Knifefish Suck So Hard They Can Make Water 'Boil'
Biologists Victor M. Ortega-Jimenez and Christopher P. J. Sanford at Kennesaw State University have discovered that black ghost knifefish can create suction with enough speed and power to make water cavitate, a form of boiling, in laboratory settings. Cavitation creates bubbles which form and collapse in the blink of an eye, producing powerful compressional waves and extremely high temperatures in the process. It results from sudden, immense changes of pressure.
Black ghost knifefish natively dwell in the rivers of Panama and South America, but are popular in home aquariums across the world. They primarily eat riverine insects and perform most of their hunting at night. Knifefish are known for their remarkable electrical senses, and sport an electric field-producing organ used mostly for navigation and communication.
Ortega-Jimenez and Sanford knew of these fascinating traits when they purchased four black ghost knifefish at a local pet store in Kennesaw, Georgia, but the duo had a sneaking suspicion that there were more abilities waiting to be revealed.
Aware that these fish were prodigious suckers, rapidly generating low pressures in their mouths to draw in prey, Ortega-Jimenez and Sanford sought to find out just how powerful this skill was. So they used a high-speed camera to observe black ghost knifefish feeding in various conditions. They were amazed at what they saw.
"We discovered that knifefish while generating suction at the tip of an 8 mm long capillary tube (1.5 mm diameter), open to the air, induce a rapid jet of water toward the mouth which in turn can generate cavitation bubbles. Approximately 3 ms after the onset of suction, we registered a maximum flow speed and acceleration of up to ~ 7 m/s and ~ 4500 m/s^2 respectively," they wrote.
That's more than 450 times the acceleration from gravity!
Two other marine animals, mantis shrimp and pistol shrimp, are the only other species known to use cavitation for prey capture, employing crushing punches to create shockwaves.
According to Ortega-Jimenez and Sanford, the knifefish' suction resulted in "a powerful jet, a hammer-like sound, and cavitation bubbles in a capillary tube." You can watch a video of their experiments below.
"We suggest that this nocturnal and riverine fish may use cavitation to facilitate prey extraction, capture and intake of small prey, hiding inside the matrix of submerged vegetation or between microcracks of rocks," the researchers say.
Source: Ortega-Jimenez, V.M., Sanford, C.P.J. Knifefish’s suction makes water boil. Sci Rep 10, 18698 (2020). https://doi.org/10.1038/s41598-020-75788-x