The squid is a swift-moving invertebrate belonging to the class Cephalopoda. While its arms, tentacles, and ability to change color are often discussed, the squid possesses a surprising internal structure known as the pen. This internal feature provides insight into the squid’s ancestry and modern capabilities, acting as a testament to its long evolutionary journey.
What Exactly Is the Squid Pen?
The squid pen, technically referred to as the gladius, is a stiff, yet flexible structure that runs the length of the animal’s body. It is not a bone, but rather a translucent, lightweight, and semi-rigid internal rod that resembles a feather or an old-fashioned quill pen. The pen is primarily composed of beta-chitin, a polysaccharide also found in the exoskeletons of insects and crustaceans, combined with structural proteins. The pen’s composition is non-mineralized, meaning it does not contain the calcium carbonate that makes up the external shells of many other mollusks.
Locating the Pen Inside the Mantle
The pen is situated internally, running along the dorsal side of the squid, which is the animal’s back. It is embedded just beneath the outer layer of the muscular mantle, the main body tube that contains the squid’s organs. The pen extends longitudinally, starting from the neck area and continuing almost to the tip of the tail fins. This positioning places the pen along the midline of the mantle cavity. Its location acts as an anchoring point within the fluid-filled body, providing a degree of internal framework.
Function and Evolutionary Significance
The pen’s primary purpose is to act as an internal support structure for the squid’s soft body. It provides necessary rigidity to the mantle, maintaining the streamlined, hydrodynamic shape essential for rapid movement through the water. The pen also serves as a crucial attachment point for powerful mantle muscles used in jet propulsion. This allows the squid to quickly draw water into its mantle cavity and forcefully expel it through a siphon for high-speed locomotion.
Evolutionarily, the pen is a vestigial remnant of the external shell present in the squid’s ancient mollusk ancestors, like the modern-day nautilus. This reduction from a heavy, external shell to a lightweight, internal structure is a key adaptation that allowed the squid lineage to become active, high-speed marine predators. The gladius offers stiffness for muscle function without sacrificing the speed and flexibility afforded by a soft body.
Preparing Squid: Removing the Pen
When preparing squid for cooking, such as in the case of calamari, the pen must be removed from the mantle tube. It is considered inedible due to its tough, plastic-like texture and chitin composition. The removal process is simple because the pen is not deeply integrated with the muscle tissue.
To remove it, grasp the small, pointed end of the pen, often found near the opening of the mantle where the head and tentacles were attached. A gentle, steady pull is sufficient to slide the entire feather-shaped structure out in one piece. If the pen breaks during removal, check inside the tube to ensure all fragments are removed before cooking.