Baby Cuttlefish Are Cute, Colorblind Killers
The business end of a cuttlefish is no place a small crustacean wants to be. Cuttlefish are hunters who creep around in camouflage—virtually indistinguishable from a gray patch of gravel or a branching green seaweed—then lash out with their tentacles, turning a passing shrimp into shrimp toast. Oh, and they're colorblind. Despite this apparent handicap, though, learning to hunt doesn't take a lifetime. Baby cuttlefish figure it out almost as soon as they hatch.
"Newly hatched cuttlefish are mini adults," says Anne-Sophie Darmaillacq of the Université de Caen Basse-Normandie in France. They behave similarly to full-grown cuttlefish, that is, and look like toy versions of their parents. Yet those grownups are long gone. "Parents die after the spawning season," Darmaillacq says. Since cuttlefish are born as orphans, they have to be able to look after themselves right away.
For this reason, Darmaillacq and her coauthors wondered how the eyesight of junior cephalopods compares to that of their adult relatives. To find out how quickly a just-hatched cuttlefish's eyes get up to speed, they collected eggs of the cuttlefish Sepia officinalis off the coast of France. (The genus name describes a cuttlefish's brown ink, not its many-colored body.)
Zero to 30 days after hatching in the lab, the tots were tested in a carousel-like device. While a cuttlefish sat stationary at the center, a cylindrical screen with vertical stripes rotated around it at various speeds. Animals that were able to distinguish the stripes spinning by would follow them with their eyes, or by rotating their whole bodies. One set of test screens had black, white and gray stripes. Another had stripes that produced different polarizations of light.
Human eyeballs don't distinguish light polarization, which is when light waves all wiggle in the same orientation as they travel, as after passing through a filter. Bees and some other animals can see this polarization and use it to navigate. Cuttlefish, too, can see light polarization, and scientists are familiar with the architecture in a cuttlefish's retina that allows this. But Darmaillacq says the ability hadn't been studied as much in young cuttlefish.
The tests in the striped carousel showed that cuttlefish who had just hatched were already great at tracking black, white and gray stripes, and got even better over their first 30 days of life. They also started life with some skill at seeing stripes of light polarization, and improved as they aged.
Watching stripes spin is less important than knowing when to pounce on a passing meal, though. In a second set of experiments, the researchers showed young cuttlefish two types of prey trapped inside glass tubes and waited to see which the cuttlefish would attack. One prey was mysid shrimp, which hide by being transparent—but they're much easier to spot if you can see light polarization. The other prey was crabs, which both cuttlefish and humans can see without the help of polarized light.
In a regular glass tube, cuttlefish eagerly attacked all the prey. But in a tube covered with a plastic film that hid light polarization, cuttlefish were more reluctant to attack the shrimp. As they grew older, they got faster at spotting all their victims, but they still didn't like to attack transparent prey unless they could see the polarized light coming off their bodies.
Darmaillacq says newly hatched cuttlefish seem to already have the cognitive skills that make a good hunter, such as learning, attention, and decision making. Her experiments also show that cuttlefish can see light polarization soon after hatching, and that skill helps them find transparent prey and decide when to pounce.
The cuttlefish's colorblindness is a deficit that's almost impossible to believe once you've watched this camouflage master in action. Darmaillacq says the ability to see light polarization may make up for the cuttlefish's missing color vision.
Polarized light helps young cuttlefish spot some of their favorite transparent snacks, which would otherwise be hidden. Additionally, "Wavelengths vary a lot depending on the depth [of the water]," Darmaillacq says. "Light polarization does not." In other words, colors can lie in the ocean, but polarization tells the truth. This means cuttlefish can see well enough that their prey—like the fish in this video from the New England Aquarium—never see them coming.
Cartron, L., Dickel, L., Shashar, N., & Darmaillacq, A. (2013). Maturation of polarization and luminance contrast sensitivities in cuttlefish (Sepia officinalis) Journal of Experimental Biology DOI: 10.1242/jeb.080390
Image: Leonard Clifford (Flickr)
Video: New England Aquarium