinguished by having eight arms.
Unique Octopus Characteristics
Octopuses have unique biological features. They are known for their intelligence, demonstrating abilities such as problem-solving, navigating mazes, using tools, and displaying individual personalities. Their nervous system is highly distributed, with two-thirds of their neurons located in their arms, allowing for independent movement and sensory processing in each arm.
Their exceptional camouflage allows them to rapidly change skin color and texture to blend with surroundings. This transformation uses specialized pigment sacs called chromatophores, which can also alter skin texture to mimic rocks or coral. As a defense mechanism, octopuses can release a cloud of dark ink to confuse predators and facilitate escape.
Octopuses possess three hearts; two branchial hearts pump blood through the gills, while a larger systemic heart circulates oxygenated blood to the rest of the body. Their blood is distinctively blue due to the presence of a copper-based protein called hemocyanin, which transports oxygen instead of the iron-based hemoglobin found in many other animals. Their arms are covered with suckers that are not only for grasping but also have chemical sensors, enabling them to taste and smell objects they touch.