Sea Urchin

Hungry Grazers of the Bull Kelp Forest

Sea urchins are part of all of the world’s kelp forests. There are around 950 species of spiny sea urchin in the class Echinoidea and they can be found in ocean waters from the edge of the tideline through the sub-tidal kelp forest zone and even deeper still. They are known as echinoderms, a phylum shared by sand dollar, star fish and thousands of other species. If you compare all three—urchin, sanddollar and sea star—you will notice their shared five-fold symmetry. The sea urchin’s beautiful, domed structure is made of calcium carbonate, but it is not a shell; it is known as the test. A sea urchin’s mouth is comprised of five radial, bony teeth in its underside that can scrape and nibble. This radially symmetrical jaw (made of calcium carbonate like the test) is called an Aristotle’s Lantern and what it likes to nibble is kelp and seaweed. Sea urchins are herbivores, like cows, but they graze, munching the primary producers of the oceans, the algae, or kelp and seaweed, as well as the residue of dead invertebrates and fish. But unlike cows, in a healthy, balanced kelp forest the urchins don’t have to move very much. 

What is a Detritivore?

Bull kelp and its understory seaweeds are constantly sloughing off at their margins, the currents and constant forces of the ocean waters pulling the degenerating tissue from the edges of the kelp even as new healthy tissue is created at the meristem (point of growth). This algal detritus floats in the water column making it murky and hard to see through–“low vis” as divers say. But these murky waters are indicators of a healthy ocean, rich in food material floating along for urchins and other “detritivores” to profit from. Urchins typically live healthy existences lodged in one place, often a niche or alcove created in the rock, eating the detritus that floats by. But urchins are voracious. They eat a lot. When a kelp forest is in balance, with ample algal material floating around, urchins have plentiful innards, or gonads (what we know as uni, the golden stuff eaten as sushi), which provide a good source of protein for their predators, the sea otter and the Pycnopodia sunflower sea star. These predators keep kelp forests in balance. If systems shift and there is not enough floating algal material, the urchin can get up and move on their suctioned tube feet that emerge between their spines, and go on the march looking for healthy kelp. This can happen because there is less kelp and seaweed around (think warming ocean), or as happened when the sunflower sea star was eradicated by disease, because there are more urchins relative to kelp abundance. 

Another key predator of the sea urchin are humans. Red urchin grow old and large—up to 200 years old—and the uni they produce hold great value to fishers. Their populations are kept in check by the market. The smaller purple urchin, however, has never had market value. Their lack of value as a commercial fishery, and in the absence of other key predators (such as sunflower sea star or sea otter) in the bull kelp forests of California’s North Coast, has allowed their populations to explode. 

Urchin Reproduction

Sea urchins are broadcast spawners. Urchins take the opposite approach to evolutionary success (r-selection) than sea otter or humans, who produce singular offspring that are cared for diligently over time so they may survive (K-selection). Urchins produce millions of egg or billions of sperm (it is impossible to tell male from female) that–cued by a full moon or before an impending storm–are ejected simultaneously into the water column through the holes in the test at the top of the dome. This creates a milky mist in the water column where a few of the millions of egg and sperm combine to fertilize, creating thousands of embryos that develop within a day or so into the larval stage of the sea urchin, the pluteus. These urchin larvae have a tiny triangulated skeletal form that looks like a miniature lunar landing module. They are part of the multitude of plankton adrift in the currents of our oceans, a favorite food for fish, shrimp and crustaceans. Within a couple of weeks, a tiny urchin grows inside the pluteus which eventually digests the outer layer that has been nurturing it, leaving the baby urchin to settle on the ocean floor, attracted to rock encrusted with pink coralline algae, and begin life as the spiky sea urchin we recognize. While only a tiny fraction of one percent of the initial spawn makes it to become an actual sea urchin, sea urchins are one of the most successful creatures on earth. The extensive fossil record for sea urchins goes back to about 450 million years. While the planktonic drifting phase is full of risks, it ensures a broad distribution of the young urchins that do survive. Once established, a sea urchin can live a very long time. Some of the big red sea urchins found along the northern California coast are thought to be more than 200 years old. If not attacked by predators (including us) or decimated by disease, an urchin can persist….forever?

This long-lived urchin poses an amazement to those of us thinking deeply about its relationship to the kelp forests. Urchins are predominantly algae eaters, but if there are no algae, no kelp, to eat, they do not die of starvation and open up space for new kelp to grow; they hunker in and lower their metabolism and wait. They use all the reserves stored in their gonads (the orange substance that tastes like the ocean, full of that umami flavor so desired in the culinary scene) until there is no gonad left, and then they continue to exist as a marginal creature, an empty, domed and spiky husk that is, nevertheless, not dead. A zombie perhaps. It might scrape coralline algae off the rocks and get a modicum of nutrition from that. But it can gain just enough nutrients from the surrounding ocean to exist in this zombie state. Expanses of these zombies are known as an urchin barrens. A tiny scrap of algae floating by, or a young bull kelp trying to grow in spring time will rejuvenate the urchin, and they will persist unless they are physically removed from a site. Regions of lush and historic bull kelp beds, have been decimated completely by these hordes of urchins. The young bull kelp reaching for the surface in spring along the Northern California coast are eaten by the predator-free purple urchin before they can reach maturity and reproduce.