Embryonic sharks keep still in eggs to survive
According to research some embryonic sharks have evolved to stay still in their eggs in order to avoid predators.
"Despite being confined to the small space within the egg case, where they are vulnerable to predators, embryonic sharks are able to recognise dangerous stimuli and react with an innate avoidance response," the researchers wrote in the online Journal PloS One.
Adult sharks have been known to be able to detect the electric signals of potential prey - which lead the researchers in the study to hypothesise that this may be useful against potential predators.
While this ability wouldn't be particularly useful to sharks that develop inside their mothers, not all shark species do that. Some, like the bamboo shark, develop outside of their mothers in an egg case, which is generally deposited on or near the substrate, where predators can get them.
The bamboo shark spends about five months in its leathery egg, though because the egg is pigmented to hide it only really becomes a risk near to the point of hatching, when it gets bigger and starts circulating sea water through openings in its shell.
This increases the risk of predators noticing it, predators it cannot actually see.
However "When exposed to predator-simulating sinusoidal electric fields, late stage bamboo shark embryos respond by the cessation of all respiratory gill movements, thereby minimising their own electrosensory and mechanosensory output in order to avoid detection. The cessation of gill movements is immediately followed by a rapid coiling of the tail around the body, with little or no discernible body movement during exposure," the researchers wrote.
Because it can sense the electric field of potential predators, the embryonic shark can stay still and thus avoid getting eaten.