Midshipman

Midshipmen belong to the family Batrachoididae of toadfishes, a group of benthic ambush predators known for their ability to produce sound with their swim bladders. There are fifteen species of midshipman, all in the genus Porichthys, Greek for "porous fish." They are distinguished by hundreds of luminous spots, called photophores, that decorate their underbellies, and are somewhat reminiscent of the buttons on a naval officer's uniform, hence the name. The glow of these jewel-like photophores comes from bioluminescent bacteria. Midshipmen likely use this bioluminescence to attract small prey, such as krill, and to hide from predators through a camouflage technique known as counter-illumination: producing light to match an illuminated background, such as the ocean surface or sky.

Our own local resident is the Atlantic midshipman, Porichthys plectrodon. Atlantic midshipmen are similar to Gulf toadfish (plus the photophores). They have large heads and mouths with eyes on top of the head and mostly brownish bodies with dark splotches and yellowish bellies. They have sharp spines on the sides (directly behind the gills) and on the first dorsal fin. These spines are venomous, and though they can deliver a painful sting, are ultimately harmless to humans. Atlantic midshipmen occur from Virginia to Argentina and exhibit variation in color pattern and meristics throughout this range. Meristics is an area of ichthyology which relates to counting quantitative features of fish, such as the number of fins or scales (the midshipman would be limited to fin counts because it is scaleless). They also show microgeographical meristic variation between the bays and the Gulf on the Texas coast. They grow up to about eight inches, are nocturnal, and as bottom-dwellers, have a preference for sandy or mud bottoms. During the day, they bury themselves in substrate. At night, they float just above the seabed, foraging on planktonic crustaceans, larval fishes, and the occasional squid.

Midshipman fish come in three genders: females, type I males, and type II males. All three types are vocal, emitting short grunts to communicate with one another, but type I males are the most loquacious by far. In the spring and summer, type I males head to shallow waters, excavate nests beneath rocks along the shoreline, hunker down, and start to sing. By vibrating muscles against the sides of the swim bladder, which in turn vibrate the surrounding water, the midshipman can hum for up to an hour at a time. This amorous serenade which has been described as sounding like chanting monks, a droning motorboat, or an orchestra of mournful oboes is so loud that it has been known to wake people at night, drown out conversations, and even vibrate houseboats. While their vocal talents may not be appreciated by everyone, the songs of type I males entice females into laying eggs in their rock nests. But they also attract an unwelcome guest...

Only type I males hum. They are the "singing males." Type II males are the "sneaker males." Type II males are the size of females, eight times smaller than type I males, BUT their reproductive organs are seven times larger than those of type I males. Since they so closely resemble females, type II males can sneak into the type I male's nest and fertilize the eggs a female has laid in and out before the singer even knows he's been cuckolded.

Although the midshipman can be kind of a nuisance, research surrounding their vocalizations could be beneficial to humans. Midshipmen are model organisms for studying both human speech and hearing because these fish have homologous brain structures to humans. One study traced the development of the midshipman's nervous system and brain and found that the fundamental structure of the brain circuit was remarkably similar to neural circuits in amphibians, birds, and mammals, an evolutionary connection that could prove important in modern medicine.

Midshipman fish can decrease their own hearing sensitivity by stiffening their inner ear hair cells while they are vibrating their calling muscles. This involuntary response is also found in crickets, monkeys, bats, and even humans (to a lesser degree), but it's not clear how this happens. All vertebrates have a nerve connection between the brain and ear that is similar to that found in the midshipman, so it is probable that they all use the same mechanism to adjust their hearing.

Humans also have a second mechanism to protect their ears when exposed to loud noise: a reflex tightens muscles in the inner ear to stiffen the eardrum and inner ear bones so they become less efficient at transmitting sound. But this response gets weaker with repeated exposure to noise, and only protects from short-lived sounds. It also cannot protect from noises that reach the ear through the bones in the head. The mechanism identified in the humming fish can. At the same time that a male midshipman's brain sends a "sing now" cue to the muscles surrounding the swim bladder, it sends electrochemical messages to the ear hair cells, essentially telling them to put in earplugs. These two types of signals happen in sync about 100 times every second. Since all vertebrate brains have similar living links to the ears, it's possible that other animals like echolocating bats, barking dogs, and human rock stars might rely on related acoustic strategies to protect and preserve their hearing when they are making loud sounds.

Midshipmen also show seasonal changes in hearing. As the breeding season approaches, sex hormones flood the bloodstreams, which in turn changes their singing and hearing. Female midshipman fish are most attuned to the males' mating calls when their estrogen levels are high. Similarly, the most successful male baritones have the highest levels of testosterone. Studying these changes could help scientists understand if a person's hearing declines as he/she gets older in part because of waning hormone levels.

Because toadfish first evolved so long ago, their particular neural circuit design is likely 400 million years old, almost as old as vertebrates themselves. Though different groups of animals evolved various designs of this basic neural prototype over the course of eons and developed diverse systems of muscles and tissues for vocal communication our own speech and song, nonetheless, owe a lot to the ancient grunts and hums of midshipman fish.

Where I learned about midshipman fish, and you can too!

Texas Marine Species
txmarspecies.tamug.edu/fishdetails.cfm?scinameID=Porichthys%20plectrodon

Texas A&M University: Galveston Campus
repositories.tdl.org/tamug-ir/handle/1969.3/19092

World Register of Marine Species
www.marinespecies.org/aphia.php?p=taxdetails&id=158784
www.marinespecies.org/aphia.php?p=taxlist

Fishes of Texas
www.fishesoftexas.org/taxon/porichthys-plectrodon

NOAA (Technical Memorandum)
www.nefsc.noaa.gov/publications/tm/tm155/tm155.pdf

Fish Base
www.fishbase.org/summary/3072

Animalia Life
animalia-life.com/fishes/atlantic-midshipman.html

Live Science
www.livescience.com/27237-fish-sings-for-mates.html

Nature: International Weekly Journal of Science
www.nature.com/news/2005/050711/full/news050711-1.html

Scientific American
blogs.scientificamerican.com/brainwaves/what-singing-fish-reveal-about-speech-and-hearing/

Wikipedia
en.wikipedia.org/wiki/Midshipman_fish