What Is the Average Life of a Sea Urchin?
I often get this question and feel obligated to help the young folks understand that when they say "sea urchin" it is no more specific than saying "bird." There are many kinds of sea urchins and they populate very different environments, from the inter-tidal to the deep sea. In fact, if you consider the deep sea, they are (along with brittle stars) one of the most numerous kinds of large animal on the planet. Needless to say, these different urchins can have very different life histories.
Furthermore, when kids say "average" they are really getting themselves into trouble! Most sea urchins are mass spawners, producing several million eggs. If even a tiny percentage are fertilized, you will have thousands of live offspring. And only a small percentage of these survive in their planktonic form without starving or being eaten until they find a place to settle as tiny adult urchins. Of these, only a handful will survive to spawn (and perhaps none). If I were being mathematically and linguistically correct I would have to answer your question, "The average lifespan of an urchin is 10-12 days!"
On the west coast the intertidal sea urchins that kids come in contact with, like purple sea urchins (Strongylocentrotus purpuratus), are thought to be able live as long as thirty years.
—Henrik Kibak