Octopuses have three hearts. When an octopus swims, two of its three hearts stop beating. The two branchial hearts, which pump blood through the gills, cease function during swimming, which is why octopuses tire quickly and prefer crawling to swimming. Only the systemic heart, which circulates oxygenated blood to the body, continues working.
Table of Contents
This is not a malfunction. It is how the system is built. And it is one of the less surprising facts about Octopus vulgaris and its relatives, which are among the most physiologically and neurologically unusual animals on Earth.
Octopus: The Distributed Brain
An octopus has approximately 500 million neurons, comparable in number to a dog. Two thirds of those neurons are not in the brain. They are distributed through the eight arms, with each arm containing its own neural cluster capable of processing sensory information and generating movement independently of the central brain.
When an octopus reaches into a crevice to search for prey, the arm is not simply executing instructions from the central brain. The arm is making local decisions, responding to tactile and chemical information from its suckers, adjusting its movements based on what it encounters, without waiting for signals to travel to the brain and back. The central brain sets the general goal. The arm figures out how to achieve it.
Researcher Jennifer Mather at the University of Lethbridge, who has studied octopus behavior and cognition for decades, has described this architecture as a fundamentally different solution to the problem of controlling a complex body than the centralized nervous systems of vertebrates. The octopus did not evolve intelligence in the way mammals did. It arrived at a comparable level of behavioral complexity through a completely different neural architecture, which makes it one of the most important animals for understanding the relationship between brain structure and cognition.
Octopus Camouflage and the Skin
The arms can continue to respond to stimuli for up to an hour after being severed from the body. A detached octopus arm will recoil from a painful stimulus, attempt to pass food toward where the mouth would be, and respond to chemical signals in the water. The neurons in the arm keep working after the connection to the central brain is gone. This distributed neural system is also found in cuttlefish, another remarkable cephalopod.
Octopus Intelligence and Play
Octopus skin contains three layers of specialized cells that work together to produce color changes of extraordinary speed and precision. Chromatophores, pigment-containing cells controlled by muscles, expand and contract to reveal or conceal color. Iridophores reflect light through structural coloration, producing iridescent effects. Papillae, small muscular projections, change the texture of the skin from smooth to spiky in milliseconds.
The combination allows an octopus to match not just the color but the texture and pattern of its background with a fidelity that makes it effectively invisible. Researcher Roger Hanlon at the Marine Biological Laboratory in Woods Hole has spent decades studying cephalopod camouflage and has documented octopuses matching backgrounds they have never encountered before, producing accurate camouflage patterns within seconds of settling on a new surface.
What makes this more remarkable is that octopuses are colorblind. Their eyes contain only a single type of photoreceptor, which should make color matching impossible. Hanlon and colleagues published a hypothesis in 2015 suggesting that octopuses may use the chromatic aberration of their eyes, the way different wavelengths of light focus at slightly different distances, to extract color information by adjusting pupil size and focal length. The hypothesis remains debated, but the camouflage itself is not. An octopus that cannot see color is producing color-accurate camouflage through a mechanism that science has not yet fully explained.
Octopuses solve puzzles, open jars, navigate mazes, and recognize individual human faces. These capacities have been documented in controlled laboratory settings and in field observations. What has been harder to document, and more controversial to claim, is whether octopuses play.
Mather and colleague Roland Anderson published observations in 1999 of octopuses repeatedly releasing pill bottles into a current and catching them as they drifted back, behavior that served no obvious foraging or survival function and that the researchers interpreted as play. The interpretation was contested, but subsequent observations of similar apparently non-functional, repetitive behavior in octopuses have accumulated to the point where the play hypothesis is taken seriously by researchers who study animal cognition.
The lifespan of most octopus species is one to two years. Octopuses are semelparous, reproducing once and dying shortly after. A female common octopus guards her eggs for weeks without eating, aerating them with her arms and cleaning them until they hatch, then dying. The intelligence that researchers have documented in octopuses is packed into an animal that lives less than two years and leaves no parental knowledge to its offspring. Whatever cognitive capacity the octopus has, it develops entirely within a single short lifetime, without cultural transmission, without learning from parents. That fact alone makes the octopus one of the most scientifically provocative animals studied in animal cognition.