In a frankly heroic demonstration of unrequited love, the male Pacific midshipman fish (Porichthys notatus) tries to attract mates by vibrating its swimbladder and humming continuously for up to an hour.
In a paper in the Journal of General Physiology, a team led by biologist Lawrence Rome of the University of Pennsylvania, Philadelphia, US, reports that the midshipman creates a mate-signalling hum by making its swimbladder muscle contract 100 times every second – or 360,000 times during its hour-long call.
“The midshipman swimbladder muscle generates more contractions per hour than any other known vertebrate muscle,” says Rome.
Making noises in order to attract a partner is a strategy used by several species of ocean fish – an adaptation, biologists believe, to the turbid and murky conditions of the water that reduce the effectiveness of visual signals.
Most species, however, make noises only intermittently – a behaviour engendered not by piscatorial eloquence but the risk of calcium overload.
Muscle contractions are triggered by the release of calcium ions into muscle fibre cytoplasm. In most muscle systems, the ions are then pumped away, back into storage, as the fibre relaxes again. In fish with vibrating swimbladders, however, contractions happen too rapidly to allow the calcium to be removed between each action.
Rome and his colleagues discovered that another noise-making fish – the Atlantic toadfish – solves the problem using two strategies. First, it makes its mate-seeking appeals only intermittently, remaining silent for long periods in between. Second, it produces a protein called parvalbumin that mops up the excess calcium and pumps it away into storage while the toadfish is silent.
Such a strategy wouldn’t work for the midshipman, for the simple reason that, for an hour at least, it never shuts up.
Rome and his colleagues found that the fish does produce parvalbumin, but in much lower quantities than the toadfish.
They also found that its swimbladder muscle contacts only once per nerve impulse (in contrast to bees, for instance, which have flight muscles that contract hundreds of time per impulse), and that it used roughly eight times less calcium per contraction than the toadfish.
Over the course of an hour, therefore, its continuous hum needs only twice the amount of calcium the toadfish uses to produce its intermittent sounds. This not only reduces the disposal problem, but also keeps the total metabolic cost of non-stop horny humming to a minimum.
“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,” says Rome.