It sounds like the drone of a guitar amplifier, but it's actually the amorous serenade of a fish called the plainfin midshipman. During the summer, this sonorous sea creature hums to attract females to its rocky seafloor love nest.
"It sounds like a drone of bees or maybe even the chanting of monks," neurobiologist Andrew Bass, who has studied these fish extensively, told LiveScience.
The hum is so loud that for years, houseboat owners in Sausalito, Calif., complained it was disrupting their sleep and drowning out conversations. Theories circulated about what was making the strange noise — sewage pumps? Military experiments? Submarines? Ultimately, scientists discovered that the plainfin midshipman (Porichthys notatus) was causing all the buzz. [See video of humming plainfin midshipman]
These fish can be found from Santa Monica, Calif., all the way up to Alaska. The name "midshipman" comes from the fact they possess light-emitting organs called photophores along their bodies for attracting prey; the photophores resemble the buttons on a naval officer's uniform.
To make their humming sounds, the fish use the gas-filled bladder that keeps them buoyant. When the fish contracts muscles on the sides of the bladder, the muscles vibrate against the wall of the bladder, which in turn vibrates the surrounding water. The result is something that sounds like a monotone didgeridoo.
And it gets even weirder: There are actually two kinds of male midshipman. There are the "singing males" that hum to attract the ladies. And then there are "sneaker males" that don't sing, but instead sneak into the singers' nests and fertilize the eggs a female has laid there. (Like many fish, midshipman reproduce by fertilizing eggs outside the body.)
The fish don't just make noise to entice a female. The males make growling and grunting sounds too, to defend their nests from intruding males.
The bizarre humming of the midshipman isn't really that unusual, according to Bass. "Sound production is extremely widespread among fishes," Bass said. Reports of fish vocalizing date back to the time of Aristotle, he added.
These fish also show seasonal changes in hearing — both males and females hear better during the summer. This makes them good models for studying human hearing loss, scientists say.