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In this issue |
How Fish Hook Fish
New York, USA
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Fish suck, or at least most do when they eat, drawing food into their mouths like underwater vacuum cleaners. Still, key details of how the complicated, kinetic mix of some 60 bones and 80 muscles orchestrate activity in the head of a bony fish to suck down meals eluded biologists for decades. Given that suction feeding is found in the majority of fish species, despite wild variations in head anatomy, it must be of immense evolutionary importance. Now, using ultrasound-emitting crystals implanted in the mouth of some hungry bass, Christopher Sanford and Peter Wainwright have for the first time captured the split-second changes fish mouth cavities go through when gulping down food (p. 3445).
Suction feeding is accomplished by an explosive expansion of the cheeks, with water flowing in like air sucked into a bellows. Changes in the fish's head structure induce the sudden suction pressure, but attempts to measure the bone movements using highspeed video recordings failed, because the skull obscured the view of these moving bone structures. Since looking at the mouth from the outside wasn't working, the researchers decided to poke around inside instead.
Eschewing Jonah-esque tactics, the scientists decided to sow six crystals in the roof, tips and joints of the mouths of the North American largemouth bass Micropterus salmoides. These 2 mm wide piezoelectric ceramic grains emit high-frequency ultrasound pulses when jostled and receive ultrasound signals as well.
By using devices called `sonomicrometers' connected to the crystals with wire, Sanford and Wainwright could precisely measure distances between all the crystals by recording the time it took for signals to pass from one crystal to another. With the aid of pressure monitors embedded between the nostrils and the eyes, the scientists caught the first glimpses of how exactly the flexing mouth cavities of the bass trigger drops in pressure.
Five voracious 25 cm long bass were tossed goldfish to eat and the team watched the pressure drop. The aggressive predators gulped down their hapless prey during a 205 ms long feeding cycle that generated a ferocious 5.2 kPa of suction. Defying any known predictions, the scientists found that the suction pressure peaked 24 ms into each gulp, about 30 percent faster than anticipated, and well before the mouth cavity gaped fully open 45 ms later. Such fast drops in pressure would be key if the fish was trying to snare an escaping victim, Sanford said, and has tremendous implications for the power output of the mouth cavitys expanding muscles.
First developed in medicine to study length and shape changes in heart muscle, Sanford explains that sonomicrometry is now helping animal functional morphologists study how muscles and ligaments change shape during bird flight and fish swimming. And now it has helped refute a few ideas of how the cheeks expand — instead of puffing up into a circular shape, they mash into a flattened ellipse. These findings contradict earlier electromyographic data of muscle electrical activity, highlighting how complex the anatomical relationships that govern mouth cavity pressure must be.
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