Crabs Color Code to Select Edible Food

Crabs living down in the ocean beyond the reach of sunlight have a color vision combining sensitivity to blue and ultraviolet light. And this detection of shorter wavelength helps them to pick between a good food and the toxic one.

A study that is published in the Journal of Experimental Biology was on how the animals that reside in the deep dark oceans respond to light. It explains how deep sea animals use their eyes and based on their sensitivity to light how they interact with the environment.

"Call it color-coding your food," said Duke Biologist Sönke Johnsen. He explained that the animals might be using their ultraviolet and blue-light sensitivity to "sort out the likely toxic corals they're sitting on, which glow, or bioluminesce, blue-green and green, from the plankton they eat, which glow blue.

 "Sometimes these discoveries can also lead to novel and useful innovations years later," like an X-ray telescope which was based on lobster eyes, said Tamara Frank, a biologist at Nova Southeastern University.

Prior to this Frank had revealed how a group of deep sea animals can see ultraviolet wavelengths despite existing in the dark regions of the oceans. Experiments to test deep-sea creatures' sensitivity to light were done only on animals that live in the water column at these deep depths.

For this study the scientists analyzed three ocean-bottom sites near the Bahamas. They took video and images of the regions, recording how crustaceans ate and the wavelengths of light, or color, at which neighboring animals glowed by bioluminescence. The scientists also captured and examined the eyes of eight crustaceans found at the sites and several other sites on earlier cruises.

Frank attached a microelectrode to each of the crustacean's eyes. She then flashed different colors and intensities of light at the crustaceans and recorded their eye response with the electrode. She noted that all of the species were extremely sensitive to blue light and two of them were extremely sensitive to both blue and ultraviolet light. The two species sensitive to blue and UV light also used two separate light-sensing channels to make the distinction between the different colors.

Johnsen said, "It's the separate channels that would allow the animals to have a form of color vision."

"During a sub dive, a small, digital camera was used to capture one of the first true-color images of the bioluminescence of the coral and plankton at the sites. In this remarkable image, the coral glows greenish, and the plankton, which is blurred because it's drifting by as it hits the coral, glows blue," Frank said.

That "one-in-a-million shot" from the sub "looks a little funky," Johnsen noted.

"But the video shows crabs placidly sitting on a sea pen, and periodically picking something off and putting it in their mouths. That behavior, plus the data showing the crabs' sensitivity to blue and UV light, suggests that they have a basic color code for their food. The idea is still very much in the hypothesis stage, but it's a good idea," Johnsen said.

"Our subs, nets and ROVs greatly disturb the animals, and we're likely mostly getting video footage of stark terror," Johnsen said. "So we're stuck with what I call forensic biology. We collect information about the animals and the environment and then try to piece together the most likely story of what happened."