While often confused, cuttlefish and squid are distinct marine animals that share a common ancestry. These soft-bodied creatures, renowned for their intelligence and remarkable camouflage abilities, are frequently mistaken for one another due to their similar appearances. Understanding their shared biological class and unique physical characteristics helps clarify how these cephalopods differ.
The Cephalopod Family
Both cuttlefish and squid belong to the class Cephalopoda, meaning “head-foot” in Greek, as their arms and tentacles attach directly to their heads. These marine invertebrates are distant relatives of snails and clams within the phylum Mollusca. Cephalopods move primarily through jet propulsion, expelling water from a siphon, and possess highly developed senses and large brains. They are considered among the most complex invertebrates.
Within this class, cuttlefish and squid diverge into different orders. Cuttlefish are members of the order Sepiida, with over 120 species. Squid fall under the order Teuthida, with over 300 species. This taxonomic distinction highlights their evolutionary paths despite their shared cephalopod heritage.
Cuttlefish: Distinctive Traits
Cuttlefish possess a distinctive internal shell, the cuttlebone, which is highly porous. This gas-filled, chambered structure allows them precise buoyancy control, allowing them to adjust their depth. Their eyes feature large, W-shaped pupils that provide a wide field of vision.
These creatures are masters of camouflage, capable of rapidly changing their skin color and texture for camouflage and communication. This ability is due to specialized pigmented cells called chromatophores, which expand and contract under neural control, along with reflective cells. Cuttlefish typically have eight short arms and two longer feeding tentacles, which are usually retracted and only shot out to capture prey.
Squid: Defining Features
Squid typically exhibit a more streamlined, torpedo-shaped body adapted for swift movement through open waters. Instead of a rigid cuttlebone, they possess a thin, feather-shaped internal support structure known as a gladius or “pen.” This flexible structure provides axial support and muscle attachment points without hindering their agility.
Squid generally have round pupils in their large eyes, developed for excellent visual acuity, particularly for detecting movement. They possess eight arms and two longer feeding tentacles used for grasping prey. Squid can also change color for camouflage and signaling, though often less dramatically or rapidly than cuttlefish.
Unraveling the Differences
Key distinctions between cuttlefish and squid are evident in their physical forms. Cuttlefish are characterized by their broader, more flattened bodies, distinct from the elongated and conical shape of squid. Their internal support structures also differ: the porous cuttlebone of the cuttlefish, used for precise buoyancy, contrasts with the thin, flexible pen found in squid, which primarily offers body support for fast swimming.
Their eyes also differ; cuttlefish possess W-shaped pupils, while squid typically have round pupils. Cuttlefish often have a long, wavy fin along the sides of their mantle, aiding in undulating movement. Squid typically have triangular or diamond-shaped fins at the end of their mantle, providing stability and assisting in rapid jet propulsion. Despite their shared classification as cephalopods, these anatomical and behavioral traits distinguish cuttlefish from squid.