Growing up is never easy, especially when the world seems intent on holding you back. As every fluid dynamicist knows, the smaller you are, the more you have to struggle to overcome viscous forces. And hatchling squid are no exception. Starting out at a diminutive 0.05 g the tiny aquanauts have to overcome enormous viscous forces, but this resistance dwindles to almost nothing once the youngsters are nearly fully grown. Joe Thompson explains that`the physics of locomotion changes dramatically over a narrow range of sizes for squid'. He and Bill Kier were puzzled how the growing squid cope with the transition from a highly viscous world to the thinner world encountered by their elders. The team knew that some creatures change their body shapes dramatically as they grow in stature, but magnified hatchlings and mature adult squid appear virtually indistinguishable. Thompson and Kier decided to investigate the developing squid's jetting technique to see if anything changes as the youngsters age().
Working with squid ranging in size from 5 mm infants up to 40 mm veterans,Thompson filmed the cephalopods with high-speed video cameras to analyse their mantle jetting styles. Filming the larger animals was relatively straightforward, but capturing the tiny youngsters' movements proved much trickier; they shot out of the camera's field of view before Thompson could record their escape jet routine. Carefully tethering the tiny squid in place Thompson was able to film the animals' pumping mantles as they attempted to propel themselves forwards. Analysing the mantle pumping rates, the team realised that the smallest squid pulsed their mantles at a much faster rate than the largest animals, with some of the smallest managing 13 muscle lengths per second while the larger squid could only sustain 3 to 5 muscle lengths per second.
Thompson explains that Danny Weiss and J. Siekman had suggested several decades ago that short-pulsed jets produce more thrust than long sustained jets, and this could explain how the rapidly pumping hatchlings overcome viscosity. But how were the youngsters able to contract their mantles so much faster than their elders? Was the squid's mantle muscle changing as they matured? Bill Kier already knew that some squid modify arm muscle tissue to contract rapidly by decreasing the length of the thick myosin filaments. Could this squid have adopted the same strategy in its mantle muscle?
The team decided to take a closer look at the developing squid's mantle muscle. Turning to transmission electron microscopy, Sonia Guarda helped Thompson painstakingly prepare and section the delicate muscle samples for the high-resolution microscope. Looking at the larger squid's muscle first,Thompson and Kier saw thick filaments ranging in length from 1.2 to 3.4 μm,but when they looked at the smallest squid's thick filaments `we had to zoom way in to see them' says Thompson. The filaments were significantly shorter,ranging in length from 0.7 to 1.4 μm.
Thompson admits that modifying the thick filament length to achieve a high shortening velocity is a novel concept for muscle physiologists familiar with vertebrate muscle, which vary muscle biochemistry to raise the shortening velocity. Although it may not be a common solution to the problem, Thompson suspects that other invertebrates may also modulate their muscle function by altering the filament structure.
