Biology
The Mechanics of Jet Propulsion in Squid and Its Energetics
Quick fact
To escape predators, some squid can accelerate from rest to over 20 body lengths per second in a fraction of a second, but the jet that powers this burst is so costly that they may need several minutes to recover before they can do it again.
Why this is interesting
Imagine if you had to move by blowing up a balloon and letting it zip across the room. Squid do exactly that, but with a highly tuned muscular system that makes them some of the fastest swimmers in the ocean—yet it comes at a steep price.
Read the full explanation
Understanding The Mechanics of Jet Propulsion in Squid and Its Energetics
Squid are soft-bodied mollusks that have traded a shell for a powerful jet engine. Their body—the mantle—is a muscular, cone-shaped sac. To move, they first widen the mantle, drawing seawater into a central cavity. Then they slam the mantle shut and contract its walls, forcing the water out through a narrow funnel (or siphon). The funnel can be pointed in different directions, allowing the squid to steer. This jet of water pushes the squid in the opposite direction, following Newton's third law: for every action, there is an equal and opposite reaction. Imagine sitting on a wheeled office chair and throwing a heavy weight backward—you roll forward. That's the basic idea, minus the chair and the weight.
A deeper explanation
The mantle operates as a muscular hydrostatic skeleton: instead of bones, it uses the incompressibility of water to transmit force. Two sets of muscle fibers work in opposition. Circular muscles wrap around the mantle; when they contract, they squeeze the mantle, reducing its diameter and forcing water out. Radial muscles run from the inner to the outer wall; when they contract, they thin the mantle wall but increase its diameter, drawing water in. By alternating these contractions, the squid produces a pulsed jet. The funnel adds directionality and can also modulate jet diameter to control thrust. Energetically, jet propulsion is surprisingly inefficient compared to swimming with fins. The jet is intermittent, so the squid accelerates and decelerates between pulses, losing energy to drag during the slow phase. Also, accelerating a relatively large volume of water to a high speed carries a high kinetic energy cost; energy is lost as turbulence and heat in the wake. The efficiency—measured as useful thrust relative to the energy expended—is often estimated to be only 30–60%, lower than the steady undulatory swimming of fish, which can exceed 80% efficiency. To compensate, squid have evolved a second swimming mode: slow, undulatory fin swimming for cruising, reserving jet propulsion for rapid maneuvers and escape bursts. This high cost also explains why many squid are ambush predators, relying on short, explosive chases rather than prolonged pursuit.