The deep sea’s twilight zone – between 200 and 1,000 meters, where sunlight barely reaches – is home to two new species of bioluminescent fish that use their bellies to control light emissions, according to new research published today in PLOS ONE.
Called mirrorbellies, these fish produce light with bacteria that grow in a pouch in their intestinal tract. The light-controlling organ on their underside, called a “sole,” is covered with pigmented scales; when the sole contracts, no light shines through, and when the sole expands, light shines through the transparent parts of the scales.
“Deep sea fishes are some of the most extraordinary creatures on our amazing planet,” says Jan Poulsen from the Australian Museum, Sydney, who has been fascinated with fish since childhood.
She and her colleagues compared sole pigment patterns and mitochondrial genomes of four specimens of a mirrorbelly that were caught on recent research cruises near American Samoa and New Zealand with long-preserved specimens caught near Australia and the mid-Atlantic ridge.
The researchers noticed three pigment patterns on the bellies of the fishes studied: a black streak down the middle, grey near the head and dark near the tail, and four dark spots. This suggested the fishes were three distinct species rather than one, and differences in mitochondrial DNA supported this conclusion. Interestingly, the two new species – the black mirrorbelly and the grey mirrorbelly – were found only in the Pacific while those of the original species were found only in the Atlantic.
Why do mirrorbellies control the light they emit? One reason is that, living in the deep-sea twilight zone, these fish would otherwise stand out against the dim sunlight that filters down. Counter-illumination lets them blend in with the sunlight, providing camouflage in a realm where there is no place to hide.
And why do the three species have unique pigment patterns? The researchers speculate the answer is communication within each species. Curiously, mirrorbellies have tubular eyes that point upwards, while the pigment patterns are obviously downwards. “How do they actually see each other?” Poulsen wonders. “Possibly with a little dance or repositioning of the body.”
Next, she would like to see these fishes for herself in the wild. “Real live observations would be incredible,” Poulsen says. “When you have worked with these creatures, it is very hard not to become partly obsessed with what is down there.”
Research Article: Poulsen JY, Sado T, Hahn C, Byrkjedal I, Moku M, Miya M (2016) Preservation Obscures Pelagic Deep-Sea Fish Diversity: Doubling the Number of Sole-Bearing Opisthoproctids and Resurrection of the Genus Monacoa (Opisthoproctidae, Argentiniformes). PLoS ONE 11(8): e0159762. doi:10.1371/journal.pone.0159762
Image Credit: Poulsen et al. (2016)