Sea urchin eggs have a "jelly" coat around them that serves them in many ways.
- It prevents too many sperm from getting to the egg at the same time, because of its viscosity.
- Proteins in the jelly initiate the acrosome reaction in sperm so they are ready to fertilize the egg.
- It provides a sort of "shock absorber" to prevent injury to the egg from wave action.
It is colorless, so it is difficult to see.