Biology
The mechanics of jet propulsion and mantle contraction in squid
Quick fact
A squid can accelerate from rest to over 10 body lengths per second in less than a second by contracting its mantle muscles to expel water through a funnel—a feat that relies on the principle of action-reaction (Newton's third law).
Why this is interesting
Imagine being able to shoot through the water by squeezing a balloon—squid do exactly that, and with incredible finesse. But there's a catch: this powerful propulsion comes at a high energy cost. Let's see how it works.
Read the full explanation
Understanding The mechanics of jet propulsion and mantle contraction in squid
A squid's body is essentially a muscular tube (the mantle) with an opening at one end that can be narrowed by a movable nozzle called the funnel. To move, the squid first expands its mantle—like a balloon being filled with air—drawing seawater in. Then, it rapidly contracts the mantle muscles, squeezing the water out through the funnel. Because the water is forced backward, an equal and opposite force pushes the squid forward. This is exactly the same principle that propels a rocket or an inflating party balloon that you release. The funnel is versatile: by rotating it, the squid can steer in any direction, including backward, which is its default swimming direction. This is an efficient way to make a quick getaway, which is critical for a soft-bodied animal with many predators.
A deeper explanation
The mantle is a powerful hydrostatic skeleton. It contains two main layers of muscle fibers: circular muscles that run around the body, and radial muscles that run across its thickness. When the radial muscles contract, they thin the mantle wall, causing the mantle to expand outward—this is the intake (filling) phase. When the circular muscles contract, they constrict the mantle, increasing the internal pressure and forcing water out through the funnel—the jet phase. This cycle of expansion and contraction creates a series of water jets that propel the squid forward. The pressure generated can be very high, allowing for rapid acceleration. However, this mode of movement is energetically costly. Each jet involves accelerating a large mass of water, but much of that energy is lost to turbulence and the cyclic nature of the thrust. In contrast, fish swim using a continuous, undulating motion that is more energy-efficient for steady cruising. Squid compensate by using jet propulsion mainly for short bursts, such as catching prey or escaping danger. They also have a pair of fins that they use for gentle, efficient cruising, reserving jet propulsion for when speed is essential. This trade-off shapes many aspects of squid biology, including their growth rates, foraging strategies, and how they interact with their environment.