Way down in a spooky part of the ocean called the twilight zone lives the lantern fish.
Aptly named, this flashy fish emits its own light; lantern fish have organs on their bodies, called photophores, that produce a molecule called luciferin that, when combined with oxygen, makes a blue-green light. The same way fireflies glow on land, lantern fish can glow underwater, an effect called bioluminescence.
Lantern fish are mesopelagic, meaning they live between 200 and 1,000 below the water's surface. In this twilight zone, there's very little light — less than one per cent of the light at the water's surface — but not complete blackness.
But why do lantern fish light up? According to Maxime Geoffroy, a fisheries researcher scientist at the Marine Institute in St. John's, the reasons are not fully understood.
The fish might be communicating with each other to signal danger lurking in the depths, he said.
"We know they produce light when they are stressed. So that's one hint that it's probably a reaction to predation or to fear.… The bioluminescence is being used to camouflage so their predators have trouble seeing them," said Geoffroy.
Lantern fish may be tiny — just four to five centimetres in length — but what this deep ocean swimmer lacks in individual size, it makes up for in number.
Lantern fish are the most abundant fish stock in the North Atlantic and all of the world's oceans. There are some 240 species globally. But Geoffroy says it is difficult to estimate their total biomass.
"It's hard ... because they are deep, they are small, they are hard to capture, but it varies between 500 million tonnes to a few gigatonnes," said Geoffroy.
To put that into perspective, he said, the total global fisheries landings vary between 90 million and 97 million tonnes per year.
These fish are plentiful and provide fuel for some of the ocean's top predators.
"They are eaten by sharks, by tuna, by whales, by dolphins, by salmon. So they are very important for the food chain, and we know very little about them," said Geoffroy.
One of the things that scientists do understand about lantern fish is that these little pelagics are powerhouses when it comes to carbon sequestration and their participation in what's known as the biological carbon pump. Avoiding daylight, lantern fish make a nightly vertical migration of hundreds of metres all the way up to the surface to feed on zooplankton that graze on carbon-rich algae.
Lantern fish return to the twilight zone, taking the carbon with them and removing it from contact with the atmosphere for hundreds of years.
"By descending at great depths … they go and eject fecal pellets at depth during the daytime. So they are not just important for the predators, but also for the climate and to help us cope with our increase in CO2," said Geoffroy.
There are only a few places in the world where there is a commercial fishery for lantern fish. The fish contain a lot of wax, so harvesters in the Gulf of Oman and Southern Africa sell their catch for use in the cosmetics industry.
Geoffroy said there are also pilot studies underway in Norway to explore using lantern fish protein as feed for farmed salmon in the aquaculture industry.
Currently, there is no commercial fishery in Canada for lantern fish, but Geoffroy said it could be on the horizon.
"Think about it — it's one of the last stocks that is not exploited and it's probably the most abundant, so it would be really surprising that there would not be projects, at least in the future, to see if it could be exploited." said Geoffroy.
The trouble is, said Geoffroy, without knowing the biomass of lantern fish, a commercial fishery could present risks to the species itself and its predators.
"How do we manage them properly if we're not able to count them properly? If you start tapping into that resource is it a good idea? More research needs to be done."