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Marine Life
The Watcher in the Egg
A new study on cuttlefish sheds light on visual learning.

Cuttlefish embryos.
Ludovic Dickel, University of Caen Basse-Normandy


Crabs and shrimp, beware: However harmless it may look, the cuttlefish embryo inside that egg has got its eye on you. A study recently published in the British journal Animal Behavior shows that common cuttlefish (Sepia officinalis; small, shallow-water animals related to squid and octopi) can learn to recognize prey even before they hatch. It’s the first time any animal has been proven to learn visually while still in the embryonic stage.

A female cuttlefish spawning.
John Neuschwander

The cuttlefish is a science experiment darling because of its large, complex brain, which is the largest of all invertebrates. But what makes it particularly suited to experiments in visual learning has to do with how it grows up. After a female cuttlefish spawns, or lays her eggs, in inky, grapelike clusters on the seafloor, she leaves them to mature on their own for about two months. During that time, the egg sacs grow thinner and clearer, losing their inky camouflage and, within a week or two of hatching, allow the baby cuttlefish to peer out of its protective bubble. By the time it hatches, a cuttlefish is completely prepared for a life of hunting shrimp and evading predators.

Until now, though, it wasn’t quite clear where baby cuttlefish were acquiring their survival skills. Was everything—prey preference, predator recognition, swimming, mating—innate, or was some of it learned? Was the baby cuttlefish, with its little black eyes, passively observing the world through the milky embryo—or was it actually absorbing information?

Cuttlefish eggs.
John Neuschwander

To answer that question, Ludovic Dickel and his colleagues at the University of Caen Basse-Normandy, in France, tried a relatively simple experiment. They divided their cuttlefish embryos into a series of tanks. In some tanks, they placed crabs where the cuttlefish could see them; in others, they placed nothing. When the embryos hatched, something amazing happened: The cuttlefish that had been exposed to crabs showed a distinct preference for eating them, while cuttlefish that had not been exposed preferred shrimp (as most juvenile cuttlefish do). In other words, the crab viewers acted differently because of what they had seen as embryos, before they hatched. And that alteration—a behavior change as a result of experience—is the definition of learning, says Jean Boal, a marine biology professor at the University of Millersville, in Pennsylvania.

“The experience was seeing a crab; the result was that [the cuttlefish] prefers crab over shrimp,” Boal says. According to Boal, the jury’s still out on exactly how the cuttlefish knows that a crab is prey, rather than a predator or neutral object. Since cuttlefish perceive prey mainly through visual recognition (“Even if you present the prey in a test tube, they’ll still hunt it,” says Boal), they may have an innate “idea” of what prey should look like. That idea, Dickel suggests, could be refined as the cuttlefish views its surroundings while growing inside the egg sac.

A late-state cuttlefish embryo.
John Neuschwander

The translucent embryo and lack of parenting make the cuttlefish perfect for experimentation: Its environment can be altered in perceptible ways, and its reactions can be observed. Though the concept of embryonic learning isn’t new—human embryos, for example, can learn the sound of a mother’s voice and the chemical combinations that contribute to food and odor preferences—the findings from Dickel’s experiment shed some light on the development of visual learning skills. Dickel says he hopes to explore how newly hatched cuttlefish learn defensive behaviors (like camouflage) and develop spatial skills. In doing so, he’s chipping away at an ancient debate, says Boal: nature versus nurture.

A cuttlefish.
Ludovic Dickel, University of Caen Basse-Normandy

“People usually talk about nature versus nurture as cut and dried—before birth [or] after birth; before hatching [or] after hatching,” Boal says. But, she adds, the reality is more complicated: “Nature and nurture are all mixed up together, and the environment is influencing development from the moment of conception.” With cuttlefish, for the first time, we can watch as an organism’s own view of its environment influences its development. Of course, the cuttlefish can also watch us.

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