They used to be called sea hedgehogs, their scientific name is Strongylocentrotus droebachiensis, but most people just call them green sea urchins.
In Alaska, there are three species of sea urchins I know of: the green sea urchin, the red sea urchin and the white sea urchin. The most common is the green sea urchin, which averages about two inches in diameter, but can grow as big as three inches and is often washed ashore as juveniles of only one inch or less. When you find the little, slightly flattened spheres of urchins on the beach, chances are they are from the green sea urchin. This species lives in temperate waters around the world, in the Pacific and the Atlantic oceans and from shallow tide pools to over 3,000 feet deep. The secret to their success is that they don’t only love to chow down on seaweeds, but will easily switch to eating other organisms if their favorite food is not available.
Less common is the red sea urchin. This species occurs from Alaska to the coast of Baja California in Mexico. It grows bigger than the green sea urchin and can get very old. The large tank at the Fisheries Research Center on Near Island houses a beautiful individual that has been there for nearly 15 years.
The white sea urchin also has a couple specimens in the touch tank. Curiously, Wikipedia does not have much to say about it, except that there are at least four species of sea urchins commonly called white sea urchin. Only one of them occurs here in Alaska.
Sea urchins, along with sand dollars, sea stars and sea cucumbers, belong to the class echinoderms, which translates into “prickly skins.” Inlaid in their skin these animals have platelets or spicules of calcerous material which provides their body structure. It is neither an internal skeleton nor considered an exoskeleton. The little spheres you find on the beach are called “tests” and are the platelets of calcarous materials which stick together when the animal dies. Sometimes, the spines are still attached, but they usually fall off when the test is completely dry.
The spines are really interesting. When you look at them closely, you can see little ridges along their sides. More intriguing is that they sit on the test in a ball-and-socket joint and are moved by tiny muscles. If the tip of a spine breaks off, it is regenerated. If the whole spine is lost, the socket is reabsorbed and the whole assembly gets recreated.
Think about this: if you lost a segment of your finger, you could regrow it. Of course the comparison lags, because you really can’t compare an echinoderm’s regenerative powers to the slow healing process of a severely injured mammal. There are no nerves or blood vessels that penetrate into the spines, so losing them would not cause the sea urchin any pain.
I just learned something new and interesting: Around the spines are three pedicellaria. These are little pincers that keep debris off the sea urchin and even pince intruders. On some tropical sea urchins these pincers can deliver poison. Our green sea urchin also has that capacity, but luckily, its poison is very mild and does not harm a person. If you touch an urchin gently, the spines will move toward your finger. If you push hard, the spines will move away and make room for the pedicellaria to come out and defend against you.
Pedicellaria are not tube feet. Tube feet are the fine, long, tentacle-like structures that stick out and wave around in the water when you watch a sea urchin. Tube feet are the animals’ means of locomotion; it walks on them. It can also attach them to bits of seaweed to cover and hide itself.
Green sea urchins live on rocky shores, preferably in kelp forests. The kelp stalks are their preferred food and if sea urchins are abundant they can literally clearcut and area of kelp. Predators of sea urchins include some fish, birds and sea otters. It is this connection between sea urchins, their appetite for kelp and the role of sea otters in controlling them that determines the diversity of our coastal ecosystems.
Kelp forests provide habitat for many organisms, including the juveniles of some commercial species of rockfish and crab. A single large kelp can house over a hundred small organisms within its holdfast, the intricate system of pseudoroots with which it attaches to the rocks.
Next week I will teach a unit about ocean acidification to a class of middle school students. We will study the effects of a change in seawater acidity on the development of sea urchin larvae. I already know the outcome: Sea urchin larvae cannot build their spines when the acidity in the water is too high. It is believed that this lowers their chance of survival, because it makes them an easier snack for small fish.
We know from history that when the sea otter population declined as a result of intense hunting, sea urchins became more abundant and grazed down much of the kelp habitat around Kodiak. In the future, with increasing ocean acidification we may come to find out the effects of a decrease in sea urchin populations on sea otters and everything else that lives around the kelp forest habitat.
As spring nears, green sea urchins are spawning in the ocean around us and tiny larvae are settling to the ocean floor to transform into small sea urchins and find some kelp nibbles to grow on. As you get out into the first sunny days of spring and summer, look for the little creatures in the tide pools and be amazed by the many ways animals make a living.