Fish
Plainfin Midshipman
Porichthys notatus · also called plainfin midshipman, singing fish, California singing fish
The plainfin midshipman is a stout, big-headed fish of the Pacific coast with two claims to fame. The males hum — a loud, continuous drone kept up for hours to attract mates, once mistaken by a houseboat community for a broken pump. And the fish carries rows of light organs along its belly, arranged so neatly that they reminded early naturalists of the buttons on a naval uniform.
Overview
Plainfin midshipmen live in the eastern Pacific from Alaska to Baja California, from the intertidal zone to a few hundred metres deep. They grow to about 15–25 cm, olive-bronze, with a broad mouth and small high-set eyes. They ambush small fish and invertebrates at night.
Buttons of light
Hundreds of small light organs (photophores) run along the underside of the body in orderly rows, like the brass buttons on a midshipman’s uniform — the source of the name. They provide counter-illumination camouflage and may also feature in courtship.
The hum
A nesting male contracts sonic muscles against his swim bladder up to about 100 times a second, producing a steady, monotone drone for hours on end to advertise his nest. Smaller sneaker males do not hum; they invest in sperm and dart in to fertilise eggs when the resident male is busy.
The Sausalito mystery
In the 1980s, houseboat residents in Sausalito were kept awake by a mysterious hum blamed on pumps and power lines. It was a chorus of courting midshipmen under the boats, amplified by the hulls.
Status
The plainfin midshipman is Least Concern, common and stable. In the breeding season a female’s hearing re-tunes to the frequency of the male’s hum, then returns to normal.
Plainfin Midshipman: common questions
How does the plainfin midshipman make its humming sound?
The male has a pair of specialised sonic muscles attached to the wall of his swim bladder, the gas-filled organ most fish use for buoyancy. By contracting these muscles very rapidly and continuously, up to around 100 times per second, he makes the swim bladder vibrate and produce a steady, droning hum at a low pitch. Unlike the brief grunts and pops most sound-producing fish make, the midshipman's hum is sustained, going on for many minutes and often for hours without a break, and a group of males calling together under a rocky shore creates a loud, buzzing chorus that carries into the air and through boat hulls.
Why is it called a midshipman?
Because of its lights. The fish has hundreds of small light organs, called photophores, arranged in neat rows along the underside of its head and body. To nineteenth-century naturalists these evenly spaced glowing spots looked like the double rows of brass buttons on the dress uniform of a naval midshipman, a junior officer, and the name stuck. The plainfin midshipman is one of the few fish that lives in shallow coastal water yet is genuinely bioluminescent, and the light is thought to help hide its silhouette from predators looking up and may also play a role in courtship.
What actually happened in Sausalito?
Through the 1980s, residents of the houseboat community in Sausalito on San Francisco Bay reported being kept awake in the warmer months by a persistent, droning hum that seemed to come from the water beneath their homes. Explanations ranged from sewage pumps and electrical equipment to more exotic ideas. Investigation eventually traced the noise to male plainfin midshipmen, which move into the shallows to nest under rocks and floating structures in summer and hum through the night to attract females. The houseboat hulls acted as sounding boards, amplifying the chorus into the living spaces above.
Do the fish guard their eggs?
Yes, the large nesting males do. A male excavates a nest chamber under a rock in the intertidal zone and calls until a female enters and attaches her eggs to the underside of the rock. She then leaves, and the male stays to guard the eggs and, later, the newly hatched young, fanning them with his fins to keep water moving over them and driving off predators. He can remain with the nest for weeks, and because the tide drops the nest above water twice a day, he is able to survive several hours of air exposure by taking up oxygen through his skin and the lining of his mouth.