Deep-sea crabs have colour vision

The bioluminescent glow of a deep sea crab

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Deep-sea crabs have a type of colour vision despite living up to 1000m below the surface, say scientists.

Investigating the "murky depths", US-based researchers recorded the glow of tiny bioluminescent species using a submersible vehicle.

The team also studied how crustaceans react to this light, and found previously unknown sensitivities to blue and ultra violet wavelengths.

They suggest crabs use their colour vision to discern edible food.

The research, conducted in the Bahamas, is published in two papers in the Journal of Experimental Biology and was undertaken with Professor Sonke Johnsen from Duke University in North Carolina, US, Dr Tamara Frank from Nova Southeastern University in Florida, US and colleagues.

Deep sea secrets

Submersible vehicle in deep water

Bioluminescent species are known to be common higher in the water column. But because it is much harder to access few studies have been made of the animals living on the sea bed.

"It's very, very hard to get this sort of data," said Prof Johnsen, citing access to a submarine, safely collecting specimens and recording the light sensitivities on a rolling ship as considerable challenges.

Descending to sites between 600 and 1000m down, the scientists observed flashes of bioluminescence where plankton collided with boulders and corals.

But they found that only 20% of the species they collected emitted light; a much smaller percentage than further up in the water column.

The team recorded the different glowing colours of the species they encountered to begin to understand the relationships between organisms living at these depths.

Grab a crab

To test how larger species perceive their environment despite the lack of sunlight, the researchers used a specialist suction arm on the submarine to carefully collect crustaceans living at the sites.

Blue-glowing plankton passes green-glowing coral A 'true' capture of the deep shows blue-glowing plankton blurring as it passes green-glowing coral

Of the eight species studied by the team, all were sensitive to blue light and two also reacted to ultra violet (UV) wavelengths.

"I personally think it's fascinating that there are animals that see UV in one of the most UV-poor habitats on the planet," said Prof Johnsen.

According to the marine expert, the species with the ability to detect two channels of colour could be using this to tell the difference between the green-glowing, often toxic, corals they live on and the blue-hued plankton they eat.

"The idea that they may be using it to colour-code their food is exciting, but of course still in the hypothesis stage," he said.

Part of the deep-sea mission's original aim was to confirm whether the sea floor was carpeted in a glowing mat where sunken debris from higher up in the water column was decomposed by bioluminescent bacteria.

Deep sea shrimp with bioluminescent plankton The natural blue of the plankton can be seen here surrounding a shrimp that appears red from the submersible's lights

But conditions in the largely unexplored depths denied the scientists any evidence to support the theory.

"The currents were too high on the bottom to allow the pile up of any mats, so this is something that has to be left for future expeditions," Dr Frank told BBC Nature.

"Wouldn't that be something if the ocean floor was covered with mats of glowing corpses and poo?"

She also theorised that the crabs' slow sight could be explained by the presence of glowing patches.

"I was surprised at how slow the isopod eyes are," she said, "By slow, I mean that... they basically operate like a camera with the shutter held open for such a long time that anything moving is blurred."

"Since the [crustaceans] are scavengers, known to specialize on decaying dead stuff, I thought that maybe this very slow shutter speed might give them the sensitivity to see these dim glowing mats."

Crabs on deep sea corals The crabs live on corals that can be toxic

Prof Johnsen agreed that further exploration of the depths is needed to appreciate the complex systems at work.

"The sea floor is three quarters of the earth's area and the water column is over 99% of the earth's liveable space, yet we know less about it than the surface of the moon," he said.

"I think people will only protect what they love, and they'll only love what they know. So part of our job is to show people what's down there."

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