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Ouch
I wonder if this helps them stay in place in the sea bed.
The shark eggs have spiral ridges on their outer layer which helps them grip better to rock crevices. The mother shark screws the egg into rock crevices and nook which prevents the eggs from getting washed away by waves.
Too interesting not to share
Should be noted that even though some sharks do lay eggs, not all of them lay spiral egg cases. https://en.wikipedia.org/wiki/Egg_case_(Chondrichthyes)