Tag Archives: Midshipman fish

Amusing Monday: The secret to the midshipman’s song

Among the wonders of nature in Puget Sound is a chunky little fish with bulging eyes called a plainfin midshipman. The species includes two very different types of males, and one type tries to attract a mate by emitting a continuous humming sound for up to an hour before stopping.

An hour-long mating call is rather remarkable, considering that most animals use short intermittent bursts of sound followed by periods of rest. Until recently, scientists were not sure how the midshipman could keep its call going so long.

When large numbers of midshipman are calling at the same time, the effect can be disconcerting. Years ago, folks living near Quilcene on Hood Canal reported an eerie humming sound that kept them awake at night. Since Quilcene is located near the Navy’s acoustic-testing range on Dabob Bay, some speculated that the Navy was up to something.

Other people living along the shores of Puget Sound have reported the same strange humming sound from time to time. Midshipman appear to be the primary prey of bald eagles that congregate along Big Beef Creek near Seabeck each spring before the first salmon runs provide larger fish to eat. Do you remember the award-winning photo by Bonnie Block featured in the Kitsap Sun?

Plainfin midshipman caught in a beach seine off Bainbridge Island while surveying for surf smelt // Photo: U.S. Geological survey

Hums produced by Type-I males can be heard great distances underwater, all the better to attract mates in murky waters. The sound is created when the fish contracts and relaxes the muscles around its swimbladder, causing the gas-filled organ to vibrate.

The contractions in the midshipman are extremely rapid, up to 100 times per second, or some 360,000 times during an hour-long call, according to Lawrence C. Rome, professor of biology at the University of Pennsylvania who has published a new paper in the Journal of General Physiology.

“The midshipman swimbladder muscle generates more contractions per hour than any other known vertebrate muscle,” Rome commented in a news release from Rockefeller University Press.

Muscle contractions are triggered by the release of calcium ions into the muscle tissue. In other species, the calcium ions are pumped back into storage before the next contraction. But the speed of the contractions in the midshipman has forced researchers to look for another explanation. The secret turns out to be the tiny amount of calcium needed to cause a contraction — just one-eighth as much as in the Atlantic toadfish, a related species.

“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 said. “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.”

One mystery still remains, he added. How do such low calcium levels cause the swimbladder to contract with enough force to generate the distinctive hum heard over great distances?

For a more detailed explanation of the physiology, review the news release or read the research paper (subscription required).

The name midshipman apparently comes from having a series of photophores — light=producing organs — along its sides used to attract prey. Someone apparently thought they looked like buttons on a naval uniform, according to an entry in Wikipedia.

Midshipman fish are nocturnal, swimming just above the seabed at night and burying themselves in the mud or sand during the day. When out of water, these unusual fish have the ability to breathe air.

While type-I males use sound to attract females, type-II males have a different reproductive strategy. Their sex organs are seven times larger than those of their type-I counterparts.