Sharks are one of many marine creatures that can sense electrical fields emanating from other animals, and they use this sixth sense to find prey. Brown-banded bamboo sharks can sense these fields even before they've hatched from their egg cases, researchers report today in PLOS ONE. Scientists hung 11 egg cases containing shark embryos 3 to 4 months old in a water tank in front of a bright light, exposing the developing sharks' silhouettes. Normally, an embryo pulses its gills actively. But when researchers turned on electrodes to produce an electric field near the egg cases, the unhatched sharks froze and stilled their gills for several seconds (see video). The researchers suggest that freezing is a response to the presence of possible predators, such as other sharks. By freezing, the embryo may reduce its heart rate, which in turn may also reduce its own electric field, making it harder for predators to notice it.
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