Unique light patterns bring fish families closer together

The two new species of bioluminescent fish belong to the Mirrorbelly family(Credit: Poulsen et al)

If you want to thrive in the dark, gloomy ocean depths then it certainly doesn't hurt to have a few tricks up your sleeve. Scientists have discovered two new species of fish that can expand an internal organ to shine a light from their bellies, a tool the researchers believe might be used to communicate with brothers and sisters.

Marine creatures that produce light are actually pretty common in the deep sea. Known as bioluminescence, this ability is the result of a chemical reaction inside the animal's body that mixes oxygen with a different versions of molecule called luciferin. It is the same process that enables fireflies to sparkle at night and is used by a range of sea creatures for a number of different purposes.

The spooky anglerfish dangles a bioluminescent lure over its mouth to attract prey, while other marine animals use bioluminescence to create a strong flash and scare away would-be predators. Some glowing fireworms use it to catch the eye of potential mates, and there are even theories suggesting that these off-shore show ponies welcomed Christopher Columbus to the new world more than 500 years ago.

The two new species of bioluminescent fish belong to a group called the Mirrorbellies and inhabit the ocean's twilight zone, an area between depths of 200 and 1,000 m (656 ft and 3,280 ft) that receives very little sunlight. Researchers from the Australian Museum in Sydney came across the new Mirrorbelly species when studying specimens they had collected near American Samoa and New Zealand, discovering differences in mitochondrial DNA and distinct pigment patterns on their bellies.

The team says that the fish gather bacteria growing inside the intestinal tract and use it to produce light with an organ in their belly called a sole. This sole is covered with pigmented scales, so when the organ contracts, the scales conceal the light within. But when the sole is expanded, the light bursts through transparent gaps in the scales.

This allows the fish some control over the amount of light shining through their bellies, something the researchers believe they may use in a couple of ways. Although the twilight zone is indeed dim, some sunlight does makes its way down. By emitting just the right amount of light, the researchers believe the new Mirrorbelly species are using bioluminescence as a form of camouflage, a technique called counter-illumination that is observed in some other marine animals.

The other reason pairs bioluminescence with the distinct pigment patterns on their bellies. Mirrorbellies have tubular eyes that are pointed upwards, while the belly patterns are pointed downwards. The researchers speculate that the fish illuminate their uniquely decorated underbellies so that other, upwards-gazing fish swimming nearby can identify their relatives.

"How do they actually see each other?" says Jan Poulsen from the Australian Museum and lead author of the study. "Possibly with a little dance or repositioning of the body."

Poulsen is now setting her sights on observing these fish in the wild to learn more about how they behave while swimming through the ocean depths.

The research was published in the journal PLOS One.

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