Archerfish mouth is the secret of precision spit

Shooting gobs of water at targets requires quick changes and fine control

archerfish spitting

SPIT POWER  An archerfish can weaponize water into a high-impact blob thanks to subtle variations in its mouth opening. 

Ingo Rischawy (Schuster lab, University of Bayreuth)

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When archerfish hunt by precision spitting water into the air, they do a lot more with their mouths than put their lips together and blow.

If a Toxotes archerfish can’t jump high enough to snatch an insect off an overhanging leaf, the fish spits a water stream upward that knocks prey off its perch. In a quirk that puts extra punch into the water blast, the fish fires the trailing end of a water stream faster than the water released at the beginning.

Archerfish in the lab have revealed that they tailor the extra punch depending on the height of their target, says physiologist Stefan Schuster of the University of Bayreuth in Germany. The fish blasted several of his hypotheses about how they manage their sophisticated spitting. But high-speed videos of the target shooters finally linked water speed control to split-second changes in the fish’s mouth openings, Schuster and Bayreuth colleague Peggy Gerullis report September 4 in Current Biology.

So far, human-made fluid spitters, such as the nozzles in inkjet printers, don’t have refinements such as variable-size openings, says Alberto Vailati at the University of Milan. He has also studied archerfish and he says the new study calls for some fish-based technological innovations.

In a color video, an archerfish shoots a water stream upward to knock prey off a leaf. A black-and-white video shows slow-motion that the shape of the water stream changes continuously as the fish manipulates it.

  Ingo Rischawy/Univ. of Bayreuth; Gerullis et al. Curr. Bio. 2014.

Spit speeds matter to archerfish because the variation produces a stream with a fat glob of water at the front that maximizes wallop on the target. (The glob forms when the faster water toward the end of the stream catches up with the slower spit at the leading edge.)

A 2012 paper indicated that fish shooting at a nearby target could time their spit so that the water glob forms shortly before the stream smacks prey. Schuster and his colleagues now report that the fish orchestrate glob formation right before impact regardless of the distance to the target.

Pondering how fish managed to form globs at the proper distance, Schuster and his colleagues tested for additives such as fish mucus, which can change water viscosity. “After very much effort, the result is absolutely zero,” Schuster says. The archerfish use slime on their scales but not in their spit.

Researchers also checked to see if the fish were making head movements that might adjust for different heights. After much video analysis, again the answer: apparently not.

Training an archerfish to perform for a high-speed camera takes about a year, Schuster says. Getting fish to spurt water at targets is the easy part, but he also needed them to spit only from the small zone where the cameras focused and to ignore the glaring lights.

Pairs of videos of fish faces and the streams they were spitting did reveal complex mouth motions. Opening and closing speeds, for example, changed depending on the height of the target. Plus, he saw evidence of continuous shaping of the water stream as the shot varied in cross section.

Now Schuster is interested in how the demands of nuanced spitting may have influenced the considerable cognitive powers of archerfish. And the new research, he says, means that archerfish need to be added to the club of animals that use tools. The fish aren’t just appropriating an object (water, in their case) to do a job; they’re also modifying it to improve its function.

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