Question

squid are the fastest swimmers among invertebrates. A cavity within the squid is filled with water. the mantle, a powerful muscle squeezes the cavity and expels the water through a narrow opening (the siphon) at high speed. using momentum conservation, explain how this propels the squid forward. How is the squids swimming mechanisms like a rocket engine?.

Answer 1

Squid have more than one way to get around. One way is to cruise at slow speeds using the paired fins at the rear of the mantle, combined with gentle, rhythmic pulses of water pushed out of the mantle cavity through the funnel. Squid expand the mantle cavity by contracting sets of muscles within the mantle, water fills the expanded space, the muscles relax, and the elastic mantle then snaps back to a smaller size, jetting water out through the funnel. The jet of water closes the flaps on either side of the squid's head so water can exit only through the funnel. This rhythmic flow of water is also the way in which squid breathe. As water passes in and out, the gills are refreshed with oxygen.

But let's back up. We said "squid expand the mantle by contracting sets of muscles." But how is it possible to make something bigger by making something (muscles) shorter? Muscles get shorter when they contract. Squid mantles are thick. Muscles within the mantle actually compress the thickness without collapsing the outer skin. The mantle then behaves like a syringe, and water is drawn or, more accurately, pushed in.

The second way squid move is with great bursts of speed and acceleration, which they do by recruiting other muscles to make dashes through the water. Essentially, they get more water into the mantle and they push it out more forcefully. For squid, this is like running the 100 m dash; they can swim at full speed only for short periods of time before they get worn out. But then that is usually all the time they need to evade a predator such as a tuna, shark or wahoo. Some squid may even leave the water and glide like flying fish when they are being chased by fast predators. Unfortunately for squid, this makes them available to squid-eating birds!