Octopuses are marine creatures, often mistakenly grouped with fish due to their aquatic habitat. Despite living in the ocean, octopuses are not fish. They belong to a distinct group of animals with unique biological characteristics. This article will clarify the differences by exploring the defining features of fish and then detailing the unique traits of octopuses.
What Makes an Animal a Fish?
Fish are aquatic vertebrates, meaning they possess a backbone or spinal column. They typically breathe underwater using gills, which extract oxygen from the water as it passes over them. Most fish also have fins, which are specialized appendages used for propulsion, steering, and maintaining balance in the water. For example, a caudal fin (tail fin) provides forward movement, while pectoral and pelvic fins assist with steering.
Fish are cold-blooded, or ectothermic, meaning their internal body temperature fluctuates with their environment. Their bodies are usually covered in scales. A notable internal feature in most bony fish is a swim bladder, a gas-filled organ that helps them control their buoyancy and maintain depth without expending constant energy. Sharks and rays, which are cartilaginous fish, do not possess a swim bladder and rely on active swimming or oil-filled livers for buoyancy. Common examples include tuna, salmon, and cod.
Unpacking Octopus Characteristics
Octopuses are invertebrates, meaning they lack a backbone or internal skeleton. Their soft and highly flexible bodies allow them to squeeze through incredibly small spaces. Instead of fins, octopuses possess eight arms, each lined with powerful suckers that aid in movement, hunting, and manipulating objects. Their main propulsion comes from jetting water through a siphon.
Their circulatory system includes three hearts: two branchial hearts pump blood through the gills, while a single systemic heart circulates oxygenated blood to the rest of the body. Their blood is blue due to the presence of hemocyanin, a copper-based protein that transports oxygen, unlike the iron-based hemoglobin found in vertebrates.
Octopuses exhibit intelligence and a complex nervous system, with a significant portion of their neurons located in their arms, allowing for independent arm movements and problem-solving abilities. They are masters of camouflage, capable of rapidly changing their skin color and texture using specialized cells called chromatophores to blend with their surroundings or to communicate. When threatened, they can also expel a cloud of ink to disorient predators.
Octopus: A Mollusk, Not a Fish
Fundamental biological differences distinguish octopuses from fish, placing them in separate branches of the animal kingdom. Octopuses belong to the phylum Mollusca, a diverse group of soft-bodied invertebrates that also includes snails, slugs, clams, and oysters. Within Mollusca, octopuses are classified under the class Cephalopoda, a name derived from Greek words meaning “head-foot,” which describes their anatomy where arms are directly connected to the head. This class also includes other intelligent marine animals like squids, cuttlefish, and nautiluses.
The distinction between an invertebrate octopus with multiple hearts, blue blood, and eight suckered arms, versus a vertebrate fish with a backbone, red blood, and fins, is significant. Fish typically rely on fins and a swim bladder for movement and buoyancy, while octopuses use jet propulsion and their muscular bodies. These anatomical and physiological disparities highlight that octopuses and fish represent distinct evolutionary paths in the aquatic world. The classification of octopuses as cephalopod mollusks, rather than fish, accurately reflects their unique biological makeup and evolutionary history.