How do cephalopods move by jet propulsion?

Cephalopods, such as squids and octopuses, utilize a highly efficient form of hydraulic propulsion to move through the water. Unlike most mollusks, cephalopods have evolved a highly muscular mantle that has largely replaced the rigid shell, allowing it to function as a primary locomotory organ.12

The process of jet propulsion involves the following steps:

• Intake: The mantle relaxes, causing its circular muscles to expand. This draws water into the mantle cavity through the neck region or aperture.1
• Compression: When the mantle contracts, the aperture closes via locking mechanisms and muscle contraction, preventing water from escaping backward.1
• Ejection: The water is forcibly expelled through a flexible, tube-like funnel (or siphon).12

Because the funnel acts like a jet nozzle and can be pointed in various directions, the animal gains significant flexibility for steering. By ejecting water through this siphon, the cephalopod is propelled in the opposite direction. In more advanced species like oceanic squids, a nonreturn valve further increases the efficiency of this system.12