For humans, sight is the most important of senses but only after we are born. Within the womb, surrounded by fluid, muscle and darkness, vision is of limited use and our eyes remain closed. But not all animals are similarly kept in the dark.
Cuttlefish develop inside eggs that are initially stained black with ink, but as the embryo grows and the egg swells, the outer layer slowly becomes transparent. By this time, the developing cuttlefish's eyes are fully formed and we now know that even before they are born, they can use visual information from the outside world to shape their adult behaviour.
Cuttlefish, and their relatives the squid and octopuses, rely on vision as strongly as we do. Their ability to change colour, shape and pattern is the basis for attack, defence and a wondrous system of communication. Right from the moment of hatching, sight is a crucial part of a cuttlefish's life, for they receive no care from their parents and have to find food solo. Now, Anne-Sophie Darmaillacq from the Université de Caen Basse-Normandie has found that they can use information gleaned from inside the egg to help them.
Darmaillacq suspended eggs laid by a captive female in a shallow tank. The eggs sat in the middle of two compartments, which has glass sides and opaque plastic floors. In some cases, the compartments were empty and in others, they contained crabs. The embryos could see what was in the compartments, but as they hatched and sank to the bottom of the tank, their views were obscured. The hatchlings were collected and after a week of hunger, Darmaillacq gave them a choice of either crab or sand shrimp.
She found that youngsters that had been treated to the sight of crabs as embryos preferred them once they had hatched. If their eggs had been suspended between compartments filled with crabs, the youngsters showed a clear preference for them, with 71% of juveniles choosing crab from the menu. In contrast, just 10% of cuttlefish whose eggs lay between empty compartments picked crabs over shrimp.
This is the first instance of embryonic visual learning in any animal. Other studies have found that the foetuses of other species, from humans to dogs to birds, can learn from chemical cues that they detect while in the womb. But the cuttlefish were clearly learning from sight, for the embryos couldn't possibly have smelled any chemical cues from the crabs, which were completely encased in plastic.
The ability to see and learn about potential prey items that wander by their eggs may be important for young cuttlefish. Female cuttlefish generally lay their eggs in shallow water, and Darmaillacq speculates that they may choose sites where hatchlings can easily find potential prey. Being able to learn the visual characteristics of the local menu would be useful to them in their search.
Reference: DARMAILLACQ, A., LESIMPLE, C., DICKEL, L. (2008). Embryonic visual learning in the cuttlefish, Sepia officinalis. Animal Behaviour, 76(1), 131-134. DOI: 10.1016/j.anbehav.2008.02.006
Image: by Diliff