This is the smalleye pygmy shark. It may be tiny, but it has a cool trick that few fish have.
You’ve probably noticed that the belly of fishes are lighter than their top sides. The typical explanation for this is countershading. We often see fish in tanks, with light coming in from all directions. But in water, the only source of light is above. This means the underside of the fish is in shadow. Having a lighter underside than top makes the fish blend into it surrounding, making it harder to see.
But the problem is that light varies in intensity as you go up and down in the water. It would be an advantage for a fish to be able to change how bright its underside was.
The smalleye pygmy shark (Squaliolus aliae) pulls this trick off thus:
Its underside lights up.
A decent number of fish are bioluminsecent, but this little shark is a bit different. Some fish, like the flashlight fish, generate a glow using bacteria. To turn the glow on and off, they cover or reveal the patch.
In this shark, the glow is generated by light generating organs called photophores. And the shark controls the brightness more directly. It’s a little bit closer to the way a cephalopod changes colour, but not as fast.
Claes and colleagues have been studying photophores in sharks for a few years now. In this new paper, they looked at how the shark controls the brightness of its photophores. It’s a neat system.
The shark switches its light “on” using a hormone: melatonin. Melatonin is a widespread chemical, and so not surprising that it would be found in these sharks.
The shark switches its light “off” using neural control: the neurotransmitter GABA will turn the light off... eventually. It’s a very slow decline, and GABA never shuts the light off entirely. The authors showed this by bath applying the GABA. Ultimately, I would like to see neural stimulation turning off the glow, but this is a good start.
I was a bit frustrated with the writing here. The story is simple, but the prose is complicated. For instance, the very first sentence starts with an exception:
Except Dalatias licha, a benthopelagic shark that can attain almost 2 m in total length (TL)...
This caveat not so important that it needs to be the first thing on the page.
A nifty finding, though. I can think of cases where a single organ has both endocrine and neural input for fast and slow changes, but I can’t think of another effector where the two opposing effects are controlled by two different organ systems off the top of my head.
Claes J, Ho H, Mallefet J. 2012. Control of luminescence from pygmy shark (Squaliolus aliae) photophores. Journal of Experimental Biology 215(10): 1691-1699. DOI: 10.1242/jeb.066704