Fish engage in plant feeding frenzy (video courtesy of R Jeffreys / NIOZ)
Deep sea fish like to eat their greens, gobbling up plants that have sunk thousands of metres to the ocean floor.
Scientists have for the first time captured footage of one of the most abundant species of deep sea fish feeding on plant material.
Though the fish were artificially fed, it demonstrates they have much wider tastes than previously thought.
That tests our ideas of how ocean food webs work and that deep sea fish only predate or scavenge upon other animals.
"Fish may take advantage of terrestrial plant remains and macroalgae," says Dr Rachel Jeffreys of the Royal Netherlands Institute for Sea Research based in Texel.
This is the first time that deep-sea fish have been observed in situ vigorously feeding on plant material
Dr Rachel Jeffreys Royal Netherlands Institute for Sea Research
"This may have implications for the food web."
Details of the fishes plant-eating habits are published in the journal Deep Sea Research Part 1.
Dr Jeffreys and colleagues at the Royal Netherlands Institute for Sea Research and Oceanlab at the University of Aberdeen, in Aberdeenshire, UK, study where deep sea creatures get their nutrition from and how biodiversity in the ocean deep is linked to the abundance of food.
As part of that research, they investigated whether deep sea fish can sense a plant food fall, and if so, how quickly they might be able to find it in the dark.
The idea that deep sea fish may feed on plants that once grew on land isn't new.
Last year, researchers described how one species of deep sea crab feeds on wood that has sunk to the ocean floor.
Also, vegetable and plant remains have been found in the stomachs of some species of deep sea fish.
But their response to plant food falls has never been investigated.
So Dr Jeffreys and her colleagues simulated a plant food fall by dropping spinach bait into the deep North Atlantic sea, 185km off the coast of Portugal.
Attached to a rig containing the bait was a video camera, which recorded any animals that ventured close.
"We were very surprised and excited by the results," says Dr Jeffreys.
Soon after the bait was dropped 3000m underwater, at least three species of deep sea fish, grenadiers (Coryphaenoides armatus and C. mediterraneus) and cusk eels (Spectunculus sp) began to attack it, eating away at the spinach.
"This is the first time that deep-sea fish have been observed in situ vigorously feeding on plant material," says Dr Jeffreys.
The discovery may also prompt a rethink of how ocean food webs work.
"Most research shows that these fish are at the top of the food chain feeding mainly as predators and scavengers, mostly on other animals such as squid, crustaceans and carcasses of dead animals.
"This highlights the variability in their diets and that they are opportunistic generalist feeders," Dr Jeffreys says.
Such plant material could come from algae in the sea, or from land plants, having been blown or washed onto the ocean surface before sinking.
That means it could form an important part of the food chain in deep-sea canyons, near seagrass habitats or in the Caribbean where the seaweed Sargassum has been seen in the deep-sea.
Carbon sink
The research also raises another tantalising possibility.
Much of the plant material that reaches the bottom ocean comes from phytoplankton, blooms of which live near the surface.
Phytoplankton are microscopic plant cells and when they die, they become phytodetritus, falling into the deep-sea, where they are thought to provide food for the majority of deep-sea communities.
A carpet of phytodetritus
Sometimes this material can form thick layers like a carpet on the seafloor.
So if deep dwelling fish regularly eat plant material, as Dr Jeffrey's research suggests they might, that means their fate is closely tied to that of phytoplankton, and events at the sea surface.
"Because these fish are eating spinach could they then possibly feed on phytodetritus and so be affected by changes in phytoplankton communities as a result of climate change?" says Dr Jeffreys.
Coryphaeonoides species are among the most abundant of all deep sea fish, with one estimate suggesting there could 37 billion individuals of one species, C. armatus.
That also means that plant-eating deep sea fish may form a massive, hitherto unrecognised carbon sink, playing a crucial role in how the world's carbon cycle works.
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This is the first time that deep-sea fish have been observed in situ vigorously feeding on plant material
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