the round roll is the basic expectation for the shape of animal eggs, and there is also a kind of egg that is very grotesque and popular on the Internet: it is simply a natural sea drill.

(photo source: Gary Florin/Cabrillo Marine Aquarium)

this is an egg laid by a Freund's tiger shark (Heterodontus francisci) with an amazing spiral egg sheath. What happens inside the drill is actually similar to other animal eggs: the baby shark's embryo lives inside, takes nutrients from the yolk sac, develops quietly, and then hatches out of the shell. These shark eggs are located in shallow water at a depth of 2-13 meters, and it takes 6-10 months for them to hatch, depending on the water temperature.

(how a little tiger shark breaks its shell from a drill bit, photo source https://boredomfiles.com/)

unlike eggshells, tiger sharks' egg sheaths are made up of keratin, just like our nails and hair. This shell and its rugged shape can provide protection for baby sharks. The drill-like shape helps tiger shark eggs better secure between the cracks in the rocks, preventing them from floating with the current, ensuring a stable hatching environment so that they are not easily found and eaten by predators. In addition to tiger sharks, some other egg-laying cartilaginous fish also have unique horny egg sheaths.

after the shark hatches, this strangely shaped protective shell will complete its mission. Sometimes the empty egg scabbard washes ashore with the waves, and the empty shell is called mermaid\'s purse. It would look cool if the wallet was designed like this.

(an egg scabbard of a tiger shark rushing to the beach, photo source: media.galaxant.com)

but as for exactly how the shape of the drill bit is produced during spawning, it is not clear yet _ (: tiger "∠) _

reference: https://www.sciencefriday.com/articles/picture-of-the-week-horn-shark-egg-case/

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