SALLY BENSUSEN / SCIENCE PHOTO LIBRARY SALLY BENSUSEN / SCIENCE PHOTO LIBRARY
Skate egg case. Artwork showing water flow over the egg case of a hedgehog skate (Raja erinacea). The egg case has a tough black outer layer which protects the embryo from predators. It also allows it to breathe a constantly refreshed supply of oxygenated water. Water enters the case through the horns at the front (centre left). The flow speeds up as it passes over the case, and this faster flow (purple) draws water out of the case. This process is augmented by the embryo itself, which inserts a muscular appendage on its tail into one of the horns, and beats it rapidly. This increases the water refreshment rate by 100 times, allowing the embryo to survive even in slow water.
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