Researchers at the University of Pennsylvania have discovered how the Pacific midshipman fish can hum continuously for up to an hour in order to attract potential mates. The study, which is featured on the cover of the January 2018 issue of the Journal of General Physiology, explains how the muscle fibers surrounding the fish's swimbladder can sustain the high rates of contraction--up to 100 times per second--that are needed to produce the animal's distinctive call. The article is titled “Small Ca2+ Releases Enable Hour-Long High-Frequency Contractions in Midshipman Swimbladder Muscle.” It can be difficult to find a mate within the dark and cloudy waters of the ocean, so the males of several fish species have evolved the ability to emit loud calls that can attract potential female partners to their nest. These mating calls are generated by superfast muscle fibers that surround the fishes' swimbladders and undergo rapid cycles of contraction and relaxation in order to make these gas-filled organs vibrate. Male Atlantic toadfish, for example, contract and relax their swimbladder muscles up to 100-200 times per second to produce short, repetitive "boatwhistle" calls interspersed with relatively long periods of silence. Type I males of the Pacific midshipman fish (Porichthys notatus) are even more remarkable, producing a continuous mating hum for as long as an hour (you can hear a brief snippet in this YouTube video (https://www.youtube.com/watch?v=vSmEsuhEDK0) a rate of 100 contractions and relaxations per second, the midshipman swimbladder muscle can therefore contract as many as 360,000 times over the course of an hour-long call. "The midshipman swimbladder muscle generates more contractions per hour than any other known vertebrate muscle, explains Lawrence C.
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