|YPM ICH 8902|
And here is a photo of a bioluminescent midshipman fish specimen:
|Photo taken from this website|
|YPM ICH 8902|
|Image from NOAA|
Another hypothesis is that the photophores function in something known as "counterillumination." This is a form of bioluminescent defense against predators that is used in many types of organisms, including crustaceans and various fishes. Essentially what happens is that an organism using this "counterillumination" will use photophores on its underside to match any dim light coming from the surface of the water. In so doing, they can make a potential shadow disappear and thus camouflage themselves. This hypothesis has actually been tested and supported! It's been showed in laboratory experiments that midshipman fishes can match the intensity and color (among other things) of downwelling light (Harper and Case 1999). This hypothesis also makes intuitive sense because most of the photophores are concentrated on the bottom of the fishes-- where they would need to be for counterillumination.
One last hypothesis is that the glowing photophores function in courtship (Crane 1965). Various authors have expressed skepticism about this hypothesis, however, because there is a population of midshipman fish in Puget Sound, Washington, that has no luminescence capability, yet they still live and reproduce successfully. If the photophores were necessary for courtship, the Puget Sound population shouldn't exist (Warner and Case 1980).
How do midshipman fish get their ability to glow? By eating tiny bioluminescent crustaceans called ostracods. Without ostracods, the midshipman fishes' photophores are useless. This is where midshipman fishes get luciferin, which is a compound that bioluminescent organisms need to emit light. Many organisms produce luciferin on their own, but others, like the midshipman fishes, have to acquire it through their diet. It was actually the Puget Sound population of midshipman fishes that clued scientists in to the fact that midshipman fishes get their luciferin from ostracods. It turns out that the Puget Sound population has no luminescence capability because the there are no bioluminescent ostracods for them to eat there (Warner and Case 1980)! They can easily luminesce, however, if they are fed the proper ostracods.
And here's one last picture of one of our specimens, species Porichthys porosissimus, collected in the Gulf of Mexico off Galveston, Texas, in 1932 by the vessel Mabel Taylor.
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*I should note that Jack is no longer a midshipman, although I think he was when that photo was taken. He graduated from the Naval Academy in 2008 and is now a fully commissioned Navy pilot serving in the Middle East.
Crane, J. M., 1965. Bioluminescent courtship display in the teleost Porichthys notatus. Copeia, 2 : 239-241.
Harper R.D., Case J.F. 1999. Disruptive counterillumination and its anti-predatory value in the plainfish midshipman Porichthys notatus. Mar. Biol. 134: 529–40.
Tsuji F.I., Haneda Y., Lynch III R.V., Sugiyama N. 1971. Luminescence cross-reactions of Porichthys luciferin and theories on the origin of luciferin in some shallow-water fishes. Comp Biochem Physiol 40A: 163-179.
Warner J.A., Case J.F. 1980. The zoogeography and dietary induction of bioluminescence in the midshipman fish, Porichthys notatus. Biol Bull mar biol Lab, Woods Hole 159: 231-246.