Sea Urchin Life Cycle

While the sea urchin we're most familiar with is hard, spikey, and lives along the seafloor, a sea urchin's life actually starts in a completely different form, floating through the water as plankton. Sea urchins reproduce via broadcast spawning: a process in which both males and females release their gametes into the water, essentially relying on ocean currents to bring sperm and eggs together to form new urchin embryos and continue the life cycle. As cells multiply, the single-celled gametes grow into sea urchin larvae, a phase encompassing both the prism and the pluteus stages. Lacking basically all features of adult sea urchins, these young planktonic urchins are pelagic, meaning they live drifting freely in the ocean.

Once further developed plutei sink to the seafloor, they undergo metamorphosis, a process in which the once planktonic urchins completely anatomically transform into juvenile urchins, finally beginning to resemble the iconic spikey, seafloor-dwelling urchins. Post-metamorphosis, sea urchins are no longer pelagic. They are now considered benthic, meaning they live along the seafloor. At this point, they are essentially fully developed, only really maturing from an already benthic juvenile to a larger, sturdier adult form.