Pulling water in at the back of the mantle and squirting it out of the funnel at the front propels cuttlefish fast, but these cephalopods, which are close relatives of squid, reserve this mode of motion for dire situations. They can only sustain this form of movement for brief outbursts, when the stakes are high, because jetting is extremely costly. But why is jetting so energetically expensive? Initially, scientists thought that jetting tiny volumes of high-speed fluid was inefficient, but Nicholas Gladman and Graham Askew, from the University of Leeds, UK, recently revealed that this is not the case. So, the duo switched their attention to finding out how efficient the muscles that expel jets of water from the cuttlefish's body are, to discover why jetting is so costly.
To pull this off, the duo had to measure the amount of mechanical energy generated by the muscle while virtually simultaneously measuring the amount of chemical energy, in the form of ATP, that the muscle used. After collecting tiny portions of muscle from young adult cuttlefish that had been reared in Leeds, Gladman and Askew then painstakingly measured the mechanical energy generated by the muscle as it contracted, as though propelling the cuttlefish through water. Once the muscle finished its simulated jetting workout, Gladman measured the amount of ATP in the tissue and the byproducts of the process that anaerobically produces ATP from ADP, to determine how much ATP had been used to fuel the muscle. ‘The challenge was to accurately measure the energetics in a way that truly reflects the muscle's behaviour during swimming’, says Askew.
However, when the duo compiled all of their measurements and compared the mechanical energy generated with the ATP consumption to calculate the muscle's efficiency, it was impressively high at 37%. They point out that this is in line with the efficiency of the powerful muscles that drive scallop swimming and is higher than the efficiency of dogfish and mouse muscles, so the high cost of jet propulsion is not down to muscle inefficiency burning through energy fast. The duo suspects, instead, that squirting a jet of water requires a large amount of mechanical energy to propel cumbersome cuttlefish through water, and this, rather than muscle inefficiency, makes the manoeuvre energetically costly.
