When you think of sea urchins, I would imagine that you picture a sharp, pointy mass of spines sitting on the bottom of the ocean, patiently waiting to impale your foot. And while you may not be entirely wrong or unjustified in being scared to step on one, our dear urchin friends exist for more than just ruining your day.
Urchins are one of the most important grazers on the reef. As they tip-toe along, munching away, they limit the growth of macroalgae, which helps to maintain the health and abundance of the corals that make up the reef.
And on the flip side, when they are introduced to an ecosystem they are not native to, they can overgraze and over-reproduce, damaging the plant productivity and balance of that system. But how do urchins get to this point? Surely, they are not born as insatiable little seaweed munchers?
Indeed, they are not. Most marine invertebrates undergo multiple stages within their life cycle and urchins are no exception. Many of these larval forms are unrecognizable compared to their adult form and also play a role in the classification of marine invertebrates.
There is even a variation of larval forms among the echinoderms, which include urchins, sea stars, sea cucumbers, and a few others. And while these larvae eventually metamorphose into the adult form that we recognize as an urchin or a seastar, they exist almost entirely as a different animal.
Sea urchins can have two larval forms. The first and most common being the pluteus. If you take your hand, place it palm up, and bring your fingers together, that is vaguely what a pluteus looks like. They are only a few microns tall and difficult to see with the naked eye, though not impossible. Each arm of the pluteus is lined with tiny, hair-like structures called cilia that help propel the pluteus through the water.
Those same cilia can also change direction, redirecting the flow of water, helping to bring tiny food particles towards the mouth. The little pluteus lives and feeds in this planktonic world and eventually metamorphosizes into a little calcareous ball of spines, also known as the sea urchin.
The other larval form is called a shmoo (or schmoo, depending on who you’re talking to). Yes, seriously. A shmoo. It is a blob, and unlike the pluteus, has no skeletal structure, no mouth, no gut. It’s just a blob, floating around.
While the pluteus is planktotrophic (meaning it eats plankton and things that are a similar size), the shmoo is lecithotrophic, surviving off a yolk until it can’t any longer and has to become an adult. Because of this, there is pressure for the shmoo to metamorphose quickly whereas some plutei can remain in their feeding larval form indefinitely.
Whether our dear urchin starts out as a sophisticated pluteus or a free-spirited shmoo, it will still grow into its familiar form (assuming it doesn’t get eaten, of course). And you will still not want to step on it. Everyone’s day would be ruined, especially the urchin’s.
P.S.: Found a Sea Urchins and want to identify it? Check out this guide.
