Squids and octopuses are related, both belonging to the biological class Cephalopoda. This group of marine mollusks also includes cuttlefish and nautiluses, and they share fundamental anatomical and physiological traits.
Shared Cephalopod Ancestry
Cephalopods, meaning “head-foot” in Greek, describes their body plan where arms or tentacles attach directly to their heads. All cephalopods are marine animals, characterized by bilateral body symmetry and a prominent head. They possess advanced nervous systems with a highly developed brain, making them some of the most intelligent invertebrates.
These creatures use jet propulsion for movement by drawing water into a muscular cavity, called the mantle, and expelling it forcefully through a siphon. Most cephalopods also have specialized pigment-filled cells called chromatophores, which allow for rapid changes in skin color and texture, aiding in camouflage and communication. Many cephalopods can release a cloud of ink as a defensive mechanism to deter predators.
Distinguishing Features
Squids and octopuses exhibit notable differences in their physical characteristics, behavior, and habitats. Their body shapes are a primary distinction; octopuses have a soft, rounded, or bulbous mantle, allowing them to squeeze into tight spaces. Squids, in contrast, possess a more elongated, torpedo-shaped body with fins on their mantle, designed for efficient swimming.
Another difference lies in their appendages. Octopuses have eight arms, which are covered in suckers along their entire length. These arms are highly flexible and enable them to crawl, manipulate objects, and explore their environment. Squids, however, have eight arms and two longer, specialized tentacles, which have suckers only at their tips and are primarily used for capturing prey.
Their internal support structures also vary. Octopuses lack a hard internal skeleton, with their only hard part being a chitinous beak. This absence of a rigid structure contributes to their remarkable ability to fit through very small openings. Squids possess an internal, feather-shaped structure called a gladius or “pen,” which provides support along their body and helps maintain their streamlined shape for fast swimming.
Differences extend to their locomotion and preferred habitats. Octopuses primarily crawl along the seafloor using their arms, though they can also use jet propulsion for bursts of speed. They are bottom-dwellers, favoring rocky crevices and dens for shelter.
Squids are faster swimmers, relying on jet propulsion and their fins for efficient movement through the water column. Most squid species inhabit the open ocean, often forming schools. This leads to variations in their social behavior, as octopuses are solitary, while many squid species exhibit schooling behaviors.