Some animals will go to great lengths to attract a mate.
But few boast the persistence of the Pacific midshipman fish, a hideous, nocturnal creature that lurks in the murky waters just above the ocean floor.
With its habitat lacking much light, the male midshipman fish lets loose a continuous hum aimed at attracting potential suitors to its nest. The mating call is quite dull, sounding like the drone of a guitar amp, though apparently it's enough to entice the females.
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But what the midshipman fish lacks in creativity, it makes up in stamina. It can maintain its distinctive call for an hour, even as the muscle fibers in its swimbladder contract up to 100 times per second.
That's 360,000 contractions during a hour-long call – more than any other known vertebrate muscle. But how does the midshipman fish keep its staying power?
Researchers at the University of Pennsylvania believe they have discovered the secret. Their findings were published this month in the Journal of General Physiology.
According to researchers, midshipman fish muscles only contract once per nerve impulse – a stark contrast from other animals, like bees, that rely on repetitive muscle action, multiple contractions per nerve impulse in extreme examples.
To trigger muscle contractions, animals release calcium ions in response to nerve impulses. The ions are typically pumped backed into intracellular storage sites, allowing the muscle to relax before contracting again.
But midshipman fish contract their muscles too quickly for this process to play out. Instead, they produce parvalbumin, a protein that mops up excess calcium from the muscle's cytoplasm and slowly pumps the calcium back into storage.
The process is similar to that of an Atlantic toadfish – a fish with faster muscle contractions but a mating call interrupted by short periods of silence. But the midshipman fish releases eight times less calcium than the toadfish's muscles, and only pumps twice as much calcium over a similar time span.
This calcium-pumping requires energy. But the low level of calcium release reduces metabolic demands, enabling the midshipman fish to continue its siren song for as long as 60 minutes.
"The small amount of calcium released per stimulus is the key element that permits the calcium pumps in the midshipman swimbladder muscle to keep up over long periods of high-frequency stimulation," lead researcher Lawrence Rome said in a statement.