The octopus, a soft-bodied invertebrate belonging to the class Cephalopoda, possesses one of the most diverse repertoires of movement in the ocean. Its eight highly flexible arms and a muscular body allow it to seamlessly shift between modes of travel, from explosive bursts of speed to slow, deliberate crawls. This versatility enables the octopus to survive in varied environments, facilitating stealthy hunting and rapid escape from predators.
High-Speed Movement: Jet Propulsion
When an octopus needs to move quickly, such as escaping a threat, it employs jet propulsion. This mechanism centers around the muscular mantle cavity, which the animal rapidly fills with water. Radial muscles expand the cavity to draw in water, followed by a forceful contraction of circular muscles.
The water is then expelled through the siphon, a narrow, tube-like structure. This expulsion generates a strong backward thrust, propelling the octopus forward according to Newton’s third law of motion. Though effective for rapid acceleration, this method is physiologically inefficient because the pressure required to generate the jet briefly stops the animal’s heart from beating.
The siphon is not fixed, allowing the octopus to articulate it and direct the jet stream in various directions. Pointing the siphon forward results in backward movement, which is the most common use for escape. It can also redirect the siphon to achieve forward movement or adjust its trajectory for steering. This rapid burst of speed is generally reserved for short-distance travel or emergency situations.
Locomotion on Substrate: Crawling and Walking
Most octopus movement occurs on the seafloor, utilizing articulated arms for crawling and walking across the substrate. This movement relies on the coordinated elongation and shortening of the arms to push the body forward. Octopuses often prefer to use a subset of their eight arms, sometimes six, for this type of propulsion.
The hundreds of suckers lining each arm create temporary anchor points through suction to gain traction. Each sucker can be controlled individually, allowing for precise grasping and release of the substrate. Suckers generate suction by contracting radial muscles within the cup-like structure, lowering the pressure inside.
Some species, such as the veined octopus and the algae octopus, exhibit bipedal locomotion, effectively walking on two arms. In this specialized gait, the octopus elevates its body, using two arms for propulsion. The remaining six arms are curled and held in positions that can aid in camouflage or mimicry.
Controlled Movement: Swimming and Steering
For slower, deliberate travel in the water column, the octopus uses controlled swimming that prioritizes maneuverability over speed. This movement is achieved through the gentle undulation of the webbing between the arms or by using the arms in a synchronized sculling motion. The arms open slowly in a recovery stroke and close quickly in a power stroke to generate forward thrust.
The octopus’s body is supported by a hydrostatic skeleton, a system of muscles and connective tissue lacking bones. This flexibility allows for fine-tuned control over body shape and arm curvature. It enables the animal to adjust the angle and force of its arm movements for steering and depth changes.
Steering during slow swimming can involve asymmetric movements, where some arms generate more thrust than others. This fluid movement contrasts sharply with the straight-line nature of jet propulsion. The ability to switch between high-speed jetting and slow arm swimming provides the octopus with tactical flexibility.
Specialized and Unusual Forms of Travel
Beyond crawling and jetting, octopuses have developed specialized movements for defensive or ecological purposes. The mimic octopus, Thaumoctopus mimicus, impersonates other marine animals by adopting specific body shapes. It can flatten its body and glide above the sand, trailing its arms to imitate a toxic flatfish like the banded sole.
In a defensive display, the mimic octopus may retreat into a burrow, leaving two exposed arms undulating to resemble a venomous sea snake. This impersonation relies on specific arm movements paired with rapid color changes.
The veined octopus displays “stilt walking” when carrying stacked coconut shells for shelter. It holds the shells beneath its body with two arms and progresses using an awkward gait supported by the remaining stiffened arms. Many bottom-dwelling octopuses are adept at burrowing, utilizing muscular contractions to displace sand and conceal themselves.