CUTTLEFISH can see and respond to threats while still in the egg. Even more impressively, they can learn to recognise predators by sight days before they hatch.
鈥淭hey are pretty smart,鈥� says Anne-Sophie Darmaillacq of the University of Caen Normandy in聽France.
Many animals are forced to fend for themselves from the moment they hatch. Some fish and amphibians can sense the outside world while still in the egg聽and start preparing for the conditions they will face. For instance, salamanders exposed to聽the scent of predators while in聽the egg will spend more time sheltering after they hatch.
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Now Darmaillacq and her colleagues have shown that embryos of pharaoh cuttlefish (Sepia pharaonis), which grow in eggs that are nearly transparent, can respond to the sight of predators and to cuttlefish ink.
Male pharaoh cuttlefish die soon after mating and the females die after laying eggs, usually placing them under rocks on the seabed. So for the 24 days or so聽it takes for the eggs to develop,聽the offspring are on their own.聽For the last few days they can see what is happening around them.
鈥淢ale pharaoh cuttlefish die soon after mating and the females die after laying eggs, leaving offspring all alone鈥�
The team put cuttlefish embryos in the final stages of development into small, sealed glass containers. These were then put inside a larger tank. When pufferfish, which eat cuttlefish eggs, were placed in the tank, the breathing rate of the embryos fell.
This is thought to be part of a 鈥渇reeze鈥� response that makes the eggs harder to spot by reducing movement. It also reduces the strength of electric impulses聽from their nerves and muscles that can be sensed by sharks.
The embryos responded in a similar way when exposed to clouds of black ink like those released by adult cuttlefish fleeing predators. But they didn鈥檛 react to clownfish, which don鈥檛 eat cuttlefish eggs.
However, when the embryos were exposed to ink clouds and clownfish at the same time on several occasions, they started responding to the sight of clownfish alone, suggesting聽the embryos had associated the two ().
It is the first time that any invertebrate has been shown to聽be聽able to learn while still an embryo, though we know that some vertebrates can do this.
For example, if frog embryos are exposed to scents they wouldn鈥檛 normally respond to聽at聽the same time as alarm pheromones, they start to respond to the scent alone.
Human fetuses may be unable to see what is going on outside the聽uterus, but they can hear noises during the late stages of pregnancy, and there is some evidence they can remember聽and learn from these sounds.
Cuttlefish are very visual animals, says Darmaillacq, so聽this early-sensing ability makes sense. They are also famed for their colour-changing abilities.聽In future, the team may look to see whether the embryos change colour to camouflage themselves in response to threats.
What isn鈥檛 clear is whether the cuttlefish embryos can tell the difference between fish species based on their appearance, or if they are responding to a more general trait, such as swimming patterns. The pufferfish tried to get into the containers holding the eggs while the clownfish kept their distance, so their behaviour was quite distinctive.
Another team has shown that a聽different species of cuttlefish (Sepia officinalis, pictured above), which normally prefers to eat black crabs, switches preference to white crabs if the embryos see white crabs while still in the egg.
