Locomotion

by Diana Li

Like many animals, squids depend on movement in order to find food, escape predators, migrate, and reproduce. At first glance, a squid doesn't have familiar features such as a fish's tail or shark's fins that would help it swim. So how do squids move, or locomote, through the water?

Humboldt Squid swimming in captivity. Video by W. Gilly.

Looking at the Humboldt squid swimming in the video, we immediately notice that it's flapping a pair of fins. Upon closer inspection, we notice its mantle and siphon contracting in order to produce jets. Just like any animal that moves, squids have to produce some sort of force in order to locomote, and that force is termed thrust. But thrust is not the only force at play. In general, another force called drag resists the animal's movements. Plus, many squid species are negatively buoyant, meaning they are heavier than seawater and would sink. Squids generate another force called lift to prevent themselves from sinking due to their weight. In order to combat drag and sinking, squids use a unique combination of jet propulsion and fin activity to swim.

To jet, a squid fills its mantle cavity with seawater through openings around the head (dashed arrows). When discussing the external anatomy during the dissection, be sure to gently poke around the mantle opening to identify the intake region. The squid then contracts the muscles in its mantle to push the water out of its siphon, sometimes called the funnel, as a jet (solid arrow). Then, the mantle expands back to its resting size while sucking water back in, and the jetting cycle repeats.

The dashed lines show where water is taken into the mantle. The solid lines show where water is expelled from the siphon. Figure by D. Li.

The siphon is visible as a small tube coming out from the mantle on the ventral side. Besides the jet of seawater, ink from the ink sac and poop from the intestinal tract are released through the siphon too (at the appropriate times). The thrust from the jet of water pushes the squid in the opposite direction, much like a rocket ship or a jetpack. But, the squid doesn't always have to move backward (fins-first). The siphon is very flexible, and it can aim in any direction underneath the squid, so the animal can travel arms-first, upward (to generate lift), downward, or at any angle in between.

Because the jet can be very powerful, jetting is a great way to accelerate and swim quickly. In fact, the escape response in squid consists of a single powerful jet that hopefully gets the squid away very quickly from a scary predator. A failed escape response for the squid can result in a delicious snack for a predator, so the jet has evolved to be as fast and powerful as possible. In theory, the jet could be even stronger if the mantle cavity could hold more seawater to be pushed out of the siphon. However, a bigger body to hold more seawater would result in increased drag and actually slow down the squid. So it's no surprise that many species of fast-moving squids are torpedo-shaped.

But what about slower, more complex movements? That's where the fins come in. A squid can flap and undulate its fins to generate lift and create fine maneuvers that provide a smaller amount of thrust. Fin shape and size vary greatly between squid species. The Humboldt squid, a strong swimmer in the open ocean, has muscular triangular fins that mainly flap, much like a bird's wings, often to add thrust to a jet. The fins of a smaller species, the bigfin reef squid, extend along the length of the mantle in a band more suitable for undulation. Because the bigfin reef squid must navigate between reefs and rocks in their habitat, fins provide more control for small movements. The shape and size of a squid's fins gives us clues to how they're used during swimming and where the squid lives in the ocean.

Sepioteuthis, the bigfin reef squid, swimming in capitivity. Video by D. Li.

During fast swimming, a squid typically wraps its fins around the mantle in order to reduce drag if it's not currently using them. Similarly, the arms and tentacles are all gathered tightly together so nothing is trailing behind. By pulling its appendages close to its body, the squid becomes more streamlined - just like the torpedo shape mentioned before - to decrease drag on its body. But drag isn't always a bad thing. The squid can splay its arms and tentacles out while swimming in order to brake and slow down, just like releasing a parachute.

Squids possess a huge range of swimming behaviors thanks to the combination of speedy jet propulsion and controlled fin activity. But did you know that squid swimming isn't always underwater? Scientists have documented squid "flight" since the 1950's and just recently provided evidence that it's more than just passive gliding in the air like flying fish do. Certain species of squid, including the Humboldt squid and the neon flying squid, can jet so forcefully near the ocean's surface that they fly out of the water. While in the air, squids typically jet air a few times and also flap their fins to keep moving. They fan out their arms and tentacles into a posture that aids in staying airborne. Some neon flying squid are able to travel over 30 meters during 3 seconds in the air - that's faster than Olympic champion Usain Bolt! Though the reason squid take to the air is still unclear, scientists believe it could be an extreme version of the escape response to get away from an underwater predator.

Click here to download a classroom activity that explores squid locomotion (without needing actual squid)! Students will use what they've learned about squid swimming to construct balloon squids and race them.

References

O'Dor, R. K. The incredible flying squid, New Scientist, 214:2865, 39-41, (2012). doi:10.1016/S0262-4079(12)61301-3

Gosline, J. M. & DeMont, M. E. Jet-propelled swimming in squids. Scientific American 256, 96-103 (1984). pdf