Longest Mating Call Ever: How Male Midshipman Fish Hum To Attract Females

Humans do a lot to find a mate, oftentimes going out of their way to please potential partners. But, however desperate your worst bar "hitting-on" story is, this fish will one-up you. The Pacific midshipman fish hums for up to an hour just to attract potential mates.

Now a team of researchers, have found the mechanism that gives these fish their amazing vocal stamina that helps them woo the ladies.

The study found that 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 mating call.

The murky ocean waters is no place to be finding a potential mate. With such low visibility, it’s a surprise that these fish even try and look for females. To counter this visibility problem, several male 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 fast twitch muscle fibers around the swimbladders. These fibers move in a very cyclic but rapid contraction and relaxation patterns to make the bladders vibrate, producing the sound. Male Atlantic toadfish, for example, contract and relax their swimbladder muscles up to 100-200 times per second to produce short, repetitive "boatwhistle" calls, but there are long periods of silence to recuperate energy.

This is where the ever persistent Type I males of the Pacific midshipman fish (Porichthys notatus) score a one-up. Their hum lasts for as long as an hour. Their swimbladders vibrate at a rate of 100 contractions and relaxations per second.

Wrap your head around this, the midshipman swimbladder muscle can 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. Rome, Professor of Biology at the University of Pennsylvania in a press release.

Muscle contractions are triggered by calcium ions that are released from storage sites in the cytoplasm of muscle fiber cells which are released when there is a nerve impulse from the fish’s brain. But, these ions need to be sent back to relax the muscle before it is released again for contraction and so on.

But with the midshipman fish. There is no time for this pumping phase to be completed before the next contraction begins.

Scientists already knew that the Atlantic toadfish produces large amounts of a protein called parvalbumin that basically cleans up the ions from the release site but this clean up still happens when the fish shuts up.

They found that the levels of parvalbumin in the Pacific midshipman isn’t as high as the toad fish. Then where does it get it’s stamina from?

To figure out the mechanism behind this amazing muscular feat, the team measured the amount of calcium released and pumped by midshipman swimbladder muscles as they repeatedly contract and relax.

The researchers found that in the midshipman muscles only contract once per nerve impulse. In nature, several species produce more than one contraction for a single nerve impulse like in the bee's wing muscles. But the midshipman counters this by releasing only small quantities of calcium, approximately eight times lesser than toadfish muscles, in response to a single nerve impulse.

Thus, to sustain an hour-long mating call, the midshipman muscle releases, and subsequently pumps, only two times as much calcium as the toadfish uses to make mating calls that last just a few seconds at a time.

"The small amount of calcium released per stimulus is the key element that permits the calcium pumps in midshipman swimbladder muscle to keep up over long periods of high-frequency stimulation," Rome explained in the report.

The low levels of calcium release also lower the metabolic demands of making a continuous, hour-long mating call.

"The combination of fast calcium pumping and small calcium release permits the midshipman to maintain the correct balance of calcium ions during its long-lasting mating call," Rome says.

The team says that further study is required to understand how these low calcium levels cause the swimbladder muscle to contract with sufficient force in the first place to even produce sound.

The study was published in the January issue of the Journal of General Physiology.